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

[NickH]

Hi Folks,

I have a problem with my Peavey Classic 50/50 amp...

It kept cutting out under very heavy load (big loud notes would fold up into silence)

I noticed that two of the valves on the second channel were glowing very bright red in an alarming manner when I pushed it

I thought it might be the valves, so I put in 8 fresh EL84's... Valves are now even brighter & now three are not happy...

It had also burned out the 10K resistor next to the bottom right (from front of amp) valve on channel number two... when I replaced it & brought it back up on the variac with a test 1k sine wave it promptly burned out the10k on the bottom left...

Normally I would blunder about testing every component until I find the dud, replace & repeat until it was fixed, but I am in kind of a hurry for this one to be working as I could do with using it on a session I am in at the moment...

Any Ideas?

Many Thanks

Nick

[oc disorder]

Ouch ..classic case lack of bias or worse shorted turns in o/p transformer.
Check pin 2 for approx -14 v DC.
Not sure which 10k you refer to but check the resistance of the primary of the output transformer.
According to the diagram I have Brown (J17) Red (J67) and Black (J45).
Measure Red to Brown and Red to Black.They should be approx the same
give or take a few turns.
Also check the diodes CR5 and CR6 (SR2873).
They go from the anodes (and brown & black o/p trans) to ground.
This is a classic redplate !
Let's know how you go...

[stokes]

First thing to do is to identify and then find out what is causing the 10k resistor to burn.You say you changed it,but as you see,just changing a part,without knowing what and why it burned,is pointless.Resistors dont burn form getting old or wearing out.Something makes them burn,specifically too much current.Find that problem first.I wouldnt be concerned with the transformer or diodes before tackling the burnt part.

[NickH]

Originally Posted by oc disorder

check the resistance of the primary of the output transformer.
According to the diagram I have Brown (J17) Red (J67) and Black (J45).
Measure Red to Brown and Red to Black.They should be approx the same
give or take a few turns.
Also check the diodes CR5 and CR6 (SR2873).
They go from the anodes (and brown & black o/p trans) to ground.
This is a classic redplate !
Let's know how you go...

Hi,

Thanks for the reply!

On this unit (and on the schematic I have here) the wires are Red (J209 & J214) , Brown (J213 & 216) & Blue (J210 & J215) to the O/P transformers (one for each channel)

I am getting:-
Red - Brown = 57 Ohms & 56.7 Ohms
Red - Blue = 71.1 Ohms & 70.2 Ohms

All the diodes on the breakaway board are testing ok with my DMM.

Cheers

Nick

[NickH]

Originally Posted by stokes

First thing to do is to identify and then find out what is causing the 10k resistor to burn.You say you changed it,but as you see,just changing a part,without knowing what and why it burned,is pointless.Resistors dont burn form getting old or wearing out.Something makes them burn,specifically too much current.Find that problem first.I wouldnt be concerned with the transformer or diodes before tackling the burnt part.

Thanks for the reply, which components would you recommend I check first? Where do I measure from to check the voltage to the valves & what should the readings be?

Cheers

Nick

[oc disorder]

I presume this was a typo ?..."8 fresh EL84's"
'cause 84's in an ax7 position......!
" I am getting:-
Red - Brown = 57 Ohms & 56.7 Ohms
Red - Blue = 71.1 Ohms & 70.2 Ohms "
Is the side that was glowing red connected to the brown tapping on the transformer ?
57ohms does seem a bit low

[NickH]

Originally Posted by oc disorder

I presume this was a typo ?..."8 fresh EL84's"
'cause 84's in an ax7 position......!

Erm no... A classic 50/50 has 8 x EL84 & 4 x 12ax7's...

Are you sure you are not thinking of a Classic 50?

Originally Posted by oc disorder

" I am getting:-
Red - Brown = 57 Ohms & 56.7 Ohms
Red - Blue = 71.1 Ohms & 70.2 Ohms "
Is the side that was glowing red connected to the brown tapping on the transformer ?
57ohms does seem a bit low

Nope each side has it's own O/T & both the working & none working sides have identical values...

Cheers

N

[stokes]

Originally Posted by NickH

Thanks for the reply, which components would you recommend I check first? Where do I measure from to check the voltage to the valves & what should the readings be?

