Electro Harmonix 300b...shorting problems?

[crispycircuit]

Greetings, I'm running a pair of Dared VP-300b mono's. One tube will draw 130v through the 1K cathoded resistor. This happens about every 4th start up. The problem follows the tube. Is anyone else having problems with these tubes? They normally run about 65v across the 1K cathode resistor. Is this too high of current for the EH tubes?

[arnoldc]

Hey, this is similar to a problem I encountered recently with my 300B!

I traced the problem to a faulty hum balance pot! I'm not sure what's the design of the Dared is but it may be worth looking at.

[crispycircuit]

Yes, it has a hum balance pot... What wattage pot did you use? I guess it's 100 ohms? It's very posible. Thanks for the reply...

[arnoldc]

It used to have a 5W Clarostat (I think) I got from Farnell. I had it replaced by an over-kill 50W TDO.

Both are 100 ohms.

ps.

When measured for resistance, it did gave the right Rk value, so I was pulling hairs trying to diagnose it.

[t_blizzard]

Your new 50W trim pot could be inductive and causing intermittent oscillations.
Any reason for such overkill on the wattage rating?

[7N7]

Originally posted by t_blizzard
Your new 50W trim pot could be inductive and causing intermittent oscillations.
Any reason for such overkill on the wattage rating?

Yes I was wondering this too.

On my p-p 813 amp I used to use 2W 100 ohm pots, but later changed to 25 ohm 5W as the 2W ones got noisy and someone gave me the 5W ones!

7N7

[arnoldc]

Originally posted by t_blizzard
Your new 50W trim pot could be inductive and causing intermittent oscillations.
Any reason for such overkill on the wattage rating?

The reason being Iis that I do not have any alternative at the moment. Intermittent oscillations? How does it manifest? How do I measure/test?

[t_blizzard]

Originally posted by arnoldc

The reason being Iis that I do not have any alternative at the moment. Intermittent oscillations? How does it manifest? How do I measure/test?

Fair enough.

The biggest reason for most oscillating circuits is how they are wired.

All wires are antennas and can couple RF energy into a circuit.

Temperature can also trigger a tube circuit to oscillate.

You will need a scope to monitor for intermittent oscillations.

[kevinkr]

Originally posted by crispycircuit
Greetings, I'm running a pair of Dared VP-300b mono's. One tube will draw 130v through the 1K cathoded resistor. This happens about every 4th start up. The problem follows the tube. Is anyone else having problems with these tubes? They normally run about 65v across the 1K cathode resistor. Is this too high of current for the EH tubes?

I have no direct experience of the EH 300B however I have had serious problems with other Russian made 300Bs which shall remain nameless. I have had intermittent grid connections which gave rise to similar symptoms to those described above, and I have also had filament to grid shorts that occurred and cleared as the filaments warmed up. (Saggy filaments)

Assuming the presence of grid stopper resistor the fact that it follows the tube is relatively conclusive, and I would replace it soon. The EH 300B is reputedly rugged and reliable so I would just try another one. Perhaps you can return it for an exchange if it is still under warranty.

Originally posted by arnoldc

The reason being Iis that I do not have any alternative at the moment. Intermittent oscillations? How does it manifest? How do I measure/test?

Arnold actually wasn't the original thread starter having a problem with these tubes.