Two issues with 6G3 build--hum and low preamp plate voltage

[pjhalliday]

I have a 6G3 build that basically works, but has what may be one or two issues. The amp has what I think is 120 Hz hum that is present even with nothing plugged in to either input that dips just a bit with the volume turned in its mid levels but similar at the top and bottom end. The hum goes away with V3 (PI) removed but is present with either V1, V2, or both removed when V3 is replaced. While trying--thus far in vain--to determine what the cause of the hum could be I found that voltages were low, around 65 V on V2 pin 1 and around 120 V on pins 1 and 3 of V1 for which I am using a 12AX7. Voltage at pins 3 & 8 is about what's listed on the stock layout in all locations. Power tube voltages are all fine as well. Finally, the voltage up stream of the dropping resistors for the gain stages is close to correct at 255 vs. 270 on the layout. I have traced the circuit several times and have not found any wiring errors. The only intentional difference between this build and the stock layout/schematic is that the first stage of the bright channel is bypassed by 2700/0.68 rather than sharing the normal channel's 1500/25 bypass. If anyone is willing to suggest someplace to look I would greatly appreciate the help. Thanks a lot.

[Bruce / Mission Amps]

I have a 6G3 build that basically works, but has what may be one or two issues. The amp has what I think is 120 Hz hum that is present even with nothing plugged in to either input that dips just a bit with the volume turned in its mid levels but similar at the top and bottom end. The hum goes away with V3 (PI) removed but is present with either V1, V2, or both removed when V3 is replaced. While trying--thus far in vain--to determine what the cause of the hum could be I found that voltages were low, around 65 V on V2 pin 1 and around 120 V on pins 1 and 3 of V1 for which I am using a 12AX7. Voltage at pins 3 & 8 is about what's listed on the stock layout in all locations. Power tube voltages are all fine as well. Finally, the voltage up stream of the dropping resistors for the gain stages is close to correct at 255 vs. 270 on the layout. I have traced the circuit several times and have not found any wiring errors. The only intentional difference between this build and the stock layout/schematic is that the first stage of the bright channel is bypassed by 2700/0.68 rather than sharing the normal channel's 1500/25 bypass. If anyone is willing to suggest someplace to look I would greatly appreciate the help. Thanks a lot.

V1 V2 V3... starting from where?
3 and 8 are cathodes of the 12AX7.... unless a phase inverter tube, shouldn't be more then a couple volts there unless the cathode is not grounded through a cathode biasing resistor... like maybe you forgot to ground the resistor....?

[pjhalliday]

To clarify, V1 being the 1st gain stages, V2 the second gain stage that both channels go through as well as the tremolo oscillator, and V3 being the phase inverter. All I meant about pins 3 & 8 is that the voltages there were in the 1.5-2 range as they should be. Reading back through I see that I mis-typed the original message--that should have read voltage around 120 on pins 1 & 6 of V1. Sorry for the confusion.

[Chuck H]

Have you tried a different tube for the PI or V2? Have you checked voltages with the tubes removed? Have you checked resistor values throughout the power supply. including plate and cathode resistors?

We should probably tackle this one problem at a time. I'd start with the low voltage.

[pjhalliday]

OK, hopefully these measurements are helpful. The rsistors listed are the ones first in line off the pins--either plate resistors or bypass resistors.

Pin	Specified Voltage	Actual Voltage (tube in)	Actual Voltage (tube out)	Specified Supply Voltage	Actual Supplied Voltage	Specified Resitor	Actual Resitor
V1-1	165	113	400	270	258	220k	225k
V1-3	1.5	1.1	0			1k5	1k6
V1-6	165	129	400	270	258	220k	240k
V1-8	1.5	1.2	0			2k3	2k3
V2-1	165	66	403	270	255	100k	116k
V2-3	1.4	2.3	0			1k5	1k5
V2-6	205	205	406	375	416	220k	234k
V2-8	2.0	2.6	0			2k7	2k8
V3-1	230	188	405	325	285	82k	90k
V3-3	20	19	0			820	785
V3-6	225	195	405	325	285	100k	103k
V3-8	20	19	0			820	783

For reference, here is the Fender layout.

Prowess Amplifiers - Fender - Schematics - Deluxe 6g3 - Layout

Thanks again.

[pjhalliday]

Also, yes I did swap tubes in the PI.

