Help with Four channel amp with both 6v6’s and 6L6’s build?

Jerry garrcia

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I made a layout with grid leak bias. If to make a switch to cathode bias, what is the best way? Using a turret board and have a regular on of switch with two terminals. Also a 2 pole switch
 

printer2

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I made a layout with grid leak bias. If to make a switch to cathode bias, what is the best way? Using a turret board and have a regular on of switch with two terminals. Also a 2 pole switch
Not too much involved. Do a cathode bias circuit with a 4.7 M or above resistor for the grid. Parallel the 4.7M with a 1M which is connected to one terminal of the switch. The cathode of the tube to the other end of the switch (or the cathode resistor) and the middle terminal to ground. When in the 1M grounded position the cathode resistor is functioning. In the other position the 4.7M is functioning and the resistor (and cathode) is shorted to ground.
 

Jerry garrcia

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I’ve been contemplating over a glass of Japanese whiskey and listening to Kurt Rosenwinkel.
Don’t really think I’ve got the 6SJ7 bias switch right and struggling with the tone circuit.
Any inputs would be highly appreciated.
76C64FDA-3682-46CD-8DF2-5C69480B2FA5.jpeg

I’m a physician so my writing is so and so…
 

Jerry garrcia

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Just realised that the cathode resistor had the wrong value 🤯. It should be 2.2K instead of the 100K that I wrote. Aiming for the instrument channel of the GA-50.
 

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Jerry garrcia

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@printer2
Been processing the bias switch. You surely mess up my already bad sleeping habits.
Came up with two ideas where hopefully one of them is what you suggested.
1)
B98F69D7-B94F-4C57-BE3F-62E91158DA0B.jpeg


2)
6EF55642-6ACC-455F-9A42-066EC9BADB29.jpeg


Must be a on-off-on switch I presume?
 

Jerry garrcia

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In neither any of the above switch layouts there is a cathode straight to ground as needed for the grid bias. Need to wake up those sleep depressed brain cells
 

printer2

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On the second drawing you have the wire to the switch on the wrong side of the cathode resistor.
 

Jerry garrcia

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On the second drawing you have the wire to the switch on the wrong side of the cathode resistor.
Aha. So if I connect a wire from the turret between the cathode and the 2.2K resistor and have a one pole on-off-on switch it’s correct?
Kind of make sense
 

printer2

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Aha. So if I connect a wire from the turret between the cathode and the 2.2K resistor and have a one pole on-off-on switch it’s correct?
Kind of make sense
On-On, do not need a center position, will not really hurt though. Actually would have the 4.7M resistor trying to sett the bias and the cathode resistor doing the same. Would it make for an interesting sound? No idea, have not tried it yet. I have not had a chance to try messing around with your tone control, the weather has been nice and I have been taking advantage of it outside.
 

Jerry garrcia

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On-On, do not need a center position, will not really hurt though. Actually would have the 4.7M resistor trying to sett the bias and the cathode resistor doing the same. Would it make for an interesting sound? No idea, have not tried it yet. I have not had a chance to try messing around with your tone control, the weather has been nice and I have been taking advantage of it outside.
Seen your idea around in some forums when I tried to look around yesterday. But not anything about it ever being built. Guess I’ll soon know. Ordered the two kinds and I’ll mess around with them when I’m back to town.
No hurry about that tone control in the GA-5/5F1 reverb. Haven’t really cranked the amp up yet to the level when it starts to go crazy. Will also mess around with the reverb mix and dwell to get that perfect spot with just a hair of reverb.

Also an other question. It’s regarding the on off option for the amp. I’ve used both a regular on-off switch and a pot with on-off. It seem like quite a few doesn’t like the pot with the on-off. Why is that? Any specific logical reasons?

I might be away for some time now.
 

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Sorry I'm late to the party, and haven't followed the entire thread, but the annotated schematic in post #17 was mine. You can probably search for a thread about GA-50 Rebuild and learn more. Notice that I changed the 6SJ7 plate resistor just for personal preference for a less aggressive channel. Of course you can experiment for yourself.

I have two GA-25's, a GA-50 and two GA-75's. If you want clean, the plate resistor change on the 6SJ7 is the point of interest, otherwise I think those old Gibsons want for nothing else. Do not do grid leak bias IMO, just counter-productive to the goal of clean-ish.

The GA-50 in 1948 was the first guitar amp with separate bass and treble controls. The GA-75 uses the same circuit. To me, it has a leg up on the 5E3, which I also have. Both GA-75's are on my 'rebuild' list, but are very much a GA-50 with a 15" speaker.
 

printer2

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Seen your idea around in some forums when I tried to look around yesterday. But not anything about it ever being built. Guess I’ll soon know. Ordered the two kinds and I’ll mess around with them when I’m back to town.
No hurry about that tone control in the GA-5/5F1 reverb. Haven’t really cranked the amp up yet to the level when it starts to go crazy. Will also mess around with the reverb mix and dwell to get that perfect spot with just a hair of reverb.

Also an other question. It’s regarding the on off option for the amp. I’ve used both a regular on-off switch and a pot with on-off. It seem like quite a few doesn’t like the pot with the on-off. Why is that? Any specific logical reasons?

I might be away for some time now.
I tried the cathode and grid swith a long time ago, I recall it doing its stuff and it adds a little different spice to the amp. I did use a different pentode so I have no idea how the 6SJ7 will perform with it. On the power switch, I guess if peoole have the room for a seperate power switch that is what people chose. It is only when you take up the usable panel space you need a switch on a pot. Stay safe there bud.
 

Jerry garrcia

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Stay safe there bud.
Back and fine. Eager to build. Since this is a different build and not the amp I started this thread about, I’m thinking of starting a new thread. The two channel amp 5C2+optional 6SJ7 grid leak/cathode biased circuit 🤔.
 
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