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DIY audio: what are you building?


Paul Spencer

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I’ll put up a schematic tomorrow

 

but for now those who would like some figures;

 

  • HT = 100V
  • bias = 3V
  • current = 2mA
  • Rp = 2.7K
  • Rk = 1.5K
  • Ck = 22uf (NP)
  • Rg = 3K
  • Cout = 1uf 400V
  • Rin = 100K
  • Vf = 5V
  • If = 125mA
  • Rout (pot) = 250K
  • Cout = 1uf (PIO)
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@mwhouston, I have some 71a's here to try in a preamp at some stage, and have been warned about their microphony and other suscpeptibilities.  Not sure I coul live with them iving in an alfoil nest... 

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12 minutes ago, acg said:

@mwhouston, I have some 71a's here to try in a preamp at some stage, and have been warned about their microphony and other suscpeptibilities.  Not sure I coul live with them iving in an alfoil nest... 

the foil is not needed and with the pot on the backend the microphonics aren’t an issue. You’ll be shocked at thier transparency if you build a preamp with them.

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6 hours ago, mwhouston said:

If you are having problems with a Xtalk from terrestrial beings try this.

902371F5-DBD5-42ED-82FA-7C2DD4449660.jpeg

Never thought of that, I will give it a try.

 

Cant get the USB inpit working properly yet, the SPDIF inputs work fine with the external clock although the display shows twice the sample rate!

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21 hours ago, mwhouston said:

You’ll be shocked at thier transparency if you build a preamp with them.

I hope so.  Along with the 71a's, there are also some 245/45, 801a, and YO186 DHT's to try out.  That latter DHT will push 85mA, which will drive anything!

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I put serious effort and money into  10Y linestage 5 years ago, but the rp on the tube was too high and I just couldn't get the last half octave in the bass.  It was slow too.  Maybe if I tried a SS B+ rather than a valve rectifier that might have fixed the speed issue.

 

Overall, I was very disappointed with my effort on this one and not built anything since.

Edited by Red MacKay
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20 minutes ago, Red MacKay said:

I put serious effort and money into  10Y linestage 5 years ago, but the rp on the tube was too high and I just couldn't get the last half octave in the bass.  It was slow too.  Maybe if I tried a SS B+ rather than a valve rectifier that might have fixed the speed issue.

 

Overall, I was very disappointed with my effort on this one and not built anything since.

 

I do not have much experience with valve rectifiers, but am put off them because of the generally high impedance of the supply which makes current harder to source making things sound "slow".  Another problem, depending on your amplifier, may have been the output impedance of your pre-amplifier...if it is too high for your amp you may notice the issues you raised.  Slow is not what I am after, and because I have in-room bass response to 18Hz and treble to 40kHz+ the pre will need lots of bandwidth to hopefully go an octave either side of those numbers.  Red, how did you load the plate in your preamp?

 

Because of the 5K input impedance of my amplifiers I am going to need low output impedance for the pre-amplifier.  Also, to get the aforementioned bandwidth a nice "tight", low Z, B+ will be necessary (which is SS rectifiers and decent capacitance) and an optimal plate load to minimise distortion and maximise gain.  The only topology that I think will really tick all those technical boxes is a hybrid mu-follower such as that which Ale Bartola champions.  It all sounds pretty simple, but like you say adds up to a lot of space and a lot of coin.  As planned, my preamp will be:

 

                            input >> 6 way input selector (remote controlled) >> 801a/45/71a/YO186 DHT gain stage, CCS filaments, hybrid mu-follower >> Slage autoformer volume control (remote control)  >> output

 

The autoformer may have to be shifted to the other side of the valve, but technically it will be capable of transferring the full output voltage without a problem whilst lowering the preamp output impedance logarithmically with attenuation.  Without the autoformer I expect circa 25r output Z.  With the autoformer at output as soon as I attenuate from maximum gain the output z will crash below 1r...which is ideal.

 

So fingers crossed it all works as planned.  Red, perhaps a nice low impedance B+ and perhaps a different plate loading will help to alleviate your 10Y problems...not sure.

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Sorry acg, I can't remember what I had the plate loaded to.

 

I remember the Rp for a 10Y is 4.5k so that means you need a line out trannie of around 14K.  Really hard to find.  I had a nice pair of Tangos that went to 10K, but as I said the last half octave of bass was missing and I just couldn't live with that.  Sweet spot for response was 210v @17mA.  DC Coleman regs for heaters.

 

However, what I heard at a mate's place about a month back, has made me re-access my preconceived bias and holyer than thou belief re valve rectification.  For the evenings listening we had two diy built classic circuit 6SN7 gain -cathode follower  linestage preamps, both identical except one had a very nice GZ32 and the other SS rectification.