[Chuck H]

The first triode on V2 (pins 1,2&3) is definitely cattywhompus. I don't even know what might cause high cathode volts and low plate volts with the right supply voltage and resistor values. Have you tried swapping another tube for V2? There's another plate load resistor in line with the 100k for that triode. It should be 15k I think, The signal for that stage is taken from the junction of these two plate load resistors. What is the value of the other plate load resistor. I'm actually thinking you have actually measured it in series with your "actual value" above. But check this part of the circuit anyway. Also check the bypass cap for this stage for correct polarity or a short.

[g1]

The first triode on V2 (pins 1,2&3) is definitely cattywhompus. I don't even know what might cause high cathode volts and low plate volts with the right supply voltage and resistor values.

Maybe caused by DC on the grid (pin2), which could be caused by a leaky coupling cap.
(p.s. yeah I know you knew that chuck )

[Chuck H]

Yes I do. And I thought if it too. I almost mentioned a lack of pin 2 and 7 in the voltage readings. But since it's a build and not a vintage amp I didn't even factor it in because I've never seen a leaky new film cap. But I don't do repairs so I probably see fewer component failures than I do mistakes and lousy tubes. I can't say it doesn't happen. It's just not something I thought worth considering in THIS case. If the tube hasn't been ruled out already I'm still on that track. But the OP should check for DC on pin 2 of V2 once the tube is deemed good!

[pjhalliday]

DC voltages on pins 2 & 6 were 0. I swapped out V2 for another tube that had some use on it and that seems to have fixed the issue with V2. Two of the three preamp tubes that I bought new for this amp were bad if you can believe that. At this point I think I just have the hum issue to sort out.

[Chuck H]

Two of the three preamp tubes that I bought new for this amp were bad if you can believe that.

I'm having no trouble believing that. It's been a very common problem lately. So much so that I factor it in when I order tubes for a project. As in 'I need three 12ax7's so I'll order five.' I still need to dig into my personal stash occasionally. The worst I've had happen was when I ordered five 12ax7's and a quad of el84's for a project. Where the el84's were fine (it seems they always are) four of the five 12ax7's were unusable. The problem seems to be expanding to big bottle power tubes now IME.

[pjhalliday]

OK, so now I'm getting more confused. I originally wired the amp with the heater supply center tap to ground and it had a pronounced and unacceptable hum. I clipped the center tap and created a virtual center tap in the standard manner and then connected that to a 240k/24k voltage divider from B+ to ground hoping that it would help the hum. Instead, it added a buzzing on top of the hum. The DC voltage to ground where the virtual center tap is connected is 42 V which seemed like a good level based on what I've read about elevating the heater supply. Does this outcome make any sense at all?

[Chuck H]

Does this outcome make any sense at all?

Yup. You need to install a large-ish capacitor across the dropping resistor in the divider. A 22uf electrolytic at 100V should be fine and if you don't have it on hand you can probably get it at Radio Shack or Fry's or some such other place. Observe correct polarity (negative to ground) Elevating the filament circuit doesn't always reduce hum. It may be 60hz you're hearing but that doesn't mean the heater circuit is directly responsible for the audible hum, even though it may be the source.

[loudthud]

An elevated heater supply only reduces 60Hz hum. It sounds like a low note played on a church organ. 120Hz hum sounds more like a buzz although many times you have a combination from different sources.

[pjhalliday]

Unfortunately, I don't seem to be getting anywhere with finding the source of the hum. In fact, I gave up in disgust for a while and finished up another amp that came out great. Here is what I think I know so far on the problem child...

- hum is present even with no cords plugged in
- by ear and verified with a spectrum analyzer it is predominantly 120 Hz
- with the phase inverter pulled there is just an extreemely faint noise from the power supply that is well below what I cosider acceptable
- pulling the first (initial gain stage for channels 1 & 2) or second (tone stack recovery and oscillator) preamp tubes has no effect on the hum
- I have swapped about five tubes through the PI with no improvement in hum
- hum is worst with gain on both channels at zero or quite low and then will fade away once either channel's gain is raised to roughly 3/4 of full or more (at which point it gets hissy, but that's not a major concern right now
- all voltages seem to measure close to what the layout indicates they should
- the layout is pretty similar to pictures of original brown Deluxes that I've seen except that the chassis is installed tweed-style so the tubes are rotated 90 degrees around the chassis if that makes sense
- not that I could not have missed something, but all wiring appears to be corrrect as do component values
- putting a 20 uF capacitor in parallel across each of the filter caps (individually) had little or no effect on the hum
- the filter caps are a JJ 40-20-20-20 can which I expect should be better than the original design, at least in theory
- except for the hum, the amp sounds quite good

If anyone has further ideas to check, I would appreciate the suggestions.

Pete