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Hammond 125D output transformer

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Have I got a lemon?  I have brought two DIY monoblocks that have IEC, RCA and speakers IO's, mains trans (250V CT and 6.3V tappings) with a hammond 125D pp 10W output transformer and 3 noval sockets.  The rest has been gutted.

 

I thought this was to be a good base to build something on and learn (while acquiring parts for a KT88 rebuild).  The original build had used a 6X4 rectifier and 2 x 6BM8's.

 

I read the hammond 125D is not HIFI and reading spec sheet of 00Hz doesn't go very low.   

 

I don't want to waste my efforts to accomodate whats there if it will be horrible.

 

Any suggestions for these?

 

I have found a few 6BM8 (ECL82) schematics and wonder if I just don't suck it and see or do I get better OT with other suggestions for valves to use.

 

I'm not looking for much power (max 10W is fine) and certainly not looking for anything complex or needing a lot of fiddling.

 

Look forward to any comments, suggestions and reference to schematics.

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So.......they are already built but have no valves..........is that it?

 

A couple of pics might give us a hint.

Especially of the wiring.

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The O/P transformer is definitley not suitable for a high fidelity amplifier.However the power transformer (if it has a  suitable current rating) and chassis may be a  useful  start for a valve amplifier based on push pull EL84/6BQ5 valves.The available chassis area will determine what can be made,as extra valves may need to be added for V.A. and phase splitter functions and also an increased space for a suitable O/P transformer.If space is limited then a solid state front end may be needed to drive the valves.

Edited by VanArn

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Hi logicprobe.

 

Chassis, IO's, mains trans, output trans and 3 noval sockets.  Haven't received them yet, nut, described as gutted.  So am assuming no wiring other than IEC socket with inbuilt fuse to mains trans.

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Well, it's gonna depend how much you paid for them!

No need for them to be too horrible.

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You could use one chassis, remove the OP trans and build a pre-amp.  This would be an inexpensive learning project.  If you like it, keep it.  If not gut it , get some new OP transformers and try some mono power amps.

Hammond is a very reputable brand.  The OP transformers you have are not junk, just not suitable for hifi.  You could probably sell them to someone for guitar amp purposes to finance valves etc.

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Hi RoHo.  Looking at power amps.  I'd use my Plinius 2.4 pre and image 402 speakers for the bedroom system (when the kids wont let me in the lounge:)

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I'd loose the tube rectifier and run SS. One 12AX7, and two EL84's push-pull.

They would run pretty hot (~350vdc HT). Use new production EL84 you should be right. :)

Though if using cathode bias it will lower the Cathode to Anode voltage.

Wire to 125D to reflect greater than 10K load.

 

Add a little bit of global feedback to improve the frequency response.

The quoted 150hz to 15khz is the transformer driven to full rated output, so it will be a bit better at lower levels.

 

Cheers, Earle.

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Thanks Earle.  I'll read up on the EL84 PP and peoples commnets as well as looking for schematics with SS rectification.  Do you onsider the 125D would be ok in this configuration?

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Thanks Earle.  I'll read up on the EL84 PP and peoples commnets as well as looking for schematics with SS rectification.  Do you onsider the 125D would be ok in this configuration?

 

No worries.

 

Maybe run the EL84's in triode configuration to keep the output power under 10 watts.

The 125D will be okay for starters, you can always upgrade them later.

You'd be surprised at how "good" some output transformers in commercially available amps.

 

I have a circuit for an EL84 amp. It used output transformers with cathode windings. I'll redraw it, and mod the supply circuit to suit full wave SS.

 

Do the monoblocks have any chokes under the chassis per chance?

 

Cheers, Earle.

 

EDIT. Circuit added.

Just realised you'll have to go with a bridge rectifier rather than full wave.

 

 

post-106277-0-72442600-1361257041_thumb.

Edited by ehtcom

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Thanks Earle.  I'll give the schematic a study as on first glance the A and B from power supply are -V and yet inputs to the amp appear A and C and are denoted +V.

 

I don't think the chassis has chokes but will say more when I get them.

 

Thanks, Chris. 

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Sorry, a bit of a rush edit job. HT "C" should read HT "B"

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P/P EL84's sound sweet!

