Help checking Tube Amp Supply

[ruffrecords]

OK, next step is replace V2 and remove V1. Is the hum back or not?

Cheers

Ian

 

[sonolink]

Ian

Without v1 the hum is there but I'd say it's within normal tube amp hum. It's very faint..

Cheers
Sono

 

[ruffrecords]

OK, it looks pretty clear that the hum is getting in at V1. What happens to the hum as you vary the volume control between V1a and V1b?

Cheers

Ian

 

[sonolink]

Ian,
With vol control down to zero hum is loud. When opening the volume hum is same up to aprox half way (maybe a bit less) and kind of changes going a bit lower. But as you keep cranking up the hum comes back. Also, as the vol increases some hiss is added. I hope this makes sense
Cheers
Sono

 

[ruffrecords]

OK, understood. I notice your layout does  not show C22. Is it fitted?

Cheers

Ian

 

[sonolink]

It is. And R1 aswell.  You can see them on this pic

 

[ruffrecords]

OK. Un-bypassed cathode resistors can be a source of hum in early stages. There's something wrong around V1; we just need to find what it is.

As it is still there when the volume pot is turned down we should first look at V1a. So short pin 2 (the grid of V1a) to ground. What happens to the hum?

Cheers

Ian

 

[sonolink]

Ian,

When shorting v1 pin2 to gnd the hum is still there BUT at half as loud and moving vol pot doesn't affect it (I guess because by shorting to gnd we're bypassing it)
Should I put a similar cap as C22 on v1a cathode then? (parallel to R11)

Thanks for your help again
Cheers
Sono

 

[peterc]

How do your DC voltages compare to the Fender schematic?

 

[sonolink]

Hi Peterc
Thanks for your input. My voltages are pretty near the Fender ones. In brackets are the Fender ones marked on the schematic:

TP23 (bias): -11 (-10.4)
TP24 (B+): 347 (319)
TP25 (1st filter): 323 (294)
TP 26 (2nd filter): 252 (232)

V1
pin 1: 160 (152)
pin 3: 1.3 (1.2)
pin 6: 157 (152)
pin 8: 1.3 (1.2)

V2
pin 1: 178 (172)
pin 3: 44.8 (41)
pin 6: 175 (172)
pin 8: 45 (41)

V3
pin 7: 344 (316)

V4
pin 7: 344 (316)

Cheers
Sono

 

[sonolink]

Tried a cap similar to c22 in parallel with R11. No go

 

[sonolink]

Tried grounding the other grids. Hope it helps

V1 p7 hum vol pot does the same
V1 p2 low hum vol does nothing
V2 p2 low hum vol does the same
V2 p7 same as v1p2

Cheers
Sono

 

[sonolink]

I strapped my scope across the speaker output and this is what I got.

 

[sonolink]

I also recorded the hum to make sure we're on the same page

http://soundcloud.com/thesoundkitchen/hum

Cheers
Sono

 

[ruffrecords]

OK, that is definitely 120Hz hum which implies it is getting in through the HT supply. The fact it becomes lower when you short V1 pin2, which is the second stage grid, is further evidence of the same. Let's try improving the HT supply to the first stage. Replace R3 with a 10K and 91K in series, with the 10K connected to the HT supply end and the 91K connected to the V1 pin6 connection. Then connect an extra 22uF 450V electrolytic from the junction of the 10K and 91K to ground. This will slightly reduce the gain of the first stage but mostly add some extra HT decoupling which should reduce the hum.

Cheers

ian

 

[sonolink]

Ian

Thanks a lot for your help

I tried what you suggested but there's no change. Are you sure this is not a grounding issue? If so how could I check that? A friend suggests it could be the OT. I have another one of better quality. Do you reckon it's worth checking?

If this is a preamp issue maybe a way to check would be leaving the preamp out of the equation? Connecting another preamp to the power amp? Maybe wiring a Tube screamer pedal or going from the fx loop send of another amp? (I have a few).

If so, I'd have to cut between C6 and V1pin1/r10 and wire the hot terminal of a guitar lead to C6 and ground the shield? The other end of the guitar would plug into the pedal output or the other amp's FX send. Would that be correct?

Please let me know what you think.

Cheers
Sono

 

[sonolink]

Ok so I tried using preamp and power amp sections with known clean sounding counterparts and no go.  Hum hum hum

 

[ruffrecords]

The only possible grounding issue I can think of is if you don't have the 0V and the chassis connected to the mains safety earth, These connections are not shown on your layout. If you look at the original schematic, the mains safety earth is connected directly to the chassis at the mains inlet. The HT 0V is connected to chassis at the input jack. If you do  not have these connections then you could get a lot of hum.

Cheers

Isn

 

[sonolink]

Ok. Here's my scheme:

Mains ground

 

[sonolink]

Output jack ground