Issue with an X88R build

[MGW]

180 or 300 Hz is more likely to be oscillation than PSU noise, IMHO. Does this happen when there is no jack in the input? Happens all the time IF the transformer is in contact with the chassis.

Odd! And it stops if the transformer isn't in contact with the chassis??

That is correct. I have a couple of theories about that are not unrelated to my removing the bell ends.

Can you take DC voltages to the ground on the PSU board at each pin of each valve? Not sure I understand what you're asking on this one.

I was just asking for voltages at each pin on the base of each *tube* (sorry!) with the meter black lead on the HV ground on the power supply board. It would ultimately be useful, but I've thought of a different tack to take (this was before the info about the transformer which I'm still pondering).

I thought that is what you may have been asking, but didn't want to assume. I can and will later today.

Does the schematic below represent your build? (It's from this thread.)

Yes, that's the one.

If so, you could pull certain tubes to disable two of the channels:

Using the numbering on the PDF below, pull U2 and U5 to disable the lead channel, and U3 to disable the clean channel. What's left is just the crunch channel. Is there still a problem? Try it with the switching set to each channel in turn. If that's OK, put U3 back in and try again, then pull U3 again and put U2 and U5 back in, etc... (You can't disable the crunch channel)

I'll give that a shot and see what happens.

[MGW]

Found one issue...my heater CT resistors are 100k not 100R. Gotta get that fixed. Though I must admit that I'm not sure what significance that has.

[vibratoking]

There are conflicting arguments that could be made rearding those 100k CT resistors. Change them to 100, I think you will hear an improvement.

[MGW]

Finally got around to messing with it. The CT resistors made no difference. They are attached to the socket of U6 and are grounded next to the B+ CT on the PS board.

Pulled U2, U3, & U5...lead and clean are dead quiet (as we'd expect). Crunch still has all the noise even with the gain pot at zero...so it seems that it has to be feeding noise in from somewhere.

Replaced U3...Lead channel still quiet. Clean has the same noise, but due to the difference in gain it isn't noticible at playing level, but only when turned up all the way.

Replaced U5, crunch and clean didnt' change. Lead was still very quiet...could hear a VERY faint buzz at full volume.

Installed U2 and all the noise showed back up.

Here are the voltages.
U1
1 238.6
3 1.755
6 190.4
8 1.6

U2
1 217.9
3 1.803
6 283.5
8 3.243

U3
1 237.2
3 1.721
6 235.1
8 1.758

U4
1 277.2
2 189.2
3 190.0
6 189.2
8 1.444

U5
1 277.2
2 188.2
3 188.5
6 188.2
8 1.463

U6
1 131.2
3 2.050
6 333.2
7 131.4
8 136.0

I'm thinking that U2 pin 8 looks a little off. THoughts?

[LeftyStrat]

Pin 6 is off, which would affect pin 8. Check the plate resistor value for pin 6 with the preamp off. Also check points A, B, C, and E from the schematic with power on and no tubes installed, they should be identical.

[Tillydog]

Pin 6 is off, which would affect pin 8. ...

I thought that it might be OK, as there's a 39k cathode resistor on that stage - no harm in checking, though....

@OP

Nothing leaps out at me without the power supply voltages.

When you had pulled U2, 3 & 5, you still had the same problem, and turning the gain control on the crunch channel down didn't affect it at all? What if you turn all 3 gain controls down? Does pulling U1 stop it?

Check that your output jack is isolated from the chassis (unscrew it).

When you said that "the noise was back" when you fitted U2, did you mean that there wasn't a problem at all before re-fitting U2, or just not a problem with *that channel*?

Can you post a recording of the noise? (e.g. phone recording) to try and decide if we're chasing power supply noise or oscillation....

I'm puzzled now!

[LeftyStrat]

Pin 6 is off, which would affect pin 8. ...

I thought that it might be OK, as there's a 39k cathode resistor on that stage - no harm in checking, though....

Oh, right, didn't notice that.

Another suggestion is to build an audio probe to try and narrow down the source of the hum.

[MGW]

Nothing leaps out at me without the power supply voltages.

I checked these with the tubes pulled and they all look good to me. All nodes are running at about 394 volts. That drops substantially once the tubes are put back in the circuit.

When you had pulled U2, 3 & 5, you still had the same problem, and turning the gain control on the crunch channel down didn't affect it at all?

The only thing it did was stop the noise on the clean and lead channels. The crunch channel still had the same issues.

What if you turn all 3 gain controls down? Does pulling U1 stop it?

Makes no difference. Pulling U1 doesn't stop it. Pulling U2 stops it, but only on the lead channel.

Check that your output jack is isolated from the chassis (unscrew it).

