Tracking back 100Hz hum in a tube guitar amp

[Andreax1985]

Hi all,

I have a small guitar amp which, even if nothing is plugged in and all preamp tubes are pulled out, has a low level 100Hz hum which bothers me. The hum level does not change if I increase master volume. Moreover, if I pull P24 signal header from the power amp section (see the attached schematics) the hum disappears. This puzzles me because the fact that the hum is there even if no preamp tube is rolled in makes me think it's coming from the power supply and not from the signal path in the preamp section: so, why if I pull the signal cable on the power amp the hum disappears?

So I decided to track the hum back, with the help of a scope. I'd like to use this thread to show and discuss my progresses into this search. You find attached to this post the schematics of the amp as a pdf file.

This is the hum probed at speaker terminals (with all preamp tubes pulled out):



This is the ripple in B+1, (+/-5V at 100Hz):



And this is the ripple in B+2, (+/-100mV at 100Hz):



I can observe that in B+2 I have a bigger peak followed by a shallower one, as if a 50Hz oscillation is superimposed to the main 100Hz ripple. Don't know why.

What do you think?

[soldar]

I can observe that in B+2 I have a bigger peak followed by a shallower one, as if a 50Hz oscillation is superimposed to the main 100Hz ripple. Don't know why.

That is nothing to be concerned about. When using a bridge rectifier it is normal that there can be slight differences in positive and negative peak rectification. If the same circuit has devices with single diode rectifiers then those peaks will be slightly shorter. Also could be that some diodes of the bridge have slightly higher resistance. Etc. Many causes but nothing to worry about.

[capt bullshot]

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.

[Andy Watson]

Pulling the pre-amp valves still leaves the B+3 power supply coupled through to the output section (via R79,17, C78,22).  The output stage is differential and should ignore this common-mode signal, so: either the signal is too large for the output stage to handle or the output stage is not balanced. The fact that the problem is still present with the master volume turned down suggest that it is in-balance in the output stage that is causing the problem. Perhaps the valves are aging? Perhaps it was never that well balacned in the first place and you've only just noticed the hum. Perhaps the main power supply capacitors are drying out - giving rise to more common-mode ripple signal.

[Andreax1985]

Pulling the pre-amp valves still leaves the B+3 power supply coupled through to the output section (via R79,17, C78,22).  The output stage is differential and should ignore this common-mode signal, so: either the signal is too large for the output stage to handle or the output stage is not balanced. The fact that the problem is still present with the master volume turned down suggest that it is in-balance in the output stage that is causing the problem. Perhaps the valves are aging? Perhaps it was never that well balacned in the first place and you've only just noticed the hum. Perhaps the main power supply capacitors are drying out - giving rise to more common-mode ripple signal.

The amp is new and I changed both filter caps and diodes in the rectifier bridge with higher quality components (Nichicon, Vishay...). I also bought different sets of EL84 power tubes  (balanced pairs, or at least so the sellers said). I tried every possible combination of the tubes I had and, if they were mismatched, the hum increased otherwise the hum settled at a lower level but was definitely still there (as you can see from the waveform in the first pic).

I'd also like to add that when I switch on the amp, this is what happens:

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)

P.S. It has been humming since when I brought it home, I've always noticed it. Other amps in the shop, same model, hummed too. But what I want to do is to understand where the hum is coming from and fix it.

[Tomorokoshi]

What are your DC levels on B+1 and B+2? How much current is drawn from each?

[Andreax1985]

How can I measure how much current is drawn?

If I recall correctly, should be around 320V in B+1 and 310 in B+2

[Andy Watson]

How can I measure how much current is drawn?

With 320V present ... very carefully

The current draw from the B+3 supply can be calculated by measuring the voltage across R10. Similalry, B+2 can be measured across R99. B+1 is a little more awkward but it appears that only the output stage is connected to B+1, so monitoring the the voltage across R8 (the bias current) should indicate the current draw on B+1.

