Help with bias voltage, it's too low

[elegost]

Hi folks,

I am making a Mojotone JCM 800 2204 kit. This is my first amp, although i've done some pedals, and other electronic stuff before.

I have it completed and am trying to confirm everything is kosher before i put tubes in and crank it up.

From what i have read the bias voltage on pin 5 of V4 and V5 should be about negative 40-50vdc.
The problem is i'm getting -17.7 vdc

I am attaching a series of pictures that as far as I can tell show that I have 120 vac coming out of the wall, 360vac coming out of the PT, but only -23 vdc after the diode that i believe is supposed to rectify the dc bias voltage. and then it's -17.7vdc by the time it gets to pin 5. I should be seeing a much higher voltage after the diode, right?


Any help or ideas about what else to check would be really really appreciated. I'm a bit frustrated right now

[Leo_Gnardo]

Bias filter caps in the correct polarity? Yes I've put 'em in back'ards, then they don't charge up too well.

[elegost]

They seem to be in right as far as i can tell, attached a couple pix below. do they look right to you? Thanks so much for the help!

[sluckey]

Replace that 220K with a 100K. Does that get closer?

[martin manning]

Did you move the bias supply tap to the other side of the standby switch as marked on the schematic? That's not a good idea... You want the bias voltage stabilized before turning on the HV DC. Are you measuring with the standby switched to play?

[Leo_Gnardo]

They seem to be in right as far as i can tell, attached a couple pix below. do they look right to you? Thanks so much for the help!

Yup that looks OK. Photos didn't come up first time I had a blink at your post. Martin & sluckey's advice next, yes make sure you have bias voltage before hi voltage is switched on, and reduce the value of that 220K resistor. You could clip parallel resistors onto it until bias voltage is in a good spot, then swap out for a single R or solder on the parallel, either way.

[elegost]

Did you move the bias supply tap to the other side of the standby switch as marked on the schematic?

Hah! yes, I was wondering about that. The layout and the schematic did not match there from Mojotone, so I went with the layout, which had the bias supply tap after the standby switch. Not surprised that's not a good idea it seemd weird to me too. I'll move that to before the standby switch after i get this bias voltage thing worked out.

I am definitely measuring with standby switched to play

[elegost]

Replace that 220K with a 100K. Does that get closer?

reduce the value of that 220K resistor. You could clip parallel resistors onto it until bias voltage is in a good spot

Awesome thanks guys i will try that!

[elegost]

Ok I brought the 220k resistor down to 73.5k and that brought the voltage up to a range of-35v to -42v, depending on the setting of the bias adjustment pot. Does that seem reasonable or should i go further?

[Leo_Gnardo]

I'm a little surprised you had to go as low as 73.5K but if that's what it takes - - - I expect that resistor or combination of R's turning out a bit of heat too. Looks like bias voltage is in the ballpark, dial it up to 42V and let's see how much bias current your output tubes draw.

[Reeltarded]

10k is short range jeees.

Assholes. lol

[elegost]

Hey here's a really dumb question.

when i set my dmm to the 600v ac setting, and put one lead on the chassis and the other on one leg of the hv leads coming out of the PT, as in the first screenshot i attached, i see 180v.

That's correct right? The HV side of the PT has a grounded center tap so between the two hv leads is 320 but between the ct and one side is 180?

[xtian]

Hey here's a really dumb question.

when i set my dmm to the 600v ac setting, and put one lead on the chassis and the other on one leg of the hv leads coming out of the PT, as in the first screenshot i attached, i see 180v.

That's correct right? The HV side of the PT has a grounded center tap so between the two hv leads is 320 but between the ct and one side is 180?

Yes, that's right, except your math. 180+180=360.

[sluckey]

That voltage should be a lot more than that! I think you have the primary of the PT wired wrong or if you're using the voltage selector shown on the schematic I think it's set for 220V.

Check the other voltages from the PT What do you have on each filament lead in respect to chassis? If it's also low them that really points to a primary side wiring error (or wrong switch setting).

This would also explain the bias voltage being so low.

[elegost]

Yes, that's right, except your math. 180+180=360.

Doh! Yup i'm an idiot the secondary on the PT is 360-0-360