Tracking back 100Hz hum in a tube guitar amp

[basinstreetdesign]

i) 0-15 seconds: no hum.
ii) 15-20 seconds: hum quickly builds up to quite high levels.
iii) 20-35 seconds: the hum gradually subsides to zero level (not audible anymore). I wish it stayed like that forever.
iv) 35-45 seconds: the hum slightly builds up again and stabilizes at a low but well audible level (~7mV pk-pk at speaker terminals, see first scope screen-shot in the OP).

I'd like to prevent iv)

Hi, Andreax1985.  This sequence of events is perfectly normal and to be expected in all tube amplifiers to one degree or another.  It happens in mine except for iv).  That part doesn't happen and I can tell you what causes it and what I did to get rid of it.

The waveforms you posted are to be expected for just the reasons that Soldar said.  The rectifier bridge and the two halves of the power transformer HV winding are not exactly matched.  But that mismatch is not the cause of your audible 100 Hz hum.  Mismatches in the output tubes and the output transformer are.  The output tubes you have may be sold to you as a matched pair but they are definitely not perfectly matched at all bias levels, plate currents or voltages.  There will be small differences.  Also the two sides of the output transformer primary winding will have some small mismatch, possibly by half a turn or because one side is a layer or two nearer to the core or whatever.  So even if you unplug P24 and short both inputs to ground, the two tubes will conduct slightly differing zero-signal currents.  Those two currents will have 100 Hz components and the slight imbalance of the OT will propagate them to the speaker.  With the circuit that you have there is nothing you can do about that.

i) happens because the outputs tubes have not warmed up yet and are drawing no B+ current
ii) happens because they are warming at different uncontrolled rates and pulling largely different B+ currents
iii) happens as the two tubes slowly approach as much balance as they can
iv) happens as the two tubes settle to their final operating point values but even so are not really well balanced.

In my amp, I have individually-adjustable bias voltages for each of the four output tubes in my stereo amp.  By fine tweaks of these I can balance each pair of tubes so that the 120 Hz (in North Am.) and most harmonics nearly perfectly cancel out in the primary of the OT and they are not heard in the speaker.  There is some thermal noise and higher harmonics left but that is it.  It is about 1 mV RMS total.  My preamp filaments are also DC and the B+ for them are regulated so when the volume is full up its tough to hear anything even when I bend down and put my ear to the speaker cone.

What you need to do is to separate the bias of your two output tubes and make them adjustable.
You can do this by changing  R8 from 120 Ohms for the both cathodes to two resistors, one for each cathode.  They should be 240 Ohms but with some room for adjustment.  Each must be bypassed with a cap about the same size as whats there, so you can keep C21 for one of them, just get another.  One of the two resistors should be composed of a 200 Ohm resistor with a 100 Ohm wire-wound pot in series.  When you first test it, put that pot at mid-position.  Then, with the amp fully warmed up, adjust it for minimum hum in the speaker.  BTW, when you wire the pot, short the wiper to the otherwise unused end of it so that if the wiper fails the pot will not go open but just to its max value.  The amp will still work but with lots of hum.
I guarantee this will work.
Tim

[Andreax1985]

Thank you for the detailed description. Before trying invasive solutions I need to understand what is happening in that circuit.

Another very strange thing: I scoped HTR GND w.r.t. chassis ground (0 V) and I measured 15V DC with a 5.20Vpk-pk (1.91 V rms) 50Hz AC riding on it.

P.S. pulling power tubes doesn't seem to change the measurements across R18 and R19.

[Shock]

Thank you for the detailed description. Before trying invasive solutions I need to understand what is happening in that circuit.

You have done a lot of invasive solutions already and failed to mention about the power transformer whine, safety circuit you removed, speaker mod, output section relocation. Aside from replacing all the tubes and swapping out some of the power supply.

Being sincere, you need to provide details of everything you have done from the start and what symptoms started appearing when and what changed and what was resolved. Add some photos of the amps current state so we can see where everything is.

My guess is some grounding or EMF issue, or as the output and power transformer are aligned in parallel they are susceptible to noise, especially if the power transformer is noisy to start with.

[Andreax1985]

Yes, I applied several mods to this amp, in order:

i) Relocated the power amp section.
ii) Relocated power transformer as far away from the output transformer as possible and rotated it 90 degrees, so to minimize possible coupling (this can only be an improvement).
iii) Tried a LOT of new/old tubes (nothing bad from this, I still have and use the original ones).
iv) Changed filter caps and diodes with better quality ones (already mentioned in this thread).
v) Fitted a new speaker (not relevant to the present discussion).

