RCA 6CA7 not biasing correctly when replacing el-34s...

JDW3

Member
Messages
3,637
I have a Metro JTM50 that I built 6 years ago. I gigged it hard using JJ and Mullard old stock el-34s.

One of the Mullards went recently. So I popped in the RCA 6CA7s that I had.

Sounds great, with one reading 33mA. But the other is reading 85mA.

Does this sound like an amp problem that may have caused the Mullard to fail? Or a case of mismatched tubes?

Nothing redplating, and the amp sounds really good. But this looks like a potential problem.

Any ideas?
 

mark norwine

Member
Messages
17,183
swap the tubes in the sockets.

Does the "high current" follow the tube or stay on the socket?

follows the tube = tube problem
stay with the socket = amp problem

report back
 

JDW3

Member
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3,637
OK, same readings. I noticed some light colored or faded solder on pin 5 on the same tube socket. Possible cold joint?

The reading across the 1ohm resistor was jumping around, going up to 300mv.

The "good" tube was reading normal and steady, about 32mv.
 

JDW3

Member
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3,637
The readings were the same to that socket, after the tubes were swapped.

The higher current stayed on that socket.
 

mark norwine

Member
Messages
17,183
pull the tubes.

take a volt meter, set it for DC volts.

measure each socket.....put your "red lead" on pin 5, black lead grounded.

You should read a negative voltage, and both tubes should read about the same. I suspect they won't, however.

Get the numbers & report back.
 

mark norwine

Member
Messages
17,183
interesting.

put the tubes back in & repeat the measurement. When you apply the volt meter, you'll hear a lot of hum & noise, but it's ok....
 

mark norwine

Member
Messages
17,183
and yet that one socket is still running higher current, right?

The fact that your bias voltage is the same on both, my first theory (leaky decoupling cap on the inverter plate) is thrown out the window.

That said, I'm pretty sure you have a socket issue. It's possible that pin 5 isn't making good contact with the tube, so even though you have good bias voltage at the socket's lug, the tube isn't seeing it.

Sockets do go bad....and a lot of modern sockets suck. Since my #1 theory is shot, I'm betting you have no contact with the tube's pin.
 

JDW3

Member
Messages
3,637
The amp sounds really good with the 6CA7s.

With the previous Mullard el-34s, that particular socket with the higher readings was also the Mullard was installed that had blue flashing or arcing inside. Not at first, but within 4 minutes of heating up. If I could remedy the amp, I'd love to reuse the el-34s/
 

JDW3

Member
Messages
3,637
So install new socket? Should I even try retensioning?

Would you say this problem could have toasted the previous el34s?
 

mark norwine

Member
Messages
17,183
are the sockets "tulip" type or "bifurcated" type?

Modern tubes & modern tulip sockets often don't play nicely together.
 

JDW3

Member
Messages
3,637
It's back to jumping and even staying at zero, where the high current was being read.
 

JDW3

Member
Messages
3,637
I'll order it. I messed with resoldering supposed cold joints last year and finally thought it was the tubes. Hopefully this will fix this amp because it sounded great.

Much thanks!!
 

Dan40

Silver Supporting Member
Messages
1,554
Most Metro kits came with the 1ohm biasing resistors...is this what you are using to measure the idle current? Any chance you have a bad one? I had one go bad on my build last year and kept giving me weird readings when biasing. I thought one of my old Mullards was shot but it was only the resistor.
 

JDW3

Member
Messages
3,637
I may have more resistors, I was reading at the socket end of the resistor. That also crossed my mind.

Maybe I should just check the resistor? Yes. The resistor itself it showing the ohms jumping all over, while the other reads 1.3ohms.

I suppose I'll have to find my components and replace.

Would something like this cause a tube to fail?
 
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