Peavey Classic 50/50 - Eating Valves (very bright red!)

[NickH]

I presume 14volts approx is getting to all pin 2's ?

Getting -17vdc on all pin 2's

Pin 4 = 33.33vdc

Pin 5 = 33.33vdc

Pin 7 = 450vdc

Pin 9 = 448vdc

Cheers

N

[NickH]

That capacitor blocks the high voltage on the cathode of the 12AT7
but allows the ac signal to pass through to the grids.
One side of it should have several hundred volts on it (DC) and the other
may have the bias after passing through R234.
Well there should be 3 others to check against.

All four show 450vdc on one side & 0vdc on the other...

Cheers

N

[tubeswell]

All four show 450vdc on one side & 0vdc on the other...

Cheers

N

Well then those coupling caps seem okay.

[tubeswell]

Getting -17vdc on all pin 2's

Pin 4 = 33.33vdc

Pin 5 = 33.33vdc

Pin 7 = 450vdc

Pin 9 = 448vdc

Cheers

N

That pin 2 bias voltage is within the ballpark. Pins 4 and 5 are the heaters, Pin 7 is the plate, pin 9 is the screen - I'm assuming 450V is the B+ voltage without the tubes in? (It should be lower than that for EL84s)

It could just be a bad batch of tubes. I have had trouble with recent batches of new Sovtek EL84s redplating within a minute on putting them in (the ones in the red boxes). Maybe try some older EL84s that you know to be working - maybe try swapping the ones in the other quad that are working (though no guarantees that you won't blow them up as well)

[NickH]

I'm assuming 450V is the B+ voltage without the tubes in? (It should be lower than that for EL84s)

Yep, sorry should have said, that's with the tubes out...

Thanks

Nick

[tubeswell]

Well on the face of that, there doesn't appear to be anything lacking in the power supply department. So how new were those tubes that went west?

[NickH]

Well on the face of that, there doesn't appear to be anything lacking in the power supply department. So how new were those tubes that went west?

1983 NOS Apparently... (Did I do wrong? )

Here are the details:-

EL84 MATCHED QUAD, FINEST QUALITY NOS 1983, SUPER TUBES on eBay (end time 20-Sep-09 17:46:28 BST)

Cheers

N

[Wakculloch]

450 on one side and 0v on the other. Where is the bias voltage?

[stokes]

Did you check the bias when you installed these tubes?It is possible that they just needed to be set properly,like they didnt match the old tubes and were running too hot.That could explain the glowing issue,but doesnt address those resistors burning.Perhaps they were running hot for so long the grid shorted to the plate and burnt the resistor.Your voltages all seem to be okay with no tubes installed,so you may have been lucky.I would advise getting some different tubes,set the negative bias voltage to about -25 to -30 which should set your bias very cold and see what you got.

[NickH]

Did you check the bias when you installed these tubes?It is possible that they just needed to be set properly,like they didnt match the old tubes.That could simply be the glowing issue,but doesnt address those resistors burning.

It's a fixed bias amp...

Cheers

N

[stokes]

It's a fixed bias amp...

Cheers

N

You see,Nick,when you say things like this is when I get concerned.A lot of people make the misconception that "fixed bias" means non-adjustable."Fixed" refers to a fixed amount of negative voltage applied to the control grids to set the bias point.It is absolutely adjustable and should be checked and adjusted if need be every time you change power tubes.R47 is the resistor to adjust.I would replace it with a 100k cermet pot to make adjustments easier.

[NickH]

You see,Nick,when you say things like this is when I get concerned.A lot of people make the misconception that "fixed bias" means non-adjustable."Fixed" refers to a fixed amount of negative voltage applied to the control grids to set the bias point.It is absolutely adjustable and should be checked and adjusted if need be every time you change power tubes.

No... I said it because it had no bias trim pot...

I simply meant it was a non adjustable fixed bias model...i.e set from the factory to run fairly cold with a wide ish rage of tubes.. are you suggesting Peavey expect end user to remove a PCB & solder a different value resister in place every time they replace the tubes?

N

[oc disorder]

Well it seems nothing is wrong...???
Perhaps they were just weak tubes something flashed over in the two valves
and burnt the grid resistors.
I recall when a local store was having problems with warranty regarding the output tubes in a classic 30 (red plating- microphonics etc) the distributor
recommended to them they replace with Electro Harmonix EL84's.
I don't know if that was off the cuff or official but it solved the problem.
I've heard JJ's are also robust.

