5F11 voltage/amps issue...not able to bias

[ron vogel]

OK, after sorting out a problem with my meter I am able to see what is really going on with my build.

Did a 5F11, used a 300-0-300 120ma PT with a 5v4 rectifier. I am seeing 340V B+...which is good. I do not have the 50v bias tap on the recto though. Tapping off the primary like the 6G3 is getting me about -18v in the circuit. I drop the 10K resistor in the circuit down, and I can get as low as -24/-25v max. However by the time I get the resistor low enough, there is too much current and the resistor smokes.

Ugh.

So, in order to avoid getting a new PT my thought is to do a SS recto and raise B+ to lower the current? In tandem, should I also relax the dropping resistors?

In the 6G3 they use a 22K resistor in the filter for the bias circuit, where as the 5F11 uses a 56K. It's looking like I should do that also...I popped the 25/50 filter cap trying to get to the -31V.

I have other issues with the circuit too, but the overall voltage problem needs to be solved first.

I don't mind a higher current set-up on the power tubes to get it to work...but I should probably raise voltage to the preamp tubes to free up a little current for the rest of the circuit. In the 5F11 schematic the preamp tubes are run at pretty low voltage, I have run 12ax7's up to about 260-270V before they go sterile...so I have some room.


I hate doing all these changes right at the beginning, but with this PT it may be an uphill battle to get to the stock circuit specs.

Any help would be appreciated...I'm no pro as you can tell. Just a tinkerer...lol

[pdf64]

'Tapping off the primary like the 6G3 is getting me about -18v in the circuit. I drop the 10K resistor in the circuit down, and I can get as low as -24/-25v max. However by the time I get the resistor low enough, there is too much current and the resistor smokes'

The 6G3 http://www.webphix.com/schematic%20h..._6g3_schem.pdf has a potential divider taken from the main B+ winding, made up of a 100k dropper (with diode in series) then 22k to ground, for the bias supply.
To increase the output voltage for the bias, it would be better to raise the value of the 22k resistor, rather than reduce the value of the 100k.
I think that increasing the main VB+ by using silicon rectifier will exacerbate the condition caused by inadequate bias voltage.
Pete

[ron vogel]

Yeah, you're right. I have the 56K in there from the 5F11 circuit...not the 22K. Seemed the circuit would get flaky when I dropped below 30K where the 100K resistor is. I'd prefer to leave the circuit as close to stock as possible. I was misunderstanding the purpose of the 25uf/50v cap and 22K resistor. Maybe I'm OK. I really want to avoid the SS recto because of the inrush issues...hence why I'm using a 5V4 vs the 5Y3.

[ron vogel]

So, as I am understanding this...

I need to get the circuit to an acceptible range by adjusting BOTH the 100K resistor and the 22K resistor. Since I am running at -15ish volts with the 100K/56K...putting in the 22K will make me MORE positive. I have been reading enough on this to make me think it's just going to be a lot of trial and error...but I will definately need to drop the 100K down to say a 50ishK 2 watt, and raise the 22/56K to maybe a 10K pot + 56K resistor. I have also seen that many in my shoes need to up the 25uf cap to a 50-100uf cap. It makes a little more sense that I popped the cap. The 56K/25Uf combo is bleeding off the bias voltage (and increasing current) as I am trying to introduce more voltage/current by lowering the 100K resistor.

[g1]

Popping of the cap is due to it's voltage rating, not it's capacitance value. It popped because there was more than 50V across it, yet you say you couldn't get more than 30V. Something very wrong here. Is the diode ok? Check your AC versus DC voltage values across the cap.

Edit: make sure the cap is not backwards, quite a common error!

[ron vogel]

The cap is backwards...I can guarantee it. I was poking through my connections last weekend and saw the + on the cap was pointing towards ground and flipped it. I just looked at the schematic...and sure enough that's what happened.

I don't follow directions well.

Looks like I go back to the 100K range for the drop, and get a handful of 22K to 56K range resistors and a 10K pot.

Makes sense that no matter what I dropped the 100K to, it would shelf off to -24/-25 after I dropped past 40-50K.

[ron vogel]

22k is the ticket...b+ up to 388. Getting a weird osscilation though unless I turn the depth knob all the way. Trem doesnt work at all...debugging sucks

[Bruce / Mission Amps]

The cap is backwards...I can guarantee it. I was poking through my connections last weekend and saw the + on the cap was pointing towards ground and flipped it. I just looked at the schematic...and sure enough that's what happened.

I don't follow directions well.

Looks like I go back to the 100K range for the drop, and get a handful of 22K to 56K range resistors and a 10K pot.

Makes sense that no matter what I dropped the 100K to, it would shelf off to -24/-25 after I dropped past 40-50K.

The + end is supposed to be at ground... I think you understand that, right?
Add up your resistance values in the bias circuit after the bias cap and diode... then divide that number into the voltage. That will tell you how much current you are sending to ground through the bias voltage divider... then solve for power (I^2R) to see what wattage resistor(s) you should be using. Pretty simple math.

[ron vogel]

Yes, I understand...just takes me some poking around to get it right. Last night I made a trip to Fry's to get a new cap (They only had a 47uF 50v)...tried a few resistors, and the bias responded like it should. I didn't go as far as doing the math yet. I am getting back to it tonight, but I am putting in some 1ohm resistors off the cathode grounds, and redoing the heater wiring.

I have a strange oscilation in the amp that is affected by all the controls; I think it's something to do with the tremelo circuit. Amp sounds great when you get the knobs in the right spot, otherwise it sounds like a runaway helecopter. I suspect that is a grounding issue though. I also hear some fizziness in the 12ax7's with the volume up, which I think is either an impedance problem coming in, or too much voltage in the preamp.

The backwards cap was really pulling down the volume, B+ is up to 388 from 340.

[pdf64]

'I also hear some fizziness in the 12ax7's with the volume up, which I think is either an impedance problem coming in, or too much voltage in the preamp'
It may be due to ultra-sonic oscillation, possibly due to 'less than ideal' lead dress.
Any place there are 2 signal inversion and lots of gain (eg 2 sections of a 12AX7 cascaded) a propensity for oscillation will exist, which needs to be carefully mitigated.
Capacitive coupling between wiring and components of the circuits is the cause, so keep inputs and outputs well seperated.
Pete

[ron vogel]

I'm redoing the wiring tonight anyway...but I don't think that's it...I only have one input on the amp, and it's done wrong. I'm seeing 305V on the preamp tubes...I've had bad luck going higher than 275-280V on them in the past.

[ron vogel]

Ok...took the thing completely apart last night. 9 hours later...all new wiring. Tremolo works now...kinda. only works right at the full end of of the depth pot. At the end or beginning of any of the pots it does something funky. Still having issues getting the bias right. If I get the power tubes under 18 watts(!) It gets these crazy oscillations. The 250k depth pot went bad somehow...replaced it and same issues. Amp sounds really angry and buzzy at 18watts. I want to get it in the 10-12 watt range. The amp is much more buzzy with the tremelo Jack shorted...the issue might be in there? Seems to me that since it affects all the
knobs and the amp won't bias the trem is malfunctioning.

[Jazz P Bass]

What voltage is on the depth pot with the Tremelo off?
It should be the bias voltage.

[ron vogel]

Hopefully I can get back to this tomorrow. I have a feeling the dropping resistor needs to be a smaller value, and the bias resistor needs to be a larger value. I have a 100K resistor before the diode, and the amp flakes out if I raise the bias resistor higher than about 22K. Stock circuit calls for 56K.