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

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

Post by 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
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by colossal »

Very well done! Your voltages look pretty much spot on too.
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by JMPGuitars »

bvayling wrote:
Thu 03/26/20 5:16 pm
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?
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by 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!
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by 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
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by 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
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by 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.
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by bvayling »

Thanks Josh. I'll have to go brush up on the math and the theory. It's been a while.
Bernard
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by JMPGuitars »

bvayling wrote:
Fri 03/27/20 8:45 pm
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. ;)
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by 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?
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by JMPGuitars »

bvayling wrote:
Sat 03/28/20 10:40 am
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.
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by 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
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by 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.
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by 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
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Re: 5 year old NOS Classic 18W Trem Combo build, DONE!

Post by bvayling »

Let's try a photo
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