Vibroverb gremlins

[mdoolin]

Thanks Chuck, I'll try a 12AY7. I do like these particular 12AT7's, Phillips JAN NOS. By comparison, the EH 12AT7 did indeed sound blah, so I put Phillips in all my amps' PI's.

[mdoolin]

Agreed. I've only replaced the electrolytics, all the blues are still present.

[Justin Thomas]

If you like the lower gain of the 12at7 you might also like an 12ay7. To my ears, in a typical type circuit for a 12ax7 triode the 12ay7 sounds really solid and tube-y.

I'll second this. Probably to do with the fact that the Y was designed to work optimally in the same circuit as an X. The T needs different circuit values to work at its best. The Y also had a certain nice top end that other 12A_7 tubes lacked.

Justin

[Steve A.]

Thanks Chuck, I'll try a 12AY7. I do like these particular 12AT7's, Phillips JAN NOS. By comparison, the EH 12AT7 did indeed sound blah, so I put Phillips in all my amps' PI's.

Because of the way it needs to be biased a 12AT7 isn't usually*** a good drop-in replacement for a guitar amp preamp tube - as mentioned earlier 5751, 12AY7's or any of the 12A_7 family (besides 12AT7) would be good candidates. Here's a chart showing the approximate gain of the 12A_7 family members:

MU FOR 12A_7 TUBES

12AX7. . . . 100
5751. . . . . . 70
12AT7 . . . . . 60
12AZ7 . . . . . 60
12AY7/6072 . 44
12AV7/5965 . 37
12AU7/6189. .20

Steve A.

*** The Picovalve was designed by Andy Marshall of THD but JCA apparently wanted to add a Bright switch to the original design and in doing so eliminated the grid resistor of one of the preamp tube stages which in my opinion gave the amp some ugly overdrive and distortion artifacts which would not clean up when you reduced amp gain or guitar volume. While my fix was to add back the missing grid load resistor one of the amp mod kit guys on eBay found that replacing that tube with a 12AT7 also cured the problem.

Here's a link to the original thread here at MEF on the Picovalve which BTW is a great platform for mods and builds with its heavy duty iron... if you can find one for under $200 (the latest one on eBay was $299 plus s/h - I still get emails when they are listed there.)


http://music-electronics-forum.com/t26180/

[Steve A.]

Agreed. I've only replaced the electrolytics, all the blues are still present.

Back in the mid-90's Dan Torres (whose chart on Mu/Gain for the 12A_7 family I just posted) used to have a bin full of blue molded Mallory coupling cap pull-outs on his sales counter, $2 or $3 apiece (for comparison his new Orange Drops were under $1.50 back then.) Some people might question the ethics of selling parts that previous customers had paid him to "upgrade" but what the heck they were damned good capacitors!

Checking for DC voltage on the output of poly coupling caps can be a good way of testing for leakage.

Steve A.

[mozz]

MU FOR 12A_7 TUBES

12AX7. . . . 100
5751. . . . . . 70
12AT7 . . . . . 60
12AZ7 . . . . . 60
12AY7/6072 . 44
12AV7/5965 . 37
12AU7/6189. .20

This is often posted but...............aren't the typical resistor values used in a Fender V1, 1.5k. 22uf, 100k, does that not equal a gain of 51?

[tboy]

I'm going to chalk it up to a bad tube.

Have you tried putting the original tube back in to see if the problem reappears?

[Steve A.]

MU FOR 12A_7 TUBES

12AX7. . . . 100
5751. . . . . . 70
12AT7 . . . . . 60
12AZ7 . . . . . 60
12AY7/6072 . 44
12AV7/5965 . 37
12AU7/6189. .20

This is often posted but...............aren't the typical resistor values used in a Fender V1, 1.5k. 22uf, 100k, does that not equal a gain of 51?

Dan's chart of MU values are for the tube itself not for circuit it was used in.

Steve A.

EDIT: As far as I know I was the first person to copy Dan's chart and post it on the internet, on AMPAGE, of course. I was always looking for content to copy and paste (or methodically type out) to post on my website, The Blue Guitar, which at that time was on GeoCities, a real blast from the past...

[Chuck H]

Have you tried putting the original tube back in to see if the problem reappears?

Ha! I can't believe that consideration got past me.?. I've always been a big proponent of cleaning/re tensioning tube sockets and removing oxide from tube pins at the first sign of pops and noise in old amps. Good catch

A brass wire toothbrush is a good tool for buffing some oxide off of tube pins. If cleaning the original tubes pins and reinstalling it doesn't bring back the problem then it could have been bad pin contact the whole time and inserting the latest tube just scrubbed off enough crud to make contact. If so it might be a good idea to clean and re tension all the sockets.

[mdoolin]

Sure enough, I put it back in, and the problem reappears.

[Familyortiz]

Is the noise affected when you tap the amp or chassis in any way? I'm just looking at mechanical possibilities like something loose or cracked.

[mdoolin]

No, and it doesn't even happen when you tap the bad tube directly. But that particular tube does it consistently. It's as though it has a bad internal ground, maybe a loose weld. Regardless, it's just a tube, and replacing it fixes the problem.