1969 Gibson GA5-T Amp PS Caps

[Phil_S]

FWIW, I'll second Martin on the probable ground loop as a source of hum. Clip the jumper and use one of the outer lugs on the terminal strip. The middle lug is grounded through the screw. Don't use that one.

Also, remove that zip tie that bundles the a/c power supply wires with the transformer wires. The AC line feed should be well separated from everything else.

Martin, what do you make of the low plate voltage on the tremolo tube? This is fed from B+2, which must be right around the schematic stated 250V. I'm thinking either the tube needs to be replaced or the plate load resistor should be dropped to 100K. Plate current seems to be about half of what it potentially could be. How's it going to oscillate if there's no oomph?

I wonder, too, if the 100K dropping resistor in the B+ ladder is excessive, but I'm out of my level of expertise here. If there was higher plate voltage on the 12AX7 preamp tube, it would hit the power tube with more signal. In turn, that might wiggle the cathode, which controls the tremolo. I keep thinking that should be about 10K, not 100K. if you want to drop 50 volts and the two triodes on the last B+ node pull 5mA, that computes to 10K as the dropping R.

He's also got too much ripple coming off the rectifier. I wonder if that tube has also seen better days. 20-10-10 ought to be adequate filtering for this sort of amp.

Texstrat, can you please measure VDC and VAC a each of the + leads of the filter caps and post the information?

[martin manning]

Using the terminal strip is fine, just don't use the center ground lug. What I'm suggesting is to move the lead from the larger cap over to one of the outside lugs, and then run the jumper between the two outside lugs only so that all of the negative cap leads are connected together. The black wire then goes to the tube pin with the red/yellow CT wire on it.

If you have more terminal strips you could use two, with four lugs each (three un-grounded), and place one on either end of the cap bundle. That would be a more secure mounting.

[texstrat]

So skip the middle lug with the jumper wire and connect the two outer lugs together with the black lead?

After replacing V1 and V4 with new tubes I checked V1 pin 3 new 0.87V previous 1.13V, V2 pin 3 new 8.20V previous 8.10V and V4 pin3 new 0.87V previous 1.13V. V4 pin 1 new 136V previous 52V.

Any thoughts.

[texstrat]

Using the terminal strip is fine, just don't use the center ground lug. What I'm suggesting is to move the lead from the larger cap over to one of the outside lugs, and then run the jumper between the two outside lugs only so that all of the negative cap leads are connected together. The black wire then goes to the tube pin with the red/yellow CT wire on it.

If you have more terminal strips you could use two, with four lugs each (three un-grounded), and place one on either end of the cap bundle. That would be a more secure mounting.

I understand now. I secured the caps with high temperature automotive silicon.

Would the ground loop be causing some of the voltage issues?

[Phil_S]

Ground loop causes hum. Any change on the tremolo with 136v?

[martin manning]

Also, remove that zip tie that bundles the a/c power supply wires with the transformer wires. The AC line feed should be well separated from everything else.

I don't think that is a problem; it's done all the time.

Martin, what do you make of the low plate voltage on the tremolo tube? This is fed from B+2, which must be right around the schematic stated 250V. I'm thinking either the tube needs to be replaced or the plate load resistor should be dropped to 100K. Plate current seems to be about half of what it potentially could be. How's it going to oscillate if there's no oomph?

I wonder, too, if the 100K dropping resistor in the B+ ladder is excessive, but I'm out of my level of expertise here. If there was higher plate voltage on the 12AX7 preamp tube, it would hit the power tube with more signal. In turn, that might wiggle the cathode, which controls the tremolo. I keep thinking that should be about 10K, not 100K. if you want to drop 50 volts and the two triodes on the last B+ node pull 5mA, that computes to 10K as the dropping R.

He's also got too much ripple coming off the rectifier. I wonder if that tube has also seen better days. 20-10-10 ought to be adequate filtering for this sort of amp.

That's why I suggested swapping V1 and V4, and I also wondered about the rectifier with all that ripple, now that we know the filter caps are good... I agree that 100k seems like a large dropping resistor, but the thing is supposed to have been working previously.

[martin manning]

OK, V4's plate looks good now; got another 6X4 to try?

[texstrat]

The hum/interference is still there. I noticed it when I turned the tremolo on, which is now working. The noise was in sync with the tremolo. Also, while I was checking voltages, moving the positive probe above the components the noise would ebb and flow with the movement.

I do not have any 6X4 or 6BQ5 on hand, but I am getting the impression that the tubes are on their last leg?

Here are a couple of pictures of the rewired ps cap ground and the component layout, maybe I need to run the leads differently to prevent the hum/interference.

Here are the voltages for the positive ps leads:

Blue
264VDC
70VAC

Red
285VDC
70VAC

Green
158VDC
70VAC

[martin manning]

Progress! The ripple voltage is still ~3x too high, though, and I'd be surprised if that isn't contributing to the hum. I guess I'd start looking for a new 6X4 and EL84/6BQ5.

[texstrat]

Progress! The ripple voltage is still ~3x too high, though, and I'd be surprised if that isn't contributing to the hum. I guess I'd start looking for a new 6X4 and EL84/6BQ5.

Yes...progress. Where is the ripple voltage X3 too high and any recommendations on a brand for the 6X4 and 6BQ5 tubes. Also, since I am placing an order for tubes, should I go ahead and pick up some new orange drops as well?

For the 12A7X's I have one Mesa Boogie and one Groove Tube. Which is the better for V1 and V4?

Thanks.

[martin manning]

Where is the ripple voltage X3 too high and any recommendations on a brand for the 6X4 and 6BQ5 tubes. Also, since I am placing an order for tubes, should I go ahead and pick up some new orange drops as well?

You want to see AC ripple voltage around 10% of the DC, so 25-30V at the 280V first filter. I don't know anything about 6X4s but I see that Triode Electronics has some NOS for $11, and several options for an EL84. I don't know which EL84's people are gushing over right now, but I've seen JJ's recommended highly, and my experience with their 12AX7's and 6V6's has been good. I don't see any reason to change the other caps yet.

For the 12A7X's I have one Mesa Boogie and one Groove Tube. Which is the better for V1 and V4?

V1 is where all the tone comes from; it's both of the pre-amp stages. Try them and find out which one you like better. V4 is not part of the signal path, and only half of it is in use. Anything goes there as long as it works.

[texstrat]

Great thanks.

Once I get the new tubes I will post the new voltages.

[Firestorm]

Here are the voltages for the positive ps leads:

Blue
264VDC
70VAC

Red
285VDC
70VAC

Green
158VDC
70VAC

Anyone else think it's odd that we're seeing the same AC voltage at each of the three PS nodes? It should drop by at least a few orders of magnitude at each filter.

[Phil_S]

Yes, it seems wrong, but I thought we need to concentrate on the ripple at node 1 before looking downstream. Now that you raise it, maybe the OP isn't measuring it correctly? What else might explain this? This is hard from far away.

Texstrat, you are taking these measurements with the black probe grounded to the chassis (and not the bolt for the green line feed ground), and the red on the + terminal of the cap?

[martin manning]

The constant 70V didn't make sense to me either... texstrat, can you recheck your measurement? I've also been trying to imagine what kind of failure in a vacuum rectifier could cause a large ripple. The only thing I can come up with is that one diode has shorted, but that would cause a huge hum and probably some pyrotechnics.