Using a malfunctioning Ampeg B15 direct?

[boroman]

If it's a post 64 B15N he could use 5U3GB instead of 5AR4/GZ34 to tame those voltages. Pretty sure it would work.

Anyway, I was recording like that with SB12 (without power tubes), and measuring all voltages it was really less than 10% voltage raise, would not worry about that.

By the way -
Seeing lots of (same era) B15Ns with so much different plate voltages (stock), makes me wonder if those PT were wired like +/-20% tolerance...I take one and it's 430V, take another one with same tubes - 505V... Quite a difference!

 

[beans-on-toast]

It's the first time I hear about it... Can you explain?

As far as I remember between 63-75 they all had same standby protection. I don't remember super early ones. I had two 61s but really don't remember.. The last years (76-) were equipped with 1/4" so no protection at all. I don't remember schematic, but I'm pretty sure I did this "cut" mod with dozens of 63-75 B15 amps and it was same style.

The early designs looped the B+ return through the four conductor speaker cable. There was a loopback at the speaker cabinet. If the speaker cab was not connected B+ was disabled and the amp wouldn’t come out of standby. Regulatory deemed this design dangerous. The revised circuit would disable the phase inverter using a low voltage in the speaker cable. An example of the revised circuit is shown here in the ‘65 B-15NF.

https://www.vintage-blue.com/assets/b15nf_schematic_4_65.pdf

Some of the later revisions that used the 1/4” speaker jack were protected. They used a 250 ohm 10W resistor connected from (ofter) the highest output transformer impedance tap to ground. This hopefully acts as a dummy load long enough for the player to realize that there’s an issue and shut the amp down. The 250R resistor doesn’t affect the output impedance because a low resistance in parallel with a high resistance is about the same as the low resistance.

 

[B-string]

Will be interesting to see what the shop finds. I have seen an output tube take out the cathode cermet resistor, this "kills" the speaker output (without damage to an output transformer). You may find less trouble than you have guessed.

 

[JohnMarkPainter]

I use a Peavey Raxx a bunch too