5 year old NOS Classic 18W Trem Combo build, DONE!

[bvayling]

Ok, this is crazy. I can't seem to be able to post a pdf or a photo that lines up properly. Let's try the old fashioned way.

Voltages with 160 ohm cathode resistor replacing the stock 125, amp powered up with pots at 0:

Power tubes: 12VDC at pin 3
347VDC at pin 7
337VDC at pin 9

I used Rob Robinette's bias calculator:

First calc tube max design dissipation:
for EL84 and 335VDC
Plate at pin 7 to cathode at pin 3 : 347-12= 335VDC
100% Max Safe Dissipation: 35.8ma

Second calc tube dissipation using Cathode Resistor Voltage Drop:
# of tubes: 2
Voltage across Cathode Resistor: 12
Cathode resistor: 160 ohm

Results: Plate Dissipation per tube: 11.9 Watts, or 99.2% of max.

I can't really tell much of a difference in the sound, but there was a marked increase in the range of the tone controls on both channels. They both actually work now. The only thing that changed was the cathode resistor.

Was shooting for 12V cathode voltage and got it.
Should I just leave well enough alone or go for the 180 ohm resistor?
What if the cathode voltage goes over 12.5V?
Thanks.
Bernard

[JMPGuitars]

I'd go for the 180 and remeasure. You're technically okay where you are, but a shift in wall voltage could put you over.

[bvayling]

Thanks Josh, I'll put that in today and see where it goes.
bva

[bvayling]

Josh, how do these numbers look. I think you are right about the 180 ohm cathode resistor.

18W Combo voltage chart 180 ohm.pdf

[bvayling]

Installed 180 ohm cathode resistor.
Voltages:
Standby On: 360V
B+ 308

V1
pin 1: 154V
pin 3: 1.25V
pin 6: 154V
pin 8: 1.25

V2
pin1: 236V
pin2: 60V
pin3: 85V
pin6: 229V
pin7: 59V
pin8: 85V

V4
pin 3: 12.5V
pin 7: 353V
pin 9: 343V

V5
pin 3: 12.5V
pin 7: 348V
pin 9: 343V

V6
pin1: 303V
pin3: 360V
pin7: 303V

bias data follows

[bvayling]

from Rob Robinette's site:

1./ EL84 12 W
Plate to cathode voltage:
Pin 7 - Pin 3
353-12.5= 340.5VDC
Calculate
100% Max safe 35.2ma

2./ 2 tubes- 12.5V across R20, R20=180 ohms
Calculate
Plate current per tube: 32.8ma
Plate dissipation per tube: 11.2W (93.3%)

I have a 200 ohm, should I try that?
That would definitely go over 12.5 at pin3 of power tubes

[JMPGuitars]

I think you're fine where you are. Your B+ is actually 360VDC, which is a little high but not so bad you HAVE to change it. If you decided to drop that, you may want to recheck the bias. If not, play and enjoy. If you're trying to nail in the 18W sound, then your B+ (pin 3 of the EZ81) should be around 345VDC.

[bvayling]

Thanks Josh. The amp sounds really good. As far as nailing the 18W sound, I’m not sure I even know what that is.

I’ve slowed the trem down with a 0.022 cap in C8 so I like that a lot. Tonight I am going to replace the PI anode resistors with carbon comp same value and see if I can hear the difference.

I think I’ll stop there and play the thing. I’ll keep the 345 B+ figure in mind.

Cheers,

Bernard

[bvayling]

So actually, I was closer to that B+ with the 160 ohm cathode resistor (354V)....

hmmm...that must be what the 12V at the cathode is all about?

[JMPGuitars]

So actually, I was closer to that B+ with the 160 ohm cathode resistor (354V)....

hmmm...that must be what the 12V at the cathode is all about?

A balance eventually happens when both are tweaked. Even though that B+ was closer to what is supposedly ideal, the tube dissipation was too high, so obviously no good.

[bvayling]

I might as well keep going to get it right.

According to current thought after some searching, it appears that 3 x 5V 5W zeners on the B+ line in series after the capcan on the way to the board will get me in the 345V ballpark.

Do I have that right?

Thanks.

[zaphod_phil]

Yes, that looks perfect.

[bvayling]

Thanks ZP!