Problem with the high hum in a tube amplifier for a guitar

amplifier hum

I'm sure it wasn't shorted..

I don't believe it's AC. I did one tiny change when building this amp.. I am not using the J175 since I couldn't find a place in Europe to buy them. Instead I use a 2n5462 JFET. Sorry I didn't wrote it before..but didn't thought it could be a problem..but how to solve this?

and how to check if the JFET is absolutly closed! when switching between ch1 and ch2/3

PS: I've tried to elevate the heaters (DC offset) ..didn't helped..

Thanks

Thomas

100 hz hum tube

If I understand right, the FET at V2a input is used for muting during switchover only. The amp should also show basic operation with the FET replaced by a short. It shouldn't be able to cause the hum interference (or at least, I don't see how).

2N5462 has a considerable higher Ron than J176, but this shouldn't matter much, I guess this is true for all J176 devices in the circuit. J176, by the way is in production with Fairchild and Philips/NXP and on stock at catalog distributors, as I see, but I don't think that it's necessary to replace a 2N5462.

You should have noticed from my previos post, that I'm not confident regarding filament AC causing the hum, but I would perform the DC test to check further (if it's possible with acceptable effort.

Otherwise, I'm slowly running short of explanations. I also mentioned possible RF oscillations, but they seem very unlikely for the present circuit. I don't know what's your oscilloscopes bandwith, but as I said, oscillations should be noticable at the affected tubes (V2a, V3b, V3a anodes or V3b cathode).

problems with pcb amps

Running DC would be a lot of work, since the heaters are running underneath the 10cm x 40cm pcb with 27 pots wired, a lot of LED's etc..but the heater wires are center tapped and twisted very tight and running along the chassis' side.

But if it should be the AC heaters, the hum would be present on ch1 too, right? And the hum is not present on ch1 which share the V2a with ch2/3..

My guess on the JFET at V2a is that when ch1 is on..the JFET should be open..letting the signal bypass the 475k res at V2a grid, since ch1 only got one further amplification within the power amp.

And when switching to ch2/3 the JFET should be completely closed! making the signal pass through the 475k res. SO, If the JFET don't completely close then the 475k would be much lower since the JFET is parallel with the 475k res..am I right??

Anyway, I tried to desolder the 15v connected to the JFET, didn't helped..so guess that the JFET has nothing to do with it.

But what about the 6426 (controlling the JFET) ..it's grounded the same place as V2b cathode cap etc...could this induce the hum? I mean, when the 6426 is open the 15v would run to ground, right? But then again, I just tried to disconnect the 15v..

My scope is a 20Mhz Leader..I actually never tried using it before..never needed it untill now..

If this should be HF oscillation, then I am not shure how to detect it..

Thanks again for your time and help debugging this monster!

Thomas

b-100r bass amp troubleshooting

I see, that it's not easy to perform the DC filament supply test. O.K.

I actually don't yet understand exactly the FET switch across 475k purpose, also I couldn't identify the source of 1'Bus in switching circuit. The FET on-state can be roughly checked with an ohmmeter. 2N5462 should show around 0.5-1k rather than 100 ohms with J176.

But as you reported, the hum wouldn't be reduced much when shorting V2a grid to ground. This should exclude the preceding FET circuit from causing the interference. When these test and the disable V2a test results are correct, the interference seems to be injected at V2a cathode respectively filament circuit.

hi gain tube amp pcb layout

I see what you mean..just can't understand why ch 1 is totally quiet, showing no sign on hum, even when scoping at the speaker out.

The tests is right..I've done them five times each:| an your right about the FET issue...when i ground the grid at V2a the hum is present..

Seems that I've got a unique problem here..

What if it could be the 1uf cathode cap (I did already replace, but if the "new" one also is defect)?

The 1'bus is connected one of the side of the RY1' supply coil..don't remember if it's the positive or negativ DC supply.

Best regards

Thomas

signal ground in tube amps

I also don't completely understand the different channel behaviour, however, I don't know about the gain differences. Also V2a gain is increased for chanel 2/3 operation.

