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Help! My valve amp just blew! Bewitch KT-88 / Puresound A30


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Hi SNA

 

My Bewitch KT-88 / Puresound A30 is hurting......hurting real bad.

 

This morning I turned it on with the radio and after about 3 minutes, the left channel started crackling and distorting.  I bolted out of the kitchen to save it but I was too late.  The two large valves were sparking and smoking and then POP!

 

I think maybe I did not let it warm up enough before turning the volume up.  It was not super loud - just at about the 10 o'clock position.  But maybe as I am new to valves I did not know to let it warm up.

 

So, can I please get some advice on valve amp specialists in Melbourne, and also a source for replacement valves?

 

Thanks

David

 

 

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Hi Fi Exchange is good - their repairer is top notch with valve amps & they have plenty of spare tubes for sale.

You have to be patient though as very busy or you could try Trevor Lees in Kew as I think he works from there as well.

Hi Fi Exchange

edit: doesn't sound like your fault but ouch I feel your pain!

Edited by evil c
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I feel for you, well actually I share your pain.  I'm going through the angst as well with a main transformer HT failure.  Luckily I have sourced a replacement transformer.

 

I think I can only blame old age, it is 60 years old!

 

Good luck with your repairs.

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Lets hope the OPTs are OK, and only a few passives are toast apart from the tubes (resistors?), but what was the cause is the question the tech' will need to work out.

 

I don't know of anyone down that way apart from Earle, and he is a long way out from there I believe.

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Here's a direct link to Lukaz's lampizator website which already includes his posted pictures of internal shots and corrected schematics. I've repaired, rebuilt two of these (one with better parts which kept going on fire and the other was to test different biasing methods as it's using an electrolytic to fix the bias - not ideal) and so far the 300B and KT88 Bewitches I've come across, heard and seen power transformers not within spec blowing up with open windings after any of the tubes short to ground or go arcy sparky. 

 

First thing I would do is pop off the bottom panel and see what the damage is, observation is key here, could be anything. Could also be a faulty or failing tube that just gave up.

Edited by DefQon
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Sorry to hear about this.

 

Any photos of the carnage?  Are the tube sockets toasted / melted ?

 

Really doubt the tube sockets would have seen any damage, we are talking about the cheapest ceramic tube sockets straight out of China, they sure wouldn't be using CNC teflon sockets in this amp at this price. 

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Opened her up.  Here's the damage:

 

post-109434-0-85881500-1426330028_thumb.

 

1. Wire round resistor - messed up.

2. Wire round resistor - shattered.  This looks to be the centre of the meltdown.

3. Resistor - messed up.

4. Charred board, side effect of # 2.

5. Bulging capacitor.

 

What is the likelihood of upstream / downstream impacts?
 

I've no idea if the tubes survived.  I'm guessing the way they were sparking they are probably shot.

 

The sockets look fine.

 

Off to the repair shop......

 

:(

 

Thanks for the feedback and advice.

 

BTW DefQon - the link was not attached as you've suggested.  I'll google it anyway.

 

David

Edited by Prana69
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Ops forgot to post link:

 

http://lampizator.eu/AMPLIFIERS/CHINA/bewitch%206550/Bewitch.html

 

Thought so. Only two reasons why a tube would fail, failed resistors around the tube area (for many reasons too) or a faulty tube. 

 

The damage doesn't seem that bad, just simply replace parts with same value but higher wattage rated and that's it. 

 

When I did my rebuilds, the stacked el cheapo ceramic 5watt 750ohm resistors (the white ones that have imploded in your picture but your values look to be 560 ohms?) should always be a minimum 15watt chassis mount resistor as these get very hot. If you're replacing it with wirewound or ceramic avoid Mills resistors, they are craptastic with there ratings and fail earlier then the spec'd power heat dissipation factor.

 

Briefly looking at the schematics, the burnt 2watt resistor you marked 3 should be 2.2kohm, cap 5 is a 220uf/50-63v rated low ESR cap which is part the fixed bias circuit, replace these with 63v rated 220uf non audiophile caps, i.e no Nichicon muse or Elna Silmic's as they will be useless here just go for some Panasonic NHG's or Rubycon ZL's (both for there ripple ESL and ESR characteristics). 

