Silkyn Super 50 blowing power tubes V7

[Amp Mad Scientist]

Power up, standby off
No power tubes, voltage to ground for pins 3 and 4, both sockets: 516v.

Power off (caps discharged)
Resistance across each side of OT (Pin 3 to CT / B+):
V7 - 19.1 Ω, V8 19.2Ω (mostly this has been 19.3 - 19.4Ω the many times I have checked it while biasing).

Pin 3 to Pin 3: 37.7Ω. (I would expect ~38.3Ω , dunno)

View attachment 87572




I replaced the coupling caps from the PI to the power tubes, after mapping the components and marking up the schematic.
The one to V8 - the yellow (C25), is different from the one to V7 (C24). Both are J223 400v.

Replaced with Mallory 0.022µf 630v. I polarity tested them at the input of another amp, but could not hear a difference.
Dont know why the pic is rotated.
With tubes back in, the voltage across them is @383v at idle (as I recall, didn't write it down) - pretty close the 400v rating, as was mentioned.

View attachment 87573

After letting it warm and monitoring @30min:
At a bias voltage of -52.8v at pin 4 both V7 and V8, the voltage drop across the OT halves is: V8 .695v, V7 .715v.
.715v/19.1Ω = 37.4ma x (B+ 494v) = 18.5w/30w = 61.6%.

I have not played it yet, do things appear to be in order?

When I eventually do, I will with the chassis out of the cab and watch for red-plating.

Everything seems OK,,,
Hopefully the problem was caused by a leaky coupling cap...and is now corrected.
Just keep an eye on DC pin 5 voltage...
and see if it starts drifting.

If the tube red plates, we would expect to see the pin 5 voltage drifting /not steady.

Hopefully it is steady.

A bad spot in the circuit board could cause bias to drift but hopefully the board is not damaged.

 

[jtcnj]

Thanks for all the help; this one was not straightforward.
Hopefully, it is resolved.

 

[syscokid]

 

[Amp Mad Scientist]

Thanks for all the help; this one was not straightforward.
Hopefully, it is resolved.

Sometimes the board is bunk...
look at DSL2000 and several Mesa amps, have issues with bad insulation on the board all the time.

So you have to watch and see, but at least you did test and check as much as possible...

Can anybody think of anything we missed?

 

[Amp Mad Scientist]

Trash scum garbage to some thesis things are great to others it's a way of life.

View attachment 87544View attachment 87545

Oh yes...
Boogie loves to lay resistors on top of circuit tracks.
And for decades, these resistors have been arching and burning...
Mesa has known about this problem for decades.....and has never corrected it.
You KNOW how many of these amps has blown up (?) because of the insulation failure(?)

Why do you think that I have said: "the design is laughable?"

I mean, there comes a point and time...

 

[jtcnj]

Played this about 2 hours both Sat. and Sun., pretty low volume -though this is when the failures occurred.
Watching for red-plating and frequently checking the voltage across the OT halves and the bias voltage.
The bias drifts a little from -52.8v to -53.0v, and the OT voltages vary about 15mv.
So far, so good.

 

[Amp Mad Scientist]

Played this about 2 hours both Sat. and Sun., pretty low volume -though this is when the failures occurred.
Watching for red-plating and frequently checking the voltage across the OT halves and the bias voltage.
The bias drifts a little from -52.8v to -53.0v, and the OT voltages vary about 15mv.
So far, so good.
View attachment 87678

That's not the kind of drift to worry about.
Might have been PI coupling caps...those little buggers can sneak up and grab ya nasty little varments they is.

2. Another thing that can do it is a high frequency oscillation...peaks the tube out but you can't hear it, beyond the range of human ears.
Uncommon but still possible.
all that you hear is increased hum but you still get some sound. The hum increases because the power supply is maxing out.

 

[jtcnj]

I'm hoping the 630v PI coupling caps solved it.
The one for V7 was different than the one for V8, though both were 400v caps.
Thanks again for all the help!

 

[syscokid]

I'm hoping the 630v PI coupling caps solved it.
The one for V7 was different than the one for V8, though both were 400v caps.
Thanks again for all the help!

Do you have some kind of dummy load or attenuator where you can input a signal and crank up the amp for a few hours?

 

[jtcnj]

Not really; I have a robrobbinette DIY resistor based box, but it's still plenty loud.

I'm going to leave it out of the cab as pictured and play it daily the next week or so at least.

 

[Amp Mad Scientist]

I'm hoping the 630v PI coupling caps solved it.
The one for V7 was different than the one for V8, though both were 400v caps.
Thanks again for all the help!

somebody already changed one...and it didn't hold.
383 VDC across the PI cap, but then (plus) the audio voltage AC goes through the cap too.
Althogether more than the 400V cap was made for.

 

[jtcnj]

So, I have put several, maybe 30-ish hours on this since amp the repairs, and at least some of it at high volume.
I am tickled to report it is running great!

I have an issue sometimes where the reverb is still engaged when turned off at the footswitch, but it is not eating power tubes.

Thanks again for all the help, and especially @Amp Mad Scientist for nailing the issue.