Redplating jcm900

[madmartigan]

Im new to this forum so hello!

I was hoping someone could shed some light on a situation.

A friend brought over his jcm900 2500 50 watt (el34). He was complaining of a sudden volume drop. I replaced the 2 el34s but when i went to bias I cant get it below 55vdc with the trim pot backed all the way down. The volume returned to normal but the tubes are redplating so hot that theyre literally smoking!

Could this be a bad ot or should I be looking at the bias supply (if I knew where to look)?

This is a 1990 model, the first year of the 900s I believe. Any help would be greatly appreciated.
Thanks

I was able to take a few measurements before the label on the brand new groove tubes turned black, and began smoldering
Hope these are the right ones:
v4 pin3 322v pin4 278v
v5 pin3 320v pin4 281v

and also measured v1 pin1 110v and pin6 151v

[dukeamps]

Im new to this forum so hello!

I was hoping someone could shed some light on a situation.

A friend brought over his jcm900 2500 50 watt (el34). He was complaining of a sudden volume drop. I replaced the 2 el34s but when i went to bias I cant get it below 55vdc with the trim pot backed all the way down. The volume returned to normal but the tubes are redplating so hot that theyre literally smoking!

Could this be a bad ot or should I be looking at the bias supply (if I knew where to look)?

This is a 1990 model, the first year of the 900s I believe. Any help would be greatly appreciated.
Thanks

I was able to take a few measurements before the label on the brand new groove tubes turned black, and began smoldering
Hope these are the right ones:
v4 pin3 322v pin4 278v
v5 pin3 320v pin4 281v

and also measured v1 pin1 110v and pin6 151v

Hi, Check that the rectifier diodes are ok.

[billyz]

Some of those were biased for 5881, would need a resistor mod to get el34's to bias correctly. If you measure -55vdc already, then you have bias voltage. Could have a faulty bias trim pot too, if it is fluctuating, check the bias supply cap, if it is bad will affect stability of the bias circuit.

[madmartigan]

Thanks for the speedy responses!

I checked the diodes, but didnt pull them off the board. Open one way, and a reading of about .550 the other way using a mm with the little diode symbol. Also checked the ones in the preamp but got readings both ways. Is that because I didnt pull one end?

Which cap is the bias supply? (still learning, and a loooong way to go)
Thanks

[dukeamps]

Thanks for the speedy responses!

I checked the diodes, but didnt pull them off the board. Open one way, and a reading of about .550 the other way using a mm with the little diode symbol. Also checked the ones in the preamp but got readings both ways. Is that because I didnt pull one end?

Which cap is the bias supply? (still learning, and a loooong way to go)
Thanks

Hi, I had a similar problem, were all of a sudden the power tubes started redplating on my jtm45/100. I traced the problem to the diodes, changed them and everything was ok. But there could also be something wrong with the bias with your amp.

Dukeamps

[madmartigan]

yeah as far as i can tell the diodes are fine

[es345]

Hi

take the EL34 out, switch on and measure Pin 5 (g1) at the sockets of the EL34. What is the value you measure?

[madmartigan]

I got a reading of 9.2 but only with the mm set to 200m vdc. so would that be 900mvdc? When it was set on regular vdc (200 or 600) the reading was 0. Does this mean I have no neg voltage?

[dynaman]

Im new to this forum so hello!

I was hoping someone could shed some light on a situation.

A friend brought over his jcm900 2500 50 watt (el34). He was complaining of a sudden volume drop. I replaced the 2 el34s but when i went to bias I cant get it below 55vdc with the trim pot backed all the way down. The volume returned to normal but the tubes are redplating so hot that theyre literally smoking!

Could this be a bad ot or should I be looking at the bias supply (if I knew where to look)?

This is a 1990 model, the first year of the 900s I believe. Any help would be greatly appreciated.
Thanks

I was able to take a few measurements before the label on the brand new groove tubes turned black, and began smoldering
Hope these are the right ones:
v4 pin3 322v pin4 278v
v5 pin3 320v pin4 281v

and also measured v1 pin1 110v and pin6 151v

Hey, you do realize that you want to start checking current draw of the output tubes with the bias voltage maxed out, right? Lowering this negative voltage will make the output tubes run hotter.

Check the 50W 2500 schematic. V6 and V7 are the output tubes.

Start backtracking through the bias circuit. Don't forget to check those oddball fuses for the EL34s.

[madmartigan]

with the bias pot maxed out its -11.5mv. With the standby switch off (amp on standby) its -49.5mv

[JD0x0]

no one mentioned this so i guess i will. My guess is that his old el34's died and the ones you replaced were bad. Ive been hearing ALOT about crappy el34's. I believe Groove Tubes are just relabled chinese shugaung/sino tubes whci are the cheapest money can buy. Im not saying they're bad, i have a set in my ODS. but maybe you just got unlucky.

also the bias may be fine. EL34's usually like to run hot and if you only have 350V on the plates then 50mA is not too hot. Im running 42mA in my OTS with about 470v on the plates

[madmartigan]

I wouldnt mind the 50 ma bias but theyre red plating so bad that the little stickers that GT started putting on their tubes turned black and started smoking! Also its both tubes, not just one tube or one socket. Altohugh i guess theres a chance that both tubes are bad.

[JD0x0]

I wouldnt mind the 50 ma bias but theyre red plating so bad that the little stickers that GT started putting on their tubes turned black and started smoking! Also its both tubes, not just one tube or one socket. Altohugh i guess theres a chance that both tubes are bad.

Im just wondering because the previous tubes didnt redplate did they? It just seemed like it was there time to go. try the previous tubes again if they dont red plate my reasoning brings me to believe that you just got REALLY unlucky with the new tubes. And if thats the case i'd at least try returning them for a fresh set or a refund

[madmartigan]

OK so I stuck the old tubes in with a bias probe and they measured 54vdc at the lowest and adjusted only up to 54.6. They did not redplate.
Stuck the new ones in again and got super redplating at 42vdc.

So you may be right. I may have 2 bad tubes. I will swap the tubes today and report back with my findings.
Thanks!

[billyz]

I'm a little confused by the references to mv and ma and VDC. which is it? what are your voltages at all the pins ? pin 5 is the bias voltage.

Just a side note about tube quality. I recently had a pair of NOS Mullard XF2's the amp had a short in the bias circuit and the tubes were pulling over 100ma per side and NO redplating. this was at 400VDC on the plate. it sounded shitty of course. But , I was astounded at the ability of those tubes to take that kind of abuse. They do not make them like they used to and I frequently get crappy tubes right out of the box these days. I have had the best luck recently with Sovtek, Electroharmonix etc. everything else has been a crap shoot, especially JJ and chinese.

-55v bias sounds more like a 5881 where as an el34 would be more like -45v. That is bias volts not ma. that would not account for the redplate though, in fact the opposite.