Biasing question on a 1987 Plexi (point to point clone)

[jack2911guitarblues]

Hi to all Fellow Builders,
Have been playing thru my self build kit for about 5 years now, and have been very happy.  Last 2 months or so,  amp seemed to have progressively less power (harder to compete with rest of band). Changed to some spare EL34’s, no different. One seemed hotter than the other to touch, and so Checked Bias today, and have about 480v on both  plates (50 watter) and then checked bias current draw and got a shock (no, not that sort of shock!)   One tube showed 36 mA drawing, and the other showed nothing.  I checked both Heater Voltages they show about 6.9volts.
Checked Cathode resistors  (1 ohm) both fine, Checked all other components in the area, seem to be fine.  Tubes show no sign of redlining, pulled tubes and checked tube sockets, they all show continuity  from bottom to top.  I had both volumes right down and the Master as well while testing the bias.
Anyone have a few tips or ideas as to the cause?  What else could I check?   Thank you kindly for your consideration.

[shooter]

testing the bias

do you know the # you're aiming for?

pull the PA tubes AND leave out til you can get the -bias AT the socket pin of the PA tube.  IF adjustable bias was done, post the "range" the pot covers( -something to -something)

[jack2911guitarblues]

Hi there,
Not quite sure what you mean there. I was trying to bias amp properly at around 70%,   So @480 plate voltage was aiming for about 38mA on each tube.  So you’re saying, pull the power tubes and retry to bias to 38mA to see if that works?   Just wasn’t sure what would cause one tube not to show any current being drawn, I thought maybe the tube itself is stuffed or a dodgy connection in tube socket. But then I’m only guessing with my limited experience. Just hoping it’s something minor!
If that is what you mean, I will try and see .
Jack

[sluckey]

Check dc voltages on pins 3, 4, 5, and 8 of both output tubes. What have you?

[jack2911guitarblues]

Thank you for the suggestion, I will try that tonight after work.
Jack

[jack2911guitarblues]

Hi Sluckey,

So just measured those DC Voltages you suggested as follows:

EL-34 A   PIN 3= 488V,  PIN 4= 484V,  PIN 5= -38V,   PIN 8= 0.00

EL-34 B.  PIN 3= 485V,  PIN 4= 485V,  PIN 5= -38V,   PIN 8= 28mA

Hope that makes some sense!

Regards, Jack

[sluckey]

OK. Now with amp turned off measure resistance from tube A to ground. Should be 1Ω.

[jack2911guitarblues]

So wasn’t quite sure which pin you were referring to,  I assume it was pin 1 (1 ohm resistor to earth)
With amp switch off:   PIN 1 was 1.6 ohm, PIN 2 was 0.7 ohm,  and PIN 7 was 0.7 ohm.
Does that help?

[sluckey]

Sorry, I meant pin 8.

[jack2911guitarblues]

I just checked PIN 8,   Both A and B tubes show 1.5 ohms

[sluckey]

OK, now pull the output tubes and repeat these measurements from the tube side of the socket. With power off, measure the resistance from each pin 8 to ground. Now turn power on and measure voltage on pins 3, 4, and 5 of each socket. This checks the sockets.

[jack2911guitarblues]

 I just checked the Voltages and Resistance  with both Tubes pulled.

With POWER OFF: 

Tube A  Pin 8 to ground resistance = 1.5 ohms,   Tube B. Pin 8 to ground resistance = 1.5 ohms.

VOLTAGES WITH POWER ON. 

TUBE A.  PIN 3 = 496v,    PIN 4 = 499v,   PIN 5 = -39v
TUBE B.  PIN 3 = 499v,    PIN 4 = 499v,   PIN 5 = -39v

Thank you,  Jack

[jack2911guitarblues]

Hi there,
Would the voltages I posted, then indicate that sockets are ok? Because they’re the same on   I both tubes? I did give the sockets a thorough cleaning and put tubes in and out about 6 times to ensure good contact
Or could problem lie with the little bias pot.  Power tubes are still pulled, so might try to now achieve bias voltage, while they’re out.
Sorry about all the questions, sorted out a lot problems myself thru logic  I guess,  but was unsure about this one!