I think howver, that a defective capacitor (either 1 u or 15 u) could only reduce the gain and not cause any hum. The 1 u/100 ohm network is shorted by the 15 u cap for channel 2/3 to increase the gain.

If the control is connected to RY1' coil, this can be to the switched lower side only, resulting in the 6426 switched off in channel 2/3 operation, putting the FET in high impedance state by apllying +15V to the gate. But switching the FET on or off should only affect high frequencies in the present dimensioning.

I'm off for today, anyway.

Good luck!

feedback problem with tube amp

Hi,
I agree with everything that FvM has said. I think the odds on problems with faulty components if you used new ones is extremely small.
I have a hunch that the problem may be associated with V3 but if it was HF oscillations you would have seen it with the scope. I think it may be a PCB layout problem, which is difficult to fix, I had not realised that you had used a PCB, it is easy to get feedback problems with tube PCB circuits because of their high input impedance. You could try removing V3 and probe around to see if you can still see the hum coming out of V2b, but I think you will find that this will kill the hum. I do not think that the FET substitution is likely to be the cause either. CH1 does have a lot lower gain I than the other two so this will be less likely to have a hum problem. You could also try bridging the 1u/100R network in V3 with a cap to see if this affects the level or frequency of the hum, if it does you are onto something.
Good luck
Bob.

Added after 2 minutes:

Hi,
I agree with everything that FvM has said. I think the odds on problems with faulty components if you used new ones is extremely small.
I have a hunch that the problem may be associated with V3 but if it was HF oscillations you would have seen it with the scope. I think it may be a PCB layout problem, which is difficult to fix, I had not realised that you had used a PCB, it is easy to get feedback problems with tube PCB circuits because of their high input impedance. You could try removing V3 and probe around to see if you can still see the hum coming out of V2b, but I think you will find that this will kill the hum. I do not think that the FET substitution is likely to be the cause either. CH1 does have a lot lower gain than the other two so this will be less likely to have a hum problem. You could also try bridging the 1u/100R network in V3 with a cap to see if this affects the level or frequency of the hum, if it does you are onto something.
Incidentally you need to have a space before and after your emoticons on this board for them to display properly, some of yours do not always display.
Good luck
Bob.

amp hum problem +pedals

Hi FvM and Bob!

Thanks for all your help! I'll try to pull out V3 and see if i can see any hum at V2b.

FvM..the 1uf/100 ohm is shorted by 15uf when channel 1 is present, not ch 2/3.

Bob..If I pull V3 I can still see the hum on the scope..scoping on "V3a grid"..


Thomas

tube amp oscilloscope troubleshooting

I see, that I was in error regarding the 15u capacitor and V2a gain switch. The gain is higher in channel 1 mode, which makes the issue more mysterious to my opinion.

I have to care mainly for customer needs today. Please keep me informed, I'll also tell if I have any new insights.

Good luck,
Frank!

how to reduce hum in tube amplifier

Thanks Frank!

Customer needs is very important!

A higher gain in ch1 makes sence, since the ch1 only have one further gain stage left..But you're absolutely right! it's extremely mysterious! I am absolute convinced that the problem is around V2a cathode since it's the only thing changing when switching from ch1 to ch2/3..

I'll let you know if any news

Btw..I can tell from my "offbord wirings" that I've grounded the ch2/3 gain pots at V2a ground point!!! ..maybe this could be a problem! According the schem I will say that they should be grounded at V1a...what do you think?

Thanks again

Thomas

Added after 54 minutes:

Frank!!!

It cured the hum! I soldered the gain pots to V1a ground and the amp does not hum! However, now it didn't had any gain or distortion...but i think this is just a poor ground connection at the gain pots.

Thanks for your insightful troubleshooting help! ..I realy learned something here! I've got this topic at three different forums and you and Bob are the only people which did respond..!

Still have to re-solder a few connections..the amp pops a bit because of some cold solder at V2..

