microphonic 6973 tube

[jstich]

Hello, I have an amp that has 2x 6973 power amp tubes ,and 2x 12ax7 tubes.It runs great, but one of the power amp tubes is microphonic. Is this bad for the circuitry, etc. of the amp? Thanks, S

[Metro]

Microphonics wont do any damage directly but can be an indication of a failing tube which if it goes might do damage.
Ive never seen an amp using 6973's, what kind of amp is it?

[jstich]

circa 1969 Penncrest .Was a 1x12 combo, but running it through a 4x12 lee jackson.These amps are known for their good tone, and they're sorta rare.Tried to sell it or part it out , but had no takers. Thanks

[tubeswell]

Replace the tube

EH market a modern production version according to this (which they claim is "real")

http://www.ehx.com/products/6973

[jstich]

Brings me to reason I want to sell it. Time and money for my other tube amps! I'll watch it and see if it dies. Thanks for the help! Sincerely, JS

[Alex R]

You get these 6973s in jukebox amps. I had some bad experiences with the EH reissues. And some good experiences rewiring the sockets and fitting EL84s instead.

[Enzo]

SOme Leslie amps too, I believe.

I have rebuilt a ton of old Seeburg amps, and they did use the 6973 a lot in one era. They could get four of them in a limited space easier than four 6V6s or other tube. Those amps ran typically 16 hours a day every day.

[jstich]

I was told I needed different transformers to run el 84's.I suppose it would be complicated. Is it easier to do in England with their different electrical system?

[Alex R]

No, both el84 and 6973 types are happy in the same range of plate impedances, and so the same transformers will do. Bias voltages are in a similar range also although you will need to make adjustments. But you do have to change around most of the connections on the valve base. That's the only complication, and it's the same whatever mains supply you have.

I had so much trouble getting both old and new (EH) 6973s to settle down and stop humming (due to current draw irregularities/imbalances) in Ami amps that I now rewire to EL84s immediately when an Ami continental amp comes in. Given the much lower cost and better reliability of EL84 use it saves the customer money in the long run.

[jstich]

How do I find out which wires to switch? Sounds complicated. Thanks

[Alex R]

You would need the pinouts for both tubes, which you can get from the web or via Duncan Amps TDSL - just google for these.

No it's not that complicated, but there's a good few opportunities to get it wrong, so like with any task you need to judge whether you have, or can get, the knowledge and skill to do it (or the sheer blind stubbornness, which is what I usually rely on). You will have to identify the wires going to the valve bases, sort out the EL84 pins they will need to go to, and desolder/resolder them onto the new pins, then reset the bias, which might need a resistor change.

NB the 6973 has a number of double connections; the grid and the screen (and I think the suppressor/cathode) can be connected to one of two different pins. You will see this on the pinouts.

Any problems, just ask.

...A simpler solution to this basically simple problem would be to get hold of a good pair of NOS 6973s! For once I would say that a matched pair is worth it as I get quite a lot of hum from imbalanced pairs of 6973s. Not a cheap option, this, I'm afraid.

[Gtr_tech]

Supro/Valco used the 6973...I had a 1-12 combo a hunnerd years ago that used a pair.

[Chuck H]

If you choose to rewire for el84's just don't use any of the "unused" tube socket pins for a mounting post. Some older el84/7189 tubes used the typically unused pins for different purposes. And you never know when you or someone else may want to plug some strange el84 type into the amp. When that happens you may not remember exactly how you wired it (I didn't). Better safe than on fire, I always say

Chuck

[Alex R]

Yes, for sure pin 1 on el84 bases is best not used as a connection point (as Laney do for instance) as it is internally connected on some tubes (usually older ones). And JJ label the other two 'unused' pins as internally connected also.