6v6 stereo power amp build .

[Helmholtz]

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

I suggest to redraw the complete amp circuit with cathode bias and grid (leak) resistors for the power tubes and post a readable copy with component labels (R1,..,C1,..) and supply voltages.

[Helmholtz]

Here:
http://www.ampwares.com/schematics/gibson/ga20t.pdf
is a working example with paraphase and cathode bias. Notice the difference? It's mainly an additional ground connection.

[Arbutt]

Thanks helmholtz. I will have a look tomorrow. I can see this has 2 x 220k going to ground. I thought the idea of having a trimmer I could tweak it up. But it looks like in going to have to go down the route of cathode bias with this amp.
Arbutt

[Helmholtz]

Thanks helmholtz. I will have a look tomorrow. I can see this has 2 x 220k going to ground. I thought the idea of having a trimmer I could tweak it up. But it looks like in going to have to go down the route of cathode bias with this amp.
Arbutt

Nothing wrong with the trimmer. But I think the connection between 2K and 220k is meant be grounded. Otherwise why split the resistor? Hereby we get a ground reference (grid leak resistor) for the PI and provide grid leak resistance for the power tubes. Of course this only works with cathode bias, where the power tube grids sit at 0VDC.

[Jazz P Bass]

Here is a good read on the different PI types.
http://www.valvewizard.co.uk/paraphase.html

[Dave H]

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

I think you need to have separate cathodes each with its own resistor then connect the nfb to the first (input side) cathode.

[Arbutt]

Morning. I decided this morning to start again with the pi stage on this amp. I have removed all the components from the pi just leaving the tube base with heaters connected.
Customer was keen from the start to use fixed bias over cathode. End of the day he is paying
I will start to look for a circuit.

If anyone has any recommendations on what to go for I would be grateful.

[Helmholtz]

I think you need to have separate cathodes each with its own resistor then connect the nfb to the first (input side) cathode.

Good point!
One of the cathodes is an inverting input and the other a non-inverting one. Connecting the NFB to the common cathode resistor will produce asymmetrical PI outputs and may give rise to instabilities because of the positive feedback involved.

[eschertron]

There's a schematics section of this forum https://music-electronics-forum.com/...splay.php?f=60

Where I found this Fender Schem Deluxe Reverb

Here's an example just to start you off. I suspect that you are making a HiFi amp, not a guitar amp, but Fender was known for their 'clean' sound. Not too far off from the HiFi sets of the era. This amp has fixed bias, a long-tail-pair PI, NFB, and 2x 6V6. The implementation should be close to what you're looking for. If you go this route, there may need to be some tweaking of the gain structure, but not much.

Read up on LTP PIs, and also negative feedback. The better your understanding, the more successful you will be getting the parts of the amp to 'fit', working well as a whole. Your project seems interesting. Good luck, and any questions will be answered!

[Arbutt]

Thanks I'm going to be using 6sl7 as the pi.
Do you see issues with this. I know the pin outs are different. I looked at it this morning. It's wasn't looking tidy anymore . Hence why I have started again.
After this is done he has a pre amp to build :-/
Thanks again .

[eschertron]

Are you re-doing the entire circuit? Share the tube complement, design goals, typical application for the amp. There's likely a schem out there that is close to what your customer wants.

[Arbutt]

I'm not going to redo the whole circuit. Just gutted the pi and clean it all up .As it was looking un tidy .
Want to re do PI so I can use fixed bias.
It's going to be a stereo power amp for HiFi use
With 2 x 6v6 pp per channel with 6sl7 as pi
Gz34 rectifier. 8 and 16 ohm out. Going to be used with an existing tube pre amp unit

[Chuck H]

Are you using two PI circuits then?

[Arbutt]

It's a stereo power amp with 2 x 6v6gt per channel

[Chuck H]

Are you using two PI circuits then?