 

Depending on where you are located I may be able to fix/test the amp for you free of charge and I've had experience rebuilding these buggers to "proper spec" and not the weird circuit schematics the amp designer made manufactured these amp's on. 

Edited by DefQon
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Hi David.

look like cathode resistor have fail . 5 watts resistor is too small for the cathode of the KT88 in the first place . so issue look like around the output stage only

good luck getting it fix

Duc

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Speaking of failed cathode resistor for the input tubes, what tubes are you running in the amp?

 

I was refer to the output valves cathode resistors have failed  and what i can see is the two parallel resistors to get the value they want to bias the output valves  . even two in parallel is still too small wattage to bias the two KT88 or 6550 . also there is other small resistor that fail which i can read but it might be in the output valves grid leak . if it haven't took the output transformer with it then this seem quite easy fix and won't cost an earth .

Duc

Edited by lovetube
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Hi David.

look like cathode resistor have fail . 5 watts resistor is too small for the cathode of the KT88 in the first place . so issue look like around the output stage only

good luck getting it fix

Duc

 

I noticed that the resistors in my KT-88 are "560" whereas in Lampizator's they are "750".  I assumed that was just due to AU v EU voltage differences.

 

 

Depending on where you are located I may be able to fix/test the amp for you free of charge and I've had experience rebuilding these buggers to "proper spec" and not the weird circuit schematics the amp designer made manufactured these amp's on. 

I am located in Mordialloc.  I love this amp and am prepared to travel to get it fixed, and properly set-up.  I am happy to pay a fair price to have it fixed.  I don;t expect anyone to work for free.

 

 

Speaking of failed cathode resistor for the input tubes, what tubes are you running in the amp?

 

I bought this from another SNA member a couple of months back. I believe he upgraded the tubes.  Here is the link to the advert which identifies the tubes:

 

http://www.stereo.net.au/forums/index.php?/topic/77157-bewitch-kt-88-integrated-valve-amplifier/?hl=bewitch

 

And just let me say, the seller was brilliant and I do not in any way want to imply that he sold me a dodgy unit. It has worked flawlessly from when I got it and he was very helpful in the initial set up.

 

Thanks

David

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Opened her up.  Here's the damage:

 

attachicon.gifBewitch KT-88 Damage.JPG

 

1. Wire round resistor - messed up.

2. Wire round resistor - shattered.  This looks to be the centre of the meltdown.

3. Resistor - messed up.

4. Charred board, side effect of # 2.

5. Bulging capacitor.

 

What is the likelihood of upstream / downstream impacts?

 

I've no idea if the tubes survived.  I'm guessing the way they were sparking they are probably shot.

 

The sockets look fine.

 

Off to the repair shop......

 

:(

 

Thanks for the feedback and advice.

 

BTW DefQon - the link was not attached as you've suggested.  I'll google it anyway.

 

David

 

Here's how I see it.

 

The output tube(s) has failed and become partial internal short circuit between the plates. (the sparks)

This then has created a direct path from HT supply voltage through the output transformer, partially shorted output tube, cathode bias resistor(s) to ground.

The cathode bias resistor has failed due to excessive voltage/current (overheated then fused). As HT voltage was present across the cathode resistor, the bypass capacitor has also failed (bulging cap). Also exessive current was drawn through screen grid two burning out the 100 ohm resistor near the switch (arrow 3).

 

Bin all the output tubes and replace them with Russian built KT88/6550 (Tungsol 6550 are very good). All KT88 tubes out of China are rubbish and have a 30% chance of failure. (Edit. The early Russian built Gold Lion KT88 had a few issues too)

 

There is a chance that the output transformer may have been damaged. It needs to be checked and measured. The good news is it was still conducting when the resistor fused, but it may have developed shorts between the windings due to insulation meltdown. It needs to be measured for resistance at high voltage. (using a megger), impedance, and frequency response.

 

Mods that need to be done. Whilst it's okay for 6V6, EL84 and EL34 tubes to use a common cathode bias resistor, this should never be done with KT66 or KT88 type tubes. (Yes, it is a design fault)

The circuit needs to be modified so each output tube has it's own resistor and bypass capacitor. 560 ohm 10 watt to 25 watt with 100uf to 220uf 100V bypass cap for each will do.

