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Old 26th July 2011, 03:57 PM   #1
Tekmek is offline Tekmek  Canada
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Default how to select transformers.

Greetings; this is my first post here but I've been reading for a while.

I am looking at acquiring parts to build my own tube amp. I have a vintage radio-phonograph that has a 10W 6v6 push-pull amp and its rich sound is just amazing, so I would like to make a stereo amp similar to that for other purposes.

I aim to reuse as much of the same tubes as possible in order to not stock too many different tubes.

The amp section of my radio uses half of a 7X7, one 7AF7, two 6V6 pushpull and the power supply is made with a 5Y3 rectifier.

I am currently looking at this dynaco schematics (for the fact that it has the same rectifier and output tubes)
I could strip out the schematics of the audio amp section of the radio as well, but think that would end up more complex (maybe not)

regardless, I am trying to source output and power transformers, and frankly have no idea how to select those, what to look for. It is my understanding that design values are adjusted according to tubes and transformers, so I'd like to choose wisely... but I am not wise in that regard!

There are tons of output transformers available out there with varying prices and specs, surely I can find the good one if I know what it is that I need to look?

Any hints on the whole process (and amp building itself) would be welcomed.

Thanks a lot.
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Old 26th July 2011, 06:14 PM   #2
Tekmek is offline Tekmek  Canada
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I realise I forgot to put the link to that schematics I was talking about. Here it is
Dynaco Push-Pull EL84/6BQ5 or 6V6/6AQ5 Tube Amp Schematic

This is not set in stones yet, but I'll want 6V6 push-pull and 5Y3 for the rectifier so I guess choices are somewhat limited.

As I said, I understand the values of everything would depend on the transformers; but how do I choose transformers?
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Old 26th July 2011, 06:46 PM   #3
Yvesm is offline Yvesm  France
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The answer is in another page on the same site:
DIY Push-Pull (PP) 6V6 / 6V6GT Tube Amplifier Schematic

Yves.
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Old 26th July 2011, 07:06 PM   #4
20to20 is offline 20to20  United States
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Your issue with reusing the tubes you have is a very minor part of the cost you'll encounter. At best your current output tranny can be reused if you can find one to match it. Best to get two identical. So you are left with only the 6V6's that have any value. The 5Y3 will have to go because you want to double the power through that tube and it is already taking all it should.

Consider finding an second amp like the one you have now and just dual them as R/L channel for stereo.

You basically will be starting from scratch on a build your own amp.

Good Luck, though!
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Old 26th July 2011, 08:49 PM   #5
Tekmek is offline Tekmek  Canada
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20to20, understand me. I will NOT use the radio as an amp. I will be making a scratchbuilt amp.

The point of reusing the same tubes isn't a cost perspective, rather a "not having to stock 20000 types of tubes" perspective. The amp sounds great with those tubes so why not reusing the same types?

I fail to see why the 5Y3 "have to go"... I have seen plans of stereo PP 6V6 with a 5Y3. I don't understand your point. Maybe I am missing something?

my question remains: how to select output tranny.

Yves, ton lien ne donne aucune réponse. Premièrement, on change de tube, ensuite, on explique aucunement comment choisir les transfo. Peut-être que je manque quelque chose???

J'avais bien vu ce schématique, que je tentes d'éviter pour raisons sus-mentionnées.

Anyone have knowledge to share on tranformer selection?
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Old 26th July 2011, 09:14 PM   #6
20to20 is offline 20to20  United States
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The 5Y3 tube is good for 125ma. You'll be biasing your (4) 6V6 tubes at @ 40ma. or higher each for a total of 160ma. or more. That's not figuring in the power requirements for the other tubes.
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Old 26th July 2011, 09:48 PM   #7
chrish is offline chrish  Australia
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Tekmek,

I think that you should not be concentrating on using the same tubes just because you do not want to stock extra tubes or that the radio sounds good. Choice of tubes is determined by the circuit you are building. Good quallity EL84 can be found on ebay from Russian vendors cheap. stocking 4 extra is way short of your '2000'.