[eblc1388]

Can you try placing a resistor of several kohms across R18 first then moves it to R19 while monitoring the output hum level on scope?

If the hum level changes to lower in the test, then you might consider adding the following modification in the attached image. The modification enable you to trim out the hum due to heater unbalance. 

[vk6zgo]

Back in the day, we used  "type 3 amplifiers" for audio monitoring in TV & Radio broadcasting sites.

http://www.oneillassociates.com.au/~poneill/pdf/TT_T3.pdf

The push pull EL34 output stages had poorer HT filtering than the lower level stages, because higher current filters cost more to make, and because push pull stages offer a high degree of rejection of common mode signals, such as HT hum, they could get away with it.

With new tubes, they were very good at cancelling hum, & R20 could often be left centred.
If, as is pretty normal, one EL34 lost emission faster than the other, the "cancellation" would begin to deteriorate, & the hum become audible.

Sometimes one output tube would completely die, & the amp would produce quite a few watts of hum.
Easy diagnosis, but we would groan if it was one of the "wall" monitors, because it was major pain lifting a quite heavy amplifier down whilst standing on a ladder.

Unfortunately, these days, you "pay your money & take your chance" with tubes, as some are old factory "seconds", others are relabelled "pulls" from operating equipment, & so on.

A very few are "new old stock".

[Tomorokoshi]

For current out of B+1, we can use the definition of capacitance:
C = Q/V

From the B+1 waveform, we see that the voltage drops by 10V. This happens in 7ms.

Using the nominal DC B+1 voltage of 320V, we can calculate the charge at the two points of the ripple waveform:
C = Q₁/V₁ and C = Q₂/V₂

With C = 47uF, V₁ = 325V, and V₂ = 315V, rearranging we get:
Q₁ = C * V₁ or Q₁ = 47uF * 325V, Q₂ = C * Q₂ or Q₂ = 47uF * 315V

The change in charge is:
Q₁ - Q₂ = (47uF * 325V) - (47uF * 315V) or simply C * (V₁ - V₂) = 47uF * 10V = 470uC.

The change in time is the 7ms directly from the waveform.

Therefore the current is:
470uC / 7ms = 67mA.

The average resistance of the amplifier circuit from B+1 is 320V / 67mA = 4776 ohms.

The power used is 320V * 67mA = 21W. Does this seem about right?

[Tomorokoshi]

Getting back to the problem of hum.

The ripple on B+1 and B+2 are not in phase due to the low-pass RC filter between them. This is also true with B+3 to the preamp. This may be part of what produces the complex output hum.

Looking at the preamp, it may be that 12AX7 V3 is not perfectly matched for each element, R17 and R79 are not perfectly matched, or C22 and C78 are not perfectly matched. If so, the changing out-of-phase ripple between B+1, B+2, and B+3, when applied to the preamp input, would produce a small differential signal to the main power amplifier.

I looked at some other amplifier designs, and they don't seem to have the split between B+1 and B+2, avoiding the phase difference between them.

This all has to do with ripple rejection. An alternative is to reduce the total ripple.

[Tomorokoshi]

From above, knowing that the equivalent resistance of the amplifier is around 4.7k, instead of having B+1 connected to the high-ripple output of the diode bridge, make another filter and put a resistor between the bridge and C17. Tune and calculate the value to achieve your target for B+1. Then reduce or remove R99 to eliminate the phase difference. You may need to change screen resistors R1 and R94.

What is the nominal DC voltage B+3?

[floobydust]

The schematic shows at least six different grounds, it's easy to mess that up in manufacturing or it's a design problem, not easy to fix.

The EL84 heater wiring is usually a big source of hum, and best if they are a twisted pair away from any sensitive nodes. I would look for H1 and H2 to have the same ACV wrt HTR_GND. A hum-balance pot would be at R18/R19 to allow adjustment on audio gear that does not have a bypassed cathode (C21).