Nick could you do two more tests ?
1. Tell us what the heater/filament voltages are (amp on standby).
2.See if the two tubes that glowed initially are the ones that don't have the 100ohm screen grid connecting them.
From schematic V209,V211 and V203,V205.
"I noticed that two of the valves on the second channel were glowing very bright red in an alarming manner when I pushed it "


For anyone following this thats query 1.
Whether it would be advisable to fit 4 more 100 ohm r's.
And as Wakculloch said why isn't the bias present on the other side of the coupling caps ? Maybe the meter couldnt respond quickly enough for the neg voltage?
Thats query 2.
Also throwing in another thought - quoting Chuck H from a previous post
"If you choose to rewire for el84's just don't use any of the "unused" tube socket pins for a mounting post. Some older el84/7189 tubes used the typically unused pins for different purposes. "

Could the Russian 6П14П NOS tubes which Nick bought fall into the above category ? The pinouts are the same on paper see attached.

Also I hear the 6П14П are slightly more rugged.14watts as oposed to 12watts.
Some quotes
"This 6P14P-EV / 6n14n-EB (Cyrillic) long life tube is good for at least >5000 hours. Produced at Reflector plant with Military specification. Good replacement for EL84 based guitar and HiFi amplifiers."
and from Wade's Audio And Tube Page ...
"I have also found that 6n14n and 6n14n-eb valves are very durable and sound at least as good as the other 6BQ5 types I've compared them to. The "EB" type has a plate design like an Amperex but is even larger and heavier. The plain 6n14n has a curved plate structure that looks like a small RCA 6L6GB. The sound of both of these was a big surprise to me. Most reviews of the sound of different tubes are concerned with guitar applications and the sound of deliberate distortion at overload with little or no feedback. This does not apply to high fidelity use so don't believe all you read about current production tubes. Current producers of tubes are doing a great service for those of us who love valves. Please do all you can to support and encourage continued production and developement."

Attached is also a PDF on the Russian valve specs and also a reduced jpg of the output section of the 50/50.

better have some breakfast !

[NickH]

R47 is the resistor to adjust.I would replace it with a 100k cermet pot to make adjustments easier.

I cant see R47 anywhere on the schem...

My guess would have been either R108 or R107... But as you seem to enjoy pointing out, tube amps are not my area of electronics expertise



N

[NickH]

Well it seems nothing is wrong...???
Perhaps they were just weak tubes something flashed over in the two valves
and burnt the grid resistors.
I recall when a local store was having problems with warranty regarding the output tubes in a classic 30 (red plating- microphonics etc) the distributor
recommended to them they replace with Electro Harmonix EL84's.
I don't know if that was off the cuff or official but it solved the problem.
I've heard JJ's are also robust.

Nick could you do two more tests ?
1. Tell us what the heater/filament voltages are (amp on standby).
2.See if the two tubes that glowed initially are the ones that don't have the 100ohm screen grid connecting them.
From schematic V209,V211 and V203,V205.
"I noticed that two of the valves on the second channel were glowing very bright red in an alarming manner when I pushed it "


For anyone following this thats query 1.
Whether it would be advisable to fit 4 more 100 ohm r's.
And as Wakculloch said why isn't the bias present on the other side of the coupling caps ? Maybe the meter couldnt respond quickly enough for the neg voltage?
Thats query 2.
Also throwing in another thought - quoting Chuck H from a previous post
"If you choose to rewire for el84's just don't use any of the "unused" tube socket pins for a mounting post. Some older el84/7189 tubes used the typically unused pins for different purposes. "

Could the Russian 6П14П NOS tubes which Nick bought fall into the above category ? The pinouts are the same on paper see attached.

Also I hear the 6П14П are slightly more rugged.14watts as oposed to 12watts.
Some quotes
"This 6P14P-EV / 6n14n-EB (Cyrillic) long life tube is good for at least >5000 hours. Produced at Reflector plant with Military specification. Good replacement for EL84 based guitar and HiFi amplifiers."
and from Wade's Audio And Tube Page ...
"I have also found that 6n14n and 6n14n-eb valves are very durable and sound at least as good as the other 6BQ5 types I've compared them to. The "EB" type has a plate design like an Amperex but is even larger and heavier. The plain 6n14n has a curved plate structure that looks like a small RCA 6L6GB. The sound of both of these was a big surprise to me. Most reviews of the sound of different tubes are concerned with guitar applications and the sound of deliberate distortion at overload with little or no feedback. This does not apply to high fidelity use so don't believe all you read about current production tubes. Current producers of tubes are doing a great service for those of us who love valves. Please do all you can to support and encourage continued production and developement."

Attached is also a PDF on the Russian valve specs and also a reduced jpg of the output section of the 50/50.

better have some breakfast !

Wow! Thanks for the very detailed post!

I will work through this tomorrow (2am again here lol) & come back with some answers...



N