 

The difference between the two was like night and day - no wiggle room for "like they are close".  The SS rectified unit was clean, images were held suspended in the soundstage, it was fast and every piece of music sounded different.  The valve rectified unit - well it had a richer tone, but that was the ONLY positive.  It was slower, shallower soundstage, congested and to be brutal - just plain boring to listen to in comparison to the SS unit!  And it didn't matter what you played for music, it all sounded like they were recorded and mixed in the same studio by the same tech.  I was in shock and would not have believed it if I didn't hear it with my own ears.

 

So, I have since been thinking of dragging the 10Y out of the shed (I just couldn't bear to pull it apart) and maybe put some ss rectification into it.  Get rid of the LCL and go CRC too.  I'll need to find a new trannie first though.  Food for thought...

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@Red MacKay, I'm told by someone in your neck of the woods with a lot of knowledge and practice in the area that winding an OPT for a 10Y/801a/VT25/VT62 is next to impossible to get right, that that family of tubes are a proper prick to wind for, and to not even bother with an OPT because I would not be happy.  This is from the guy that wound a SE output transformer for me so a 6c33c could drive a 1r speaker load, so he knows how to get around problems!  Choke loaded might be a little better, but really, for this family, in a lot of circumstances, the mu-followers are probably going to get the best results from the tube.

 

Have a look here to see something similar to what I am planning, and what I mean by the advantages of the hybrid mu-follower load for this tube:

 

http://www.bartola.co.uk/valves/2017/03/26/vt-25-dht-preamp-update/

 

Bandwidth is 3Hz - 800kHz at full gain of the tube.  No OPT, no choke, and at 8v output is about 0.05% THD, all 2nd harmonic.  Bloody excellent numbers for what it is.  

 

 

 

Regardless of all that, I think you should certainly resurrect your 10Y preamp with some expensive SS Schottky silicon diodes and personally I would retain the LCL filter over RCR.  You will probably need to drop the B+ voltage a little once you go to SS rectification, but that is a simple series resistor.  Go for it!  Photos? 

Edited by acg
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4 hours ago, acg said:

I'm told by someone in your neck of the woods with a lot of knowledge and practice in the area that winding an OPT for a 10Y/801a/VT25/VT62 is next to impossible to get right, that that family of tubes are a proper prick to wind for, and to not even bother with an OPT because I would not be happy. 

100% correct.  My mate Russell and I thought we knew better and could pull it off - but we were proved very wrong!

 

Disappointingly, it also didn't image very well and had zero depth. ...But talk about tone, timbre, clarity and sheer beauty of sound?  In spades - very nice.

 

Another thing that we found was that the use of flash plastic caps in the B+ led to rubbish sound.  Only when cheapie electros replaced the plastic caps did the sound sq improve in leaps.  Who'd a thought!  I was always schooled that electros were rubbish.  Wrong, very wrong.  Now I say try something before you dis it.  This was learned the hard way.

 

That info from the crazy Brazilian' site looks very tasty...  I have not revisited there for many years.  Thanks acg.

Edited by Red MacKay
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4 hours ago, Red MacKay said:

For the evenings listening we had two diy built classic circuit 6SN7 gain -cathode follower  linestage preamps, both identical except one had a very nice GZ32 and the other SS rectification.

 

Was that the night we were at Paul's, Red?

 

Try these... https://au.rs-online.com/web/p/rectifier-diodes-schottky-diodes/8099036/

 

MIght do what you're after and a lot cheaper than a decent tube rectifier...

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31 minutes ago, unclekanus said:

@Red MacKay You could try a pair in a hybrid arrangement with your tube rectifier. Not tried it myself but there is info around online and some say it's a best of both type result.

 

I use two SS diodes with a valve rectifier to make a bridge in my main preamp.  Not Schottky diodes though.  Maybe worth a change...

Edited by aussievintage
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8 minutes ago, aussievintage said:

 

I use two SS diodes with a valve rectifier to make a bridge in my main preamp.  Not Schottky diodes though.  Maybe worth a change...

They are the best ss diode I have tried. Make sure they are rated for double what your B+ is or just use the 1200v ones anyway.

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7 hours ago, acg said:

@Red MacKay, I'm told by someone in your neck of the woods with a lot of knowledge and practice in the area that winding an OPT for a 10Y/801a/VT25/VT62 is next to impossible to get right, that that family of tubes are a proper prick to wind for, and to not even bother with an OPT because I would not be happy.  This is from the guy that wound a SE output transformer for me so a 6c33c could drive a 1r speaker load, so he knows how to get around problems!  Choke loaded might be a little better, but really, for this family, in a lot of circumstances, the mu-followers are probably going to get the best results from the tube.

