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Thread: 6v6 stereo power amp build .

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    6v6 stereo power amp build .

    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
    Click image for larger version. 

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    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

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    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.

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

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    Supporting Member eschertron's Avatar
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    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'.

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    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
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    Many thanks this is all I have on the power stage. I have not used cathode bias . It's fixed bias now
    Click image for larger version. 

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    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.

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    Quote Originally Posted by Arbutt View Post
    Many thanks this is all I have on the power stage. I have not used cathode bias . It's fixed bias now
    Click image for larger version. 

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

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    Last edited by Helmholtz; 11-12-2018 at 05:39 PM.
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    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.

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    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.

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    Last edited by Helmholtz; 11-12-2018 at 05:46 PM.
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    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

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    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.
    Click image for larger version. 

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ID:	51072 this is correct sorry it's upside down
    Below are incorrect
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    Last edited by Arbutt; 11-12-2018 at 06:09 PM.

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    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.

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    If it still won't get loud enough, it's probably broken. - Steve Conner
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    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


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    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

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  14. #14
    Supporting Member eschertron's Avatar
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    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.

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    If it still won't get loud enough, it's probably broken. - Steve Conner
    If the thing works, stop fixing it. - Enzo
    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


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

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    Quote Originally Posted by Arbutt View Post
    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.

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    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.

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    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

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    Quote Originally Posted by Arbutt View Post
    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.

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    Last edited by Helmholtz; 11-12-2018 at 10:26 PM.
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    Supporting Member Jazz P Bass's Avatar
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    Here is a good read on the different PI types.
    http://www.valvewizard.co.uk/paraphase.html

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    Quote Originally Posted by Arbutt View Post
    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.

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    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.

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    Quote Originally Posted by Dave H View Post
    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.

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    Supporting Member eschertron's Avatar
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    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!

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    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
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    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 .

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    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.

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    If it still won't get loud enough, it's probably broken. - Steve Conner
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    We need more chaos in music, in art... I'm here to make it. - Justin Thomas
    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


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    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

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  28. #28
    Bent Member Chuck H's Avatar
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    Are you using two PI circuits then?

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

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

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    Bent Member Chuck H's Avatar
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    Are you using two PI circuits then?

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    Yes chuck I have a pi circuit for each channel .

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    Bent Member Chuck H's Avatar
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    Thank you. I just wanted to be clear about it because so far "the" pi has been referenced. Which doesn't imply two of them.

    I think the suggestion of using a LTP type phase inverter was a good idea. You can probably keep the NFB loop values the same, using the 1.5k cathode resistor value for the shunt resistor between the tail and ground.

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    "Never bet your life on somebody else doing their job." SoulFetish's good friend

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    Thank you Chuck . I will look at one later and post up when I want to use. And see what you guys think. I want to get this thing finished and off my bench.
    Mr a

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    Morning . Had a read last night on AC and DC long tailed pair pi. On valve wizard web site As jazz p bass said. I sort of understood it .
    I've decided to go for AC long tailed pair.
    I'm going to start it later today- tomorrow hopefully .
    If anyone has any suggestions
    http://www.valvewizard.co.uk/acltp.html
    Many thanks
    Mr a

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    Supporting Member eschertron's Avatar
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    We've been talking some time about the amp, as if it was only one channel. I never thought to ask if the "a little distorted" sound was on only one channel, or on both. I'm assuming both channels were wired up exactly the same way during any/all of the modifications that were made.

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    If it still won't get loud enough, it's probably broken. - Steve Conner
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    MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


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