UPDATE : Patrick sent along a picture of his amp and it’s actually the ’67 (?) medalist style. More info here : See Figure 7. More on that later today.
Reader Patrick has a GA5T that is not tremeloing for him. He’s debugging as we speak so we are going to follow along here.
First, he provides this schematic as being the most like his 5T :
First, lets make a few points. This schematic is for a pre-crestline 5T, so your box should look something like this :
Pre-Crestline Skylark
If your amp is dark brown with front facing controls, you have the wrong schematic.
Now, the tremelo on this amp is highlighted below.
Pre crestline 5T tremolo
What it is doing is creating a very slow (in electronics terms) oscillator that probably cycles between 0 and 2.5 volts or so, with the speed varying as you turn the knob. This is fed into the cathode of the ‘voltage amp’ stage (the second preamp). It’s very hard to read the voltages on the schematic, but it looks like a normal read on pin 3 of the voltage amp side is 2.3 or 2.5 V. To this we add the oscillating signal so the bias voltage changes from 2.5 to 5.0 during the cycle. The higher bias voltages (on the cathode of V1, which is pin 3) result in lower output into the power amp during the higher part of the oscillator cycle.
Ok. So that whole oscillator circuit exists to make V1, pin 3 steady at 2.5ish Volts when the Trem is off and between 2.5 and probably 5 when it is on. We are told that the amp sounds great but the trem has no effect.
Required disclaimer here :
You are not a pro and neither am I. These things work with real killing voltages/power. Do not make a mistake. One hand in pocket, use chopsticks to move things. You need to be serious about safety. Ok? You die = you problem.
Ok. Now, some things are easy to test.
Is the switch working? That’s an easy power off test. What about a tube swap on V4? I’m surprised they used the 12AX7a there since that leaves a whole triode section unused. But you should have easy access to other tubes to try out.
Next : What is the operating voltage on V1 pin 3 when the Trem is off? I think it should be near 2.5VDC. Does it change at all when you turn the knob? If you are only looking with a DMM, this will probably manifest as a failure to lock a voltage, but if the DMM autoranges quickly you may get to watch the voltages oscillate. Otherwise it’s time to break out the scope.
Lets try that and see what we come up with.
Here is a link to the style of GA5t amp that I have, http://www.0rigami.com/gg/skylark_4.jpg
This bit I found on harpamps.com about how the tubes are divided up, ”
The GA-5T is a push-pull 6AQ5 amp with a 10″ so-called Gibson Ultrasonic speaker. A 6X4 is the rectifier. It uses 2 6EU7 tubes: 2 halfs for the preamp, 1 half for the tremelo oscillator, and 1 half for the phase splitter. It has a tremolo frequency control, no depth control, and a volume control that I believe serves as the power switch as well.”
Ok.. whoa! You have a post crestline Skylark. Just before the first transistor versions. That schematic you have is from the Gibson Master Service Manual which (I thought) was released in ’64. So.. something is fishy. However, you have a tone control and one exists on the amp, so I suspect you have the right schematic and I was confused. That tone knob was not seen on Skylarks prior to ’65.
That blurb you found is for the brown crestline style. They have two 6AQ5′s in push-pull configuration. Yours is a single 6BQ5 output tube (smaller package 6V6).
I never noticed it before but your amp is straight up SE (like the boxy blondes of the 50′s and very early 60′s) but with a tone knob. I’ll bet she rocks, depending on the condition of that speaker.
I’ll post your pics tomorrow but you probably need to do the tests I suggested above for us to go to the next step.
The research I have done on this amp I have found that the schematics and actual layouts don’t always match.
If you look at picture DSC06419 you will see the black Sprague cap. It is not soldered to the socket closest the volume knob. Does this cap have any effect on the tremolo? Also, I am not that familiar with tube amp, diagnosing them, so looking at that picture, how are the pins oriented clockwise/counter clockwise. etc.?
“The research …”
Oh yeah.. that’s a guarantee. Even if it is bone stock, it’s probably not an exact match for any given schematic. Shouldn’t be too different though, a value here or there. Either out of debugging or just from what was on hand to fill an order.
I’ll post up your picks later. I’ll be honest though, if you’ve never done this.. sit down and read anything you can about safety. You will get shocked, face it and do your best to mitigate the effects before hand. These amps are very small but the exact wrong move can still be fatal.
Get out a magnifying glass. The bottom of the tube socket has the numbers on them but it’s usually hard to read. Clockwise from the gap if memory serves.
I agree about the shock factor. I plan to build a capacitor draining device tomorrow.
Thanks.