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

[melvin]

.. 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]

.. 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]

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..

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]

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...

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

[NickH]

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.

There is definitely something significant about the new tubes...

I dug out some old, but known good (when last used) PV EL84's I had lying about.

Tested each channel seperatly, brought them up slowly on the variac & they both tested fine under heavy load for 15 mins each.

Just for giggles, (someone's going to shout at me for this.. I can feel it..) I got one of the good NOS tubes and swapped it for the one in V205. When it brought it up on standby it was ok, but when I added a sine wave test tone & raised the volume slightly, the sine wave was horrendously square on the o-scope & the corresponding R216 grid resistor started to brown...



N

[NickH]

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.

6.93VAC

Cheers

N

[NickH]

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?

Sorry, forgot this one...

It's a Fluke 179.. Does that help in any way?

Cheers

N

[stokes]

You shouldnt "bring the voltage up" in an amp with the tubes installed or a signal going thru the amp.You have dc going thru the tube with little or no voltage/current in the heater,not good.
It certainly sounds like,as I said before in one of my responses,that the tubes were responsible for the resistor burning.As you can see the "known to be good" tubes didnt cause a problem.I would suggest getting some "real" EL84/6BQ5's and stay away from those Russian 6n14n "subs" or whatever they are.I would also change that resistor that started to brown.

[NickH]

You shouldnt "bring the voltage up" in an amp with the tubes installed or a signal going thru the amp.You have dc going thru the tube with little or no voltage/current in the heater,not good.
It certainly sounds like,as I said before in one of my responses,that the tubes were responsible for the resistor burning.As you can see the "known to be good" tubes didnt cause a problem.I would suggest getting some "real" EL84/6BQ5's and stay away from those Russian 6n14n "subs" or whatever they are.I would also change that resistor that started to brown.

Sorry, yes, bad punctuation let me down there, should have been much clearer..

I brought it up on standby to make sure nothing bad happened with the heaters, once it was at full 230v and it had warmed up for a couple of mins I switched on the signal generator & brought the volume up slowly over 30 seconds or so until it was running under heavy load. it was then ran for 15 min in that state...

Quite right, already swapped that resistor out...

And I couldn't agree more about the Russian tubes, I really screwed up there.. I thought I was buying straight EL84's I had no idea they were 6П14П's, and when they came through, I thought they were just a different name for a EL84 (a bit like the ec83/12ax7 thing) the sad part is, I only got them on a whim instead of my usual ones I get from PV, because I thought it would be interesting to see if they sounded different..

The guy didn't mention that they were 6П14П's in the auction text (although if you zoom in on the photograph you can see that they have 6П14П stamped on them...) Was he being misleading by selling them as EL84's or is this kind of thing pretty standard & it was something I should have known?

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

Cheers



N

[oc disorder]

"It's a Fluke 179.. Does that help in any way?"
No that should be fine ..another one of life's little mysteries !
I'm getting lost in the posts here !
"I got one of the good NOS tubes and swapped it for the one in V205"
YOU DID WHAT????!!! thats the shouting out of the way...
Obviously one or all of the following applies
1.You've been ripped off
2.As Stokes was saying these valves need different biasing to the modern
ones Peavey set it up for.
The resistor stokes was refering to is on the PSU schematic.near where it says bias.
If it were me I would fit a 100k so I could adjust it easily.
But then there are heaps of competent guitarists using the Classics "out of the box" with the factory bias without any problems.

The elevated heater ac voltage 6.93VAC call it 7.. (I presume with the tubes plugged in) indicates the whole power supply is elevated slightly pushing everything harder.
"It was V209 & V208" well I guess that eliminates the lack of 4 individual screen resistors

You may like to check this out
Weber Bias Calculator
There are some posts relating to this scattered in the forum.

I wonder if Peavey got the 230 tapping right in the Classic series.
Know of 2 power transformer burnouts and they do chew through the tubes.
So I guess you will be buying some more tubes ?

[stokes]

They do set factory bias settings on the cold side,Mesa for one sets it so cold it is damn sterile,so that most tubes can be safely used,but wont necessarily sound their best,a little tweaking with an adjustable bias pot lets you dial in the sweet spot.Mesa and GT sort the tubes so they are guaranteed to be "safe".I have found that tubes of the same "grade" can vary by as much as +/- 10-20ma's,they will still fall into that "safe" range,i.e. 20 to 40ma's,but obviously tubes biased at 40 ma's at a particular plate voltage are going to sound much hotter than the low 20 ma in the same amp.Then you run into the occasion we see here,where the tubes are so hot.......So it is always best to check before just plugging in any old tube and thinking you are okay.An adjustable pot is the way to go in any fixed bias amp,in my experience.
I assumed that 6.9 vac was with no tubes,which would be normal,if it was with the tubes in then as oc said it surely compounded the issue.I have never used these oddball Russian tubes,so I cant say if the seller was being misleading or if they just bias so much hotter that simply testing and checking the bias would have headed off the problem.I buy, use and keep a large stock of NOS tubes,and I dont consider anything as new as 1983 as being "old stock",but I put every tube thru a number of checks before using them in my own or anybody elses amp.Hopefully your amp is okay and there has been no serious damage,and a new quality set of tubes is all you need to get you back on the road again.Always check the bias at least,before you plug'n'play.

[Enzo]

AT the risk of arguing, unless it is missing, the bias was not the problem. And PV certainly NEVER expected someone to take the amp to a sefvice center to have resistors changed with each new set of tubes. The cool factory bias level will let all but the weirdest tubes work just fine in stock condition.

When your tubes are glowing cherry red, it is not because the bias is a couple ma too hot. it is because something is killing the bias or it is not getting where it needs to be. In other words a gross problem, not one of adjustment.

In many cases, a bad tube causes trouble, damaging the circuit, then new tubes don;t work in the damaged circuit either.

Did we ever figure out which resistors we were talking about? the burning 10ks? The part numbers are not on the circuit board, but they ARE on th parts layout drawing which is page 4 of the pdf posted above.

This amp has little in common with the Classic 50 power amp, this one features driver stages for example.

[stokes]

As best we,who dont have the amp in front of us,can tell it is the 10k grid stoppers on a couple of the power tubes.His last post says the old tubes didnt cause a problem when he put them back,he then put in one of the "new" problem tubes,that didnt cause a problem the first time and it redplated and burned the grid stopper again.It possibly is a case of a bad tube,yes,they do bias very cold so most tubes will "function",but I think it is foolish not to check before assuming any tube will work okay.Caps feeding the power tubes checked fine,so the only logical place the grid stoppers got the overage is the plate,no?PV may not intend that you must take it to re-bias,but if you get tubes that dont perform,what else you gonna do?

[NickH]

Did we ever figure out which resistors we were talking about? the burning 10ks?

R239
R227
R216

Many Thanks

N