Red plating EL84's in Liverpool clone...

[v00d00blues79]

Ok guys... i've done some troubleshooting and it seems for some reason i'm getting red plates on my power tubes... unloaded voltages are as follows:

pin 3: 0v <--- i know i have good solder connections on the cathode resistor. values: 50R 10w Ohmite wire wound and 220uF/63v

pin 7: 366v
pin 9: 366v

i don't know where else to look... i'm going to try and work on this tomorrow as i'm tired and am most likely missing something simple, but any suggestions would be greatly appreciated. i can't have the power tubes in there for more than a second or two before the plates on all four start glowing like cherries...

Also, i'm using the low voltage secondary on the PT and the 5.5k primary on the OT. These are the Transformers from Moose... The Heyboers.

Thanks,

Andy

[Chris G]

Hi Andy,
You should have some neg. voltage on pin3 (-10vdc) on mine, I would check the polarity of the cap in the bias supply make sure the + goes to pin3 and the -negative goes to ground.........also check the ground of the 2 220k resistors in the pi.

Also check the connections on pin3 of all the power tubes........make sure nothings touching.

Keep us posted.......I will post the voltages in my pool clone later tonight or friday afternoon.
Thanks
Chris

[Moose]

If you've got zero volts at the cathode resistor, you aren't biased at all. That resistor has to be porked, or something else is wrong.

First things first, check that resistor to make sure it's 50ohms to ground, not 5 ohms or zero or something. You should be able to check this with the cap in place and the reading will still be darned close.

[tubedogsmith]

What are you using for screen resistors?

[v00d00blues79]

Thanks guys for the responses. I will have check the resistor value when i get home this afternoon. as for screen resistors, i am using whats listed on the schematic. 1.5k i believe.

Also, the cap is orientated correctly, i will have to check on the 220k's in the pi. I think this may be where the trouble lies... its too bad i didn't catch this earlier though... i now have a quad of el84's that are worthless...

Thanks,

Andy

[Funkalicousgroove]

First off, if you're getting 366 unloaded that means you're probably getting 330 or so on the plates loaded, which is standard in an AC30, but still above the max ratingfor that tube. The EL84 is a 12 W tube, and I'm guessing with your current setup you're probably running them around 15w. Some EL84's will work fine like that others won't. The easiest solution(Provided you don't have the socket wired wrong) is try a different quad of tubes, or increase your Cathode resistor to 80 ohms. Also, if you're not putting all 4 power tubes in it will red plate because the 2 will draw about twice the current.

Double check to make sure that you've wired the socket correctly, and don't use the un-used pins to jumper, some of them are connected internally.

[v00d00blues79]

i am using all four tubes at once, and none of the extra pins are used for jumpers or whatnot... i'll try increasing the cathode resistor values like you suggested. the tubes in question were a quad of JJ EL-84's that i had been running in my 36w amp, so they were already taking a bit of a beating... i'm going to have to pick some more tubes up this weekend, and maybe keep an eyeball out for some NOS tubes at the Ohio Guitar Show...


Thanks,
Andy

[Funkalicousgroove]

when you find that they're redplating is that at Idle or with a signal? You should be using 100 ohm 2 or 3 watt screen resistors, you could go up as high as 300 ohms but it makes little difference. the 220K resistors to ground after the last coupling caps shouldn't cause it to redplate, if they're the wrong value they would only affect the intensity/Impedance of the signal from the preamp.

[v00d00blues79]

the red plating is at idle. also, i am using 1k/3w screen resistors. at least thats what is shown on the schematic. i can take them to 100R if that will help. Also, just to make sure, which pin are the screens? pin 9? or pin 2? I know the pinout to a 12a*7 from heart, but el84's give me fits on remembering the pinout.

Thanks,

Andy

[Chris G]

the red plating is at idle. also, i am using 1k/3w screen resistors. at least thats what is shown on the schematic. i can take them to 100R if that will help. Also, just to make sure, which pin are the screens? pin 9? or pin 2? I know the pinout to a 12a*7 from heart, but el84's give me fits on remembering the pinout.

Thanks,

Andy

Hi andy,
I am useing the same resistors on the screens 1k/3w........pin9, control grid is pin2 should have the 1.5k's there.

Here are my voltages on my pool.......no load on input..........8ohm dummy load on the out.......controls to 0

B+1=335vdc
2=312.5vdc
3=260.1vdc
4=244.5vdc
5=231.1vdc

Plate voltage=325
screen voltage=307
bias voltage=-9.42 vdc

Hope this helps
Chris G

[MarkB]

Chris

That's a positive voltage on the cathode, not negative. The negative bias exists because the grid, at 0VDC, is negative with respect to the positive cathode bias.

[MarkB]

Andy


I just had to delete my post when I reread your initial post. I think I understand now. You are getting 0VDC on your cathode because there is no tube in the socket. No tube, no connection between the cathode resistor and the plate connection coming from B+. No current through the cathode resistor, no voltage drop. However, your initial problem was too much current.
Whenever you have redplating, you have too much current going through the tube. A bigger cathode resistor will cut down on the current and lower the plate dissipation. Put in a bigger cathode resistor and recheck the tubes.
It sounds like you don't know how to figure plate dissipation and set the bias properly. Go to Randall Aiken's Tech Info page at aikenamps.com and read up on biasing. There's a section in one of the articles on biasing cathode biased amps.[/i]

[Funkalicousgroove]

here's what you need

[v00d00blues79]

I know how to calculate plate dissipation and set the bias, i just couldn't figure out why after i rechecked everything i wasn't getting a bias current. I found the culprit. I forgot to connect the 220k resistors in the PI to ground. Now i've got a problem of the output being too low and only one side is drawing too much current and red plating. I will be checking the coupling capacitors for DC leakage. I'm also thinking that i might split the cathode resistors for the tube pairs to 100R/5w resistors... sometimes troubleshooting shouldn't be done waaaaay late into the night.