Cheers

Nick

Like I already said,the 10k resistor that burnt is your main concern.I am going to say something here now that may seem offensive,but I dont mean to be offensive,more concerned for your safety.You have something drawing major current thru that amp,the glowing tubes and burning resistor are a sure indication of that.If you dont know what that resistor is,and dont know how and where to check voltages,you are likely in over your head.Tube amps have lethal voltages present,even when unplugged and turned off.In this case you're troubleshooting with a "live" amp,in other words,with the amp turned on.Its no place to "be in a hurry".You do sound like you know some about amps,but a few things you say have me concerned.If you have a schem,try scanning and posting it here and I can try to figure out which resistor is burning and we can take it from there.Forget the OT for now,and concentrate on that 10k.

[NickH]

You are quite right to be concerned, tube amps are lethal... And I am far from an expert on repairing tube amp's... (hence why I am posting) I usually get there in the end...& enjoy trying to figure it out on my own from the schematic, a bit of trial & error & a lot of coffee... But like I say, I could do with this one one the current session & a few pointers would be much appreciated...

Attached is the schematic...

Many Thanks

Nick

[oc disorder]

"Are you sure you are not thinking of a Classic 50?"
Whoops yes I was.
In that case I now understand the dual readings
....................T1............T2
Red - Brown = 57 Ohms & 56.7 Ohms
Red - Blue = 71.1 Ohms & 70.2 Ohms

Ok so its unlikely both are faulty this is the way they are wound.

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

I don't have the schematic for the 50/50 only the classic 50
which is probably very similar to the 50/50 only 2 x the output section.

In regard to the burnt 10k's you should try to identify where its position is in the schematic.
The only one visible to me on the C50 is a dropping resistor in the HT feeding
2 halves of a 12AX7 V1a & V2a.
This has to pass through a connector in the C50 if its a similar setup in the 50/50 maybe checking the connectors closely may be a good start.
Something funny going on in the power supply ...!
Edit : Just found a schematic it appears the 50/50 uses 10k grid resistors
so if it is these burning out my only suggestion is to check the 2 coupling caps
C202 and C207 .33uF 400v from the AT7.

[NickH]

Hi,

The two 10k's were feeding pin 2 on the bottom two EL84's on Channel 2...

Sadly the board is not printed with component numbers, but I think they are R238 & R239.

I am now quite suspicious of C216... If that failed, could that cause my overvoltage problem?

(I will check pin 2 for the 14v, but it's 2am here in the UK & I am dog tired, going in there now with probes would probably result in me being laminated to the wall... That stuff I prefer to do while very awake & with one hand in my pocket )

[tubeswell]

Originally Posted by NickH

Hi,

The two 10k's were feeding pin 2 on the bottom two EL84's on Channel 2...

Pin 2 on an EL84 is the control grid pin, and having looked at the scheme those 10k resistors are just grid stoppers. But if they are blown - there won't be any bias on those tubes - You could put new ones in, but you might have the same problem. Something must've been hell of a powerful to melt those resistors (and I'm guessing thermal runaway on the grids? - Long shot), which is possibly attributable to loss of bias (as one of the other posters has already mentioned).

You need to start isolating the problem, so maybe pull all the tubes and, see if you get a B+ on Pins 7 and 9 (it will be higher with the tubes pulled). Check your PT secondary AC voltages with the rectifier disconnected (but be careful), then check the B+ voltage at the first filter cap. Then replace those grid stoppers and check that you have a -ve bias supply voltage (before you put the tubes back in).

[oc disorder]

well its 11 am here and I agree you should tackle it fresh in the morning.
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.
If it is leaking or not blocking the DC the positive voltage would pass through to the grids and you can wave goodby to the 2 EL84's.
Notice at that point there is also a relay for parallel..I presume that switches
the two o/p sections together havn't sussed that out yet but worth bearing in mind.

[stokes]

Very unusual for those grid resistors to burn.Those Peavey's are a pain in the ass to work with.Like oc said check that c216 for dc leaking.Is it possible you connected that ribbon cable connector wrong,or something shorted in that connector? Saw a similar problem when someone connected one of those ribbon cable plugs wrong and put high volts on his heaters in a Mesa amp.

[Steve Conner]

That's a good point, if those coupling caps had gone leaky, it might explain the burnt grid stopper resistors. The tube itself is incapable of burning out that resistor, unless something really wild happened like the screen shorting to the control grid.

And the burnt grid stoppers would explain the glowing tubes: loss of bias due to the resistors burning open.

[NickH]

Originally Posted by oc disorder

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]

Originally Posted by oc disorder

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]

Originally Posted by NickH

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

Cheers

N

Well then those coupling caps seem okay.

[tubeswell]

Originally Posted by NickH

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]

Originally Posted by tubeswell

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]

Originally Posted by 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?

