EL84s glowing red

[twalker]

I picked up a 60's Lafayette 236 amp at a yard sale. I added some fresh tubes and two of the el84's are glowing bright red after about 10 minutes of running.

It's a stereo, cathode bias amp with two el84's on each side. The tubes that glow are in the first socket of each side.

I measured the resistors in the output section and the power filter section and they show correct values with the schematic.

Anyone have any suggestions on how to fix this or what to look for?

[alexistheo]

I'd first check for leaky coupling capacitors. Since it's cathode biased, the grid should be at 0V. You'll have to check this when the amp is powered up, so BE CAREFUL. Read the thread on safety first.

If the caps are leaky, replacing them is no big deal. It's a common problem with old paper caps.

[SY]

Leaky caps are a strong possibility. Leaky coupling caps and leaky cathode bypass. Replace 'em all. To confirm, monitor the cathode voltages as the amp warms up and until you see the red color.

See if the glow follows the tubes when you swap them around. If so, replace the tubes, too. If not, I'd replace them anyway, but don't put the new ones in until the over-dissipation problem is sorted out.

[kevinkr]

Something else to bear in mind since the amplifier is japanese and of 1960's manufacture is that the original 6BQ5 fitted in these amplifiers were a little tougher than current ones. Japanese 6BQ5 based amps destined for the US market seem to have run their output tubes very hard in order to maximize output power.

I have a 1961 vintage Realistic Stereodyne 40 which at the rated line voltage of 117V (I have an original Sams Photofacts as confirmation) runs the 6BQ5s in pentode connection at 390V on the plates and 50mA cathode current per tube. Allowing for 11mA screen current this still handly exceeds the rated plate dissipation and voltage for a 6BQ5. Cathode current per pair measures 100mA! The amplifier actually produces 20Wrms per channel, a bit extreme for a single pair. (Confirmed by measurement.)

The power supply caps, coupling caps, cathode bias resistors, grid resistors, and cathode bypass caps have all been replaced. All voltages and currents are close to spec. The amplifier actually does work.

I bought a matched quartet of current (2006) production JJ 6BQ5 for this amplifier, and several of them glow quite brightly. Some of the originals that are still good do not glow at all at these excessive plate currents.

I just ordered some Russian NOS 6P14P-ER from a fellow in the Ukraine, these are supposed to be more nearly equivalent to a 7189 and might be sturdy enough to handle the excessive voltage and current they are being run at. I'm still waiting for them to arrive.

Finally I am going to reduce the VP slightly by adding a 47 - 100 ohm power resistor from the power transformer center tap to ground.

Obviously check your coupling caps and cathode bypass components , but in several Japanese models as well as 7189 based Scott amplifiers current production 6BQ5 do not thrive..


Good luck!

[twalker]

Thanks for the help. The problem went away as soon as I replaced the coupling capacitors. The best $1.50 ever spent.

[richwalters]

Originally posted by kevinkr
.

I have a 1961 vintage Realistic Stereodyne 40 which at the rated line voltage of 117V (I have an original Sams Photofacts as confirmation) runs the 6BQ5s in pentode connection at 390V on the plates and 50mA cathode current per tube. Allowing for 11mA screen current this still handly exceeds the rated plate dissipation and voltage for a 6BQ5. Cathode current per pair measures 100mA! The amplifier actually produces 20Wrms per channel, a bit extreme for a single pair. (Confirmed by measurement.)

Yes SNAP....(this system sounded darned good) but long ago the 6BQ5 tubes on mine failed because the glass envelope cracked in the same place (ajacent to slit in anode) due to repeated thermal stresses....poor electrical designs..not as bad as what went into colour TV's of the same vintage ??? Later, I sold the unit with a lower voltage transformer.

richj

[Geoff H]

Kevin, thanks for the warning on 6BQ6s. I'll make sure to get good ones if I build a tube amp. Do you, or anyone know if the same can be said for EL84s. Electro Harmonix claim theirs are close to the original Mullard tubes.

That could be a solution to the glowing cheeks.

Geoff.

[kevinkr]

EL84 = 6BQ5 and yes the issue is the same for any that are run significantly beyond their ratings - stay within the design limits and you shouldn't have any problems. These aren't defective it is just simply some designs like my old amp run them way beyond their nominal ratings in the quest for higher power ratings.

Stay in the low 300V range and below 11W plate dissipation and they are generally quite reliable.