[shooter]

Power tubes are still pulled, so might try to now achieve bias voltage, while they’re out.

should be fine;
I typically do an end-to-end reading so I know the adjustable "range" at the socket

[jack2911guitarblues]

Sorry about the delay in responding. Took me a while to get my head around all the technical stuff.
Done a bit of reading since then, and understand a bit more about the info you guys were asking me.  I did the voltage checks that Sluckey asked me to do.
So just to reiterate, with the el34’s out of the circuit I get around 490 volts on Pins  3 and 4 of both tubes,  and I get a negative bias voltage of 42 on Pin 6 of both tubes.  On the tube side of the chassis, I get the same results, correct HT voltage and a negative Bias voltage on both sockets.  So this shows Bias circuit is working properly yes?
Shooter asked about the Range of adjustable pot. I got -30 v  to. -59 v.
With el34’s back in,  one el34 on Pin 8 shows about 38mA current drawing, and the other el34 shows 0.00 current drawing!
ALSO I switched the the tubes around and the result also switched. The other side now showed 0.00 mA.   So it seems to me that maybe it could be just the tubes that are cactus?  So while voltages seem ok, it’s a bit scary putting in new tubes.  My current set and an older set of JJ’s, both have burnt sides on them, and one of the two tubes of both sets, felt a fair bit hotter than the other.
Sorry to ramble on,  feel I’m getting there, just need a little more clarity
From you experts.   Thanks in advance

[shooter]

I got -30 v  to. -59 v.

I just checked PIN 8,   Both A and B tubes show 1.5 ohms

these along with the VDC for plates and screens tells me you're about as good as it gets, without tubes.

here's what I do;
connect a meter across the cathode resistor (pin 8 to ground)
I would set the bias to ~~~  -45
install new tubes
power-up and get the meter reading in volts DC (which is = to tube current in mA)
***if new, power down, wait a few, move meter + to the power supply tap that feeds the PA tubes, set meter for VDC***  (real experienced, you can move from the cathode to the tap with power)
Now power up, let it stabilize, record value, power down.

the math power in W = voltage plate * current from cathode resistor
so 461 * .027 = 12.5W per tube. (ballparked)
repeat the cathode voltages check, redo the math for the second tube

check back with values and smoke report 

NOTE: 2 meters are real handy for above work.

[sluckey]

Put two new tubes in it.

[tubeswell]

Check that you haven't got an open screen resistor on the socket of the tube that isn't drawing any current

[jack2911guitarblues]

Thanks for all the valuable information, and sharing your knowledge with others!
So just going to order some new tubes and then hopefully get it back up and running.
(Any recommendations for an alternative tube other than JJ’s similarly priced?)
To tubeswell, thank you and I did check screen resistors as well, readings were good, and when I swapped el34’s around the weird mA readout on pin 8 went to the other tube. So pretty sure it would be the tubes.
One set is only 8 months old which surprises me,  cos  we only jam once every 3 weeks at the moment.  An amp tech did the Bias about 5 months ago, and told me  “he  likes to run tubes a bit harder”.  Anyway,   When I checked a couple of months  later, the readings were about 53mA per tube!  So  I dropped it down to about 34mA.Could that  high current drawing have damaged the tubes?
Anyway, we’re forever learning and enjoying it!
Hope you guys and your families are staying safe in the current crisis! We’re deep into the isolation thing as well here in Australia.

[shooter]

high current

suspected in almost all fails involving electronics 

On my rally car tires were rated 40,000 miles, mine failed at 17,000 cuz I liked to drive hot, cost of doing business 

IF it drifts up over time at prober bias and just idling, I might suspect caps or wrong values somewhere