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]

Originally Posted by 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.That could simply be the glowing issue,but doesnt address those resistors burning.

It's a fixed bias amp...

Cheers

N

[stokes]

Originally Posted by NickH

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]

Originally Posted by stokes

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]

Originally Posted by stokes

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]

Originally Posted by 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 !

Wow! Thanks for the very detailed post!

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



N

[melvin]

Originally Posted by NickH

.. 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

.. yes, as necessary. even if the tubes have Peavey written on the side. And Peavey's factory may be a bit more sophisticated than end user thinks. Tubes draw different amounts of current with given bias voltage and besides, resistors drift. learn how to check the bias (current). it's probably the first thing a tech learns...with your amp you'll get twice as much practice...

Of course, I don't think Peavey really meant for end user to open the amp at all. I think end user should find a good tech in his/her area. It might even save you money (not to mention time), considering you have 8 el84's to fry. Also, learning how to repair overdesigned tube amps during a recording session with the amp in question doesn't really sound like fun. The best players have really good techs for a reason.

Furthermore, "blundering about testing every component" probably wouldn't work in this situation anyway. even if you had all the time in the world...

[NickH]

Originally Posted by melvin

.. yes, as necessary. even if the tubes have Peavey written on the side. And Peavey's factory may be a bit more sophisticated than end user thinks. Tubes draw different amounts of current with given bias voltage and besides, resistors drift. learn how to check the bias (current). it's probably the first thing a tech learns...with your amp you'll get twice as much practice...

Of course, I don't think Peavey really meant for end user to open the amp at all. I think end user should find a good tech in his/her area. It might even save you money (not to mention time), considering you have 8 el84's to fry. Also, learning how to repair overdesigned tube amps during a recording session with the amp in question doesn't really sound like fun. The best players have really good techs for a reason.

Furthermore, "blundering about testing every component" probably wouldn't work in this situation anyway. even if you had all the time in the world...

Hi,

Thanks for the reply & your concern...

Not sure what makes you think that I don’t know how to check the bias... Admittedly I have to do this amp on the oscilloscope because my bias tool is for 6L6’s etc.. And I am not that great at judging the notch every time....

Don’t worry I have missed the deadline for this session, not a major deal.. I have plenty of other amps.. .

I can’t help thinking that I have given a few people the wrong impression here.. My first posts were hurried & short on details because I was really pushed for time (and I have had about 4 hours sleep per night this week)... In the past Enso has got me out of the crap when I have been in a hurry, with sometimes the vaguest of details supplied to him, I posted in the off chance it was something very common & he could give me one of his “check pin x for 450v & if its lower change C42” replies, which would have been just the ticket for that situation...

I am not a guitar player (I am a producer) & the cost isn’t really relevant in this situation... So please don’t worry on my behalf... I am lucky enough to endorse Peavey & they would happily take care of it for me if I so wished, I just enjoy doing it myself & the learning experience... Call it a hobby if you will...

I am also quite self effacing, which doesn’t really help me in a forum situation, I will try not to be when posting here in future... Seems it’s too easy to misinterpret..

Now we have all that out of the way, can we move on?

Cheers

N

[NickH]

Originally Posted by oc disorder

Nick could you do two more tests ?
1. Tell us what the heater/filament voltages are (amp on standby).

21.79VDC rising to 34.35VDC over about 5 mins..

Originally Posted by oc disorder

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.

It was V209 & V208.

Many Thanks!

N

[stokes]

Originally Posted by NickH

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

No,Nick,I dont "enjoy pointing out" anything.I just dont want to see anyone get hurt following tech advice on a forum that they may not fully understand.I meant the 47k resistor marked R108.Sorry for the typo,but as you can see it was after midnight,I have a day job as well as my side line of amp repair/building I spend about 6 hours a night on,then I come here to try and help where I can.Peavey or any amp manufacturer doesnt expect any "end user" to alter anything in their amps,but tubes dont always draw the same current,even from the same production run and amps need to be checked and adjusted each time you change power tubes unless you buy tubes that are "pre-sorted" to match a certain current draw.
When oc asked the heater/filament voltage he was refering to the ac voltage used to operate the heaters,not the dc voltage that the heaters are "floated" on.It should be 6.3 vac or there about.

[NickH]

Ahh, fair enough...

Originally Posted by stokes

When oc asked the heater/filament voltage he was refering to the ac voltage used to operate the heaters,not the dc voltage that the heaters are "floated" on.It should be 6.3 vac or there about.

Ahh.. Gotcha! will re test



N