 

The power transformer needs to be checked as well as the rectifier tubes. They too were stressed during the meltdown.

 

I'm happy to do the work, but must stress that I'm super busy and will take at least 3 to 4 weeks to get around to repairing it.

 

Cheers, Earle.

Edited by ehtcom
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I noticed that the resistors in my KT-88 are "560" whereas in Lampizator's they are "750".  I assumed that was just due to AU v EU voltage differences.

 

 

I am located in Mordialloc.  I love this amp and am prepared to travel to get it fixed, and properly set-up.  I am happy to pay a fair price to have it fixed.  I don;t expect anyone to work for free.

 

I bought this from another SNA member a couple of months back. I believe he upgraded the tubes.  Here is the link to the advert which identifies the tubes:

 

http://www.stereo.net.au/forums/index.php?/topic/77157-bewitch-kt-88-integrated-valve-amplifier/?hl=bewitch

 

 

My last unit was 750ohms same as Lukaz's one, so it's kind of odd seeing your's 560ohm's but the original spec on another schematic I've seen says 390ohm's.  

 

Mordialloc is very far from where I'am, Earle might be closer to you then I'am to you and he has the equipment and full inventory to do further testing so that's an option.

 

 

Here's how I see it.

 

The output tube(s) has failed and become partial internal short circuit between the plates. (the sparks)

This then has created a direct path from HT supply voltage through the output transformer, partially shorted output tube, cathode bias resistor(s) to ground.

The cathode bias resistor has failed due to excessive voltage/current (overheated then fused). As HT voltage was present across the cathode resistor, the bypass capacitor has also failed (bulging cap). Also exessive current was drawn through screen grid two burning out the 100 ohm resistor near the switch (arrow 3).

 

Bin all the output tubes and replace them with Russian built KT88/6550 (Tungsol 6550 are very good). All KT88 tubes out of China are rubbish and have a 30% chance of failure. (Edit. The early Russian built Gold Lion KT88 had a few issues too)

 

There is a chance that the output transformer may have been damaged. It needs to be checked and measured. The good news is it was still conducting when the resistor fused, but it may have developed shorts between the windings due to insulation meltdown. It needs to be measured for resistance at high voltage. (using a megger), impedance, and frequency response.

 

Mods that need to be done. Whilst it's okay for 6V6, EL84 and EL34 tubes to use a common cathode bias resistor, this should never be done with KT66 or KT88 type tubes. (Yes, it is a design fault)

The circuit needs to be modified so each output tube has it's own resistor and bypass capacitor. 560 ohm 10 watt to 25 watt with 100uf to 220uf 100V bypass cap for each will do.

 

 

I've attached a partially edited schematics to show damage. I'll correct myself with my original post saying burnt resistor 3 from the image posted should not be 2.2kohm's which is connected in series with another same value resistor between the two input 6n9p tubes, (goes to show how much you pay attention to things when you make posts late at night). Resistor 3 is indeed 100ohms and connected to the output tubes 6550/KT88's screen grid. I may have missed out some other stuff.

 

Funny thing you bring up the modern Russian reissue tubes being sturdy that were used throughout this amp which has failed. In my experience the modern and as you pointed out as well the early GL reissues are rubbish and have high failure rates, no where near the quality of CCCP and Russian NOS let alone from other common tube brands (RCA, Sylvania, Mullards, Amperex etc). Not all Chinese tubes are rubbish, the newer grade A Psvane's are very good for the money they go for and are spectacular in the sound department, there are some issues with some of them but not all. The stock Chinese tubes that come free with amp's are not anything special but are good as baseline to compare to other tubes when you tube roll, they can still take a fair bit of abuse too.

post-140825-0-30449700-1426386388_thumb.

Edited by DefQon
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Here's how I see it.

 

The output tube(s) has failed and become partial internal short circuit between the plates. (the sparks)

This then has created a direct path from HT supply voltage through the output transformer, partially shorted output tube, cathode bias resistor(s) to ground.

