#124 started

[martin manning]

What happens if you plug into the return jack and short that input to ground?It should go silent then. The hum you are hearing could be due to a ground loop between the amps. The intermittent nature of the hum and output suggests it's something temperature dependent and/or a bad connection in the preamp.

If the power amp is working you can plug a cable into the return and use the other end to probe for signal, but be very careful. I would only probe low voltage nodes, meaning between coupling caps and grids.

[norburybrook]

the hum wasn't a ground loop between amps as I could get it to stop by plugging a cable into the input of the bypassed pre amp #124 amp.



Can I probe on the cathode caps connection on the board rather than the valve? obviously I need to stay clear of the high voltages on the plates

I have no signal if I plug into the input of the amp at the moment so how would I get a signal to check?


marcus

[martin manning]

the hum wasn't a ground loop between amps as I could get it to stop by plugging a cable into the input of the bypassed pre amp #124 amp.

Hmmm... is your return jack wired to disconnect the preamp, i.e. switch contact connected to send tip? Plugging into the return should take the preamp out of the picture.

Can I probe on the cathode caps connection on the board rather than the valve?

Sure, or the grid wire connections on the board or pots.

[amplifiednation]

the hum wasn't a ground loop between amps as I could get it to stop by plugging a cable into the input of the bypassed pre amp #124 amp.



Can I probe on the cathode caps connection on the board rather than the valve? obviously I need to stay clear of the high voltages on the plates


marcus

Yes you can probe the plate voltages on the board too! It 100x easier.

If you could post a couple pics of the tone stack that would be helpful. I'm sure you've read about poking around with a chopstick? This would be a good time to do that. It's easier to identify subpar connections. Did you Add the 100R resistors on the heaters? Or are you grounding the Center tap?

[larsvictor]

A new day with new problems...

the hum now is volume dependent

you mean the preamp volume potentiometer, right ? (not the master volume).

I have to think about it...

Another point:
The input wiring seems different from the original. You omitted the FET circuit. I can see two grey shielded wires running to V1a. I suppose you wanted to have 2 different grid stoppers ? This is not a classic fender wiring of the 2 input jacks. I cannot see it clear on the pics, but this could be wrong. If you have 2 identical switched input jacks one of the signal paths to the grid stopper is always grounded. That means you have a connection of the grid to ground via the other grid stopper.

Lars

[norburybrook]

Ok.found it !!!!!!!!!!!!!

I had somehow wired the wiper from the volume pot to the ground as well , I've absolutely no idea why I'd do that maybe something to do with the screened cable.

anyway................

Still a bit of hum.

when I connect the NFB the hum gets worse and stops being master volume dependent.


I have clipping when playing hard that stops when I insert a cable from the send to return

also when the NFB is connected the presence control does affect the clipping somewhat. at higher levels on the presence control the clipping gets worse.

The preamp boost doesn't seem to be working.




lars I've removed the second input for now, wanted to take that out of the equation

M

[larsvictor]

I have clipping when playing hard that stops when I insert a cable from the send to return

Could be a bad solder joint at the send and/or return jack. In the schematic the switched jack is the return jack, not the send jack. That offers a signal access without interrupting the signal path to the power amp.

[amplifiednation]

Maybe take a new set of pics? Just some quick shots with your phone would be fine

[larsvictor]

Yes, some better focussed pictures could help.

The 22M Resistor from treble 3 to the R/J switch is very close to the treble pot and there is no isolation around the leg. If the leg touches the housing of the potentiometer the PAB switch would have no effect anymore.

[norburybrook]

thanks guys.

Quick question while I've been going over everything.

the Capacitor for the NFB on the layout is showing the + end going to the presence pot and -end going to the 390R.

the schematic shows it the other way round.

I have an electrolytic in there so which was should it be?

I'll take some more pics.

marcus

[larsvictor]

The polarity of the NFB cap is correct. Did you ground the 390R and the "-"-terminal of the NFB cap ?

[norburybrook]

Lars,


Well spotted on the in-isolated resistor grounding on the treble pot.

pre amp boost is now working. thank you.


so are you saying the layout is correct and the schematic is wrong? so the -ve end of the cap/390r should go to ground with the send return?


I've put a 1.0 uf tantalum in there as it was easier to do so for the minute with the -ve to ground.


Ok this terrible hum that's worse when NFB is connected and the clipping.

the clipping is really bad when the presence pot is full up. putting a jumper across the send return stops it.


would changing the send return jacks to the schematic rather than the layout make any difference?



M

[martin manning]

...so are you saying the layout is correct and the schematic is wrong? so the -ve end of the cap/390r should go to ground with the send return?

Correct.

...putting a jumper across the send return stops it.

That suggests the switching contact on your loop jack is bad. The layout wiring for the loop jacks is correct. Only the return jack has a switching contact in use. The send is a plain mono jack.

[norburybrook]

Ok then that's all as it should be.


So I'm left with two issues before I can start tweaking.

1; HUM!!!!!!! I've still got a volume independent hum.

I've re soldered every earth , I've tightened all the physical screws , I had Dremelled the anodising of every earth point prior to this.

removing the NFB makes the hum lessen but then causes clipping/distortion

heaters are grounded with a center tap at ground 5


2: clipping; turning the presence pot to 10 causes bad distortion/clipping

having a cable in the send return stops this somewhat.

I have resoldered all connections on the send and return jacks and checked continuity to the boards and they're all fine.


M

[martin manning]

Re 1, can you isolate the source of the hum by pulling V1, then V2?
Re 2 and reducing clipping "somewhat," might be an ultrasonic oscillation kicking in.