Bias cap value?

[daz]

When i built the amp i'm messing with lately (my 1st build) it was a 18 watt. I later radically changed it to a sort of mini JCM 800 and converted it to 6V6. I can't remember where i got the bias circuit from but it doesn't look like any i have been able to find in a 6V6 amp. One thing i'm wondering about is the 22uf bias cap. Should i be using a larger value? And aside from that, as long as the circuit allows me the proper bias is that all that matters ?

[R.G.]

Size isn't all that matters. What matters is low noise and ripple. Any noise or ripple on the bias voltage is fed directly into the output stage. If it's a push-pull stage, this ought to cancel out, but that only happens if everything is perfect.

So a 22uF may be good enough, may not. It depends on where you're getting your raw AC voltage for rectifying to bias, whether you half-wave or full-wave rectify to bias (most are half wave), and how much total loading on the bias filter cap or caps you have.

In the end, yes, a quiet bias voltage is all that matters. How you get there is a smaller consideration.

[pdf64]

A drawback of big caps in a bias supply is that it may result in long time constants, taking too long to charge up, potentially leaving the power tubes under biased for a while at power up.

[daz]

Size isn't all that matters. What matters is low noise and ripple. Any noise or ripple on the bias voltage is fed directly into the output stage. If it's a push-pull stage, this ought to cancel out, but that only happens if everything is perfect.

So a 22uF may be good enough, may not. It depends on where you're getting your raw AC voltage for rectifying to bias, whether you half-wave or full-wave rectify to bias (most are half wave), and how much total loading on the bias filter cap or caps you have.

In the end, yes, a quiet bias voltage is all that matters. How you get there is a smaller consideration.

So no reason then not to go to a larger cap, say a 100uf ? As to the circuit i don't have a schematic but heres a very professional drawing i just did.

[daz]

A drawback of big caps in a bias supply is that it may result in long time constants, taking too long to charge up, potentially leaving the power tubes under biased for a while at power up.

Sorry, you posted about when i did and i didn't see your explanation. So is the 22uf good then?

[pdf64]

The time constant thing becomes an issue when the bias is derived from the HT winding, as per a JTM45, as the feed / dropper resistor is >100k.
I think that 100uF would be ok in regard of the feed resistor in your amp, tau of ~3 seconds RC pad corner frequency upper and lower cutoff frequency calculation filter calculate time constant tau RC voltage power calculator capacitance resistance - sengpielaudio Sengpiel Berlin
However, I'm not keen on reservoir etc caps being charged via a pot wiper.
I would move the rectifier and cap to the other (input) side of the pot.

[daz]

The time constant thing becomes an issue when the bias is derived from the HT winding, as per a JTM45, as the feed / dropper resistor is >100k.
I think that 100uF would be ok in regard of the feed resistor in your amp, tau of ~3 seconds RC pad corner frequency upper and lower cutoff frequency calculation filter calculate time constant tau RC voltage power calculator capacitance resistance - sengpielaudio Sengpiel Berlin
However, I'm not keen on reservoir etc caps being charged via a pot wiper.
I would move the rectifier and cap to the other (input) side of the pot.

Will do, (wiper swap) but what about the 100k u mentioned? Should i change that 15k to 100k?

[Dave H]

Will do, (wiper swap) but what about the 100k u mentioned? Should i change that 15k to 100k?

No don't do that if 15k gives the correct voltage. 100k would be used when the bias is derived from a higher voltage like the full HT winding. I wouldn't worry about the 22u. If it doesn't hum then it's not a problem.

[Chuck H]

Daz,

Bias supplies perform voltage division and work on a ratio throughout the circuit and not only on the pot. That you thought it may be possible to replace the 15k resistor with a 100k demonstrates that you didn't understand this. How, then, you came to a circuit that provides you with a working bias voltage I can't imagine.

We still don't know the voltage source for this circuit. Are you working with a bias tap or is this circuit connected to the HV winding?

Please tell us your bias current. If you don't know how to determine your bias current then I will assume, based on prior experience, that you have simply determined that the amp "works" with this bias supply and have taken it no further than to "listen" to the amp for adjusting the bias. If this is the case the amp may (is likely) incorrectly biased.

It is a "functional" but crappy design. That is, it works just fine with no tonal detriment, but there are some liability issues WRT failure mode that it ignores. I'm not sure how you intend to "wiper swap" in the design you have and maintain circuit function. Pots are notoriously unreliable. Bias supplies are VERY important to the well being of your tubes and potentially your output transformer. Ergo, having the bias rely on a pot is bad. Having the bias rely on a pot wiper is worse. The adjustability should be arranged in the circuit so that the pot wiper cannot cause a "no bias" condition. So...

Provided you actually have the correct bias voltage range and using most of the parts you have, rearrange your circuit like this:



You'll need a 22k resistor and you'll get rid of the 27k. This should give you about the same bias voltage range you have now.

