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Vintage threads from the first ten years

Re: amp with shorting noise

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7/17/2000 11:24 AM
Graywater	Re: amp with shorting noise Tristan, As I read the symptoms you posted, I'd first clean all of the tube sockets, all of the controls, and the jacks - especially the input. Unless there's a "sizzle" to the sound it's probably not arcing. "Scratchy" and "static" noises are usually dirty contacts somewhere. Out of curiosity, what's your tube line up? GW

7/17/2000 7:50 PM
Tristan	Graywater, by tube line up I assume you mean tubes in the amp? there are 3 12AX7's in the preamp, then two 6BQ5's make up the power section is class B. there is also a 6CA4, I think that has something to do with the vibrato... and there is another tube that has had the markings worn off.... might be a rectifier, but I'm not sure. -Tristan

7/17/2000 8:38 PM
Graywater	Tristan, I sincerely doubt that the amp is Class B, more likely Class AB1. The 6CA4 is your rectifier. As to the other tube, how many pins does it have and is it tall or short, etc.? GW

7/17/2000 8:57 PM

I meant that the 6BQ5's make up the class B section, and the 12AX7's make up the class A part
the "mystery" tube has 9 pins, and it is a little taller than a 12AX7, but the same size around.
looks like a dual triode to me, though the two sections are different. there are two heaters.

I was tapping the first preamp tube (12AX7)while the amp was on and it went all staticy and did that shorting out noise again. Could there be a bad connection and it stops only when the heat from the tube expands the connector and makes it actually work?

thanks,
-Tristan

7/17/2000 9:51 PM

Sounds like you have a microphonic preamp tube. I would swap in some known good ones.

The last tube may be a 7199 -- it has one triode and one pentode in a bottle. (commonly used for the PI section) Although, I am not sure why it would be over on the rectifier side of the power tubes. How many wires (inside the tube) go each tube section tristan? See if you can figure out what the pinout is by looking at the routing.

--or look inside the amp and see how it is wired, and if that gives you any clues to what is up.

BT

7/17/2000 10:59 PM

I tried another 12AX7 that works in an amp I am building, and the same thing happened. I think it is a dirty tube socket, but I cleaned it with rubbing alcohol and nothing changed.

3 wires go into the smaller section (not including the heaters) and four go into the bigger one.
then there are two wires that go to the heaters.
probably is a triode/pentode combo....
this tube is over near the rectifier side because that is where the reverb/vibrato controls are, and this tube has something to do with them.

thanks,
-Tristan

7/18/2000 12:54 AM

Tristan,

I'm afraid you don't understand the classes of tube operation. In Class A plate current flows for the full 360 degree cycle. In Class B each tube of the push-pair conducts for 180 degrees of the cycle - with a sine wave each tube conducts "half" of the waveform. In Class AB each tube conducts less than 360 but greater than 180 degrees, there being some "overlap." In AB1 no grid current flows in the output tubes and in AB2 current flows. Class B is almost never used except in cathode driven Music Mans and tube PA that worked of a battery supply and really had to conserve current (and probably a few funky ones here and there). With Class B there it's hard to eliminate the crossover notch and associated distortion.

DeOxit D5 works well for tube sockets but I generally flush it off for higher voltage applications - alcohol won't dissolve corrosion, just gum and grease.

GW

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