The hum I'm mentioning in this thread was present since DAY 1, when I brought home this amp and has nothing to do with the mods I have applied. I had problems in the past, but of different nature and I solved them. I did not mention the details of the mods because I didn't want to put too much on the plate and I don't want to derail this thread from its logic path, which now is very promising. I think we are close to indentify the hum's source.

[basinstreetdesign]

The sheer amount of different GND symbols in the schematic is confusing.
Looks like the EL84 tubes lose their GND (GNDb) when you pull P24. So no more power available to them, resulting in a silent output.

Yup that looks like it.
And I agree there are way too many different circuit points labelled "GND" of one sort or another.  I still stand behind my earlier post, though.

[Shock]

There is a slightly better service manual here.

[Andreax1985]

i) 0-15 seconds: no hum.
ii) 15-20 seconds: hum quickly builds up to quite high levels.
iii) 20-35 seconds: the hum gradually subsides to zero level (not audible anymore). I wish it stayed like that forever.
iv) 35-45 seconds: the hum slightly builds up again and stabilizes at a low but well audible level (~7mV pk-pk at speaker terminals, see first scope screen-shot in the OP).

I'd like to prevent iv)

Hi, Andreax1985.  This sequence of events is perfectly normal and to be expected in all tube amplifiers to one degree or another.  It happens in mine except for iv).  That part doesn't happen and I can tell you what causes it and what I did to get rid of it.

The waveforms you posted are to be expected for just the reasons that Soldar said.  The rectifier bridge and the two halves of the power transformer HV winding are not exactly matched.  But that mismatch is not the cause of your audible 100 Hz hum.  Mismatches in the output tubes and the output transformer are.  The output tubes you have may be sold to you as a matched pair but they are definitely not perfectly matched at all bias levels, plate currents or voltages.  There will be small differences.  Also the two sides of the output transformer primary winding will have some small mismatch, possibly by half a turn or because one side is a layer or two nearer to the core or whatever.  So even if you unplug P24 and short both inputs to ground, the two tubes will conduct slightly differing zero-signal currents.  Those two currents will have 100 Hz components and the slight imbalance of the OT will propagate them to the speaker.  With the circuit that you have there is nothing you can do about that.

i) happens because the outputs tubes have not warmed up yet and are drawing no B+ current
ii) happens because they are warming at different uncontrolled rates and pulling largely different B+ currents
iii) happens as the two tubes slowly approach as much balance as they can
iv) happens as the two tubes settle to their final operating point values but even so are not really well balanced.

In my amp, I have individually-adjustable bias voltages for each of the four output tubes in my stereo amp.  By fine tweaks of these I can balance each pair of tubes so that the 120 Hz (in North Am.) and most harmonics nearly perfectly cancel out in the primary of the OT and they are not heard in the speaker.  There is some thermal noise and higher harmonics left but that is it.  It is about 1 mV RMS total.  My preamp filaments are also DC and the B+ for them are regulated so when the volume is full up its tough to hear anything even when I bend down and put my ear to the speaker cone.

What you need to do is to separate the bias of your two output tubes and make them adjustable.
You can do this by changing  R8 from 120 Ohms for the both cathodes to two resistors, one for each cathode.  They should be 240 Ohms but with some room for adjustment.  Each must be bypassed with a cap about the same size as whats there, so you can keep C21 for one of them, just get another.  One of the two resistors should be composed of a 200 Ohm resistor with a 100 Ohm wire-wound pot in series.  When you first test it, put that pot at mid-position.  Then, with the amp fully warmed up, adjust it for minimum hum in the speaker.  BTW, when you wire the pot, short the wiper to the otherwise unused end of it so that if the wiper fails the pot will not go open but just to its max value.  The amp will still work but with lots of hum.
I guarantee this will work.
Tim

Hi Tim I wish to thank you so much. You really nailed the problem. Following your instructions I've completely solved the problem and made the hum disappear. Thank you.

Why tube amp producers still use shared cathode resistors is beyond me.

[basinstreetdesign]

Hi Tim I wish to thank you so much. You really nailed the problem. Following your instructions I've completely solved the problem and made the hum disappear. Thank you.

 

Why tube amp producers still use shared cathode resistors is beyond me.

