Silvertone 1391 w/no power

[BigE]

I've got a Silvertone 1391 that just has no power and sounds really distorted.   I've basically replaced everything but about 3 resistors because it was so roached on the inside.  However it still has no power and really distorted.

To top it off there isn't really any schematics for the thing.  The closest you can get is the attached 1431 which is very close, but not exact.  It does however have the same tube configuration.  I've tried  a different 6V6 but not the other tubes because i don't have any replacements to try.  It's almost has to be a tube since virtually everything except the pots and a couple resistors have been swapped, rite?

Here's my voltages:
6AU6:
2 = 0v
5 = 1.2v
6 = 25v
7 = 0v

6V6
5 = 0v
3 = 275v
4 = 240v
8 = 12v

Thanks in advance.

[jjasilli]

Use flow chart or checklist.  E.g.: http://geofex.com/  -Tube Amp Debug Page  > Order of Suspicion ; then:  Low power or volume, or volume drops off

[sluckey]

6AU6 plate voltage is extremely low.

[eleventeen]

Are you sure the plate resistor for the 6AU6 is supposed to be 330K? That seems awfully high.

[mresistor]

Have you tried swapping tubes? maybe pull them and see if PS nodes check out. Eleven - Maybe RP is that high because it's a grounded cathode config.
Got any pics of the innards?

[jjasilli]

Everybody's advice is good. But you need the right test equipment (simple tube tester would at least indicate internal shorts, etc.) + spare tubes to diagnose amp issues.  Lacking that, probably the best thing to do is pull the tubes & measure all those voltages again.  With no load you should read higher voltages than specified on the schematic.  If yes, then a tube is bad.  Likely the 6AU6 is bad, because the power tube voltages look reasonable.


OTOH, if voltages remain unreasonably low with the tubes pulled, then there is likely a fault in the B+ supply.  Such fault may be a high voltage short to the chassis, in which case the chassis is LIVE and dangerous.

[mresistor]

I found a thread about this amp with pics  http://www.thegearpage.net/board/showthread.php?t=344845

and they say the 1331 is very similar to this amp. 

[mresistor]

BTW Doug - it would be nice to replace the blurry Silvertone 1331 image in the schematics library with a copy of this one.

[PRR]

> Are you sure the plate resistor for the 6AU6 is supposed to be 330K? That seems awfully high.

It has to be high. You can barely get enough gain with one pentode into a power tube. If we need 8V rms at the 6V6 grid, and pentode gave gain of 100, this is 80mV input sensitivity. We want closer to 20mV. So the impedances are raised until stray capacitance sucks-off the treble.

Read the Resistance-Coupled Amplifier Charts (500KB PDF). 6AU6 is Chart 2. It shows plate-loads up to 1Meg. We are really limited by 6V6 G1 resistor must be under 500K. Using 180V with 0.47Meg and 0.47Meg, we can get gain about 190. Note that restricted to 0.5Meg on 6V65 G1, gain falls a trace from Rp=0.22Meg to Rp=0.47Meg. Actual results will differ because Silvertone used grid-leak bias (which may be why they used a different Rg2), and our voltage is not on the table (and in this case it matters somewhat). 330Meg is certainly in-the-zone.

> 6AU6 plate voltage is extremely low.

1.2V is certainly wrong.

Pull the 6AU6 and see that plate voltage shoots up near B+. If not, replace that 330K or inspect solder joints. Then replace 6AU6-- it is still cheap.

PLEASE put a proper modern 3-pin plug on that thing!!

[mresistor]

Also remove the death cap on the fuse holder (if it's there) when replacing the power cord with 3 wires.

[BigE]

Sure enough.  Remove the tube and voltage shot up to 180v.  Tube back in... 2v.  Another tube is on order.

Already removed the death cap when I 3 pronged it.

I'll post results when the new tube gets here for posterity sake.

Thanks guys!

[sluckey]

Sure enough.  Remove the tube and voltage shot up to 180v.  Tube back in... 2v.  Another tube is on order.

Check the value of that 330K plate resistor.

[jjasilli]

Good point!