Soundtronics 70's 4 x 8417

[BruceJ]

Acquired this amp sometime ago. Unfortunately there are no schematics to be had.  As of right now it has very (very) low distorted volume. Have to turn it up to about 10 to hear it. The PI voltages seem a bit off. This also seems to be the only spot someone has done any work at. Everything else appears to be Original.  I did pull all the Output tube and tried them in the amp individually as it can be done in this amp. Any thoughts would be appreciated. I am also aware of the 8417's availability. I would like to get this amp up and running before converting to 6550/KT88's.

Bruce

TUBE     1   2   3   4   5   6   7   8   9
8417    -13   H   600   300   -13   NC   H   0   
8417    NC   H   600   300   -13   NC   H   0   
8417   -13   H   600   300   -13   NC   H   0   
8417    NC   H   600   300   -13   NC   H   0   
12AX7 296 0   3.12   H   H   0   0   0   0
12AX7 196    0   1.4   H   H   189   0   1.4   H
12AX7 NC   NC   NC   H   H   304   0   2.6   H
12AX7 148    0   1   H   H   187   0   1.5   H

[eleventeen]

One of these?

http://z4.invisionfree.com/Marshall_Law/ar/t1924.htm

Looks like a quality build from the gut shots.

The first 12AX7 volts I note you have "0" listed (meaning there are wires going to those pins but zero volts) versus the second 12AX7 where you have "NC" listed meaning, no wires go to those pins. Is there a plate resistor missing from the "0 volt" tube section? My thinking is, if there are wires going to half a 12AX7, we should see B+ on pins 1 or 6. You seem to understand that.


Otherwise your 12AX7 voltages don't seem astronomically off. There are several different types of PI. Some (most Fenders) have big volts on the cathode(s), about 100 volts. Other do not, they have typical 1-2-3 volts on the cathodes.

I am not that familiar with 8417's, but -13 bias volts is kind of low relative to what a 6550 would want to see.  But, if that was really low for 8417's those tubes would be redplating with gusto. *If* you want to change to 6550's, you'll need to calculate and then change the cathode resistors under the tubes; or, go to "fixed bias". (which is a confusing term, typically "fixed" bias means "variable" bias, of the type found in most Fender amps....) To do so, you'll most likely need to build a simple bias rectifier. Easy enough, but it would use enough parts so that you'd want to add another terminal strip.


Your third pix...judging by the type of resistors shown, I assume this is the place where you are saying "work was done"....as these are "recent" resistors obviously different from the others in the amp. I take it you suspect that a repair was attempted there but did not succeed?

[BruceJ]

You, one of those you pictured.

First 12AX7 is Input tube. Second 12AX7 is 1/2 Reverb Tube. The Fourth 12AX7 is the PI I suspect.
Yes that is what is pictured and those components are "modern". This is where I suspect the issue lies. Since the 8417 hasn't been used very much in Guitar amp I thought some of the HI-FI guru's could shed some light. I know Bogen used these for a time. It's possible someone has seen this config at some point.

The -13 is correct bias for these tubes. I am familiar with how to rebias for 6550 Tubes when and if necessary.

I am also going to draw this circuit out. That may help both myself and anyone reading this post.

[PRR]

> 8417 hasn't been used very much in Guitar amp I thought some of the HI-FI guru's could shed

8417 is a high-gain cheap 6550.

Only difference is bias, drive, and IMHO a higher likelihood of bad power tubes.

If you can possibly set the bias down to -50V area, I'd put a fresh pair of guitar-grade 6L6 in it and do debugging that way. (Or EL34, but check the G3 jumper.)

Yeah, yeah, 600V is over the book-rating for 6L6. They will take it plenty long enuff to find all the other problems and save-up for 6550/KT88.

The four*8417, OT, and 70V tap strongly remind me of my eight*8417 2*150W Bogen PA/industrial amps. Enough to make me think they were buying iron from Bogen. The chassis and small-bottle lineup is very different.

[eleventeen]

"If you can possibly set the bias down to -50V area, I'd put a fresh pair of guitar-grade 6L6 in it and do debugging that way. (Or EL34, but check the G3 jumper.) "


Truly excellent idea. They'd surely have to be pretty robust 6L6's with 600 v on the plates though.

[BruceJ]

Thank PRR for the suggestions. Have to be gone this weekend but I will get back on it on Monday. I think I have some 6L6's laying around. I also have not checked locally to see if anyone may have a tube tester. Not likely but worth a shot.


Bruce

[Crazy Eric]

Any more traction on this issue?  A schematic maybe?