The cathode bias resistor has failed due to excessive voltage/current (overheated then fused). As HT voltage was present across the cathode resistor, the bypass capacitor has also failed (bulging cap). Also exessive current was drawn through screen grid two burning out the 100 ohm resistor near the switch (arrow 3).

 

Bin all the output tubes and replace them with Russian built KT88/6550 (Tungsol 6550 are very good). All KT88 tubes out of China are rubbish and have a 30% chance of failure. (Edit. The early Russian built Gold Lion KT88 had a few issues too)

 

There is a chance that the output transformer may have been damaged. It needs to be checked and measured. The good news is it was still conducting when the resistor fused, but it may have developed shorts between the windings due to insulation meltdown. It needs to be measured for resistance at high voltage. (using a megger), impedance, and frequency response.

 

Mods that need to be done. Whilst it's okay for 6V6, EL84 and EL34 tubes to use a common cathode bias resistor, this should never be done with KT66 or KT88 type tubes. (Yes, it is a design fault)

The circuit needs to be modified so each output tube has it's own resistor and bypass capacitor. 560 ohm 10 watt to 25 watt with 100uf to 220uf 100V bypass cap for each will do.

 

The power transformer needs to be checked as well as the rectifier tubes. They too were stressed during the meltdown.

 

I'm happy to do the work, but must stress that I'm super busy and will take at least 3 to 4 weeks to get around to repairing it.

 

Cheers, Earle.

 

Hi Earle

 

Thanks for the offer.  Given this amp really needs a major over-haul, I will definitely be coming to see you.  3-4 week turnaround is fine.  I will PM you to get your shop details.  I'm in Mordialloc so a cruise down the P link will be easy.

 

Thanks everyone for your feedback, troubleshooting and insights.

 

David

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Must say, I have never seen a single resistor shared for cathode bias use :blink:  Maybe In just don't look hard enough :unsure:

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  • 3 years later...

Just read your story (April 2nd, 2018) from 2015 about your Puresound A30 going Bang !. Well, I had the very very same problem in 2017. I was fortunate enough to email and Speak over the phone to the proprietor of Puresound Mr Guy Sergeant. 

My situation started when I could hear crackling through the speakers, Guy said that this was down to the Cathode resistors going Noisy. He asked me what 6550 tubes I was running. I purchased the amp secondhand with Tung sols. He told me that the original design used Electro Harmonix and from his experience, the Tung Sols took more current. Anyway, a couple of days later one of the 6550s starts glowing cherry red. I switch the mains off and isolated the thing. I used to work for the GEC many years ago working on valve amplifier public address systems. Anyway, Guy told me what voltage should be across the 250-ohm resistor. (35 to 38 volts) anyway, I measured mine and it was 38 but the Resistor was only 8 watts and was running really hot and at more than 50%. The bypass capacitor had failed and it was only rated at 50 volts. So, I replaced the resistors with a 250 ohm 50 watt, which I fitted on stilts to allow cooling, and I replaced the capacitor with a 100-volt type. Whilst I had the thing open I fitted the triode / UL switches which were not fitted on the earlier Amps. In addition I fitted inline fuses to the 6550 B+ power rails just in case an Tube decided to go short and take the transformer with it !. Guy is a really helpful man and very pleasant. He told me that his Amplifier is made in the same Factory as the Bewitch class AB push-pull but has different transformers and biasing. Just don't mess inside these things if you're not experienced with High voltages and capacitors needing to be bled to prevent your Death !. I have two of these Amps, one running 4 Electro Harmonix 6550, and the other using clear topped Svetlana. Primary tubes being all Raytheon, RCA, or Phillips JAN military

IMG_1704 (1).JPG

Edited by Bryan Powell
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Got mine second hand as my first tube amp without knowing what I was buying. 

 

Few months later I got a couple of Krunch noises.

 

Thanks to this thread I've sent it to Earle Weston to kindly fix for me. 

 

Basically it will then be an English design with reliability by Earle.  Was sounding pretty good with vintage US tubes and KT88s.

Edited by eman
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  • 2 years later...

A very interesting old-ish thread.

I'm the very happy new owner of this same BEWITCH 6550 amp.

