High-pitched piping whistle in C30

[tubeswell]

I started getting this about 2 weeks ago and thought I'd cured it, but now wonder if its possibly due to microphonic caps - I put in some disc ceramics in the tone stack 6 months ago. It was intermitent but now seems to be on all the time (when I switch the amp on without a guitar plugged in). Not a loud sound, but very high-pitched almost like the upper register of a dog whistle. It gets fainter as I knock the treble and mid controls to zero. The bass EQ doesn't have any effect on it. Its in both channels. I'm confident its not a microphonic pre-amp tube - I know what they sound like, and its too faint. Any ideas?

[Enzo]

REverb involved? Turn to zero the control. ANy help? I think the pan cables aer just a four pin Molex on the board, unplug it. ANy help?

Does whacking the amp on the top with your fist cause it to react? Turn down the reverb first please. We don't want KA-RASH.

And come on, your amp has been apart more often than a hooker's knees, so tug on all the little jumper wires to see if one is off.

WAIT, it is there with no guitar you say, but does plugging in a guitar make it stop?

Microphonic tubes, caps or anything else should react to mechanical shock. Rap hard on the front of teh chassis with a knuckle, does it affect the noise?

You KNOW it is not a microphonic tube, but have you subbed V1 anyway? Of you flick each tube with your finger, does it affect the noise on any of the tubes?

[tubeswell]

Okay... seems like it could have been a bad output tube (touch wood). I did some subbing with old spares and the noise was gone. I realised this after I swapped V1 and it made no difference, (the reverb on or off, the guitar in or out, whacking the top of the amp - all made no difference) but after I had the amp running for about a minute or two the piping noise (like a high note on a Bosun's Whistle) faded away (just as I was about to record it for posterity, DANG!). It was only there when I first switched the amp on so it (surely?) must have been some mojo to do with warming it up with a bad output tube. So until next time...

Anyhow I FINALLY managed to score some silver mica caps from Oz this week (after how many months was that?), so I'll put them in and see what gives.

(And to think - I never needed to get between hookers knees this whole time)

[Amp Kat]

I didn't think it was any of the mods we had done as they were pretty straight forward or the Cathode switch which is used in many amps but I'm glad you found it and hope it stays clear. You'll like the silva mica caps as they are a little smoother than the ceramics but take into account the mods we did were aimed at getting you somemore grind and the silva mica's are a step in the other direction so do a comparison check and tell us what you think.

[tubeswell]

Okay... seems like it could have been a bad output tube (touch wood). I did some subbing with old spares and the noise was gone.




Anyhow I FINALLY managed to score some silver mica caps from Oz this week (after how many months was that?), so I'll put them in and see what gives.

...Ha Hum - well I switched it on again this morning from cold start and there was that piping noise again. So I swaps the tubes and it still was there, but after a while it faded away, so I guesses to myself that it probably can' do any harm to take it apart yet again and put those silver micas in (since I finally gone to the trouble of getting them). And bingo! after start-up the noise was gone. Hmmm... does this mean it was those disc ceramic caps after all?

The Silver Micas definitey aren't as edgy sounding as the disc ceramics, but (still nicer than Polysytyrene) and better than disc ceramics on the whole methinks because they takes away the excessive treblyness when the presence control is dimed. I put a 330 (Treb) and a 390 (boost) in the Tone Stack, and an 820 in the coupling cap between V1A and V2A, a 22 in the treble bleed before VR3, and another one to ground at the input for RF bleed.

[Enzo]

Glad it sounds good, but you just did one of my pet peeves. I take nothing away from silver micas, nor from your efforts, but when you change all the values at the same time as you switch to micas, it is hard then to say the mica made the difference. For example if you increase the treble cap from 270 to 330, then of course it will sound less shrill or edgy. Same with doubling the value of C4 from 470 to 820. The value change will have a large effect on tone.

Unless we change parts part for part with same values, we cannot point to the mica as the difference.

I like mica caps myself, but we gotta look objectively.

[tubeswell]

I should've explained that I was replacing the discceramaic caps with teh same value Silver mica caps. When I first did the Blue Guitar tone stack mod about som many months ago, I put 330 disc ceramic in for the treble cap and 390 disc ceramic in for the boost. THis took the brightness off the original amp and left plenty of sparkle. The then I tried a 330 and 390 polystyrene and found them noticeable duller than the disc ceramics. The silver micas maybe aren't quite as dull as the polystyrene but are less edgy than the disc ceramics to my ear.

The 820 silver mica coupling cap I put in was a replacement for an earlier 820 polystyrene coupling cap, which I went back to after working on a mod I was working through with Amp Kat to put in a SLO100 O/D setup to get a more grainy sound. I think an 820 here compensates for keeping the 1.5k/22uF cathode setup in V1A. The SLO10 setup had a .002uF couping cap there bypassed with a 470k resistor. The SLO100 mod to the O/D channel gave a good O/D channel but a lousy clean channel. I didn't like the effect that the 1.8k/1uF cathode resistor/bypass cap setup for V1A did to the overall tone (it was too bright) on the clean channel, although it was good on O/D. So I went to a switchable 1.5k/22uF to 1uF setup for V1A to have that choice. which I really like on clean but took something away from eth O/D SLO100 mod. Then I decided to try the Ahola value for the coupling cap between V1A and V2A (i.e. the 820pF, but I only had polystyrene in that value. Since having the sliver mica in there I can't sya for sure what had what effect now, but, I really like the sound I am getting now on clean and O/D. I have stuck with the 1.8K/1uF setup for V2A (from the SLO100 mod), but I changed the Cathode resistor for V2B from a 39k back to a 4.7k/and included a 10uF switchable bypass there for extra gain when i want super sustainy, or even a bit of brown sound when I choose to run the O/D channel cleaner.

[tubeswell]

Problem fixed - Thanks Tmenzo

That's a neat trick with grounding each gain stage's grid with a high voltage cap to ground (when you don't have a scope). Wow! After some experimenting and deduction, (and of course Tmenzo's terrific guidance), turned out to be the coupling cap from V1B on its way out. I changed C6 for a new cap and the problem has disappeared. Thanks once again

[Amp Kat]

Nice job dude check your home email. I sent you some pictures.