Trainwreck Express Build

[Littlewyan]

No difference Going to try lifting the 2 100K Resistors a second to see what that does.

[Littlewyan]

No difference . The pulsing carries on even with all knobs set to 0 so must be a loop.

[M Fowler]

Ah good idea! That would make sense I'll give that a go. The Capacitor Grounding didn't seem to make a difference by the way.

The capacitor isn't grounded it is connected to 100k plate (anode) resistor on one end and to the junction of 150k and pin 2 of V2 (grid). The other end of the 150k is grounded.

[Littlewyan]

No sorry I meant the Filter Capacitor's Grounds

[M Fowler]

One more source of buzz can be the bias caps 10uf/250v on your amp.

[Littlewyan]

Wahey I've managed to make the buzz quieter by changing V1 to a JJ. Its still there but quieter.

[Littlewyan]

Ok changed V2 for the tube that was in V1 and its got a bit quieter again I think. Now when I get the pulsing I find the only knobs that affect it are the Volume and Treble. Mid and Bass make no difference at all.

[Littlewyan]

Success! In a way! I made the Bass and Middle leads a bit longer so they can be pushed into the edge of the chassis, shortened the treble wire by like an inch and went direct from the board to the pot. Put a sheet of foil on top of the amp, knobs maxed out, noise pretty much gone. If you put your ear to the speaker you can just about hear it, but considering its on full volume I wouldnt class that as an issue . I do however have some oscillation that I can hear, sounds like someone tapping really fast. I'll upload a clip.

[Littlewyan]

Heres the clip

[RJ Guitars]

Congrats... When I read your previous post and you mentioned that oddity about the treble pot... I was wondering about that wiring on the volume control and tone pots. Lead dress around V1 and the signal wires in and out of the pots is critical on these amps. Great work and thanks for sharing.

Are you using snubber resistors going into each tube?
rj

[Littlewyan]

Thanks RJ With regards to the oscillation I'm getting this may be dirty mains, as I'm sure I had this with my Marshall Amp once and once I moved it to the other side of the room it was fine. About to give the amp a blast full volume, girlfriend is about to have a shock!

[Littlewyan]

Snubbers? Do you mean grid stoppers? If so I've got a 2.7K on V1B but thats it.

[RJ Guitars]

Yeah grid stoppers... Transmutilated my terms there. Don't be afraid to try one on V1A as well.

[Littlewyan]

Ok would it make the amp quieter if I reduced the 2.2K (sorry got it wrong) Grid Stopper to 820 Ohm?

[RJ Guitars]

I doubt you'd be able to see it with high end electronics much less hear it with your ears... Merlin suggests an absolute minimum of 100 ohms (but use something) and it's probably going to go without any impact using a value up to 68K... I'd say try a 10k and listen to it. It's simple enough to take out if it does bad things but it will help you kill high frequency oscillation's that you don't hear directly but cause you grief. In the trouble shooting stages this is a useful tool...