• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Could someone look over my schema?

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I've just finished this hifi amp. I tried to keep it simple with the best sound possible (mostly by listening). I've put all the voltages there to show it all.
Is there anything too unusual my design? Thanks for any feedback, good or bad. I've re-uploaded the schematic hopefully without the errors.

An externally hosted image should be here but it was not working when we last tested it.
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several drawing errors - see attachment
remove 22uF cap from NFB to ground - otherwise FB will not work
increase bypass cap on cathode of 6v6 - otherwise sound will be thin
add grid stoppers - to prevent parasitic oscillations, solder as close as possible to socket pins


  • scmzs_1.jpg
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The 470k is meant to be 470 ohm. Sorry I've corrected it, the schema was just a quickie one to put up.
The OT is connected between the plate and screen grid, that's correct. I've copied that from the amp the OTs were removed from years ego. Not sure why is it so but it works fine.
The NFB... oh yes, somehow I forgot about the cap. I was building without NFB at first. Maybe that's why the amp sound somewhat dull. Glad you've mentioned that.
It's been quiet few years since I build anything and seem to have forgotten a bit. Also have noticed that there were a tons of good schemas of all kinds on the Net still about 10 years ego, but now seem to be much less and mostly guitar amps so I'm having hard time to find suitable references.
The amp all works fine,......

Is that because it makes some sound? Look at the diagram of any amp and you'll see the B+ goes into the OPT on one end first and to the G2 before it goes to the plate FROM the OPT other end. You are filtering the output from the plate as it is now drawn. The signal on the G2 is driving the output.
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The B+ is correct. It can be done that way, I've copied this design from a physical amp some years ego. An old technician back than explained to me why is that an advantage, wish I could remember tho.
The main error I had in this design was forgetting the bypass cap in while adding NFB. With the cap removed it's just fine now.But I'm still tweaking it a bit, than I'll draw the chema properly.
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I don't know which diagram we are now working from, but certainly the original diagram had the OPT between g2 and anode, and supply rail to the anode. Someone pointed out that the supply rail should go to g2 - then we have a pentode output with g2 at supply rail voltage.

Leaving it as it was you get a much smaller output from g2 alone, with the anode current wasted. This is not an advantage, unless perhaps you want a low power practice amp?
You mean the supply rail should go to g2? I must admit I don't fully understand why is it connected this way but it is exactly what I've copied from an old amp. It has plenty of power I'd say more than my other 6V6 SE amps I've build but that could be also due to the OTs which I must mention were NOT grounded either at primary or secondary. (Ignore the + and - I got on the OT, those are just red and blue wires on the OT primary)
It confuses me too the way it is, it is the only amp I've build like this. Others are build the common way - anode to primary on OT, the other OT side to ground.
Maybe I should to take a pic?
A good diagram of what you actually built would help. You don't ground the primary of an OPT. Taking output from g2 would give audio but much less than from the anode; given efficient speakers or a small room you might not notice the difference. I don't take a huge amount of notice of what someone hundreds/thousands of miles away assures me he has done; experience tells me that what people have done and what they think they have done is not always the same thing, however confident they are.
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