It's got shoulder washers on both sides of the chassis. At one point I had cliff jacks on it...made no difference either way.

When you said that "the noise was back" when you fitted U2, did you mean that there wasn't a problem at all before re-fitting U2, or just not a problem with *that channel*?[/guote]
When I pulled U2 the noise stopped...when I put it back the noise was there again. Wouldn't have expected anything different. I guess that means that the noise is getting into the circuit at or before the second stage. Given that is it present, at least to some degree, on each channel, I would think the issue is in a place in the circuit that impacts all of the channels. That leaves ground, or input as I see it.

Can you post a recording of the noise? (e.g. phone recording) to try and decide if we're chasing power supply noise or oscillation....

I'm puzzled now!

So to update further...I made some changes last night. Bell ends are on the trafo. Nylon shoulder washers are isolating the trafo from the chassis. The 60hz issue with the input line proximity to the chassis is no longer an issue...I'm guessing bells made the difference there or it could be trafo orientation. The nasty buzz seems to be only slightly present...but the 60hz component is still quite loud. I replaced the 100k filament CT resistors with 100R...made no difference that I could hear. I attached the noise. Lemme know what ya think.

[Tillydog]

The 60hz issue with the input line proximity to the chassis is no longer an issue...I'm guessing bells made the difference there or it could be trafo orientation.

Pretty sure that was sorting out the correct grounding at the input (added a grounded jack & fixed bad solder joint on PCB). I'd still try unscrewing the output jack, just in case it is inadvertently making contact somewere. Previous results are not a reliable guide, since you have had the other fault at the input - need to try again.

The noise definitely sounds like mains, not oscillation.

Next check: What happens to the noise when you flick the mains off? Does it fade away or stop abruptly?

[MGW]

Noise stops abruptly when the mains shut off. Also of note, the hum increases and decreases with the gain knob. With the gain off, the hum is all but gone. So wouldn't that indicate that it is something to do with the input stage and/or gain pots?

Time to probe!!!

Ok...that might have sounded a little gross...

[Tillydog]

Noise stops abruptly when the mains shut off.

If the noise stops abruptly it's coming from the power supply or the heaters (rather than ground loops, etc.).

ISTR you have a standby switch, also - might also be worth checking what happens to the noise when you switch the standby off - if the noise still stops abruptly, then it's coming from the B+ supply.

Also of note, the hum increases and decreases with the gain knob. With the gain off, the hum is all but gone.

Wow, that's progress! What did you do? Only pulling U2 would stop it before...

If it now responds to the gain pot, the noise is coming through the amp - the nearer the input you get, the more sensitive the circuit will be to noise.

You could try moving the connections between the transformer and the busbars around to see if it changes the level of hum.

What sort of level is the noise at, compared to the guitar signal? If it's generally low, it might be time to try an elevated heater supply again or fit a 'humdinger' pot to fine tune the balance of the heater supply.

Time to probe!!!

Well, when you've finished, wash your hands before touching the amp again

[MGW]

We have progress. I wired in a quick and dirty humdinger...didn't do much. Elevated the heaters and woohoo...significant improvement. It is quite usable. The buzz when the gain pots is turned down is gone. There is a faint buzz, however, it is quite a bit lower than the thermal noise and hiss from all of the gain. I think there could still be some improvement, but that is just being a bit nit picky. It is definitely in gigable condition.

The remaining noise is not 60hz it some other harmonic of it...it's higher than 120hz though. I may still do some checking with a probe and see if I can track that down.

Thanks for the help.

[Tillydog]

Woo-Hoo!

Well done

[MGW]

I do have a question though. With regard to the elevation voltage what is appropriate? Right now it is at 30v. Would it be better to raise it to 50v?

[Tillydog]

In theory, the heater elevation voltage just needs to be high enough to keep the most negative part of the AC on the heaters (6.3V RMS = +/-8.9V peak) above the most positive of the preamp cathodes (don't try and beat the cathode followers, though!), so about 3-4V on the cathode, plus a few volts of signal, plus the 8.9V on the heater = roughly 15V, so there shouldn't be much to gain by going from 30 to 50V.

However:

In practice: Try it and see!

Two pet suggestions of mine (others may feel differently, YMMV, etc.):

Make sure that the elevation voltage is well smoothed, too - a decent electrolytic from the elevation voltage to ground.

Try a small value cap (100n < C < 1u) directly across the heater connections on the first preamp tube - sometimes the rectifier noise fom the HV gets coupled into the heater windings in the transformer (unless there's an inter-winding screen, etc.) and this gives the noise somewhere to go.

Chances are that you will be chasing small contributions from many sources now. Some tubes will be better in V1 than others - don't forget to put the metal covers back on the amp - that will kill some of the hum.

Enjoy!