If you run it with V3 pulled, the output tubes are still high impedance inputs connected to B+3, as far as signal is concerned. Master Volume is weird because it only adjusts diff-mode impedance.
I'm saying with V3 pulled and Master Volume set to min. you would only be hearing imbalance in the output stage with tiny ripple on B+3 as common-mode.

I would needs pics of the grounding, there are at least two critical ones. Set your scope to line trigger to make it easier to trace the hum.


Interesting the input stage uses LND150 N-Channel Depletion-Mode DMOS FET, 500V IDSS 1mA as a triode.

[spec]

Hi  Andreax1985,

It is very difficult to diagnose faults without having the faulty unit to examine, but from the symptoms that you describe, and looking at the first scope trace, it seems that the two EL84s in the push-pull output stage are oscillating at a high frequency, which would be the root cause of the problem. EL84s are high-gain pentodes, and are firery little devils, prone to parasitic oscillations.

If you are wondering why parasitic oscillations could cause hum, I can assure you that it is often the case and the reason is quite simple, but perhaps cover that in another post.

In order to stop the oscillation, try this:

• Are the EL84 valve holders of good quality and are the EL84s fitting snugly and fully home in the valve holders?
• Burnish all EL84 pins. Likewise burnish the valve holder sockets (even though the EL84s and valve sockets are new)
• Are all the solder joints on the valve base pins well made, especially if PCB type. Consider re-flowing the joints anyway
• Are R11/R12 (3K3 grid stoppers), connected directly to the grid pin on the EL84 valve holders with a short self lead on the grid side of the resistor?
• Change R11/12 to 5K6 and use good quality metal film resistors, connected directly to the valve holder pin, as described above

For initial fault-finding, you could simply try connecting a 100k metal film resistor in series with the power amp input connector pin.

If the problem does prove to be parasitic oscillations, we can discus a complete modification scheme, which will probably also involve some decoupling/frequency shaping capacitors.

[Andreax1985]

Hi guys, thank you very much, you put a lot on my plate... a lot of things to try as well. Please don't stop adding your thoughts, in the meantime I'll try some of the things you suggested and I'll keep you posted.

[IanMacdonald]

Don't overlook the possibility of transformer-to-transformer magnetic coupling. Is the hum still there with the output valves pulled? (If it's this it will also be present during warm-up)

[rf-loop]

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).

Investigation and finding problem need take this timetable to account.

This is perhaps miss but least good  to think if....

You told these EL84 are matched pair. But do they have also matched filaments? (personally I suspect that matching do not include filaments) Very different cathode temperature also may lead to some imbalance. And this your timetable about hum....  at least not completely incompatible with cathode heating related residue imbalance. (im not sure if you can also watch it with eyes looking glow color when you turn it on, in worst case you may see other EL glow fast bright and after then decay and other EL filament brightness rise (if difference is big). )

Least check it, connect two meters (or scope channels) to record voltages over both filaments when it start and warming. I suspect that in start from gold they run very different and even other one may go totally overvoltage ( in worst case) until also other filament warms up in worst case. As we also know  when filament warms up its resistance drop and then other get low power... and again think your time table.

Even if this is not any part of problem in this case but this kind of design is not "best possible" at all.

As someone have previously noted, transformers magnetic fields can also be source of problems, this is why we many times see that specially E type transformers have weird positions and angles. If transaformers are toroidal then it is more easy.

Then, filament current is also near 0.7A so this need keep away from sensitive signals, so it need least be twisted pair and better is far away from sensitive parts as also someone have previously noted.
Preamp filaments are DC and end amp is AC. But why designer have selected series mode for drive filaments. It can do, specially for 12AX7 but it is not necessary and with EL84 end tubes this method is - imho, bad.

World is full of good and top class and state of art tube amplifiers. Why this designer has drawn his own magpie's nest. It is intersting if can see how it have mechanically assembled and how these all GND's are connected.

[Andreax1985]

Yes, I'm starting to believe that hum has something to do with unbalance between power tubes.

