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

[bvayling]

Hi all,

I bought this kit from Graydon 5 years ago and just now finished it! Sounds great and looks great in the cream tolex Swanson cabinet I commissioned Jeff to build at the same time I bought the GDS kit.
Many thanks to Graydon for all the gracious help on an old sale, it makes a big positive impression on this customer! Thanks also to you on the forum for help with the grounding. It's the only "non standard" thing on the amp. Btw, it's quiet as a mouse (not the signal!), no noise, hum, etc.

I posted a photo of the final check voltage chart on the album page. (I think I just found a way to put it in the thread here, we'll see.)
Please let me know if any of you see anything there that might be a concern.

Cheers all, I've got to go crank this rig up!

Bernard

18 W Trem final voltage check web 1.jpeg

[colossal]

Very well done! Your voltages look pretty much spot on too.

[JMPGuitars]

Please let me know if any of you see anything there that might be a concern.

Why aren't your power tube cathode voltages the same?

[bvayling]

Pardon me, they actually are. Both 11.4
I was using Graydon's chart in the assembly manual while I was taking measurements and mistakenly used his value for one of the tubes while I was copying it down.
Thanks!

[JMPGuitars]

That makes more sense.

11.4 is ok, but it's very slightly low. Ideally you want to be between 12 to 12.5VDC. If you're happy with the sound, you're probably fine. A small increase in the cathode resistor would correct this if ever you need to.

Congrats on getting it up and running! I hope you'll post a demo in the Hear It! section.

Thanks,
Josh

[bvayling]

Thanks for the feedback, but I have nothing to compare it to. The amp sounds fine but there is always room for improvement.

What would you say one would gain with a 12.5 cathode value, and what kind of resistor value would get there?

Thanks.

Bernard

[JMPGuitars]

12VDC is the goal, 12.5VDC is acceptable. The idea is biasing the tubes into a more idea spot. 11.4V isn't terribly far off, but it's likely pushing the limit of the tube. You might notice less noise, and a better distortion tone.

But here's some math including assumptions: 11.4VDC cathode voltage across a 150 ohm resistor is 11.7watts per tube, which is probably safe, but very close to the 12W limit of the EL84 tubes. 12 to 12.5VDC across a 180 ohm cathode resistor is between 10.3 to 10.7 watts per tube, which is a much safer number. OTOH if you're already using a 180 ohm resistor measuring 11.4VDC, you're fine. That would put you at 9.8W per tube.

The way to adjust the cathode voltage is to change the value of the cathode resistor. I personally find the best results at 180 ohms, but YMMV.

[bvayling]

Thanks Josh. I'll have to go brush up on the math and the theory. It's been a while.
Bernard

[JMPGuitars]

Thanks Josh. I'll have to go brush up on the math and the theory. It's been a while.
Bernard

Or be lazy and use an online bias calculator like I did.

[bvayling]

The stock R20 value of 125 ohm is installed right now. I’m ordering higher values from Mouser.

Another couple of things I’ve noticed on this amp:

The tone controls on both channels seem to have pretty limited variation in the tone. Is this normal?

[JMPGuitars]

The stock R20 value of 125 ohm is installed right now. I’m ordering higher values from Mouser.

Another couple of things I’ve noticed on this amp:

The tone controls on both channels seem to have pretty limited variation in the tone. Is this normal?

Yikes. 11.4VDC across a 125 ohm cathode resistor is putting the tubes at 14 watts each! Yeah, you definitely need to go higher.

I would check the wiring and verify the correct values for the tone knobs. The tone knob on the first channel should have a pretty serious effect on the tone and drive.

[bvayling]

I just got a box from Mouser with some larger value cathode resistors. Before I change anything, I'd like to review the procedure and make sure my reasoning is correct. I did a lot of searching and looking at the build, layout, etc.

Using Rob Robinette's calculator in the first section, I plugged in EL 84 with my plate voltage of 337VDC, read directly from pin 7 of the power tube. After pushing the calculate button this yielded a 100% safe dissipation of 35.6ma.

I then went to the next section "Tube Dissipation Using Cathode Resistor Voltage Drop", entering in 2 for the number of tubes, 11.4VDC for the voltage across the cathode resistor (this value is from pin 3 and I'm not sure this is exactly what is called for, however I reasoned that it is the voltage that is coming from that resistor). I then entered the 125 ohm value for the resistor. After pushing calculate, the results yielded plate dissipation per tube of 14.5 watts, which is 120.8 % of the max.

Do I have the methodology correct? At least in the ballpark? I found other stuff in the search about accounting for the actual current drop in the resistor and screen grid current but wasn't really sure if I was doing that correctly. I did wind up with numbers not too far off (cathode current of 36.1ma and W=12.13) but am unsure of the methodology.

Anyway, thanks for being around and having a look over my shoulder!

Cheers,

Bernard

[JMPGuitars]

The amp is cathode biased, so that is the correct method to figure out tube dissipation. I think you still have to do the first part either way.

I usually have good results with 180 ohms.

[bvayling]

Just installed 160 ohm cathode resistor to replace the stock 125.
Here is the new voltage chart:

18W Combo voltage chart 160 ohm.pdf

[bvayling]

Let's try a photo

IMG_0709.JPG

[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