JTM45 Build with 6P3P output tubes and GZ34

[dai h.]

FWIW here's a couple of diagrams I have from Brian Wallace (Marstran/Wallace Amplification) and SDM (he designed the efx. loop used in Metropoulos amps).



p.s. if you have a standby maybe check the bias voltage and range before you apply B+

 

[StarTrucker]

yes your schemo is fine except for the values. If it helps you to visualize what's happening, check the DC on the grid as you make your adjustments. More negative DC voltage on the grid means less plate current which is a colder bias. Red plating means there is not enough negative DC on the grids. Adjust the resistors in your bias supply until you get in a healthy range where 25k on your pots is about the right voltage on the grids for proper bias.

Shrink the 180k to allow more voltage into the circuit so you can put it on the grids of your output tubes. El34s like about -40ish volts with the big B+ on the originals. Your number will be a little less if you have a lower B+ than stock.

If you don't do the math, you can turn your amp off, set your bias pots to 25k, and swap your 180k for something smaller, fire it up, and see what your negative DC is after the 220k resistors. Get it to about -40v and then dial in the exact number on each tube with the 25k pot.

Pick a power supply voltage first though because the JTM45 uses a tap straight off the AC secondary to get the original voltage for your bias supply. If you get your bias resistors right then change your transformer it will be all out of whack again.

Uncle Doug on YouTube is a great resource for tube amp theory.

 

[Spencerleehorton]

So if I lower the 180k to about 150k this would be better you guys think?
And keep the 56k and 50k trimmers or change the 50k trimmers to 25k?

 

[Spencerleehorton]

That first picture drops the 180k to 150k and changes the 50k trimmers to 100k, so I’ll try that first as that is a little change

 

[StarTrucker]

Trimmer size doesn't really matter all that much, but yes going to 100k will increase the bottom leg of your voltage divider (4k7 + pot) and give you more voltage on the wiper. Harder to dial in correct bias on large value pots though because every little move you make on the adjustment screw is a big change in resistance and voltage on the grid.

 

[merlin]

Measure the voltage across the first cap, it should be at least -40V to -50V. Reduce the 180k (your drawing shows 220k?) until you get at least this much voltage (I suspect you need about 100k). 50k trimmers are fine.

 

[Spencerleehorton]

what voltage should be coming into the 180k resistor before the diode?

 

[Spencerleehorton]

I e reduced the 180k to 100k and get -77.8v at the diode
When I test the 1ohm resistors on pin 1 and 8 I get 250ma now!!!???

 

[Spencerleehorton]

Right, nearly success!! The trimmers need wiper and leg connected together rather than all individual!!
I’m able to dial down to 70mA now so do I need to change the 100k up or down now?
I will try putting in a 150k

 

[Spencerleehorton]

68k seems to do it!! Phew
Sounds amazing now, really savage fully dimed!!! See ya later eardrums!!!

 

[Spencerleehorton]

Now to work out the dissipation, good old uncle Doug!!

 

[Spencerleehorton]

Uncle Doug’s method

 

[Spencerleehorton]

Thank you everyone, hopefully I’ve sorted it now.

 

[Spencerleehorton]

leaving this here incase i forget how i did it or may help some other poor soul!!!

 

[StarTrucker]

This is a good resource for finding your ideal plate current for most tubes. I picked al EL34 here

https://robrobinette.com/Tube_Bias_Calculator.htm

Class AB Fixed Bias row at 70% is what you're looking for


This is what it's doing :
you're looking for current in amps

(Current in amps) * (B+ Voltage) = (Dissipation )

EL34 dissipation is 25 watts
Shoot for 70 percent of total dissipation when biasing power tubes in guitar amps

(Current in amps) * (414v B+) = (25 watts) *.7

(Current in amps) * (414v B+) = 17.5

17.5/414 = .0423

Current in amps = 42.3mA

Same procedure as what you did with Uncle Doug, but a different way of looking at the math.

25 watts of power is how much electrical power the plate can handle before it burns up because of heat. With more voltage on the plate, it has to flow less current in order to be safe inside. This is why you have to adjust bias with different plate voltages. 70% lets it run for long periods of time without pushing the limit of how heat the plate can dissipate.


Glad you got it sorted out. Enjoy your new amp! I'm sure your neighbors will too

 

[Spencerleehorton]

Still got to put in the PPIMV and the fx loop!!!

 

[Spencerleehorton]

What I have noticed, on the half output setting (triode?) when I turn volumes up above half and play a note there is a low end oscillation? Is this the mismatch OT causing this?

 

[StarTrucker]

Check wiring to the triode switch. Doubt it's the OT if you have correct voltage in both modes. Check bias in both modes. Something wrong with the screen wiring perhaps?

 

[Spencerleehorton]

Bias in low mode is a little more 45ma compared to 40ma in hi mode.
I’ll check the wiring but it’s a bass rumble oscillator which shakes the chassis!
Really strange?
For the PPIMV will it work with the dual bias?

 

[StarTrucker]

send schemo of the whole amp. Bass rumble like a hum?