I'm running 6550EH tubes and it sounds great!  Nice tone, great imaging and low fatigue listening.

One new improvement - to my ears - is that I am no running all 4 x driver tubes as 6N8P (6SN7 equiv.) rather than the original 1 x 6N9P (6SL7 equiv.) + 1 x 6N8P per side.  Yes, there is less overall Gain - which suits us - but a meaningful increase in sonic openness and palpability.  I welcome expert opinions on this simple tube reconfiguration.

 

Sadly - shipping an amp of this weight to AU for repairs is a bit prohibitive, so its out with the soldering iron...  With this in mind, I would also like to ask if Earl (or any of the other gurus) would be willing to share some of the  before and after tweaks and improvements to this amp, that were performed for the OP.  I'm also very interested in upping some resistor values, in the interests of safety.

I'd also like to be able to tweak the circuit to be able to run KT120 tubes, but am aware that this will probably require the introduction of a manual bias system - hopefully not too difficult.

I know that this is a very complex (evidently, awarded a 1950s French patent!) little circuit and that even messing with the Negative Feedback has an impact on the driver balance.

I welcome more conversation around these aspects of the BEWITCH 6550 amp.  Appreciated.

Edited by SONDEKNZ
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14 minutes ago, SONDEKNZ said:

A very interesting old-ish thread.

I'm the very happy new owner of this same BEWITCH 6550 amp.

I'm running 6550EH tubes and it sounds great!  Nice tone, great imaging and low fatigue listening.

One new improvement - to my ears - is that I am no running all 4 x driver tubes as 6N8P (6SN7 equiv.) rather than the original 1 x 6N9P (6SL7 equiv.) + 1 x 6N8P per side.  Yes, there is less overall Gain - which suits us - but a meaningful increase in sonic openness and palpability.  I welcome expert opinions on this simple tube reconfiguration.

 

Sadly - shipping an amp of this weight to AU for repairs is a bit prohibitive, so its out with the soldering iron...  With this in mind, I would also like to ask if Earl (or any of the other gurus) would be willing to share some of the  before and after tweaks and improvements to this amp, that were performed for the OP.  I'm also very interested in upping some resistor values, in the interests of safety.

I'd also like to be able to tweak the circuit to be able to run KT120 tubes, but am aware that this will probably require the introduction of a manual bias system - hopefully not too difficult.

I know that this is a very complex (evidently, awarded a 1050s French patent!) little circuit and that even messing with the Negative Feedback has an impact on the driver balance.

I welcome more conversation around these aspects of the BEWITCH 6550 amp.  Appreciated.

There must be a tube amp guru somewhere in NZ.

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The few that remain are booked months ahead with PRO-AUDIO / GUITAR AMP repairs and dealer warranty work.

 

You can't blame them at all, as it's their bread and butter - but it just means that us tube-loving audiofools have needed to get a bit more hands-on...

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  • 5 months later...
On 15/03/2015 at 12:02 PM, ehtcom said:

 

Here's how I see it.

 

The output tube(s) has failed and become partial internal short circuit between the plates. (the sparks)

This then has created a direct path from HT supply voltage through the output transformer, partially shorted output tube, cathode bias resistor(s) to ground.

The cathode bias resistor has failed due to excessive voltage/current (overheated then fused). As HT voltage was present across the cathode resistor, the bypass capacitor has also failed (bulging cap). Also excessive current was drawn through screen grid two burning out the 100 ohm resistor near the switch (arrow 3).

 

Bin all the output tubes and replace them with Russian built KT88/6550 (Tungsol 6550 are very good). All KT88 tubes out of China are rubbish and have a 30% chance of failure. (Edit. The early Russian built Gold Lion KT88 had a few issues too)

 

There is a chance that the output transformer may have been damaged. It needs to be checked and measured. The good news is it was still conducting when the resistor fused, but it may have developed shorts between the windings due to insulation meltdown. It needs to be measured for resistance at high voltage. (using a megger), impedance, and frequency response.

 

Mods that need to be done. Whilst it's okay for 6V6, EL84 and EL34 tubes to use a common cathode bias resistor, this should never be done with KT66 or KT88 type tubes. (Yes, it is a design fault)

The circuit needs to be modified so each output tube has it's own resistor and bypass capacitor. 560 ohm 10 watt to 25 watt with 100uf to 220uf 100V bypass cap for each will do.

