Bugs in a scratch built JTM45

[big_teee]

I think your amp bias is layed out for EL34s.
It looks like the resistor value to ground needs to be bigger.
Instead of 15k to ground from the 50k pot, it probably could be double that.
Others like Enzo can verify that.
You are shunting to much of the minus bias voltage to ground.
T

[mort]

How much of a bias difference should there be between 66's and EL34's?

I have a pair of EL34's... I'll try those and see if the limiter glows. Oh btw, when I added the 66's to the recto, that's when the bulb lit up on the limiter.

edit: still glows with el34's

[big_teee]

I would try to get the bias voltage higher with the tubes out.
You could try to change the 15k to ground to a 56k to ground.
Then measure to see if the voltage will come up.
Are you using the 50k linear bias pot like the layout?
Most of the marshall schematics that do the either or with EL34s, and KT66s, they double the resistor for KT66s.
My JCM800 has a 50k pot and a 56k Resistor in series to ground.
It will bias EL34s, or 6L6s, which is more in line with the KT66s.
Once you get the bias voltage higher then you can limit the Output tube current, which will probably cure the fuse blowing.
T

[mort]

Yep, 50k linear. I just tried a 23k and got a couple more volts out of it, -39.3. I'll try a 56k real quick

[mort]

Ok, now we're up at -43.9. Should I go to 100k?

[big_teee]

Ok, now we're up at -43.9. Should I go to 100k?

Yup, Now your cooking!

[mort]

with el34's back in the limiter glowed brightly for a couple seconds and then settled to dimly lit. It's a 75W bulb and would have dimly lit a closet. There's still a problem, right?

[mort]

up to -47.3 w/ 100k

Still have dim bulb and still hums loud with v3 installed

[MWJB]

There is no distinction between negative voltage requirements for EL34 & 6L6. Either power tube type wants to see 11% of the power tube plate voltage as -dc at the junction of the 220K grid loads (as a start point).

You appear to be missing a bias electrolytic cap right after the diode (should be a cap to ground at each end of the 10K following the diode), your bias rectifier is not rectifying corrrectly. 50VAC rectified should give you -70vdc, then the bias voltage divider with a 10K dropper & 65K pot & resistor in series should give -60vdc at the junction of the 220K power tube grid resistors. If you just keep making the load resistor bigger, you are not ever going to realise the max potential dc and your bias pot will have no sweep.

Did you disconnect the NFB loop from the speaker out? See if this cures the loud hum? If it does, swap the power tube plate wires over.

[mort]

There is no distinction between negative voltage requirements for EL34 & 6L6. Either power tube type wants to see 11% of the power tube plate voltage as -dc at the junction of the 220K grid loads (as a start point).

You appear to be missing a bias electrolytic cap right after the diode (should be a cap to ground at each end of the 10K following the diode), your bias rectifier is not rectifying corrrectly. 50VAC rectified should give you -70vdc, then the bias voltage divider with a 10K dropper & 65K pot & resistor in series should give -60vdc at the junction of the 220K power tube grid resistors. If you just keep making the load resistor bigger, you are not ever going to realise the max potential dc and your bias pot will have no sweep.

Did you disconnect the NFB loop from the speaker out? See if this cures the loud hum? If it does, swap the power tube plate wires over.

The Marshall schematic shows a cap on either side, but the weber layout does not. I had followed the weber bias diagram, but I can put another cap on it.

What is the NFB? I only have two wires to the speaker out.


Will have to pick up in this later this evening. Thanks ya'll.

[MWJB]

NFB is 'negative feedback'. From the presence pot follow the yellow wire from the RH tab that runs to the 27K resistor that is vertical on the board, to the left of the phase inverter section. This 27K is your "NFB dropper" resistor (presence pot serves as the "NFB load" in your amp). There is a wire at the other end of this resistor that connects to the hot speaker terminal. Remove this wire from the speaker tap, or if easier lift one end of the 27K resistor, keeping the NFB wires in place. Do not leave the wire connected to the speaker tap & the other end of the wire just flapping around, as this will short the OT secondary if it touches ground/chassis.

[Jazz P Bass]

The Negative Feedback comes off of the output transformer secondary.
From where it says 'to 16 ohm tap' to the 27K resistor to the tail of the phase inverter.
Usually if the output transformer primary is miswired the amp will squeal, not hum.
It's worth a try to disconnect it.
Have you monitored the bias voltage for a steady Vdc.
More important, no Volts AC.

[mort]

OK, I added an 8uF cap between the 10K resistor and diode and now I have -54.2 vdc going to pin 5 of the power tubes. There is a 100uF at the opposite end of the 10K... should these caps be the same value or does it not matter?

Next, I lifted the NFB resistor and voila, no BBBBRRRRRR. It plays and makes "decent" guitar sound, with little hum at moderate volume. The OT primary is just red, brown, and blue. So I'm swapping the brown and blue with each other? I wonder why that would make a difference... I wired it like the layout shows....

And I'm still getting the glowing bulb with power tubes installed. It's a little bit dim but gets bright when I play guitar through it. This means that if I took the limiter off I would still be blowing fuses?

[Jazz P Bass]

You cannot always count on the OT wiring colors
What you had was 'positive feedback'.
The lamp will glow with tubes in.
It is probably idling at 75 watts from the mains.

[mort]

Well it's a wrap. It's off of the limiter and sounding great. Looks like all that grounding paid off too. It only makes any hum on the treble channel, hi input, with the volume up... which I'm guessing is to be expected. Sounds nice and full with some sweet 'trilly' overtones when dialed in right. I'm quite pleased, and very grateful for the help you guys have given

So to sum up the problems:

Bleeder resistor value raised from 15k to 100k
8uF cap added at diode
Backwards cap corrected on V1 P3
Soldered missed connection at diode
Swapped OT secondary leads


Thank you gents!