For advice on how to select a transformer, you can either go with a proven design and use the specification or you can look at the power tube data sheets and look at the recommended load, voltage and current. Here is a link to online data sheets http://tubedata.tigahost.com/tubedata/. While you are there, you can look up your 5Y3 and see why 20to20 is right, the 5Y3 cannot handle the current.

For the power transformer you will need something that provides the correct voltage and enough current for the B+ and also has secondary taps with correct voltage and sufficient current for heaters and bias.

Cheers,

Chris
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Old 26th July 2011, 10:01 PM   #8
rknize is offline rknize  United States
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To answer your main question about "speccing" an output transformer, you need to know 4 basic things to start:

1) Output topology (SE, PP, triode, pentode, ultralinear, etc).
2) Input impedance.
3) Output impedance.
4) Power handling.

There is nothing wring with the 6V6 tube type. It's a nice tube when used within it's limits. The schematics you posted indicate that you are looking at a PP pentode/ultralinear amp. That answers 3 of the 4 above:

1) PP pentode/UL type
2) 8k primary impedance is customary for the 6V6 (see the tube datasheet).
3) This depends on your speakers (4/8/16 ohm, etc). Some have multiple taps.
4) About 12-14W is typical for a 6V6 PP amp.

The Hammond 1608A noted on the schematic you posted is a tad undersized, IMO (10W rating). The 1650G is a bit better if bass response is important.

Hammond Mfg.-"Classic" Push-Pull-Easy Wire-Tube Output Transformers - (1608A-1620A, 1645A & 1650A Series)

Edcor gives you more bang for the buck. The XPP and GXPP are on the budget end of things while the CXPP series will give you more power at the bottom end:

EDCOR Electronics Corporation. Push-Pull Tube Output Transformers

For example, these GXPP OPTs are good for this application. Just chose your output impedance:

EDCOR Electronics Corporation. GXPP Series Output Transformers

For the CXPP, they offer one with multiple secondary taps or chose one with a single to save a little:

EDCOR Electronics Corporation. CXPP Series Output Transformers

Of course, these are on the cheap end of the output transformer cost spectrum. You can spend plenty more on higher-end stuff just like anything else. There is also cheaper stuff out there for guitar amps that could work too.
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Old 26th July 2011, 10:06 PM   #9
20to20 is offline 20to20  United States
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Quote:
I aim to reuse as much of the same tubes as possible in order to not stock too many different tubes.
I misunderstood when you said "reuse" the tubes. I thought you meant you would build an amp with the parts you had already.

As far as stocking 2000 tubes... The MORE tubes the MORE FUN!!

Good Luck!
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Old 26th July 2011, 10:53 PM   #10
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by Tekmek View Post
There are tons of output transformers available out there with varying prices and specs, surely I can find the good one if I know what it is that I need to look?

Any hints on the whole process (and amp building itself) would be welcomed.

Thanks a lot.
You are going about this mostly the right way. So many people pick the tubes first when what really matters is the output transformer (OPT) I think the OPT is the most importent part of the amp and it's quality is what determines the sound. Building an OPT is as much an art as a science. The Dynaco amp is a good one. The OPTs are very good and "clones" of the old OPS are available. There are many people who know these amps and you can get help.

Don't worry about "having to stock so many kinds of tubes" First off why stock them? When you need a replacement buy a replacement. That said never build anything that uses out of production tubes. The new production tubes are getting better and better so don't buy then and keep them for 25 years waiting until they are needed. Tubes are actually the least expensive major component of the amp. The metal chassis will cost more and the transformers much more.

What tubes ans transformers sound best? You can't answer that. One does not listen to parts, yo have to put the part into an amp first and if you do then the part that "fits" best will sound best. Also amps that use negative feedback are less sensitive to component swaps, the Dynaco use NFB.

A stereo amp even one with only 15WPC is not a small project. It's almost worth learning to play guitar soyou can build some 5W simple amps. It is not hard to build one fo $100 but a good stereo amp is 4X as expensive to build.

People here are correct about the rectifier tube. Your stereo amp will need twice the current (at least) of theat mono amp and will need a larger rectifier tube. and NO you can't use two of the small ones because yo'll find you don't have enough current at the 6.3 heater circuit. The larger power transformer will have a 5V secondary for powering the larger 5V rectifier tubes. Or you could build two mono amps.
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