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Schematic help please


2015-09-28 7:26 pm
The idea is to build as simple an amp as possible using EL34 as the power valve in single ended operation Driven by one side of the 6N1P I have a power transformer with two 6.3V 1.5A windings and a 250V 340ma winding without a centre tap. I also have a couple of output transformers that were recovered from an old Blaupunkt radiogram with no data or idea what drove them. These are fairly small physically so I am not wanting to put too much through them. I also have an EZ81 rectifier. So I have cobbled up a schematic based on a couple of different ones I have found and would appreciate suggestions as to whether it is likely to work or whether I have done something really dumb!

I may have an issue with heater power with this so I could use a solid state rectifier or get a dedicated transformer to add to heater power. I really need some help with this!




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Myself i would skip the ez81 and use 4 1n4007 diodes as a conventional bridge.
I would also not paralell the filament windings, if they are the slightest dissimular
a huge current will load them. Use one only in this case.
And you don't need DC powered filaments, just make sure they have some ground
reference, midpoint of winding or 2 resistors will do.
It would be MOST helpful if you could offer more information about those salvaged OPTs. Size - weight - hookups - resistances - and ideally, measured voltage ratio(s). I suspect that EL34 is a poor choice for this job. It could produce a lot more distortion than necessary if it's biased low enough to work with OPTs that were intended for use with smaller power tubes.
I see at least five things that need attention. 1) The SS diodes in the bridge are backwards. 2) The EZ81 filaments should be on their own 6v winding with the cathode tied to the filament. 3) Put a catch resistor (around 470K) from the volume control wiper to ground. 4) Supply the plate power for the input tube from the main B+ source through a decoupling resistor and another filter capacitor. Not from the plate of the EL34. Doing so gives you feedback which may oscillate. 5) Like Peter says, establish a pseudo center tap on the other filaments with two resistors who's center point goes to ground.


2015-09-28 7:26 pm
Thanks for the input folks. I think I now understand the transformers they are likely to be for use with Philips AD3800M or similar speakers that are 800 Ohm so the output transformers are a bust for now unless I can find similar speakers though that is not what I wanted to do. I will re-work the schematic over the next few days and repost it for another round of comment.

In respect of some of the other comments, I rather naively took the B+ supply from the output tube plate as I had seen that done before so I will change that so it comes direct from the power supply. Given the other issues with the mains transformer I will abandon the valve rectifier for a solid state version and create a centre tap for the heaters.

So I am now on the lookout for some budget output transformers preferably to match EL34s as I have some. Any idea where to look and what I should accept? There seem to be a lot for guitar amps though the frequency range seems to be limited m 70Hz to 16kHz, I guess these will sound rubbish for music. Any suggestions or offers of suitable transformers gratefully received:)

Thanks for the help!


2015-09-28 7:26 pm
Soul merchant, that was one of the schematics I was looking at but not copied ! Maybe I should pay more attention to it. I am in Edinburgh, Scotland ( there is another in Texas :) there was a company in Galashiels winding toroids but for power not audio but I think they are gone now.


Caber :)


2012-05-27 7:34 pm
While your circuit definitely needed more development it is not far from the RH universal amp which uses EL34 tubes. The driver is the 12at7 which is dirt cheap. You can. If your power transformer has no 5v filament tap, double up or parallel the 6CA4 rectifier and get the current you need , if you have extra 6.3 volt filament supply. As far as Output trannys, Edcor has some really good sounding 10 and 15 watt SE opts that are the most reasonable priced, if you live in the states. Good luck!