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

[FvM]

cause of humming in tube amplifiers

Hello Thomas,

when not using split capacitors as with V1 DC filament supply, you should have one or two smaller capacitors to ground to block AC common mode voltage. It could also cause interferences otherwise. I would double the capacitor to 10000 uF to get below 0.5 V AC RMS voltage with 1.2 A current.

Regards,

Frank

 

[Thomasdj]

100 hz hum tubeamp

Hi Frank,

I did the DC conversion as mesa and used a 4700uf and 2x2200uf as split caps (it's all I have) and 2x150ohm 5w resistors from +/- to ground. I measure +-3,9v DC without load and think it's a bit too high. At the V1 I measure +-3,5v DC which is also too high. I think the transformer could be over-dimensioned to 8v AC (haven't tested it since it's original).

What do you think, - do I have to worry about the extra voltage?

Thanks

Thomas

Added after 32 minutes:

Offcourse! Forgot to say I measured these voltage in "high power" mode.

 

[FvM]

tube amp ac hum

Hello Thomas,

the voltage is higher without load current. 7.8V would be 20% too high, but not immediately blow the filaments. I expect the voltage to O.K. with the 1.2 A load.

Regards,

Frank

 

[Thomasdj]

tube amplifier hum tube

Hi Frank!

Ok, now it has DC heaters..It didn't do much/anything to the noise. However, when i scope at the output I can see that something has disappeared, but "nothing" is better.

What do you think? should I pull out the extra bridge rectifier or leave it as is?

Best regards

Thomas

 

[FvM]

amp 100hz hum

Hello Thomas,

when DC supply it doesn't give perceptible or measurable reduction of hum, it doesn't serve a purpose. I would go back to AC filament supply. You have shown by your test that it is sufficient.

Regards,
Frank

 

[Thomasdj]

easy tube amp

I'll do that Frank,

I think I'll run a few more test to find out exactly where this hum is coming from. I still haven't checked the output (anodes) on the preamp tubes while the amp is on standby which could be interesting. The high B voltage could cause some hum if it's too close to a signal wire, i guess.

In comparison to other high gain amps (also the mesa dual rec 2ch version) I've noticed a 0.005uf cap going from V2a anode to ground..I do not know what it's good for, but think it's there to prevent oscillations, am I right?

I can shape the hum with the EQ's and with the tone control on the guitar and when I switch between the pickups, the hum also change. It's not something that I can't live with but I'm convinced that the amp could be more quiet.

 

[FvM]

weird humming coming from tube amp

Hllo Thomas,

I don't know if there is a real danger of oscillations with these preamps. An anode capacitor would work as a lowpass and may be simply intended as such,

Hum observed with a guitar plugged in to the amp isn't a problem of the amp normally. I would basically refer to hum with input shorted or unconnected.

Regards,
Frank

 

[Thomasdj]

tube amp hum guitar

Hi Frank,

I resolodered my guitar and put a nice little FET-circuit in and the noise is gone..I'm now convinced that the amp is as quiet as it can be!

However, I've observed an issue with the power. When I turn the amp off standby it has more output power for about a minute or so and then the power decreases to a lower steady level. Is this normal? I ask because there is a big difference, - sounds like it's half of the power.

Best regards

Thomas

 

[FvM]

tube amp curcuit humming

Hello Thomas,

seems like your work is nearing completion. Very good!

A gain drop after releasing standby isn't normal operation, I think. Can it be a problem with the solo circuit? If it isn't so, you should check again bias points (anode voltage) of the individual stages and detect unexpected variation during the first minute of operation.

Another option would be to check which stage gain is actuality changing, either by measurement of AC level with a constant input signal. As you have no signale generator, you can connect a MP3 or CD player. Or you can connect the test signal at a a a different point in signal chain, e. g. at effect return and check, if the gain variation is before or behind this point.

Regards,

Frank

 

[Thomasdj]

elevating tube amp heater center tap

Hi Frank,

I've built this square wave generator. Now what to do:|

I've measured with the scope at the speaker out and when I hit standby off I measure 1,2v from max to min at the wave, and after one minut I measure 10mv. After 15 min I measure 10mv..

The B+ voltage is steady at 443 voltage, but when I hit the B+ wire with the probe it's very sensitive and the volume came up and then slowly decrease again (within 10 sec.).

what could it be?

Thanks

Thomas

 

[Thomasdj]

tube amp ground switch

Btw..the measurings are for channel 1! (V1/V2a and V5).

I haven't tested the amp for an hour, but when we tried to record the amp we observed a power drop after about 1½ hour. I thought it was because of heat stress but now I'm not sure..

 

[FvM]

ldr resistor channel muting

Hello Thomas,

The generator should be a useful tool. You may need an additional attenuator not to overdrive the input stage before gain control.

A drop from 1.2V to 10 mV is bad. I also don't understand why the amplifier reacts on touching B+ node with a probe. Do you observe the gain drop similar for all three channels?