Please answer to the above issues in bold print before proceeding.

[daz]

Thanks for that, i'll rearrange it like that. And no, i don't understand it at all so i took that from some other amp i guess. Although i DID expect that if i used 100k in place of the 15 i'd have to change other values. I just didn't know if theres some reason that should be 100k. It's been 10-15 years since i changed it to fixed bias so i can't recall where i got it. Might have been someones suggestion who's almost as stupid as me.
I measure it using 1R resistors from cathode to ground and at that point i set it to 17mV. The source is the HV winding and i tap off one side before the diodes that rectify the HV rail. I see by your drawing now what PDF64 meant by the wiper thing.....if the pot loses contact the tubes will go nuclear and with your drawing i see the bias V only changes but the circuit isn't broken.

Daz,

Bias supplies perform voltage division and work on a ratio throughout the circuit and not only on the pot. That you thought it may be possible to replace the 15k resistor with a 100k demonstrates that you didn't understand this. How, then, you came to a circuit that provides you with a working bias voltage I can't imagine.

We still don't know the voltage source for this circuit. Are you working with a bias tap or is this circuit connected to the HV winding?

Please tell us your bias current. If you don't know how to determine your bias current then I will assume, based on prior experience, that you have simply determined that the amp "works" with this bias supply and have taken it no further than to "listen" to the amp for adjusting the bias. If this is the case the amp may (is likely) incorrectly biased.

It is a "functional" but crappy design. That is, it works just fine with no tonal detriment, but there are some liability issues WRT failure mode that it ignores. I'm not sure how you intend to "wiper swap" in the design you have and maintain circuit function. Pots are notoriously unreliable. Bias supplies are VERY important to the well being of your tubes and potentially your output transformer. Ergo, having the bias rely on a pot is bad. Having the bias rely on a pot wiper is worse. The adjustability should be arranged in the circuit so that the pot wiper cannot cause a "no bias" condition. So...

Provided you actually have the correct bias voltage range and using most of the parts you have, rearrange your circuit like this:

[ATTACH=CONFIG]40884[/ATTACH]

You'll need a 22k resistor and you'll get rid of the 27k. This should give you about the same bias voltage range you have now.

Please answer to the above issues in bold print before proceeding.

[daz]

Well, i wired it up like you suggested Chuck and i'm getting no current reading at all. Must be the source since you didn't know what that was but as i said after u asked it's taken from one side of the HV before the rectifier. I checked and rechecked to be sure i wired it correctly and the components were all good so it's not that unless i did some big time spacing after numerous checks.

[R.G.]

There are a couple of trade offs in any technical design. The charge up time on a cap which is charged from a transformer winding is heavily dependent on the source impedance, and when doing this through a capacitive voltage divider, the actual impedance, not the equivalent-resistance fall back matters.

Caps charge and discharge according to V = Idt/C. I is the charging voltage through the limiting impedance.

For charging up, this will be the transformer winding, stray inductance, rectifier resistance, and any divider impedance (capacitive or resistive).

For discharge, the rectifiers switch off the transformer and divider, and the run-down impedance is just the load impedance. So the charge rate on a cap through a rectifier from a transformer is nearly always much faster than the run-down rate. While very big caps will cause slowness problems, it's not clear that a change from 22uF to 100uF would. Gotta do the detail.

I did a different solution. I charged the cap to a much higher voltage and used a smaller cap so it charged faster but had more ripple, then wiped off the ripple with a zener shunt regulator. You get fast and low ripple, and pay for it with the usual miseries of zeners, heat and uncertainty.

[Chuck H]

Daz,

The circuit as I have drawn it works. Recheck again and then recheck

If you have NO current then there may be another issue? Did you check bias voltage? Did you confirm component values you've posted prior to changing them? There's the possibility that if the component values in the original were different (like that 15k is actually a 150k) then your amp may be in perpetual cutoff, ie: no current.

Confirm component values for the before and after circuits and recheck design.

[daz]

Daz,

The circuit as I have drawn it works. Recheck again and then recheck

If you have NO current then there may be another issue? Did you check bias voltage? Did you confirm component values you've posted prior to changing them? There's the possibility that if the component values in the original were different (like that 15k is actually a 150k) then your amp may be in perpetual cutoff, ie: no current.

Confirm component values for the before and after circuits and recheck design.

I'll check again, but if i missed something it was a major case of space.

EDIT: rechecked and i just cannot find anything. I'll have to just stare at it endlessly i guess, but i'll get back when theres just no use in looking further or i find it.

[Justin Thomas]

I liked this because the first time I "designed" a bias supply it hummed like mad without even being attached to a speaker. When I went in for a closer look, BANG!!! No, that 100uF/100V cap most certainly did NOT enjoy having 350V on it when the bias pot was turned to full CCW... a face full of electrolytic cap spew and several hours of ringing ears later...

Justin