Gibson GA50T frequency pot

[pontiacpete]

Hi All,
I have one of these that is having a problem with the tremolo. The frequency pot turns 360 degrees so I was thinking of changing it . But it is a dual gang pot. I have a feeling this might be a reverse taper style pot. I tried a linear and the trem works but the speed is fast at the minimal setting and nothing at the fastest.
Is this meant to be a reverse taper?
If so where can one be found?

Thanks
pete

[nosaj]

Hi All,
I have one of these that is having a problem with the tremolo. The frequency pot turns 360 degrees so I was thinking of changing it . But it is a dual gang pot. I have a feeling this might be a reverse taper style pot. I tried a linear and the trem works but the speed is fast at the minimal setting and nothing at the fastest.
Is this meant to be a reverse taper?
If so where can one be found?

Thanks
pete

Dumb question. Does the pot turn 360 or the knob?
Older pots can be taken apart and fixed sometimes.
nosaj

[pontiacpete]

It definitely spins without knob.
I will try to repair it if can't locate a new one. That is if it is a reverse taper.

[Dave H]

The dreaded single pixel. Is this it?



It looks like it's a Wein bridge. The Wein bridge frequency is proportional to 1/R [f=1/(2piRC)] I think it would work better with a reverse log pot.

[pontiacpete]

Thanks for posting that? I wonder what happen there

Also thanks for confirming the taper. I don't understand this kind of oscillator. Gotta look up wein bridge

[Tom Phillips]

...I have a feeling this might be a reverse taper style pot. I tried a linear and the trem works but the speed is fast at the minimal setting and nothing at the fastest.
Is this meant to be a reverse taper?
If so where can one be found? ...

A dual linear pot of the 250k value shown on the schematic should work at the extreme high and low settings. It just won't produce a nicely useful frequency change rate as it is rotated. Since you found that "the speed is fast at the minimal setting and nothing at the fastest." then there is something else (or something additional to the original pot) wrong with the circuit. (Or the replacement pot installation may have a wiring mistake. Or not be 250kΩ within ~± 20%. Always good to double check.

I suggest a two pronged approach. You could:
1) Leave the dual linear pot in the circuit for now and troubleshoot the oscillator circuit
2) Work in parallel to look inside the original pot and determine if it can be repaired. It would be good to post photos of the pot repair process.

Tom

[nosaj]

It definitely spins without knob.
I will try to repair it if can't locate a new one. That is if it is a reverse taper.

If you get another dual gang pot you could take both apart and put your reverse taper Carbon elements in the new pot chassis.

nosaj

[Dave H]

If you can't find a reverse log pot you could use a log pot with the outside connection wired to the oposite end. The high speed will now be at the other end but the taper will be the same.

[pontiacpete]

The 'original' pot looks like it had been opened up previously.
I put it back in and found it does the same thing the 250KL pot I put in, ie, tremolo only shows up when turning all the way to the minimum setting and it's fast. As it's increased oscillation seems to get slightly faster then disappears.
The wiring in the pictures is a bit different from the schematic above. The 9.1K goes directly to ground and the .1 on the wiper of the trim pot(which is a 5K pot not 1K)is omitted.
I tried rewiring it to the schematic and there's no oscillation.
Now I'm even more confused. Is the 'original' pot the right taper afterall? Or what's goin ?
I provided pictures of the oscillator tube and some shots of the pot installed.
Thanks for the help. I greatly appreciate it.

[Tom Phillips]

The 'original' pot looks like it had been opened up previously.
I put it back in and found it does the same thing the 250KL pot I put in, ie, tremolo only shows up when turning all the way to the minimum setting and it's fast. As it's increased oscillation seems to get slightly faster then disappears.
The wiring in the pictures is a bit different from the schematic above. The 9.1K goes directly to ground and the .1 on the wiper of the trim pot(which is a 5K pot not 1K)is omitted.
I tried rewiring it to the schematic and there's no oscillation.
Now I'm even more confused. Is the 'original' pot the right taper afterall? Or what's goin ?
I provided pictures of the oscillator tube and some shots of the pot installed.
Thanks for the help. I greatly appreciate it.

The pictures are very helpful and tell the story that the amp has been worked on several times during its life. This is not surprising and it doesn’t look like anything has been permanently destroyed. I assume that the trim pot in the V7A cathode circuit is the one shown attached to the rear of the two gang tremolo speed pot. The missing 0.1μF cap would for sure have prevented the LFO from operating properly. Who knows what else has been screwed up by previous repair attempts.

If the amp was on my bench I would do the following:

1. Trace out the tremolo circuit to verify proper connections. I like to use a highlighter to mark up a copy of the schematic as I check.
2. In the process I would verify the actual values of the components. The CC resistors will probably be found to have drifted substantially upward in value. I see several 5% tolerance resistors in the photo. Those have already been changed out since the amp was built because the manufacturer would not have used 5 percenters. For a first pass I wouldn’t bother to change any that are still within ±10% of the value shown on the schematic.
3. Other standard troubleshooting will apply such as the voltages in the circuit and a known good 6SL7 to if the circuit doesn’t work properly after the checks and circuit corrections.

The LFO must be sensitive to the bias set point of V7A or the 1000Ω trimmer pot wouldn’t be in the circuit. I don’t find any instructions for setting that. I would just change it back to 1000Ω and experiment with different set points after the rest of the circuit is verified.

Edit: Added an image I found showing an original GA-50T frequency pot installation.

[pontiacpete]

Thanks Tom!
Interesting things you mention. Yes, someone replaced all the components but I didn't think the 5% might have been part of the refurb. They certainly are very old, looks as though they are from the 50s but they are still measuring within range. The only 2 caps that are original are the big .25uf metal box types screwed down to the chassis(I didn't post a picture of these). Anyway all looks good but one thing which your picture seems to agree with me on, the position of the 9.1K on the cathode of the V7A. In your picture it clearly looks to me like it goes to ground and not the pot. It works better like this. Oh and I added in the .1 at the grid of V7A to ground and the 270Ks. This helped
The other thing you pointed out was the most obvious and of course I over looked was the 6SL7. This amp had a 6SN7. That helped things along.
So now it's working where I think it should be, a little thumpy but it's the best I was able to get with the 1k trim pot.

One more thing I think helps is removing the 15k resistor at the switch at V7Bs cathode. I've seen this before and never understood why it's there. The trem never really turns off with it, so I removed it.
Thanks again!!

edit: oh, I added 2 more solder spots on the top of the pot like the other tech had done. THis secured the top and now it's not spinning like it was.

[Tom Phillips]

...one thing which your picture seems to agree with me on, the position of the 9.1K on the cathode of the V7A. In your picture it clearly looks to me like it goes to ground and not the pot. It works better like this...

I think the amp in the photo has the 1k pot (wired as a rheostat) connected to the V7A cathode then the rheostat is connected to ground through the 9.1k resistor. I.e same series circuit as shown in the schematic but with the position of the two parts swapped. I'm not sure what you mean by "It works better like this"

Glad you got the amp sorted out.

Tom