6p14p-k 80s reflektor

[mike-p]

Hi all,

Having got my 36w marshall style (aka the hodge podge mess!- see my other thread) Actually going pretty nicely with a few niggles.

Bought a couple of used,but tested, quads from Ukraine plus 10 untested. Out of those 18 I only have 5 which don't gradually increase voltage on the grid until I turn off power before they become toast! Seller says he sells plenty to replace el84 in guitar amps and has had no complaints, wonders whether the EV variety might suit me better and I see a lot more mention of these than the K type?

The seller tests up to 275 on the plates, I have 330 which seems OK, all my amp voltages are where they should be, the tubes which work seem to work fine but my first suspicion is, of course, that I'm doing something wrong... seems unlikely I have so many dud valves.

Cathode bias 150ohm 100uf bypass cap - 10v
Pin 9 300v
Pin 7 325v

Any thoughts appreciated.
Thanks.

[bepone]

Send a pic of the socket.
Explain what are you doing with "testing"? Plug pairs in the amp? Like 18W?

How much in kilo-ohm you have from pin2 to the ground? Is it pin 2 connected or is in the air?

[mike-p]

470k grid to ground.

It's a double power section 4xel84 18w lite 2b marshall style.

I'm replacing one of my functioning tubes at a time and then watching the grid voltage slowly climb over a few minutes, I usually cut power at 0.1 odd volts by which time there may be a slight hum.

Each pair has a shared grid so each pair has pin 2 - 8k2 - coupling cap from PI - then 470k to ground.

The good valves are running happily with 20mv on the grid rising to maybe 40mv after running for a while.

[bepone]

grid? i dont follow. cathode voltage is important, and to put 1 ohm in cathode to mesure current.

usually im doing it in tube tester not in real amp.. are you measuring vacuum quality via grid stopper current draw? or ?

[Stevem]

When you start to get a light hum then the tube is starting to red plate from idling too hot/ pulling too much current.

What screen voltage are these tubes spec'ed for and what is the screen voltage your applying?

[mike-p]

Thanks for all replies.

https://soviet-tubes.com/product/6p14p- ... t-pentode/

Found the above.

My voltages are as in my above post -

Cathode bias 150ohm 100uf bypass cap - 10v
Pin 9 300v
Pin 7 325v

Maybe this is just too high for the K type?

[bepone]

My voltages are as in my above post -

Cathode bias 150ohm 100uf bypass cap - 10v
Pin 9 300v
Pin 7 325v

Maybe this is just too high for the K type?

i was using this tubes without problems.. all normal measurements here dissipation is ok.

[bepone]

The good valves are running happily with 20mv on the grid rising to maybe 40mv after running for a while.

what means 20mV, and 40mV? milivolts? where?

[mike-p]

Millivolts on the grid, pin 2.

150 ohms is a shared cathode resistor between two pairs of tubes but I think you understood that.

Is there something I'm missing as a beginner that can cause the tubes to red plate if the voltages and dissipation are all good?

[bepone]

Millivolts on the grid, pin 2.

150 ohms is a shared cathode resistor between two pairs of tubes but I think you understood that.

Is there something I'm missing as a beginner that can cause the tubes to red plate if the voltages and dissipation are all good?

Red plating means current through the tube * Voltage Uak, must be below max dissipation (from datasheet 12W-14W). And if the tube is left like this long time, over max dissiation. then you have red plating, which can be dangerous but if short time, tubes can survive.

In cathode bias, which is not good at all for testing tubes, if one of the tube fails from the pair, or doesnt pull current from some reason (socket or any), then the other tube will pull its nominal current, but bias created from cathode resistor will be only half *per example, instead 10V on the cathode, will be only 5, and this 5 V will push extreme current through the tube, which will be causing redplating.
So probably you have tested tube by tube, without second pair inserted, and when you insert only one, you probably kill it.

Dont test tube like this, you need proper tester, or test all together in the amp and see.. insert 1ohm resistor in each cathode and measuring each tube you can find matched pairs if you want to do this.

All the time measure cathode voltage, not g1 voltage. So you can react fast. Then if the tube is in short or out of order, in 20-30sec you will see the voltage on cathode raising over 10V 15-20-25 you can switch off the amp mark bad tube

[Roe]

I have a similar issue with similar tubes in two ac30s with 50R cathode resistors and 320+v plate voltage. most tubes draw too much current and redplate. but these tubes would be better for another amp with colder bias, such as the 82R cathode resistor used on early ac30s.

[mike-p]

Thanks all. I'm interested that on non technical sites running tubes "hot" seems to refer to either plate voltage or bias or both, I imagine the tubes are more affected by bias than high voltage and my bias seems just right?

I tried putting a troublesome quartet back into the amp but using the vvr to lower the b plus to 250v, they seem to be fine idling and a short play cranked right up. What is the disadvantage of lower b plus? I can't discern a big difference in sound but I find it hard to compare tones when volume is high!

Bepone I don't quite understand testing using 1 ohm resistor on the cathode, can you link me to more information?I am testing with four tubes inserted at all times

[bepone]

I tried putting a troublesome quartet back into the amp but using the vvr to lower the b plus to 250v, they seem to be fine idling and a short play cranked right up. What is the disadvantage of lower b plus? I can't discern a big difference in sound but I find it hard to compare tones when volume is high!

Bepone I don't quite understand testing using 1 ohm resistor on the cathode, can you link me to more information?I am testing with four tubes inserted at all times

on 250V is better sound,
1 ohm resistor put on pin 3 socket from EL84, and after that connect cathode wire to 120R