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Sets, Parts and Service Information Wanted (private buyers only, no swaps) If you need help obtaining components, sets or equipment, post a message here. Private buyers only - no traders. No swaps.

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Old 13th Jan 2021, 6:46 pm   #1
Skywave
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Default WTB: O/P xfmr. for SE EL34 amp.

Wanted to buy: an output transformer for a single-ended EL34 amplifier running in class-A.
Maximum primary d.c. current in the region of 100 mA (approx.) with an HT in the region 250 ~ 300 v. Primary impedance close to 2000 ohms; secondary load 3 or 8 ohms, ideally user selectable (so usual 3 taps).

Al.
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Old 13th Jan 2021, 7:34 pm   #2
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Default Re: WTB: O/P xfmr. for SE EL34 amp.

Hi Al, are you after a cheap and cheerful one or do you want to go into the Hi Fi realms.

For a good one it needs to be quite large and can be wound with a UL tap for G2.
It would also be sectionally wound on both Pri and Sec for close coupling and reduced leakage inductance.
This was probably what was fitted to the Pye Super BB and was, I think, about the same size as the mains transformer

Ed
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Old 13th Jan 2021, 7:53 pm   #3
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Arrow Re: WTB: O/P xfmr. for SE EL34 amp.

Thanks for the quick reply, Ed.
A 'cheap and cheerful' type will do. I intend to build a basic amp. for testing several used EL34 valves. The object of testing will be to determine basic functionality, mutual conductance, power output, freq. response and distortion. Briefly, to answer the question, are they any good? The stated electrical parameters for the transformer were obtained from the Mullard spec. for EL34.

Aside.
I did (albeit briefly) contemplate using an HT mains transformer, until I realised that the standing d.c. current in the primary winding will simply saturate the core!)

Al.
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Old 13th Jan 2021, 8:30 pm   #4
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Default Re: WTB: O/P xfmr. for SE EL34 amp.

Would there be any merit in testing them by using them as an RF amplifier, at a lowish frequency, say, 1 to 2 MHz? Or are there some characteristics that are only visible/ measurable at audio frequencies?
I was thinking that way would avoid an output transformer, unless you really want a nice audio amp.

Cheers

Aub
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Old 13th Jan 2021, 9:27 pm   #5
Ed_Dinning
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Default Re: WTB: O/P xfmr. for SE EL34 amp.

Hi Al, you could avoid the transformer by using a resistive load in the anode circuit and a meter and/ or scope to monitor what is happening, unless you need to hear it

Ed
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Old 13th Jan 2021, 9:34 pm   #6
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Default Re: WTB: O/P xfmr. for SE EL34 amp.

Wouldn't you then need about twice the HT supply volts and a separate screen supply not to mention a beefyish load resistor?

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Old 13th Jan 2021, 10:44 pm   #7
Skywave
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Arrow Re: WTB: O/P xfmr. for SE EL34 amp.

Right now, today, I do not have an immediate need to use an EL34. However, one way or another, these valves will be used one day - maybe by me, maybe by someone else.

For myself, (a) one day I might decide to build an A.F. power amp. for my own use. (b) I do have a regulated HV PSU which uses a pair of 12E1 valves for which I do have two spares. But if push ever comes to shove, this PSU could be possibly re-configured to use EL34 valves instead. Or a completely new PSU design. (c) On infrequent occasions, I do the odd repair for private customers: an EL34 (or two) may be required. That has happened in the distant past.

I may choose to try to sell a few. In which case, being able to state that one (or more than one) valve has been tested assists the buyer and protects myself. And in such an event, said buyer will almost certainly intend to use such a valve (or valves) in an audio application.

Hence for both ownership / usage categories above, in most cases the need for a suitable output transformer arises.

Finally, I will add that a transformer for a push-pull configuration instead of SE mode is another option. Simply a question of what is available, if any.

Al.
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Old 13th Jan 2021, 11:35 pm   #8
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Default Re: WTB: O/P xfmr. for SE EL34 amp.

We're in the parts wanted section, so we should assume that Al has decided that he wants one, in this case in order to try out an idea.

Can we please keep this thread on the topic of finding him one?

Thoughts on whether it's necessary or whether it works or not should form another thread in a section where anyone in future, thinking of similar experiments, might find it.

David
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Old 14th Jan 2021, 10:10 pm   #9
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Arrow Re: WTB: O/P xfmr. for SE EL34 amp.

I've started a new thread in the section Vintage Test Gear & Workshop Equipment. Perhaps some of the posts above should be moved to that thread.

Al.
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Old 14th Jan 2021, 10:13 pm   #10
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Arrow Re: WTB: O/P xfmr. for SE EL34 amp.

Quote:
Originally Posted by Ed_Dinning View Post
Hi Al, are you after a cheap and cheerful one or do you want to go into the Hi Fi realms?
Ed
Ed: you have a P.M.

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