Btw..the PCB-layout is my own design and this is the first clone of the mesa dual rec 3ch version (at least the first on the internet) ..guess I've been pretty lucky

I'll let you know when the amp is stable!

Thank you very much!

Thomas

faulty amplifier parts causing hum

I fear, that no hum could come from a floating V2a grid (no ground connection at all), that shuts down V2a, a similar effect as when pulling up the cathode. But the ground wiring seems to be the key problem.

grounding problems with tube amp

I was too quick!

When plugging in the guitar I've got almost no gain at ch2/3.

I had to change the 100ohm/1uf at V2a, think I must have ruin it when I desoldered and so on..

The hum is back and has became a bit noisy too..I think the noise is comming from the 0.022uf coupling cap which i changed (cut it's leads and wired another one, whcih now has 2cm bare leads) ..so i guess I'll run through the tests again and see if something new happens..

Thanks

Thomas

Added after 54 minutes:

Ok..did the tests and nothing has changed..same results and same hum

ac hum heater tube

Hi Thomas / Frank
Glad you seem to be getting somewhere now you seem to be heading towards the star-ground configuration. However, if you have got no gain it does suggest that you may be shorting out the signal path somewhere, I hope this is not what is happening. The popping could possibly be due to cold joints on the grounds.

Channel 1 has the higher AC gain in V2 when this is selected but surely channel 2/3 has an extra high gain stage added in series with it for adding audio distortion effects, this would give Channel 2/3 the higher overall gain or do you not agree Frank, have I missed something fundamental here? The problem is clearly not with this part of the circuit though, this has been proven by the tests that Thomas did.has done. I still think we have a ground loop problem with the circuit from the latest tests that Thomas has done.

Thanks Frank for your input, I have learned a thing or two from your posts too. I think a lot of us came into electronics from fiddling about with amps etc while at school like you did. I first remember playing around with an old valve(tube) radio that I found on a junk heap back in the 1960s and managed to make it work again. High value screen resistors if I remember correctly. I came back into electronics when I had to develop a bit of lab apparatus for a final year project at university, I then studied electronics and never went back to doing chemistry again as a main job!

Best wishes
Bob.

caps affecting amp hum

Hi Bob!

Well, the pops are gone..we are totally back to zero!

Grounding V2b grid killes the hum...and the hum is present when grounding V2a grid! but only on channel 2/3..which is a mystery since ch2/3 has it's lower gain at this point! ..I've changed all components between V2a and V2b, does nothing!

I simply can't believe that this could be a heater issue since the gain stage is lower than on ch1 which is absolute quiet and sounds great

Perhaps this is a ground issue...but can't find out where since the hum is only present at Ch2/3 and not when shorting V2b grid to ground or when V2a is "disabled" as Frank described it!

Guess we need an army of electronic engineers to solve this problem!

Thanks

Thomas

Added after 23 minutes:

What about V1b..! ..it shares the same ground point as V2a but not V2b..could this inject some hum. Thinking, if RY1' is at the normaly open position (ch2/3) then the input to V1b is floating (0.047uf)..

killing tube amp hum

V1b istn't really floating, the input has a parallel resistance comparable to the grid node resistance of other stages in active operation, e. g. V2b.

A remark regarding channel gains: I did some rough calculations. Basically a 12AX7 stage has a gain of around 100 maximal. This means, that connecting a 1u or 15u capacitor at V2a cathode effectively doubles the gain from 50 to 100, using a 100 ohms resistor in series with the 1u has only minor effect. So the mid range gain of V2a should be equal for channel 1 or 2/3 mode, but channel 2/3 mode has a higher low frequency roll-of due to 1u cap.

The channel 1 respectivly 2/2 gain are around 40 and 130, so channel 2/3 gain is higher, but not that much. I would prefer however, to check the actual gains with an oscilloscope and a test generator. This would allow to state e.g. a hum level of xx mV referred to V2a input respectively output.

