Push-Pull from Stereo SE amp - diyAudio
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Old 26th July 2011, 08:55 PM   #1
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Default Push-Pull from Stereo SE amp

I was recently given a RCA New Vista Victrola console amp chassis. It has two EL84's and is in single ended stereo configuration. I was wondering if it was possible to make it into a mono push-pull amp. Like this:

run the tubes with a standard phase inverter and connecting the secondary's of the transformers together. is that possible or do you have to use one OPT for push-pull.
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Last edited by benproiii; 26th July 2011 at 09:17 PM.
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Old 26th July 2011, 10:51 PM   #2
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Greetings from FixitLand!

You will need a proper push-pull output transformer. The characteristics of PP and SE transformers are such that they are not interchangeable. A Hammond 1608 (8KCT) or 1609 (10KCT) would work well. I used 1609s on my EL84 amp.

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Old 27th July 2011, 02:10 AM   #3
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Hmm,well each SE transformer is gapped,to handle the DC,so that shouldn't be an issue?
You'll have to mind the phase of the output trans. secondaries,so they're both in phase.
Or perhaps connect the secondaries in 'anti-series'. for a sorta bridged output. -(basically grounding the neg end of each 8ohm winding,and connecting the load/speaker across both + output terminals)

At the moment I can't see any major reason why it shouldn't physically work? (Anyone see anything?)
How it will sound is anyone's guess. Might be interesting for a guitar amp!?
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Old 27th July 2011, 11:08 AM   #4
DF96 is offline DF96  England
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For best push-pull you need tight coupling between the two halves of the primary. You won't have this. It might work, but not as well as a real p-p. Also, you will need a 4 ohm speaker to provide the correct load as both valves are driving it in parallel so each see half the load.

A lightly-loaded SRPP with low mu valves might give you slightly higher distortion than a simple grounded cathode with active load (i.e. take output from lower anode instead of upper cathode) but the difference will be small. SRPP is good for its particular purpose, but no advantage otherwise.
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Old 28th July 2011, 12:17 PM   #5
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I'm thinking of running the tubes and OPT's in parallel for more power, how much power does a parallel SE EL84 amp make.
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Old 28th July 2011, 01:02 PM   #6
TheGimp is offline TheGimp  United States
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You can parallel them or you can bridge them (Series). Either way you will get about twice the power out. Parallel should drive a speaker with half the immpedance, bridge should drive one with twice the impedance of the original.

In the schematic shown you are set for 8 ohm speakers. optimally you would want to drive 4 ohms in parallel and 16 ohms in bridge (series).

I would expect parallel to produce twice the power of the single se which is around 3-5W for the EL84. So you will get around 6-10W which is about half of what you will get with a properly designed PP amp with EL84s.
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Old 28th July 2011, 05:36 PM   #7
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It's a slick idea. Unforch, it only works for RF amps with air core OPTs. Otherwise, it's not gonna work since the two halves of a proper PP xfmr need to be coupled as tightly as possible. It's not like two xfmrs on the same core, and you can't separate them like that.
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Old 2nd April 2014, 03:47 PM   #8
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by Miles Prower View Post
the two halves of a proper PP xfmr need to be coupled as tightly as possible.
Why ?
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Old 2nd April 2014, 07:07 PM   #9
rmyauck is offline rmyauck  Canada
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I read the Decware site about 4 yrs ago and the owner I believe Steve said he tried SE OPT transformers in series for a PP amp and it worked really well with matched transformers which wasn't practical $ wise as he would have to buy many and sell off the rejects probably at a loss.
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Old 2nd April 2014, 11:17 PM   #10
Bigun is offline Bigun  Canada
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I still don't see why tight coupling is needed. It seems to me that using two SE amplifiers bridge mode is no different than having them in parallel - they are both push-pull and both have one input inverted. I can see that the two 'channels' should not be different and that closer matching might reduce distortion.
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