 

The power transformer needs to be checked as well as the rectifier tubes. They too were stressed during the meltdown.

 

I'm happy to do the work, but must stress that I'm super busy and will take at least 3 to 4 weeks to get around to repairing it.

 

Cheers, Earle.


@ehtcom and others.

Since my last post, I have returned our new BEWITCH 6550 integrated amp precisely to its original design.

Despite some judicious parts replacement - better coupling caps, etc. - the amp is now fully configured as per the schematic.  Sounding better than ever for it too!

 

We have no plans to roll-in KT88 tubes, as it will likely put the transformers under additional stress.  The stock NOS Russian 6550C tubes sound great.

I wanted to address the potential risk around the low-ish resistance value of the Shared Cathode Resistor.   It has been made clear on this thread and others that each individual 6550 should have at least a 500-Ohm resistance in the Cathode Resistor position.  Opinions seem to vary about exactly how much is a safe minimum, but I think we can all agree that 375-Ohms resistance shared between two 6550 tubes working in triode, is not enough to be safe.

 

In the present Shared Cathode Resistor configuration (Position R13 on the schematic below) each pair of output tubes shares a cathode resistor - two paralleled 750-Ohm / 5W sandcast resistors, effectively presenting a 375-ohm resistance to each pair of 6550 tubes - with a total of 10W power handling.  Interestingly, the BEWITCH 6550 factory schematic specifies 250-Ohms and the actual circuit-board labelling asks for 390-Ohms, so it seems that the factory view has moved on this issue also.

 

Until I figure out a way to install an individual Cathode Resistor / Bypass Cap for each individual tube - which could be months or years away - it is in my thinking to immediate reduce the risk of failure by: -
Replacing existing two paralleled 750-Ohm / 5W sandcast resistors, with two paralleled 2.4K-Ohm / 12W wirewound resistors, effectively presenting a combined total of 1200-Ohms resistance to each pair of 6550 tubes - with power handling increased to 20-24W.

What do others think of this workaround?  Will it reduce my risk?  I'm really hoping the increase in resistance value won't change the sound of the amp.  I'm not too bothered if we lose a few watts in total amp output power - due to increased burn-off - as we use efficient speakers anyway.

One further question I have is - will I also need to vary/increase the value of each of the two stock 220uF Shared Cathode Resistor Bypass Caps?


I really appreciate some expert thoughts here.

(The board photo below is generic and features an older PCB.  Mine is fiberglass...)

BEWITCH 6550 SCHEMATIC, Standard.png

BEWITCH 6550 INTERNAL - Shared Cathode Bias Resistors, Stock.jpg

Edited by SONDEKNZ
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On 25/02/2021 at 3:59 PM, SONDEKNZ said:


@ehtcom and others.

Since my last post, I have returned our new BEWITCH 6550 integrated amp precisely to its original design.

Despite some judicious parts replacement - better coupling caps, etc. - the amp is now fully configured as per the schematic.  Sounding better than ever for it too!

 

We have no plans to roll-in KT88 tubes, as it will likely put the transformers under additional stress.  The stock NOS Russian 6550C tubes sound great.

I wanted to address the potential risk around the low-ish resistance value of the Shared Cathode Resistor.   It has been made clear on this thread and others that each individual 6550 should have at least a 500-Ohm resistance in the Cathode Resistor position.  Opinions seem to vary about exactly how much is a safe minimum, but I think we can all agree that 375-Ohms resistance shared between two 6550 tubes working in triode, is not enough to be safe.

 

In the present Shared Cathode Resistor configuration (Position R13 on the schematic below) each pair of output tubes shares a cathode resistor - two paralleled 750-Ohm / 5W sandcast resistors, effectively presenting a 375-ohm resistance to each pair of 6550 tubes - with a total of 10W power handling.  Interestingly, the BEWITCH 6550 factory schematic specifies 250-Ohms and the actual circuit-board labelling asks for 390-Ohms, so it seems that the factory view has moved on this issue also.

