Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J3ona1 on June 11, 2025, 04:23:26 pm
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Hi Everyone,
I'm putting together a schematic for an amp that gives me the preamp of a 6G3 Deluxe and the phase splitter/power amp of a Canadian amp called the Celestion from the Johnson sound system in Brandon Manitoba. It also has 6sj7 tremolo that's applied to the gridleak resistors for the 6l6's (not pictured in the schematic) . Would you be willing to review the schematic and let me know if you see any glaring errors or design issues. Thank you folks.
- David
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why are you running the 6L6 as a triode?
Unless that is a mistake, Maybe consider a triode/pentode switch
https://el34world.com/Forum/index.php?topic=28185.0
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I will run it as a pentode, sorry for leaving out the screen and suppression grids. I will put a 470R->1k 5watt on the screens. I dont want to choose between the two modes. Thank you for pointing that out scstill.
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in PS the fuse should be before the switch
the PT needs to have a 6v secondary with CT for 6v filaments
As a suggestion, you might consider using a schematic drawing tool for edits and to be easier to follow.
I use Scheme-it from digikey its free and does a decent job
https://www.digikey.com/en/schemeit/home
After you redraw with your tool of choice double/triple check your schematic against the one you are cloning.
Then post it back here...
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Will make these changes. I'll also add the trem circuit along with the rest of the schematic in scheme it and report back. Thanks scstill!
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Heres the schematic after putting it into an editor and adding the trem circuit. Let me know what you think. Thanks in advance for the time!
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Do you have the original schematic from the "Celestion Johnson" amp you are cloning?
Why do you have double .02uf coupling caps from the PI?
The 6L6 grid leaks need to be grounded
Why separate 6L6 cathode resistor and bypass? Often they are shared.
Any plan for v1b don't see it in circuit...
Hopefully others with more knowledge will weigh in
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Here's the original schematic, hand drawn by a tech in Canada that restored one of the amps. I didn't have a plan on using the other half of the 12ax7, though I am toying with the idea of using a one tube reverb driven by a 12dw7. I would us the 12ax7 triode for a tremolo oscillator, but I wanted to try the 6sJ7 trem from the original amp design. I can use a shared cathode resistor and bypass cap. There's no other reason for the separate resistors other than being able to use tubes that aren't well matched.
As for the double coupling caps I got them from the original schem and I also feel like the first .022 cap should've done the job of blocking DC and rolling off bass. Would that also mean that the .022uF on the grid of the lower 6L6 is also redundant?
With reference to the 6L6 grid leak resistors, it seems like they're dependent on the wiper of the intensity pot for their connection to ground. Bad idea considering that pots break/get dirty?
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that hand drawn Johnson has the 6L6 sharing a cathode resistor/bypass cap
Yours seems to show independent one for each tube.
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The only thing I can think of is that the second .02 cap is to deter the trem's oscillator signal from going to ground through the 470k resistor/ or toward a previous stage rather than going to the 6L6. Here are versions with the changes. One with the double caps and one without. I've also added the reverb with the 12dw7 tube.
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You might consider studying this grounding scheme and possibly implement in your design
https://el34world.com/charts/grounds.htm
Note for example how the power supply grounds are tied to the stage that it powers
Are you going to put your components on a board? terminal strip? or Point wired?
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Also on your input to the 12dw7 you possibly have an extra .022
if you take the input after the first coupling cap you might be able to eliminate the 2nd one at the 12dw7 grid.
Hoping that some others weigh in our your schematic.
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Thank you scstill!! I will definitely separate the preamp grounds from the grounds in the power amp. I'm hoping to use this schematic so that I can get a custom board made here. I'll only have terminal strips for chassis grounds and extra tie points.