First amp build! Acceptable voltages?

[mclemaudio]

I just finished my first amp build following the TMB Modern Classic layout (thanks JMP!). I mistakenly wired the input jacks backwards, but after realizing and fixing that, everything seems to be working! Now, the V1 plate voltages are 25-30V too high and the EL84 screen voltages are 15V too high. Is that cause for concern? And what would be the reason for that? I've only been able to test it thru a 15" bass speaker at apartment level so I haven't been able to really get a feel for the tone and response yet.
Thanks everyone for their contributions to this site, its been really fun!

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[JMPGuitars]

I just noticed, you need to replace the cathode resistor for the power tubes with a minimum of 5W. I didn't look close enough before, sorry.

What value cathode resistor did you use? If you used 180Ω, you should probably raise it up a little. I'd get a 180, 200, and 220 to be safe if you don't already have them. Using this calculator, you want to target about 85% dissipation: https://robrobinette.com/Tube_Bias_Calculator.htm

You also should share the bottom row of voltages missing from your chart. The two voltages you mentioned aren't that important if it sounds good. Power tube plate voltage needs to be higher than screen voltage, and it is.

Thanks,
Josh

[Crabman]

Your voltages in the power section look good. The schematic actually has the voltage at the B node and the screen backwards. It should be lower at the screen than at the filter node. Based on expected current flow, If the reservoir cap has 345 volts I would expect to see around 340 at the B node and 335’ish volts at the screens.

Voltages at your TMB tube are higher than expected. With a 27K dropping resistor at that final filter node, you would expect to see lower voltage than what you have.

A good starting point would be to measure the voltages at all the filter nodes A through D and post those here. Also make sure that all the values for resistors in the filter chain are correct and also the anode dropping resistors at the TMB tube are correct.

[JMPGuitars]

His B+ node voltages aren't posted. I imagine they start closer to 360 if he didn't use any zeners.

[mclemaudio]

Thanks guys!

Ah I see. A = 357.2V, B = 338.8V, C = 298.7V, D = 245.4V and the heaters are 3.24V.
The voltage at the cathode resistor (which is 180 ohm 5W) is 13.3V.

[JMPGuitars]

that's about what I expected. It's up to you- if you like how it sounds, you can skip the Zener diodes. If you want it to give you some dirt earlier, then I'd drop the B+ voltage about 10 to 12 volts. More info is available in my signature.

If you choose not to do drop the B+, I'd suggest increasing the cathode resistor value as I mentioned previously (you might need to do this either way). Remember, target dissipation is 85%.

Thanks,
Josh

[mclemaudio]

Am I doing something wrong on the bias calculator? It's saying dissipation is 66%, right? But based off my voltage readings it should be higher than 85%, no?

Screenshot 2026-05-18 at 8.17.47 AM.jpeg

[JMPGuitars]

Am I doing something wrong on the bias calculator? It's saying dissipation is 66%, right? But based off my voltage readings it should be higher than 85%, no?
Screenshot 2026-05-18 at 8.17.47 AM.jpeg

Pick EL84 as the tube type, not 18w. Your dissipation is closer to 100%.

[mclemaudio]

Oh duh, ok cool!
Thanks for the quick help. I'll go ahead and order some parts, I won't be able to get to my studio and really put the amp through its paces until this weekend. So i'll report back next week!

[Crabman]

Regarding elevated voltages at the TMB input tube.

The voltages you supplied from the filter nodes indicates that what you are observing at the TMB tube is what would be expected.

From a purely theoretical perspective:

You observe 299 volts at C and 245 volts at D. The 27K resistor between these 2 nodes has therefore dropped 54 volts. According to Ohms law, the current passing through the resistor is:

I = V/R
I = 54 V / 27000 ohm
I = 0.002 A (2 mA)

This is exactly what you would expect since around 1 milliamp of current will pass through each half of the 12AX7 when operated under “normal” conditions (i.e. 300 V, 100 K anode resistor and center biased).

In this case you measured a cathode voltage of 1.26 volts on pin 3. This translates to about 0.82 mA when you read it off a corresponding 12AX7 anode load line graph (100 K and 245 volts). Using Ohms law to calculate the voltage drop across the anode resistor:

V = IR
V = 0.00082 A x 100000 ohm
V = 82 volts

Expected anode voltage is therefore 245 - 82 = 163 volts.
You measured 162 volts on pin 1 …. looks good to me.

On pin 8 you measured a cathode voltage of 0.78 volts. This translates to about 1.1 mA on the load line graph:

V = 0.0011 A x 100000 ohm
V = 110 volts

Expected voltage on pin 6 is therefore 245–110 volts = 135 volts.

You measured 134. This look to be right on the money to me.

There may a number of reasons why the expected values are a tad lower. These calculation just confirm that from a mathematical standpoint things make good sense.

We can go through the Power dissipation calculations for your EL84s if you want. Like Josh said you are up around 97.5 % on the Robinette calculator. I calculate it out to be just a bit lower.