 

Until I figure out a way to install an individual Cathode Resistor / Bypass Cap for each individual tube - which could be months or years away - it is in my thinking to immediate reduce the risk of failure by: -
Replacing existing two paralleled 750-Ohm / 5W sandcast resistors, with two paralleled 2.4K-Ohm / 12W wirewound resistors, effectively presenting a combined total of 1200-Ohms resistance to each pair of 6550 tubes - with power handling increased to 20-24W.

What do others think of this workaround?  Will it reduce my risk?  I'm really hoping the increase in resistance value won't change the sound of the amp.  I'm not too bothered if we lose a few watts in total amp output power - due to increased burn-off - as we use efficient speakers anyway.

One further question I have is - will I also need to vary/increase the value of each of the two stock 220uF Shared Cathode Resistor Bypass Caps?


I really appreciate some expert thoughts here.

(The board photo below is generic and features an older PCB.  Mine is fiberglass...)

BEWITCH 6550 SCHEMATIC, Standard.png

BEWITCH 6550 INTERNAL - Shared Cathode Bias Resistors, Stock.jpg

Way too high in value. That will put the output tubes outside their operating curve. To play it safe, run two 1K 12W resistors in parallel. I'm pretty sure I ended up repairing the amp above and somewhere, maybe even on SNA, there are some pictures of the internals. 

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@ehtcom

Many thanks for digging these photos out.  Appreciated.
A VERY impressive repair!  You certainly know your stuff.  Good on you for keeping photos of your work.

There is not a lot of unused space under the amp, so I am impressed with your design choice to utilize that last bit of vacant real-estate for the new individual Cathode Bias Resistors - and their heat sink.  Very nice.
As will be evident from my questions, I am still learning about how tubes operate.  I had largely come to the same conclusion: that for our purposes, simply increasing the Shared Cathode Resistor power handling - to include 2 paralleled MILLS 12W resistors for each 6550 pair - was the main change needed on our BEWITCH, as you have recommended.  It is comforting to get your consensus on this.  Thank you.
We have had zero problems with BEWITCH amp hum, but I suspect your installation of hum-bucking resistors may have been related to some of the other non-standard changes that were asked of this repaired BEWITCH.

I was very surprised and curious to see the KT120 tubes in action here and wonder if they improved sonics at all?  I was a bit worried that the stock transformers would not be able to handle them.
Were there any other changes required just to accommodate the operation of KT120 tubes?
What are the smaller rectifier tubes being used there?  I have read that the stock BEWITCH is quite tolerant around operating with many different rectifier tube variants.

Edited by SONDEKNZ
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Through this learning journey, I have read a lot of discussions around whether or not different types of resistors sound different. I appreciate there are wildly disparate opinions across the multitude.

 

My experiments with our BEWITCH 6550 amp and in our system - easily revealed to both my wife and I, significant difference between CARBON FILM and METAL FILM resistors in the GNFB position - R14 - given same values, wattage, etc.

 

On this basis, I am keep to experiment and learn more, so I wanted to ask for opinions from those with more experience.

 

QUESTION: Which resistor positions are seen as most impacting on sound quality?

 

I have provided the original schematic for reference to this circuit, bearing in mind that some of the factory-fitted resistor are materially different, from those on the schematic. This amp is so quiet and well-behaved, that for any changes, I intend to stick with the current factory-fitted resistor values - and simply try different types of resistors.

 

I am thinking that R15/R16, R11/R12 and R9/R10 are the critical SOUND IMPACTING positions, but very happy to be corrected here and welcome recommendations.

BEWITCH 6550 SCHEMATIC, Standard.png

BEWITCH 6550 INTERNAL - Component PCB Labels JOINT A B.png

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First I would get rid of that blu tak that is on caps and some resistors.

 

Edit: as far as resistor choices go, I like Shinkoh Tantalum, Allen Bradley Carbon composition and AN non magnetic Tantalum are OK.

Granted the Shinkoh are hard to get in many values and are not cheap, the Allen Bradley's are getting harder to source in some values and as they absorb moisture over time the values might be higher than spec, so some folk bake them in the oven before use to remove moisture.

