6v6 stereo power amp build .

[Arbutt]

Hi.
I am rebuilding a 6v6 pp stereo power amp for someone. It was a kit and he did it but not did it right. I have rebuilt it with a couple of changes
It is just a power amp now with 6sl7 as pi

I have been running it for a week seems ok
But I'm not happy if you turn it up sound just a little distorted
On testing with signal gen an scope sine wave is prefect.

I believe that the pi circuit needs a tweek

The circuit for pi I have used before a few years ago and it's ok. I should know I use the amp at home. Can't remember where I found it. On line
Variable bias 25ma per tube
Ht voltage 287v
Pin 2 -148v 100k
Pin 5 - 180v 100k
Pin 3 and 6 -1.6v
The nfb I'm using is from the original circuit 22k and 50pf cap connected to the 1.6k . The amp is definitely better without nfb switched on
Looking at other circuits they don't seem to use an cathode bypass cap across the 1.6k

Looking for feed back on any tweak. If any more info is required let me know
Many thanks
Mr a

[Reader]

Before making circuit modifications it would be a good approach to be extra careful to verify that the original circuit is operating properly and is configured according to the original design. Since the amp was a kit would you please post the model of the kit information? Someone will then be able to find a copy of the original kit schematic. If the kit had known defects then someone may know about them and the remedies.

Also important information is that you said "On testing with signal gen an scope sine wave is prefect." This is a clue that the problem may be elsewhere in your set up than the 6V6 Power Amp. We should consider that.

[Helmholtz]

Please post complete power stage circuit. The grid of the triode on the right needs a ground reference.

[eschertron]

The PI topology is some sort of paraphase, but looks different than what I have seen.

I hesitate to offer suggestions before we see additional info as requested above, but I will anyway
With a few resistor changes, a long-tail design might be done that improves the behavior as you 'turn it up'.

[Arbutt]

Many thanks this is all I have on the power stage. I have not used cathode bias . It's fixed bias now

[Arbutt]

I don't know the kit or model or by whom. I have asked the owner and forgets. He assembled the kit. I told me it wasn't working well . I wanted to just repair it. But he wanted to start again.

[Helmholtz]

Many thanks this is all I have on the power stage. I have not used cathode bias . It's fixed bias now
[ATTACH=CONFIG]51069[/ATTACH]

So the right side PI grid is at the negative bias voltage of the power tubes?? Can't work. Need the complete final circuit.

[Arbutt]

I have a variable bias circuit is connected to both 4.7k resistors. I have 1ohm resistors on both 6v6 cathodes. Bias is 25ma on both tubes.

[Helmholtz]

The right grid of the PI wants to be at 0VDC. I don't see how this is accomplished. From what you describe I assume the right half of the PI to be inoperative.

[Arbutt]

I don't know hence im asking you guys.
The circuit for the pi I saw online but can't find it since.
The amp sounds good and works well . But when turned up just has a little distortion.
When I scope the output with signal gen . Looks ok to me. I have finished today. I will draw out the complete circuit later and post up.
Many thanks
A

[Arbutt]

Sorry helmholtz. You were right I forgot to write in the gnd reference on the 1st picture.
I have quickly drawn out what the circuit is . Not brilliant but that is my drawing
Thanks again.
this is correct sorry it's upside down
Below are incorrect

[eschertron]

So you have a mashup of fixed and self bias, looks like to me. The resistor/pot/resistor/ground/resistor string in parallel with the bias control does not belong here, and will not work the way you want it to. Unless the coupling caps are after the resistor string, the power tube grids are getting competing bias control info.
Even if you keep the paraphase, the PI's second grid reference needs to be redone. The way you show it, it is interfering with the power tube bias.

[Arbutt]

Ok looks like I've fluffed it up. If I remove the fixed bias and fit cathode will that cure it or improve matters.?
Thanks for the help
A

[eschertron]

I'd take a look at some different paraphase PI circuits. Maybe moving those resistors to before the coupling caps will fix it? But at that point, it's still needlessly complicated, because that string was a compound of signal reference (for the PI) and bias (for the power tubes). You simply don't need the bias part of the function.

Having said that, I'm still wondering if the paraphase topology is part of the distortion problem.

*edit: probably not a good idea to not decouple the PI feedback, you'd need additional coupling caps. sigh. Start with finding schems of fixed-bias amps with that kind of PI. I had an old Baldwin organ amp like that. Can't draw the circuit from memory, though.

[Arbutt]

Ok so to redesign the pi circuit . Any reason not to go with a paraphrase type ?