Tubes Asylum

RE: Running hard?

172.11.32.65

Chip647

In Reply to: RE: Running hard? posted by Triode_Kingdom on March 05, 2025 at 22:55:27

The data sheet I was looking at said "both plates (both units operating) max 2 watts". So while not exceeding max, ts is the only place where you could say relatively "running hard".

Follow Ups:

Here's one way to fix it... - Triode_Kingdom 10:37:23 03/06/25 (2)
"The data sheet I was looking at said "both plates (both units operating) max 2 watts"
In that case, you're absolutely correct, it's operating on the edge. The Philips data sheet doesn't show that limitation, and maybe the AR designers didn't see it either. In any event, the intention was obviously to squeeze as much drive capability out of the 6DJ8 as possible. At this level though, they should have used a different tube.
Here's the simplified schematic with all the extraneous circuitry removed:

I modeled this in SPICE and made a few changes to the cathode followers. Simply increase the value of R13 to 180K, and increase the value of R14 to 39K. Both new parts should have a 1W rating. This will reduce the dissipation in the followers to about 70% of the original value and extend their lifespan considerably. The reduction in current of course means less drive capability, but it will still drive 18V RMS cleanly into a 10K load.
This change will also reduce standing DCV on the cathodes to about +100V. The heaters are elevated to about +122V, so this is well within the H-K ratings for the tubes.
As a final note, this has virtually no impact on line stage gain. The difference is about 0.1dB.

Edits: 03/06/25 03/06/25
- RE: Here's one way to fix it... - Chip647 11:18:07 03/06/25 (1)
  That makes sense.
  
  Isn't a cathode follower the only "acceptable" use of a 12AU7 (5.5 watts both plates combined max). Would seem to be a better choice.
  - RE: Here's one way to fix it... - Triode_Kingdom 11:57:10 03/06/25 (0)
    Yes, a 12AU7 would probably work well. I would need to toss it into the model to determine appropriate values. Other tubes might work as well, such as the 6BQ7A. The idea was to do the minimum necessary to make the original tube type work. The high Gm of the 6DJ8 is beneficial in this circuit.

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