I assume a yet undetected wiring error, PCB short or similar, that causes the hum interference. You may check if the bias point (anode voltage) of any preamp stage changes unexpected when switching the channels, preferably with a high impedance (10:1) oscilloscope probe.

ground drop for amplifier hum

Hi Frank

I'll do my best! ..I don't own test generator and never tried to realy use the scope before..but I'll do my best with the anode voltage and let you now the results.

Thanks

Thomas

Added after 23 minutes:

Well, don't know if it's usefull..but when I scope on pin6/V2b anode I can see that the signal is jumping up and down on the O'scope-screen. Is this a kid of intermittent voltage drop i see? or just a bad probe? I've got no guitar plugged in / input shorted to ground.

When the amp is on stadby the signal is stable

tube amp filament hum

You are apperently measuring with AC coupling? In standby, I expect zero anode voltage at all stages, so actually nothing should happen. I think, to check the bias point, a DC measurement should be taken, it would show about the anode voltage values written in the schematics. However, additional AC measurement is interesting also. How much is the signal step (jump) in V or mV?

Unfortunately, you have no signal generator. A fixed frequency, e. g. 400 or 1000 Hz sine generator with a linear, roughly calibrated poti given 0 - 1 Vrms and an additional decade attenuator (1:1, 1:10, 1:100, 1:1000) would help a lot. It can be made from a few 10$ material, including a case, I think.Variable frequency wouldn't mean much extra effort.

eliminate hum noise in tube amps

Yes, I should make a generator..I also build pedals ofcourse..so it wuold be usefull.

I've got the scope set to AC, 20mv/DIV and 2ms/DIV and I measure +-10mv, peaks at +20mv. When measuring at DC the signal disappear from the o'scope-sreen which is limited to 5v/DIV. Am I doing something wrong here??

Actually I just played through the amp. The hum is present when I turn the Master volume pot and increase a little with the bass pot..but it could be that it's not noticed when playing since ther amp is loud!I'll check this tommorow in the rehersal on the big speaker cabs..could it be that I just ask for more than is capable?

I also got another hum that starts and increase with the gain pot. Turning down the guitar volume makes this hum disappear. Touching the guitar decreases this hum.

Btw...I'm from Denmark and buy all my components from Germany Do you know any good online store?

Thanks

Thomas

tube heater hum connection

When measuring at DC the signal disappear from the o'scope-sreen which is limited to 5v/DIV. Am I doing something wrong here??

Click to expand...

You're missing a 10:1 oscilloscope probe apparently. It would extend the viewable range to 400 or 500 V, provided the probe stands this voltage (it would normally). Connecting the oscilloscope input directly to anode voltages may be slightly above permitted input voltage (300 V is allowed even with my GHz oscilloscope). If no probe suitable probe is available, you can perform the bias point check with a multimeter.

Hum disapearing when touching parts of the guitar usually is a matter of amplifier grounding, or shielding of pickups and associated electronics, maybe also cable quality.

I get my parts (for protype manufacturing) mainly from professional catalog distributors as Farnell or RS Components, also DigiKey. And manufacturer contracted distributors for some special parts. Farnell has the mentioned J176 on stock, as an example. But they aren't cheap generally.

can amplifier tubes cause hum

Yeah...I only got the 1:10 probe..the inpu on my scope shows max 600v (P.P+DC), so guess it's ok.

I think I'll replace the 100K anode resistor at V2a which could have been damaged during this troubleshooting... It's more a hiss-sounding hum than mmmmm-sounding hum.

Anyway, this is my second tube amp build (first was a very simple one channel 15w class A PP) and in these few hours you have been helping me debugging this build, I have learned more than I did the last year! ..and I realy appriciate it!

I'll check the bias voltage at the anode with the multimeter and report the findings.

PS: It's not J176 but J175..but guiss I could use the J176/J174 if the gain on ch1 is too low..

Added after 1 hours 16 minutes:

Guess what Frank!

Replaced the 100k res at the D voltage/V2a anode and it actually decreased the hum and hiss a lot!

I measured the anode voltage but didn't changed remarkable when switching between the channels..it dropped 1 volt and got up again..