1954 Gibson GA-40 tremolo

[Diverted]

A friend of mine has a 1954 Gibson GA-40 with a weird early three-position tremolo speed switch he doesn’t like.
Looking at it it, would it be possible to use a two-gang pot to give better speed control?
The amp is not in collector condition. It’s had its transformers replaced and is in pretty rough shape but sounds good; so the goal is to make it a more usable player’s, not collector’s, amp.

Here’s the schematic:
https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga_40_54_early.pdf

[PRR]

This would make a lot more sense if there were a cap from V4b grid to ground. As is, it looks wrong. No reason to have two resistor strings.

[Diverted]

It’s correct; the amp matches the schematic in the pic and in the link.

[Diverted]

No reason to have two resistor strings.

The total resistance of the string is about 3.3m. Would I be able to use a single gang 3m pot here, with wiper going to v4b grid, low side to ground and high side to c18?

[2deaf]

No reason to have two resistor strings.

The total resistance of the string is about 3.3m. Would I be able to use a single gang 3m pot here, with wiper going to v4b grid, low side to ground and high side to c18?

A dual ganged 1M linear pot and two 820K resistors would probably work pretty good.

There are two strings of resistors so that the voltage divider for the grid of V4B is always 50/50 while the series resistance for C18 changes.

I'll draw out what I mean within the hour.

[Diverted]

Thank you!

[2deaf]

Here ya go.

[Diverted]

Thanks, 2Deaf. I should apologize; just noticed in looking at annotated schematic I posted that I used 330. It's actually 330K all around; was scribbling on it on my phone and room was tight so no K.

And, just a side point: The "slow" level on this tremolo is still quite fast. If I were to slow the baseline down, would I up the value of those 820Ks?

Thanks for that great schematic. I'll post some pix and clips once I get to it.

Ted

[Bill McKenna]

Hi there tube amp experts,I have this exact amp on the bench now and have questions about the circuit. I can't understand what Gibson was thinking with this switch. The 2 sides are wired mirror image, Resistance to ground of each side are  1.6M,(fast) 1.3M (med) 1M (slow) on to the grid of V4b?  the other side going to the 6V6. is the reverse to ground? 1.6M (slow), 1.3M (med) 1M (fast).  And reads 2.6m to ground. ( the sum of the entire  chain).
On this example the tremolo works but there is no change in the speed. Its always a pretty fast trem speed on all settings? Tubes and wiring have been tripled checked.    Can anyone explain how this resistance shift to the grid of v4b is causing oscillation?  Usually I expect a chain of 3 Caps .01 or .02 doing this? There appear only coupling caps in this one?  Thanks in advance for any help on what the heck Gibson was thinking Perhaps I should rip it out, but its a pretty nice example and would love to preserve the 50s oddness :)thanksBill

[sluckey]

V4B provides 180° of phase shift. When combined with the phase shift of the other R/C networks, that's more than enough to sustain oscillation.

[PRR]

I still say there's no frequency control unless there is a second cap to make a Wien.

[Bill McKenna]

Thanks PRR and Sluckey,So the thought is that Gibson made something that didn't work right and shipped it for several years? or there is a missing cap in the various schematics online? Seems hard to imagine? I have verified everything is as shown in the schematic, and that changing the resistance to Grid of V4b but no change in either intensity or speed. Which seems reasonable considering the design. Curious if anyone here has one or experience with a properly working 55 GA 40?  Looking through the archives I see this amp has been discussed a number of times, however the Trem was less the focus. Mine sounds great other than the trem only being on/off and at an unusable speed and level.  I have to think that the 6V6 is doing the right thing as I am hearing strong pulse in the output.
I have been mostly able to self support over the past few years but this thing has me stumped :)
ThanksBill

[bmccowan]

One of those threads referenced is an amp I worked on. It took a lot of head scratching to figure out that damn trem circuit. Its not a great trem, but the amp sounds really good straight up. And besides that, it just looks so cool! I am snowed-in in a camp in Maine right now, but when I get home I will check mine out and see what I can offer.