Amp Advice

[Dmoney]

Hello all...

I made this recently... a 50watt 'crunch only' SLO based amp.
I've attached the schem of what I've made, the sepia bits are the lead channel parts I have left out. I also sacked off the loop.

The amp has less gain than I would imagine, I've done some reading up and I'm wondering if I should change the 500K gain pot to a 1M? I vaguely remember something about the lead channel loading the clean channel gain stage, or something... I really can't remember in detail. I don't understand that fully, but I think a 1M pot at full rotation would get me more distortion than the 500K at full rotation...

am i right? I'm on nightshifts and im no pro anyway!


Getting some noise in the early stages too. It's in the high F's. The 'bright switch' in the schem reduces it. I also had a go at putting the 68K grid stop fron the 1st stage directly onto the 2nd stage grid... the noise was still present there (with max gain and max master vol) but at a much lower level. I also had a go at putting an 80uf filter cap in parallel with the 47uf filter for that stage, but that had no effect. I've tried moving wires around the pot and the noise gets no better and no worse. hmmmm.

[Dmoney]

just stuck in a 1M gain pot in place of the 500K. had the desired effect. woop!

[Dmoney]

Here it is...
still needs a little tonal tweaking but it sounds good. might just need a *touch* more drive.

I have noticed that the PT has a kind of ticking noise coming from inside it when the amp is running and this is getting into the audio somehow.
With the amp in standby there is no noise. Standby in this amp is a DPDT switch cutting either side of the main secondary before the rectifier. The rectifier is a 4xUF5408 full wave with snubbing caps. With the standby in the operational postition something starts to have a highish pitched hum, and its getting amplified.

Toe-Knee suggested trying a shielded socket on V1.
Any more ideas?

[Dmoney]

actually!

the noise isn't coming from the PT. it sounds like the filter caps are vibrating, its like a fast ticking noise. I don't think its coming from the switches. I can't tell if its coming from all the filter caps. It's really hard to pinpoint. It's not the IEC arcing...

hmmmm

[Dmoney]

Some more pics. I only have a camera on my macbook currently! photobooth is not the way forward!

So... I tried poping out the HT fuse, and that stops the noise....
So I'm assuming something after the fuse is the issue. So, not a problem with the first stage of filtering, something from the choke onwards???? then again, with the fuse out the power supply isn't loaded right? so it might not mean much.

here are some gut shots. still a bit of a work in progress. I HATE noise!

[Dmoney]

more...

[HTH AMPS]

shielded wire from the input jack to the grid of the first stage (minimum).

if you still have problems, a 100pF cap on the first stage between the plate and cathode should kill any HF squeeling.

after that, I'd start using shielded wire on subsequent stage's to the grids.

[Dmoney]

all done. all checked. though the cap I used is 10pf. moving wires around makes no difference either. the gain hiss is low and there is very little if nothing in the way of heater hum. holding a shield over v1 makes no difference. you can hear it, like an almost mechanical noise when the amp is on and no input, and with all the preamp valves pulled. it sounds like its coming from the filter caps but im not sure

[HTH AMPS]

the sound you describe is the same one I've experienced when the filter caps are running close/slightly-over their rated voltage.

might be a bad-cap right out of the box - did you form them or just turn the amp on with full voltages?  you're generally ok not to form new caps, but its possible you have a bad one.

[Dmoney]

ok. cheers.
I don't think the voltage will be an issue. I went with the 220uf 350v in series for the first 2 stages. the others are 47uf and either 450 or 500s.

I might check some joints, failing that what is the best way to root out a bad cap?

[Frank]

Didn't see a PSU on the schematic but I assume there's a standby switch - does this ticking happen on standby as well as when the amp's running? Is it worse on standby?

Ticking might mean something is arcing, maybe a bad cap with a tiny fault internally.

[HTH AMPS]

are the caps definitely orientated correctly?

[Dmoney]

the caps are correctly placed. the sounds doesn't happen when in standby. the standy cuts the ht winding right at the output of the pt.

[HTH AMPS]

the caps are correctly placed. the sounds doesn't happen when in standby. the standy cuts the ht winding right at the output of the pt.

I still say 'bad cap' - replace them one at a time is the easy way.

[Dmoney]

I THINK I've narrowed it down.
If I pull the fuse after the 1st filter stage, I don't get the tick.

If I disconnect the OT primary CT from the output of the fuse
Disconnect the voltage divider for the elevated heater ref after the second stage, and disconnect the link to the 10K/3W dropping resistor and fire up the amp..

I get the ticking. defo not from the choke, and I'm pretty sure its from the 2nd stage filter caps. The ticking is quieter with the divider disconnected, but it's still there. So... I'm guessing it's one of those two caps... now I just need some spares