Changing bias circuit in JTM-45 to accommodate EL34s

[dehughes]

Hello all,

I have a Germino Classic 45 (JTM-45 clone-sh) and I really like how it sounds with EL34s. Unfortunately, I need to find high idle current EL34s in order to have them bias up in my amp, and that's proving somewhat hard to do. So, I'm contemplating changing the bias circuit to allow me to use EL34s and KT66s/5881s/6L6s.

The bias circuit uses a bias pot (50k) to adjust the idle current. Now, in order to change the sweep to allow me to bias the EL34s hotter, what resistor would I need to change? I assume I'd need to lower the resistor to the bias pot, yes?

I'm a cathode biased guy, so this is somewhat new to me and I don't want to screw it up.

Thanks.

[martin manning]

What does the bias network look like starting from the rectifier? For example, a Marshall JTM 45 RI schematic shows: Rectfier/HT, 220k, diode, 15k, 56k, 22k trimmer, ground. If you can't get enough idle current maybe there is a resistor between your trimmer and ground? If so that's the one you want to reduce.

[Jana]

If it's the standard marshall bias circuit, I would change the resistor before the diode (the one coming from the HV AC). Adjust the value of this resistor so that with the trimmer in approximately in the middle of its sweep and with a typical set of EL'34's you are close on your bias. That way you should have a decent range of adjustment either way (hot or cold) for various tubes.

[plexitone]

You can either increase the resistor before the diode or decrease the resistor that goes to the bias trim. Most people change the latter when adjusting for different tubes.

[martin manning]

If it's the standard marshall bias circuit...

Evidently not, because if the trimmer went to ground like the standard circuit, there would be no problem getting the idle current up. There are several ways to get there, but it'd be best to find out what's in there before making a recommendation.

[Jana]

That's why I had the "If."

[dehughes]

What does the bias network look like starting from the rectifier? For example, a Marshall JTM 45 RI schematic shows: Rectfier/HT, 220k, diode, 15k, 56k, 22k trimmer, ground. If you can't get enough idle current maybe there is a resistor between your trimmer and ground? If so that's the one you want to reduce.

Yup, that's basically it, except the trimmer is 50k. All else is just that. There is no resistor between the trimmer and ground.

So I'd either increase the 220k OR reduce the 56k resistor, yes?

[tubeswell]

And remember that for the EL34 pinout, Pin 1 should be connected to Pin 8 on the socket(s)

[ToneMerc]

So I'd either increase the 220k OR reduce the 56k resistor, yes?

Well, you could decrease the 220K or the 56K or both.

You just need to convert your circuit to JTM50(EL34, 3.4K OT) bias specs. I'm going to assume that you have the high voltage (460V plate) "classic" vs the low voltage(410V) model which is relatively new.

The stock JTM50 bias topology is: 220K>diode>15K>47K>27K trimmer>ground. Thus, it leaves possibly 74K between ground the 15K resistor.

To get your bias circuit closer to EL34 operating range, you can try this.

220K>diode>15K>27K>50K trimmer>ground or 180K>diode>15K>56K>50K trimmer>ground

FWIW, here's how I setup my JTM45(440V) bias circuit which biases KT66 to 6V6.

200K>diode>15K>56K>20K chassis pot>25K trimmer.

I have 67 "Blackflag" JTM50 in queue, just waiting on a period correct steel chassis and I'm going to set it up the bias up the same way.

220K>diode>15K>33K>20K chassis pot>25K trimmer>ground.

hopefully this helps

TM

[dehughes]

That helps a ton, ToneMerc.

However, I do have the newer, low-voltage PT, so my plate voltage is around 420vdc with EL34s biased up at 31ma, given a 5AR4 rectifier. With a solid state plug, I can get about 37ma, but again, that's with the trimmer maxed, and with the highest-idle-current EL34s I could get (JJ EL34s from Eurotubes).

[vibratoking]

I have 67 "Blackflag" JTM50 in queue, just waiting on a period correct steel chassis and I'm going to set it up the bias up the same way.

That sounds like an interesting build. May I ask where you are getting the chassis?

[ToneMerc]

So you want to get about 31mA with the SS rect then, correct? I would change the 56K to maybe a 33K or 39K. Now you could most likely fine a value that will allow you toggle between both tube types if you want to experiment.

TM

[Andy Le Blanc]

Put a couple extra cathode resistors on the power tubes.

you'll get a few more volts bias on the tubes, might not have to frig with
rest of the adjustable bias circuit, and degree of extra safety with some cathode bias.

Hold on... you want it the other way? less bias voltage more tot. current?

[dehughes]

So you want to get about 31mA with the SS rect then, correct? I would change the 56K to maybe a 33K or 39K. Now you could most likely fine a value that will allow you toggle between both tube types if you want to experiment.

TM

What if I just replaced the 56k resistor/50k trimmer with a 100k trimmer and called it good? Wouldn't that give me the fullest range, covering both KT66s and EL34s?

[paulster]

You probably want to change the bias pot to 100K to give you more sweep range, and then the resistor before the diode sets the extent of that sweep so you might want to change this too to get a good usable range for the tubes you want to use.