High Pitched Squeal - SVT

[Groover]

I have the following tubes in my SVT (HD)

---------------------

V1 = 12ax7 NOS RCA Long Black Plate (Square-Getter, White Letters)
V2 = 12ax7 RI Tung-Sol
V3 = 12ax7 NOS RCA Long Black Plate (D-Getter, Orange Letters)
V4 = 12ax7 RI Tung-Sol
V5 = 6c4 NOS RCA Long Black Plate (White Letters)

PI = 5751 GE 5-Star Gray Plates (White Stars, White Letters)

Driver Tubes = 2 x 12Bh7a NOS RCA Long Gray Plates (D-Getters, Orange Letters)

Power Tubes = Matched Sextet Sveltana Winged 6550C

----------------

These are all brand new tubes. When I crank the volume up all the way, I get a high pitched squeal. I've tried Channel 1, and Channel 2. Both do it at high volume, but Channel 1 seems a little more squeally than Channel 2.

From what I understand, this is a sign of a bad/microphonic pre-amp tube.

I'll do some tube swapping this weekend to isolate it, but in your opinion, which one is the culprit? Being that it's on both channels, would it possibly be the 6C4?

Other that that squeal at high volume, the amp seems very quiet. No buzz, hums, or popping.

[metron]

Originally Posted by Groover

From what I understand, this is a sign of a bad/microphonic pre-amp tube.

I wouldn't make this assumption first. Microphonic tubes will usually chime or ring. I had an amp that squealed because somewhere in the circuit there were oscillations that were ultimately unrelated to the tubes.

How long have you had the amp? Did it squeal before you replaced tubes? Has it always done it since you changed tubes?

[BassmanPaul]

6C4s are known to be prone to microphony. If you tap each pre-amp tube with a pencil and you can hear the "clunks" through the speakers that tube is microphonic.

Paul

[Groover]

Following up here...

Ended up being this tube:

V1 = 12ax7 NOS RCA Long Black Plate (Square-Getter, White Letters)

It was microphonic. Once I swapped that, the squeal was gone. I also confirmed by tapping on the tube. It sounded like tapping on a hot microphone.

[Seans]

Don't jump to conclusion's with mic'phc tubes, switch on the amp and gently rotate each tube, if you hear a scratch like sound this could be the cause, then clean all the points on the bases with a tooth pic type brush and some deoxit ( you should do this anyway as a matter of course). 12dw7 and 12ax7 can do some odd things if they don't have good contacts and a bad contact can set up a reaction like this.

[MAMMOTHvolume]

Originally Posted by Groover

I have the following tubes in my SVT (HD)

---------------------

V1 = 12ax7 NOS RCA Long Black Plate (Square-Getter, White Letters)
V2 = 12ax7 RI Tung-Sol
V3 = 12ax7 NOS RCA Long Black Plate (D-Getter, Orange Letters)
V4 = 12ax7 RI Tung-Sol
V5 = 6c4 NOS RCA Long Black Plate (White Letters)

PI = 5751 GE 5-Star Gray Plates (White Stars, White Letters)

Driver Tubes = 2 x 12Bh7a NOS RCA Long Gray Plates (D-Getters, Orange Letters)

Power Tubes = Matched Sextet Sveltana Winged 6550C

----------------

These are all brand new tubes. When I crank the volume up all the way, I get a high pitched squeal. I've tried Channel 1, and Channel 2. Both do it at high volume, but Channel 1 seems a little more squeally than Channel 2.

From what I understand, this is a sign of a bad/microphonic pre-amp tube.

I'll do some tube swapping this weekend to isolate it, but in your opinion, which one is the culprit? Being that it's on both channels, would it possibly be the 6C4?

Other that that squeal at high volume, the amp seems very quiet. No buzz, hums, or popping.

does it do this without your bass plugged in and a jack in the input socket?

[Groover]

Thanks again for replies. Pretty sure it's the tube in this case. The contacts were all deoxit-ed as I had this amp completely apart to restore/clean/check. With that tube in there even at lower volumes, I could hear what sounded like little chaîns rattling when I'd hit a note with force. At first I thought it was the ghost in the house again.... But I took the tube out, tried a differnt tube, and both problems (squeal and chain sound) were gone. Put the "bad tube" back in again, and both issues came back. That's when I also did the tap test, and that one tube had the big clonk sound when tapping (the others didn't). The bass was plugged in, and amp was off standby.

[MAMMOTHvolume]

Originally Posted by Groover

Thanks again for replies. Pretty sure it's the tube in this case. The contacts were all deoxit-ed as I had this amp completely apart to restore/clean/check. With that tube in there even at lower volumes, I could hear what sounded like little chaîns rattling when I'd hit a note with force. At first I thought it was the ghost in the house again.... But I took the tube out, tried a differnt tube, and both problems (squeal and chain sound) were gone. Put the "bad tube" back in again, and both issues came back. That's when I also did the tap test, and that one tube had the big clonk sound when tapping (the others didn't). The bass was plugged in, and amp was off standby.

glad you sorted it normally best way to do this is to give all the valves a wiggle in their sockets untill it kicks into oscillation

[nysbob]

Mine will do it if I switch the ultra hi switch on. Probably a tube, but I don't care about the the ultra hi switch being on, so no problem!

[Seans]

Funnily mine has just developed a microphonic reaction with the fan, so I go through the tubes and find its the 6C4, so one is on order, but now I go back again its still the 6c4 but I notice the centre 12dw7 is scratching, so I clean up the contacts on that one and guess what, all microphonoics are gone and the 6c4 too is quiet.
So as I posted a few days ago, I got caught out by my own post!! serves me right for not doing some research first, oh well I have a spare GE JAN 6C4WA for when the original goes.