Dr Z redplating

[Randall]

As stated, grid voltage is 0v.

The hi/lo power switch mutes the grids of two of the 6v6s thru 100K ohm resistors. The wiring has not been altered.

Both cathode resistors measure the same 151 ohms.

Plate voltage is 409v.

[Dave H]

That amp has a half power switch. I'll bet the cathode resistors aren't wired as one per side, but rather have a series connection for two tube operation.

Yes, that would make sense (It's like the Matchless DC30). Randall says in the OP that it came in with two dead 6V6s. If all four share a common 150R cathode resistor the good pair are sure to red plate if two are not conducting. Also operating it with only two 6V6s with the switch in full power mode would also cause red plating.

[eschertron]

IIRC from the other thread about this amp, the cathode resistors were assigned to each pair of tubes. Could there have been a single 150R cathode resistor (for all 4 tubes) from the OEM, and someone split the cathodes without upping each single pair's cathode resistor to 300R?
@Randall, any evidence that could have been done to this amp?

[Randall]

This amp has not been modded. The wiring is impeccable. It is stock with two 150 10 watt cathode resistors, one for each pair of 6V6s.

[nosaj]

This amp has not been modded. The wiring is impeccable. It is stock with two 150 10 watt cathode resistors, one for each pair of 6V6s.

The sign of a good tech is one who leaves no trace he's been there

nosaj

[John G]

Hi,
I have attempted to draw up a schemo from the image that Guitar Player published in their review.
There are many guesses................
This may or may not be of help??
John

[Randall]

OK, I reached out to their repair guy and received this:

Randall,
Install a quad of JJ 6V6s ( we do not recommend using Russian, Chinese or NOS in this amp )
Replace the (4) 3W screen resistors with 5W 500 ohm
Replace the (2) 10W 150 ohm bias resistors with 10W 180 ohm

Interesting. But does this seem like enough, increasing screen resistors by 40 ohms and cathodes by 30 ohms? I replied by asking if this was standard procedure on this amp, and what should I expect to see for current at idle, but I haven't gotten a response yet.

[nosaj]

OK, I reached out to their repair guy and received this:

Randall,
Install a quad of JJ 6V6s ( we do not recommend using Russian, Chinese or NOS in this amp )
Replace the (4) 3W screen resistors with 5W 500 ohm
Replace the (2) 10W 150 ohm bias resistors with 10W 180 ohm

Interesting. But does this seem like enough, increasing screen resistors by 40 ohms and cathodes by 30 ohms? I replied by asking if this was standard procedure on this amp, and what should I expect to see for current at idle, but I haven't gotten a response yet.

Should have asked them what stock was also.
nosaj

[The Dude]

......we do not recommend using Russian, Chinese or NOS in this amp......

Translation: Our amps are designed to run the piss out of tubes.

[Randall]

I'm thinking of upping the cathode resistors to 200 ohms for a little extra pad.

[Chuck H]

Or use the recommended power tubes. You can always bump the cathode resistance later if they become unavailable. This way you'll be delivering to you customer a fix that is in keeping with the manufacturers intention and design. Something I've often interpreted as your intent.

[Gaz]

Translation: Our amps are designed to run the piss out of tubes.

I was going to say my guess is that it just eats tubes in it's stock form. Every cathode bias Dr. Z. redplates at idle, and biased over 100%. They stop redplating with signal, but still it seems crazy to design amp in mass this way. Anyone whose experimented a lot with cathode biasing will tell you the extra "tone" from biasing that hot is not worth all that money that could have otherwise been spent on pizza.

[Randall]

"Or use the recommended power tubes. You can always bump the cathode resistance later if they become unavailable. This way you'll be delivering to you customer a fix that is in keeping with the manufacturers intention and design."

I sure am going to use the recommended power tubes. But, I am going to go with 200 Ohm cathodes now in hopes that it will save another failure later. I don't want to hear that later it still ate a quad and died at a gig, and have that be interpreted as a failure on my part. I'm not sure an extra 20 ohms will make that much of a difference, but that is what I ordered.

[Dave H]

6V6GTA total dissipation rating is Plate 14W + Screen 2.2W = 16.2W

Remedy Plate voltage 409V, Cathode voltage 21.5V.

Total dissipation (plate + screen) is 27.8W = madness

Edit: LTspice sim results -

Cathode resistor 150R total diss. = 27.1W
Cathode resistor 200R total diss. = 23.3W
Cathode resistor 250R total diss. = 20.4W
Cathode resistor 300R total diss. = 18W

[Randall]

I just had a conversation with the customer saying I will do the upgrades, and explaining that I don't think there is anything wrong with his amp except the design, and I can't give my usual warranty if it blows up next week or next month. He repsonded by saying he's used it 3 -4 nights a week for the last 2 years, so he will take that chance.

I gotta say, this has really tweaked my sense of what I thought was supposed to be acceptable amp design and tube operation. What were they thinking, "hey let's run those tubes at almost twice their design parameters"? I must admit, and I did tell him this, it is difficult sending an amp out of my shop fully expecting it to blow up again.