Tonehunter JB Special - 50 cycle hum

[Guitar-Tube-1969]

Hi guys,

I bought a Tonehunter JB Special a while ago which did a great job until now. Now there is a 50 cyle-like hum I cannot get rid off. It starts after warming up of the tubes (no standby-switch). Unfortunately I don't have a schematic.

Attached are two photos and an audio file with that hum (vid is too large...).

Tubes:
V1: 12AX7
V2: 12AX7
V3: 6V6
V4: 6V6
V5: 5Y3

What I've tried so far:
1. A different set of output tubes (6V6)
2. A solid state rectifier instead of the 5Y3
3. Swap V1 / V2
4. Try a different V1

Nothing changed regarding the hum.

I detected a black colour on pin4 of V3 but it looks like that black glue that the builder (Ralf Reichen) used for fixing the various components. In addition I checked if there is contact between the upper and lower side of the pin and it is.

Any idea?

Thanks for your help - Axel

[jeff]

Filter caps?

What I've tried so far:

2. A solid state rectifier instead of the 5Y3

Wouldn't advise replacing 5Y3 with SS rect. Voltages will rise.

[PRR]

If it is 100/120Hz buzz, it is filter caps.

If it is 50/60Hz hum, it comes direct from the power lines or AC filaments or their CT.

[Guitar-Tube-1969]

Thanks to you both!

I'm not sure how to detect what type of hum it is because I have no reference.

The loudness of the hum (or buzz) changes when using the master volume - does this give a hint?

Is it possible to measure the filter caps without soldering them out (have a good digital fluke which can measure capacitance)?

Visually the big filter caps (22uF) looks good but the smaller one on the left side does not. Could this one be the cause?

Thanks in advance - Axel

[shooter]

does this give a hint?

means the problem is BEFORE the MV

have a good digital fluke

mine will measure frequency.  you can also measure the VAC on the filter caps, you want something small-ish, like =<4vac at 1st filter cap (PA tube tap), something like =<200mVac at last cap (1st preamp tube tap)

[rake]

Is that an aluminum chassis? I recently worked on a Deluxe clone in an aluminum chassis with hum and it turned out the lug bolted to the chassis for the center taps was corroded under the lug and lost good contact.
a little trick I learned from old motorcycles is to use stainless steel hardware with a stainless steel washer between the lug and the chassis. Good luck and happy hunting.

[tubeswell]

If the amp worked good but suddenly spontaneously became hummy, 1st thing to try is a tube swap

[Guitar-Tube-1969]

Is that an aluminum chassis?

No, it is steel :-(

[Guitar-Tube-1969]

If the amp worked good but suddenly spontaneously became hummy, 1st thing to try is a tube swap

What I've tried so far:
1. A different set of output tubes (6V6)
2. A solid state rectifier instead of the 5Y3
3. Swap V1 / V2
4. Try a different V1

I now ordered three new 16uF filter caps, that bipolar cap and a new 5Y3. Hopefully one of this causes that hum.

[Guitar-Tube-1969]

I've detected that by pulling V1 the hum is still there, by pulling V2 it is off. Give this a hint?

[PRR]

Without a schematic or a clue, we can only guess. Maybe "V1" is the 2nd stage and "V2" is the first stage? Several commercial amps or pedals have the physical layout backward from the functional layout.

Tube and filter-cap voltages may find a "WTF?" reading. If a filter cap is at 290V and a plate is at 289V (only 1V dropped in 100k resistor) then we suspect tube or (since swapping didn't fix) a bad joint or failed part.

[Guitar-Tube-1969]

I tested a new 5E3 => no effect
I changed the bipolar cap and the three 16uF filter caps => no effect

shit

[tubeswell]

Re-tension the tube socket pin clamps (with something small like a jewellers screwdriver. And don’t electrocute yourself)

[Guitar-Tube-1969]

Re-tension the tube socket pin clamps (with something small like a jewellers screwdriver. And don’t electrocute yourself)

what is the purpose of this?

[Guitar-Tube-1969]

Following the advices in Uncle Dougs vid (see below), I measure the following:


Output  | Resis-  | Voltage | Plate    | Plate   | Plate
Tube    | tance   | Drop    | current  | Voltage | Dissipation
        |   (R)   |    (V)  | PC=V/R   |  (PV)   | PD=PV*PC 
----------------------------------------------------------------
- outer | 238 Ohm |  9,78 V | 41,09 mA | 370 V   | 15,20 W
- inner | 241 Ohm | 10,39 V | 43,07 mA | 371 V   | 15,98 W
                                                   15,60 W (avg)


That looks good so far, right?

Dougs vid:

[tubeswell]

Re-tension the tube socket pin clamps (with something small like a jewellers screwdriver. And don’t electrocute yourself)

what is the purpose of this?

You may have an intermittent/bad pin contact in V2 (or not)
(Going by your earlier report the hum goes when you pull V2. You also said you swapped V1 and V2 but didn’t confirm whether you replaced the tubes or merely swapped them around with each other. So I haven’t got much to go on. Re: PRR’s comment about bad connection as a likely source of noise, an obvious point to rule out is the pin clamps on V2)