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]

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]

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]

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?

" 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]

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]

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.