@floobydust , @eblc1388 : I measured H1 and H2 AC voltage w.r.t. HTR GND using a multimeter. That is, If I'm not wrong, the AC voltage across R18 and R19. Well, I read 12.40V across R18 and only 2.0V across R19. Is this normal?

[Electro Fan]

Hi Andreax1985,

Just wanted to say I admire your tenacity in tracking down the hum and fixing it.  Tube equipment (both for musical instruments - like guitar amps - and for hifi systems) can provide outstanding sound, so it would be great to reduce or eliminate any extraneous noise, especially persistent hum.

I really like the way you have been diligently trying to diagnose the issue with your study of the circuit, your use of your test equipment, and your Q&A in the thread.  I don’t know the answer to the problem but I have a hunch you will figure it out and in the process you are going to come out of this as an accomplished tube amp designer. 

Personally, I’m intrigued by two theories:  1) that it’s a GND issue, and 2) that it has something to do with tube performance, possibly matching (but it could be something else in the circuit, of course).

I don’t know if you have access to a good tube tester but some of the best tube testers ever made were produced by Hickok.  Of all their testers, one of their very best models was the 539C.  The 539C has become a bit of a collector’s item and it’s therefore somewhat pricey - but just in case you are interested in learning about them and how they work (in case the 539C or one of Hickok’s other models might help you with tube testing) below is some info.

Good luck and please keep us updated as you get the issue resolved. 


https://static1.squarespace.com/static/57ae25c75016e1690c41b053/t/57ae303803596ed24fa8f9c2/1471033401729/Hickok+Tube+Tester+539C+Manual.pdf

https://stevenjohnson.com/hickok/data/hickok-539b-539c-test-data.pdf

https://www.ebay.com/itm/HICKOK-MODEL-539C-TUBE-TESTER-Tested-in-excellent-condition-/183654239996

[floobydust]

It's an awkward circuit because the output pentodes are AC driven filaments, with a bridge rectifier and filter caps for the preamp triode DC driven filaments, all driven off a single transformer winding.

The whole point of R18/R19 and R29/R30 are to keep balance between the AC and DC filament loads.
Look for symmetry, so the same voltages across R18 and R19 as well as between HTR GND and GND HDC should be 6VDC with little ripple.
I think you found a problem with 12.4V and 2V instead of 6VDC.

Most likely, one output tube might have a heater-cathode short/high leakage current there, which is imbalancing HTR GND.
I would check R18, R19 are the same value, same for R29, R30 and that C11 is OK. A hard heater-cathode short can cause current to flow through the DC bridge and near R4, C13 is mystery part NT2 which might be for protection against that. It would all cause hum and upset bias at R8/C21 as well.

The last Vox I was in, an antique AC30 with cracked coupling capacitor, covered in nicotine and booze. That was a weird repair.

[eblc1388]

... the AC voltage across R18 and R19. Well, I read 12.40V across R18 and only 2.0V across R19. Is this normal?

It isn't, with such a big difference. Pull both EL84 to see if the voltage returns to approximately half the filament supply voltage. But judging from the large difference, it probably won't help much.

Look more closely to the schematic to see if there is any unexpected connection to the GND on this low voltage part of the circuit that breaks the symmetry.

 

[floobydust]

CAREFUL
Pulling the output tubes leaves almost no load on the HV power supply and B+ will rocket up.
There are 350V and 450V caps, careful they do not get overvoltaged with little or no load there due to tubes being pulled.
The two power transformer primaries are in series?

Also, after removing power there will be dangerous HV stored in the filter caps, I don't see any bleeder resistors and the JFETS might take a while. Use your multimeter to check the voltages on the caps before going back in.

[eblc1388]

That's a very valid point.

Andreax1985 can you confirm whether the unit has only one or two separate power transformers. If there are two, pulling the EL84 tubes would cause voltage increase of B+ and should not be attempted.

 

[Andreax1985]

Hi! I can see only one power transformer.