I would expect a single stage have an unstable bias point, e. g. due to a floating grid. My suggestion measure anode voltages was unclear, as I see. I meant the voltage at the individual anodes, not the anode supply. The measurement should reveal unstable bias point of amplifier stages.

You should check, if you are able tó measure your generator signal along the signal chain, with AC coupling and a sensitive oscilloscope Y setting. This could also help to identify the fault location.

Regards,
Frank

 

[Thomasdj]

tube amplifier power supply hum

Hi Frank,

I have a 1k trim pot at the output of the oscillator (set to 28mv). I haven't directly tested if it's the same for each channel. I mostly play on channel 2 which also do this, so I'm pretty sure that it's the same for each channel..

I've just measured the voltage and thay all looks fine, - about 40 voltage lower than shown in the schematic, but I think it's because I test the amp with EL34 tubes and without the rectifier tubes. Could also be that the values in the schematic is measured without load..

When touching the anodes of all the preamp tubes, I can kill/lower the signal!! If I tap at the anodes, the signal raises!! There is something I haven't mentioned..sorry for that. The tube sockets are connected via short wires and terminals (solder pins) to the PCB. So, the wires aren't soldered..could this cause the problem? However, V2a is soldered directly to the PCB and the same happens when I tap/touch its anode...
It's the same with any of the preamp tubes..also the phase inverter. It seems like everything is very sensitive when touching it with the probe and the signal power goes up and down. It's not a problem to trace the signal but I can't tell if it goes up/down beacuse of the influence of the sensitivity issue..

Not sure what you ment by "floating grids".

Thanks

Thomas

 

[FvM]

100hz hum

Hello Thiómas,

the same effect when touching pins at any tube probably indicates, that the instable operation is ia present at another circuit location. If there is a bad contact, it could be temporarily changed by the voltage transient caused from charging the probe.

Floating grids simply means a grid without correct connection.

Actually, I don't know which circuit stage could cause the problems. If transients from connecting the probe modifies the circuit operation, the measurement may be performed with the probe in place. It should be possible to detect in which stage the gain is lost.

What do you mean with the wires aren't soldered?

Regards,
Frank

 

[Thomasdj]

getting rid of hum in a tube amp

Well Frank,

I think these pictures says it all and properly this is the answars to the "bad contacts" you mentioned...

Hopefully I will be able to just solder the lug and pin together..

I'll try this tomorrow and see if it makes any changes.

Thank you very much Frank

Thomas

 

[FvM]

polish bf245

Hello Thomas,

After thinking a while about not soldered, I expected something like this. The connection hopefully isn't an issue with these elements, but each additional connector has a risk of failing.

As a general consideration for further fault-finding: Each stage has a nominal gain, that can be estimated from the schematic, it can be compared against the signal level measured at each stage. Position of gain and equalizer controls should be defined (on 0-10 scale, or simply min, max, center).

Regards,
Frank

 

[Thomasdj]

causes of extreme low hum in tube amp

Hi Frank,

I've recieved the potentiometers yesterday and now the EQ's, gain and master controls are working perfectly! I am very satisfied with the sound, headroom and the tube breakup, which is happening around 60% master volume.

I've soldered all the lug/pin connections together which actually stabilized the amp (no scratching and so on)..

I still observe the "volume drop" issue, but here is something new; When I switch between the channels I get the same problem! I am thinking this could be a switching issue! As far as I can tell, there are two critical transistors, - Preamp pt2 schematic - up right corner at RY2b and Preamp pt1 at V2a grid.

What do you think?

Thanks

Thomas

 

[FvM]

18v supply tube amp

Hello Thomas,

I like to remind my previous suggestions to search the volume drop systematicly by measuríng the signal at each stage with a generator at the input, at best during the time when the drop takes place.

The mute switch at RY2b could reduce the signal level only if it becomes rather low-ohmic. Not impossible at all. The transistor at V2a has already shown it's capability to shift the V2a bias point due to leakage currents (if I understood the effect correct). This could be detected by a DC level change at anode and cathode (small, but significant change) as well.

Regards,
Frank

 

[Thomasdj]

tube amp hum replace tube

Sorry for that one Frank..

I tried to trace the signal and guess what! When I was moving the o'scope probe to test the anodes, I found out that the +15 wire connected to the FET (preamp pt2 upper right) was extremely sensitive! If I touch the wire with the probe or a plastic pencil, I get full signal which decreases as I move the probe/pencil away from the wire.

What can it be Frank?

Thanks

Thomas

btw..None of the gain stages are losing signal/amplification..

 

[FvM]

elevating heater tube amp

Hello Thomas,

what is connected to the gate of the said FET? It should be connected to the mute bus, that's the 68k/1u connection at switching part 1. This results in +15 V idle level at the gate, keeping the switch in an open position. Could there be a misunderstanding of the schematic or wiring error, so the the gate hasn't +15 V level in normal operation but is effectively floating?

Regards,
Frank