Why? - money!   If there is nothing to calibrate then production is greatly speeded up.  And that is CASH in their pocket.

[vk6zgo]

i) 0-15 seconds: no hum.
ii) 15-20 seconds: hum quickly builds up to quite high levels.
iii) 20-35 seconds: the hum gradually subsides to zero level (not audible anymore). I wish it stayed like that forever.
iv) 35-45 seconds: the hum slightly builds up again and stabilizes at a low but well audible level (~7mV pk-pk at speaker terminals, see first scope screen-shot in the OP).

I'd like to prevent iv)

Hi, Andreax1985.  This sequence of events is perfectly normal and to be expected in all tube amplifiers to one degree or another.  It happens in mine except for iv).  That part doesn't happen and I can tell you what causes it and what I did to get rid of it.

The waveforms you posted are to be expected for just the reasons that Soldar said.  The rectifier bridge and the two halves of the power transformer HV winding are not exactly matched.  But that mismatch is not the cause of your audible 100 Hz hum.  Mismatches in the output tubes and the output transformer are.  The output tubes you have may be sold to you as a matched pair but they are definitely not perfectly matched at all bias levels, plate currents or voltages.  There will be small differences.  Also the two sides of the output transformer primary winding will have some small mismatch, possibly by half a turn or because one side is a layer or two nearer to the core or whatever.  So even if you unplug P24 and short both inputs to ground, the two tubes will conduct slightly differing zero-signal currents.  Those two currents will have 100 Hz components and the slight imbalance of the OT will propagate them to the speaker.  With the circuit that you have there is nothing you can do about that.

i) happens because the outputs tubes have not warmed up yet and are drawing no B+ current
ii) happens because they are warming at different uncontrolled rates and pulling largely different B+ currents
iii) happens as the two tubes slowly approach as much balance as they can
iv) happens as the two tubes settle to their final operating point values but even so are not really well balanced.

In my amp, I have individually-adjustable bias voltages for each of the four output tubes in my stereo amp.  By fine tweaks of these I can balance each pair of tubes so that the 120 Hz (in North Am.) and most harmonics nearly perfectly cancel out in the primary of the OT and they are not heard in the speaker.  There is some thermal noise and higher harmonics left but that is it.  It is about 1 mV RMS total.  My preamp filaments are also DC and the B+ for them are regulated so when the volume is full up its tough to hear anything even when I bend down and put my ear to the speaker cone.

What you need to do is to separate the bias of your two output tubes and make them adjustable.
You can do this by changing  R8 from 120 Ohms for the both cathodes to two resistors, one for each cathode.  They should be 240 Ohms but with some room for adjustment.  Each must be bypassed with a cap about the same size as whats there, so you can keep C21 for one of them, just get another.  One of the two resistors should be composed of a 200 Ohm resistor with a 100 Ohm wire-wound pot in series.  When you first test it, put that pot at mid-position.  Then, with the amp fully warmed up, adjust it for minimum hum in the speaker.  BTW, when you wire the pot, short the wiper to the otherwise unused end of it so that if the wiper fails the pot will not go open but just to its max value.  The amp will still work but with lots of hum.
I guarantee this will work.
Tim

Hi Tim I wish to thank you so much. You really nailed the problem. Following your instructions I've completely solved the problem and made the hum disappear. Thank you.

Why tube amp producers still use shared cathode resistors is beyond me.

They didn't in the days when tubes were the default device----- see my link to the schematic of the "Type 3" Amplifier in Reply #9.

Modern versions of tube amplifiers seem to be designed more from voodoo than proper engineering design principles.

There were thousands of similar amplifiers to the "Type 3" in service as monitors throughout the world.
The consequences of PA tube unbalance was well known, & "matched pairs" of audio output tubes was the stuff of fantasy to those of us who worked on these devices.

Two new tubes out of stock, adjust the balance pot, & "Bob's your Auntie".

[Shock]

From what I understood there were other owners without problems, so seems it stems from a tolerance issue. Perhaps a properly matched pair of reasonable quality new tubes would have resolved it also.

[Andreax1985]

Now that I solved the 100hz hum problem... just for the sake of curiosity... I'm wondering: how much white noise is to be considered normal? When volume and gain controls are set to zero the amp is dead silent. But if I go past 12 o' clock with both, white noise level gets considerable (as is for every other amp same model as mine I have tried, so it's not a "problem" of my amp but, rather, a design issue).