(but that's roughly 15 watts)

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P/P EL84's sound sweet!

(but that's roughly 15 watts)

Circuit above is Triode connected, ~8wpc

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I know, I know...........was just stating my preference, isall.

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I know, I know...........was just stating my preference, isall.

Sorry LP, thought your were pointing out that 15wpc is too much for the 125D (rated at a maximum 10 watts).

 

Cheers, Earle.

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Nah..........I could see you had it sorted and was pining for my old amp that I've been too busy/lazy to rework.

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I really like the sound of EL84's. I've built a few and plan to build another one day for my workshop amp.

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My Motown CD's sound fabulous through them and into shitty speakers.

They were clearly mixed for small radio speakers.

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Well it looks like an EL84 PP monoblock.  I actually have a little Chinese stereo amp using 2 x EL84 and 5670 per channel (Mengyue MINI).  I like its sound in the bedroom set up on the Luxman bookshelf speakers, but, it doesn't really do it in the main setup on either the Image 402's or the ESL57's (hard to compare with the plinius or AWA KT88's)

 

Thanks for the schematic Earle.  I'll continue to look over it and refer to Jones books I have and Linde's - Audio Valve Amplifiers.  The latter has a very similar schematic to what you provided.

 

I'm sure i will have a number of questions to get this project right from the start (at last with what I have there).

 

Would you be willing to "Mentor" through this?

 

P.S.  I am electrically competent (No electrician or electronics engineer though).  I sell and install electrical immobilisers and stimulators into abattoirs in my own business.  I've just done a recap on my Plinius MA102 monoblocks and am redoing the soft start circuits (the triacs have blown the tracks off the boards).  My other hobby is high voltage generators with a tesla coil (2 microwave oven transformers in antiparallel and a doubler circuit giving 10 kV 100Hz at about 0.5 amps).  It does about 1.2m arcs.  Built a walton cockcroft for 25 kV which will be the primary supply for a Marx generator designed for 500 kV.  With the right information and some references I can manage to learn most things.  But, other than the basics of valve amps it is proving difficult to understand the designing from scratch.

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At least you're used to working with high voltages and know how lethal they can be!

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But strangley the worst shock I've had was when I use to race speedway bikes in New Zealand.  Wet day, very wet leathers and holding the spark plug cap on from the exit of the final corner to get across the line.  Needed the points for my partner and I to win the pairs race meeting.

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Well it looks like an EL84 PP monoblock.  I actually have a little Chinese stereo amp using 2 x EL84 and 5670 per channel (Mengyue MINI).  I like its sound in the bedroom set up on the Luxman bookshelf speakers, but, it doesn't really do it in the main setup on either the Image 402's or the ESL57's (hard to compare with the plinius or AWA KT88's)

 

Thanks for the schematic Earle.  I'll continue to look over it and refer to Jones books I have and Linde's - Audio Valve Amplifiers.  The latter has a very similar schematic to what you provided.

 

I'm sure i will have a number of questions to get this project right from the start (at last with what I have there).

 

Would you be willing to "Mentor" through this?

 

P.S.  I am electrically competent (No electrician or electronics engineer though).  I sell and install electrical immobilisers and stimulators into abattoirs in my own business.  I've just done a recap on my Plinius MA102 monoblocks and am redoing the soft start circuits (the triacs have blown the tracks off the boards).  My other hobby is high voltage generators with a tesla coil (2 microwave oven transformers in antiparallel and a doubler circuit giving 10 kV 100Hz at about 0.5 amps).  It does about 1.2m arcs.  Built a walton cockcroft for 25 kV which will be the primary supply for a Marx generator designed for 500 kV.  With the right information and some references I can manage to learn most things.  But, other than the basics of valve amps it is proving difficult to understand the designing from scratch.

 

The high voltages wont worry you then.

More than happy to help where I can.

 

If you have a basic understanding of electronics, this book is pretty helpful.

 

http://www.parts-express.com/pe/showdetl.cfm?partnumber=500-108&utm_campaign=bazaarvoice&utm_medium=SearchVoice&utm_source=RatingsAndReviews&utm_content=Default

 

Cheers, Earle.

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The book has mixed reviews but I'll certainly get it.  Nothing like a bit of light reading before bed :-)

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