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  1. That's cool! An AC30 has no negative feedback...

    That's cool! An AC30 has no negative feedback though right?

    My goal with this is to just knock a little volume off the top. I'm not trying to turn this into a bedroom amp.
  2. Understood. Thanks. As long as this type of MV is...

    Understood. Thanks. As long as this type of MV is safe for my application, I'm gonna give it a try.
  3. Okay sorry. I'll just try it and see what happens.

    Okay sorry. I'll just try it and see what happens.
  4. Ok fair enough. I don't pretend to know this...

    Ok fair enough. I don't pretend to know this stuff.

    I thought it was like how much or how little cancellation happens depends on where the wiper is at. Turn it down...lots of cancellation. Turn...
  5. That whooshing sound is all of that going right...

    That whooshing sound is all of that going right over my head! :happy:

    I'm looking at it like it's taking a feed from each side of the phase inverter and summing them in the pot. They'll cancel...
  6. Isn't that sort of what the Type 3 does? It mixes...

    Isn't that sort of what the Type 3 does? It mixes the out of phase signals to cancel them out and reduce volume?
  7. Ok thank you. I mainly just want to make sure...

    Ok thank you. I mainly just want to make sure it'll work. It seems like it's safe since no components are actually removed. If the pot fails I still have bias! I don't mind the reduced presence/NFB...
  8. New question if you don't mind.... I think I...

    New question if you don't mind....

    I think I might put a PPIMV on this thing. I like using it loud and proud in the studio, but maybe sometimes I might wanna gig with it.

    I've put LarMar PPIMVs...
  9. These are Sovtek 6550WE tubes. The spec sheet...

    These are Sovtek 6550WE tubes. The spec sheet says 42 watts, so that's how I did my figures.
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    I'm no expert but I just built one of these amps....

    I'm no expert but I just built one of these amps. If it were me I would check everything that's involved to get -35vdc at the grids. The AB868 calls for -35vdc at the grids. I understand suspecting...
  11. Yeah thanks. I was thinking the medium bias had...

    Yeah thanks. I was thinking the medium bias had about the same sound as "hot" and it makes my nerves feel a little better with the tubes being a little cooler. Lol. Thanks for checking it out and all...
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    https://robrobinette.com/images/Guitar/Deluxe_Mode...

    https://robrobinette.com/images/Guitar/Deluxe_Models/AB763_Deluxe_Reverb_Annotated_Schematic.jpg

    I guess first thing is to see why you can't get -35v at the grid leaks.
  13. Update.... All seems to be going well so far....

    Update....

    All seems to be going well so far. Nothing has exploded and the amp sounds pretty righteous to me.

    Here's some sound samples if you're interested.

    1978 Gibson Les Paul Custom -...
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    I'm no expert by any means, so anything I might...

    I'm no expert by any means, so anything I might suggest is just thinking out loud right along with you. But I am a guitar player, and I too tinker with amps, and the distortion you're getting is IMO...
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    Have you tried injecting a sine wave instead of...

    Have you tried injecting a sine wave instead of banging a guitar chord? Might be easier to trace if you're not having to play guitar at the same time.
  16. Awesome! I love the 2203 circuit...maybe even...

    Awesome! I love the 2203 circuit...maybe even more than the 1987/1959 circuit for most things. I love P90s too. We probably have mostly the same kind of taste in guitar tone.



    Cool man!!!
  17. There is still a fairly large number of 6550...

    There is still a fairly large number of 6550 equipped JCM 800s out there. Those started in 1981 and many of them over here originally had 6550s in them. It's not uncommon at all to see them on the...
  18. Yeah, I think it's pretty well documented that a...

    Yeah, I think it's pretty well documented that a long run of Marshalls from the early 70s and into the 80s came to the US slated for 6550s. Dealers in the US got tired of warranty work on broken...
  19. From previous experience, I'd say the 6550s are a...

    From previous experience, I'd say the 6550s are a little tighter, more bass, more highs, normal mids. Cleaner headroom. EL34s are mild in the lows and highs and rage in the mids. Good compression and...
  20. Thank you. Yeah my thought process was to get the...

    Thank you. Yeah my thought process was to get the negative readings at the grids close first, without the tubes installed just to keep everything safe. Then from there I put the tubes in and fine...
  21. Bingo. I strapped a 56k across the 27k to get the...

    Bingo. I strapped a 56k across the 27k to get the bias drop to about 18k. That gets my 6550s nailed down at 53-55ma with the bias trim pot in the middle of it's range. Good call.
  22. I think what I'm going to try is...with no power...

    I think what I'm going to try is...with no power tubes installed...juggle resistance values until I can get around -50vdc at the grids with the bias pot near it's middle setting so I have wiggle room...
  23. Lol. Deal.

    Lol. Deal.
  24. Also, I'd like to offer my sincerest thanks to...

    Also, I'd like to offer my sincerest thanks to all of you that have offered your expertise and help so far. I'm learning a lot from "problems" like this. I wish there was some way that I could...
  25. Thanks man. I too am glad I sort backed my way...

    Thanks man. I too am glad I sort backed my way into it working...so far. I still haven't fine tuned it or played it. But what I've got looks promising.


    I've done the opposite to what I'm doing...
  26. Okay, thank you.

    Okay, thank you.
  27. It's entirely possible, and likely, I just did...

    It's entirely possible, and likely, I just did something wrong before. I put the bias circuit to 6550 specs and got nothing. I put it back to EL34 specs and everything works. Who knows.




    ...
  28. Well friends, I'm not exactly sure what's...

    Well friends, I'm not exactly sure what's happened, but with the amp's bias supply back at EL34 specs, these 6550s are working. Working too well actually. They are pulling the juice! I can't get the...
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