 

For cathode bypass cap try Elna Silmic ll

 

Of course the coupling caps C3 and C4 (0.47uf) have a noticeable effect on the sound, so they are worth thinking about.

 

Edit: BTW, where are your 0.47uf coupling caps? all I see are some 0.01uf/3000vdc bypass caps, are the others on the other side of the PCB?

Edited by muon*
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3 hours ago, muon* said:

Edit: BTW, where are your 0.47uf coupling caps? all I see are some 0.01uf/3000vdc bypass caps, are the others on the other side of the PCB?

 

Ditto this. The circuit diagram calls for a 0.47uF 630v coupling cap. Why is a 0.01uF 3,000v in use in there?

Edited by xlr8or
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4 hours ago, muon* said:

First I would get rid of that blu tak that is on caps and some resistors.

 

Edit: as far as resistor choices go, I like Shinkoh Tantalum, Allen Bradley Carbon composition and AN non magnetic Tantalum are OK.

Granted the Shinkoh are hard to get in many values and are not cheap, the Allen Bradley's are getting harder to source in some values and as they absorb moisture over time the values might be higher than spec, so some folk bake them in the oven before use to remove moisture.

 

For cathode bypass cap try Elna Silmic ll

 

Of course the coupling caps C3 and C4 (0.47uf) have a noticeable effect on the sound, so they are worth thinking about.

 

Edit: BTW, where are your 0.47uf coupling caps? all I see are some 0.01uf/3000vdc bypass caps, are the others on the other side of the PCB?


Hi Muon,

Thanks for sharing your thoughts.

 

I'm with you on the SHINKOH resistors, but never actually tried any myself.  They are non-magnetic too, so I'm keen.

Because all of these exotic resistors are very pricey, I was hoping that we could explore the most impactful resistor positions first - as 40+ SHINKOs (or similar) is probably a serious overcapitalization.

 

The coupling caps that you can't see are hidden beneath their bypass caps.  They are the original SOLEN .47uF - cheap and cheerful - that I have upgraded with other amps, with great results.  I will likely upgrade to CORNELL-DUBILIER 940C caps - as per the current Coupling caps Bypass caps - in the C3/C4 positions - but at the moment, I am trying to focus my efforts on the best RESISTOR approach.

 

I'll do some research on the ELNZ SILMIC II.  Cheers!

For the Shared Cathode Bypass resistors, I will upgrade to 02 x MILLS 12W (paralleled) in each position, as per earlier in this thread.

The Blu-Tac...LOL!  All I will say is don't knock it unless you have tried it.  And if you have tried it, I know you won't knock it.  It's amazing stuff that I have used on numerous amps to transform the sound to my liking.  As with all of these things, YMMV.

PS:  The Blu-Tac under those GNFB resistors is temporary, as I try different values...  I LOVE Blu-Tac!  -SONDEK

 

 

Edited by SONDEKNZ
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5 minutes ago, SONDEKNZ said:

The coupling caps that you can't see are hidden beneath their bypass caps.  They are the original SOLEN .47uF

Ah!

 

I love a good mystery, but like it being uncovered even more in this case :thumb:

 

I love Shinkoh's :D

20191002_103249.jpg.8cf6c3a8d1b4c3ff7518664a3347ff58.jpg

Edited by muon*
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IME Shinkoh resistors should be used judiciously and mixed with other types to get a balanced sound. I settled on using Shinkoh as grid leak resistors and that's it. The rest are Takman carbon film and a couple of Amtrans carbon film in my 2A3 SET amp.

 

 

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2 hours ago, SONDEKNZ said:

@muon*

What schematic position did this (hidden) SHINKOH fill?

It's a critical one for stability that doesn't appear in that schematic posted further up.

 

55 minutes ago, Gryffles said:

IME Shinkoh resistors should be used judiciously and mixed with other types to get a balanced sound. I settled on using Shinkoh as grid leak resistors and that's it. The rest are Takman carbon film and a couple of Amtrans carbon film in my 2A3 SET amp.

 

 

Yes, mixes well with Allen Bradley also :)

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34 minutes ago, muon* said:

Yes, mixes well with Allen Bradley also :)

Yes, I'd imagine they would!

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