 

Have a look here to see something similar to what I am planning, and what I mean by the advantages of the hybrid mu-follower load for this tube:

 

http://www.bartola.co.uk/valves/2017/03/26/vt-25-dht-preamp-update/

 

Bandwidth is 3Hz - 800kHz at full gain of the tube.  No OPT, no choke, and at 8v output is about 0.05% THD, all 2nd harmonic.  Bloody excellent numbers for what it is.  

 

 

 

Regardless of all that, I think you should certainly resurrect your 10Y preamp with some expensive SS Schottky silicon diodes and personally I would retain the LCL filter over RCR.  You will probably need to drop the B+ voltage a little once you go to SS rectification, but that is a simple series resistor.  Go for it!  Photos? 

There is a lot of sand in this kitty’s play box. Would you hear the tube? 

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10 hours ago, Red MacKay said:

Another thing that we found was that the use of flash plastic caps in the B+ led to rubbish sound.  Only when cheapie electros replaced the plastic caps did the sound sq improve in leaps.  Who'd a thought!  I was always schooled that electros were rubbish.  Wrong, very wrong.  Now I say try something before you dis it.  This was learned the hard way.

 

I believe you.  There are huge electros the size of my forearm in the power supplies of the bass channels of my 6 channel SET amp.  Treble channels are not as tight, with less overall capacitance, and only then are the final filter caps film.

 

393418174_BottomFloorDone2.jpg.7a9eb9e60ab6737092301713c66ed91d.jpg

 

7 hours ago, unclekanus said:

@Red MacKay You could try a pair in a hybrid arrangement with your tube rectifier. Not tried it myself but there is info around online and some say it's a best of both type result.

Yep.  If I were to ever pursue tube rectification, Stephie Bench is a great source of information.  See here... https://jacmusic.com/techcorner/SBENCH-PAGES/sbench/reg1.html

 

Low Z, tube regulated power supplies are possible but just take some effort.

 

 

5 hours ago, mwhouston said:

There is a lot of sand in this kitty’s play box. Would you hear the tube? 

Yet to be tested by me, but the sand is only to provide an optimal load for the tube in which to operate, so I reckon you may actually hear more of the tube and fewer of the of the power supply issues.  Either way, in some circumstances such as mine, it is pretty much the only way to drive distortion and output z down to a suitable level while achieving maximum gain.   We will see when it is built.

Edited by acg
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21 hours ago, acg said:

 

I do not have much experience with valve rectifiers, but am put off them because of the generally high impedance of the supply which makes current harder to source making things sound "slow".

Hi acg,

I have valve rectification on my DIY pre-amp and two power amps  and they certainly don't sound slow, whatever else you might say about them.  With you and your 6C33Cs,  they draw a lot of current compared to almost any other valve so maybe supply impedance is more of an issue for you than others.  I must read up on PS impedance.  I do suspect that operating conditions and impedance matching are just as, if not more, important than rectifier type.  Along with good, solid power supply design.

I do have shunt regulators on the pre-amp and driver stages of the power amps  - I assume this would lower the supply impedance, no? 

Rod

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31 minutes ago, RoHo said:

 and they certainly don't sound slow...

Don’t know if this is relevant, but Allen Wright suggested to me to add a small resistor on each of th eleads of the secondary to avoid the pounding the 1st cap gets, particularily if it big.

 

dave

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30 minutes ago, RoHo said:

I must read up on PS impedance.  I do suspect that operating conditions and impedance matching are just as, if not more, important than rectifier type.  Along with good, solid power supply design.

It shouldn't need impedance matching.  A power supply is a voltage source, and an ideal voltage source has zero impedance.  Just seek the minimum, and that should be easily achieved with cap bypassing.  The regulator, rectifier and transformer are all bypassed by the final filter caps (and their smaller bypass caps)

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Just now, planet10 said:

Don’t know if this is relevant, but Allen Wright suggested to me to add a small resistor on each of th eleads of the secondary to avoid the pounding the 1st cap gets, particularily if it big.

 Yes, the resistor simulates the internal resistance of a valve rectifier and prevents the large ripple current that might result without it.  Probably best to match the size of the resistor to the valve rectifier characteristic if you can.    

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20 minutes ago, aussievintage said:

It shouldn't need impedance matching.  A power supply is a voltage source, and an ideal voltage source has zero impedance.  Just seek the minimum, and that should be easily achieved with cap bypassing.  The regulator, rectifier and transformer are all bypassed by the final filter caps (and their smaller bypass caps)

I tend to use polyprop, 20-47uF, caps for the final filter.  With their low ESR should I still bypass them with a smaller cap?

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