Gibson GA-8T 120hz.Hum & No Tremolo

[misterc57]

Schematic attached. Seems to be a lot of articles on this issue which I have yet to read but I will. So this is a common issue?

I am bringing back to life a 1961 Gibson GA-8T. I acquired it with no OT and a rats nest of PS caps installed. I took out the rats nest and installed new PS caps to spec consisting of a 22 uF, 10 uF, and 10 uF, all rated at 450V. I installed an old Hammond organ OT that was used to output a push pull pair of EL84 (the GA-8T uses a pair of 6BM8).

I am having two problems with the PS. Voltages are much higher than specs and there is a loud 120 hz hum. The output tubes had a 1.75K and 22uF/160V in the cathode bias. I have tried different value cathode resistors and have recorded the following V at pin 8 of the rectifier tube which schematic shows should be 270 VDC

cathode resistor - VDC
1.75K - 362V
370R - 311V
220R - 293V

The lower the cathode R value, the louder the hum. I increased both 10 uF PS caps to 20 uF but still have loud hum.

My tremolo is also not working but I want to focus on the PS first and try to get tube voltages close to the schematic with no hum. Any help and guidance is greatly appreciated.

Thank you! MarkO

[misterc57]

attaching 6BM8 tube sheet

6bm8ecl82 (1).pdf

[Helmholtz]

Are the output tubes balanced, i.e. drawing similar idle currents?

Try the OT resistance method to measure idle currents.

[Jon Snell]

Measure the voltage on pin 3 of V2 and V3. They should be similar.
If they are not either the coupling capacitor is leaking and unbalancing the output valves or the valve with more than about zero volts on its grid is probably faulty.
If it were heater cathode leak, you woud have 60HZ ripple not 120HZ.
The HT can have 50 volts of ripple on it and if the valves are balanced, will not produce hum.
ECL82s or 6BM8s are fairly innexpensive.

[misterc57]

Output tubes are probably not matched. Not even same brand.

Here are some readings. Not sure I did it correctly, I measured from rectifier pin 8 to plates (pin 6). This was done with a 220R on the cathode bias.

[g1]

Here are some readings. Not sure I did it correctly, I measured from rectifier pin 8 to plates (pin 6). This was done with a 220R on the cathode bias.

You need to do the voltage and resistance measurements from center-tap of OT primary, not from rectifier pin 8. And those measurements are to each power tube plate, so you end up with current per tube, you do not divide by 2 at the end.

[Enzo]

Seems to be a lot of articles on this issue which I have yet to read but I will. So this is a common issue?

Common? I can think of no other issues more common than hum in a tube amp.

Hum comes from a whole passel of sources, each having its own cure. Hum in a tube amp is not a very specific problem.


The lower in value you make the cathode resistor in the power tube circuit, the hotter the stage runs, so any hum will likely be increased by that.

Your power supply voltage seems high, yes. The chart also calls out 280vAC on the plates of V4, what do you have? If you have 350vAC there or something, it isn't the poor power tubes' fault. And whatever that is, how much ripple do you measure on the B+, at pin 8 for example?

Jon asks after the grids of the power tubes - pins 3. Turn the trem to zero and ignore it. Now there SHOULD be zero volts DC on those pins 3. Is that the case?

Is your rectifier tube a 5Y3?

Iis that giant black beauty cap near the end of the strip board the 0.25uf from the trem plate? In my experienec those caps haven't aged well, it could be leaky and not only killing trem, but also putting unwanted DC on your grids. Worth a check.

[misterc57]

Pin 3 of both output tubes are close, both reading around 0 volts within mV.

Updated readings using OT center tap to plates (pin 6). Looks like I need to find a matched pair of 6BM8. Should resistance be different on each side of the OT?

I am ignoring the last column of data.

[Enzo]

Many OTs have uneven RESISTANCE on the primary halves. The transformer works on turns ratios, not resistance so it is OK.

[Helmholtz]

Updated readings using OT center tap to plates (pin 6). Looks like I need to find a matched pair of 6BM8. Should resistance be different on each side of the OT?

As Enzo said, somewhat different OT DCRs per side are quite common (especially with smaller OTs).

An idle current of 55mA is certainly way too much.
Cathode biased tubes can be biased up to 100% plate dissipation, i.e. 7W here.
To calculate plate dissipation, multiply idle current with plate voltage minus cathode voltage.

The different plate currents could be the cause of the 120Hz hum.
Also high idle current increases ripple hum.

[SoulFetish]

Hey dude, in you photos, it looks like the HT center is connected to an unused pin on the rectifier tube socket (This is common in Gibson and other vintage amps). However, It also looks like there is a black wire connected to the same lug that is just floating unconnected at the other end. I'm willing to bet your hum is due to either the Transformer CT wire not connected to the power supply ground, or there is a break in the ground continuity at the grounded side of the 22µF/10µF junction.
Check the grounds.

But, these tubes can be noisy as well. I would definitely grab another pair of 2. You can chase noise around your amp for days, only to find out that it was caused by one or more of your tubes. It's should be the simplest and first thing to eliminate when troubleshooting noise in a tube amp.

[misterc57]

Hey dude, in you photos, it looks like the HT center is connected to an unused pin on the rectifier tube socket (This is common in Gibson and other vintage amps). However, It also looks like there is a black wire connected to the same lug that is just floating unconnected at the other end. I'm willing to bet your hum is due to either the Transformer CT wire not connected to the power supply ground, or there is a break in the ground continuity at the grounded side of the 22µF/10µF junction.
Check the grounds.

But, these tubes can be noisy as well. I would definitely grab another pair of 2. You can chase noise around your amp for days, only to find out that it was caused by one or more of your tubes. It's should be the simplest and first thing to eliminate when troubleshooting noise in a tube amp.

Thanks for taking a close look. That floating black wire is left over from the rats nest that was in there. It is actually connected to ground at the tube pin side and I am using the other end to clip to ground when testing. All of my large PS caps negative side are at a new terminal strip with a good ground connection.

Getting matching tubes are my next step.

MarkO

[SoulFetish]

Is the chassis grounded at only one point?

[misterc57]

Is the chassis grounded at only one point?

Edit...

No. I think I have 3 ground points. And I have some questions about them.

• pin 3 of the rectifier tube which has 2 wires from the PT (I think the center taps for filament and HT) and the floating black wire. There is also a white wire connecting the grounds of the input jacks and pots. I am not sure how pin 3 is grounded (perhaps from the input jacks connected to the chassis)
• there is a chassis lug attached to the PT bolt where my mains ground wire is connected, and another "mystery to me" wire coming from the PT.
• I installed a new terminal strip to another PT bolt where my 3 large PS caps are grounded.

[misterc57]

2 6BM8 tubes arrived (labelled C and D). Tried them and still not getting a match. Tried some different combinations with the original tubes (labelled A and B).

I am still struggling to get a match. Tested value of a tube does not stick with the tube, it changes depending on the paired tube.

Hum still present on every test. I need suggestions please.

Thank you.