bias cap

[C Moore]

All else being equal...what affect does the Mic value of the bias cap have on a guitar amp.
For example, a BF Fender Sup Rev calls for a 25-50. I THINK many Fenders of that era used a 50-50, with many going to a 100-100 when it comes time for a "cap job".
So.....would you hear/feel a difference in a Super Rev if you could toggle between a bias cap of:
25-100 or 100-1000 .?

thank you

[vibratoking]

So.....would you hear/feel a difference in a Super Rev if you could toggle between a bias cap of:
25-100 or 100-1000 .?

Not with the same resistor values.

[pops]

Amps with tube rectifiers need a smaller cap after the rectifier than solid state rectifiers where it doesn't make a difference.

[gui_tarzan]

I've discussed that here and on other forums and the consensus is that the stock 25/50 or 50/50 should be updated with a 50/100 or 100/100 and that it will not alter the tone. That makes sense, it's on the negative voltage part of the circuit, not the tone circuit.

[C Moore]

I've discussed that here and on other forums and the consensus is that the stock 25/50 or 50/50 should be updated with a 50/100 or 100/100 and that it will not alter the tone. That makes sense, it's on the negative voltage part of the circuit, not the tone circuit.

OK.....Thanks.

[C Moore]

Amps with tube rectifiers need a smaller cap after the rectifier than solid state rectifiers where it doesn't make a difference.

I am not sure I follow what you are saying.
This is before the Tube Rectifier, and it is a SS Rectifier.
But I might be misunderstanding your advice.
I am looking at this schem.....
thanks

[NickC]

I agree with gui_tarzan, and have increased bias cap value from 25/50 up to 50/100V or 100/100V ratings in Fender circuits. The bias cap in not in the signal path and has no effect on tone. So there is no point in having two values there to toggle back and forth.

[Leo_Gnardo]

The 50V rated bias caps were often handling more than 50V. (And in some tweed Bassmen I've seen, 25V caps were installed and somehow didn't fail for a long time. But I wouldn't recommend that.) As long as the voltage rating is higher than the voltage produced by the circuit, you're in the clear for that. 100V seems popular these days.

There has been some discussion tending toward "smaller caps are better" but I don't buy it. Too small, and you haven't adequately filtered hum out of the bias supply, and the hum shows up in the amp's output. Typically a 47 to 100 uF cap is satisfactory in a single-cap circuit. You CAN get away with smaller caps in a two-stage filtered circuit. Marshall for instance usually has a pair of 10 uF.

The idea behind "smaller uF values are better" doesn't have so much to do with straight up tone as it does with how the amp "breathes" or reacts to large signal level changes. Those who would have us avoid large value caps are trying to promote the idea that the bias supply voltage drops when the amp is taxed with a big signal, then bias supply must be replenished and that takes "too long with a big cap." Well you can balance that notion against the hum factor. If all you do is play earsplitting loud and background noise makes no difference, then go ahead and put in small bias filter caps. Many of my customers are playing jazz, light rock, country, and an amp that always buzzes when it should be quiet bothers them. They get the big bias filters, and to this date, nobody has ever complained to me about a "lack of dynamics" or "sterile sound" the small-cap fans would have us believe is a problem.

[C Moore]

I agree with gui_tarzan, and have increased bias cap value from 25/50 up to 50/100V or 100/100V ratings in Fender circuits. The bias cap in not in the signal path and has no effect on tone. So there is no point in having two values there to toggle back and forth.

That was a rhetorical question...I was not asking about having a switchable bias supply.

Lots of things are not in the signal path:
speaker magnets
wood to build a cabinet (marine ply vs ply vs pine etc)
rectifiers
But they all affect how an amp sounds.

So if your grid bias is not 0Hz -25V, but rather fluctuating between 22 and 28 .........
Can that effect the sound of the amp.?

[Milkmansound]

A higher value filter cap in the bias supply can reduce noise on the output.

For fixed and cathode bias definitely use 100V

[C Moore]

So why did Fender use 25 Mics in the Super Rev.?
I am not a Fender expert...but was that not, kind of, a uniquely low value for a BF Fender.?
Thank You

[rp]

So.....would you hear/feel a difference in a Super Rev if you could toggle between a bias cap of: 25-100 or 100-1000 .? So why did Fender use 25 Mics in the Super Rev.?

IME very little, if any, audible difference concerning the mFs. As to voltage rating maybe because their tech department called Astron who they bought 10,000 caps a year from and asked if they could exceed the rating and were told yes. Caps were physically annoyingly big back then and expensive, Fender prolly had their reasons with fit and price. You are free to call whatever brand you are using and ask if you can get away with the same thing, but in 2014 I would just use 100V.

I read somewhere that too large a cap(s) on a fixed bias supply will delay the bias voltage on the power tubes on boot and for a few seconds you have no bias voltage. I could see this being true with ss rectification but you probably need to be using like 10,0000mF caps on the bias. Anyone verify this?

BTW to the OP you are aware on cathode bias outputs changing the value changes the frequency response, just like on preamp tubes? And the feel too once you get really big. 1000mf supposedly will give a near fixed bias quality though I never bothered tried it, I like the bouncy feel of cathode bias. I'm more curious to try a cathode biased output stage using a diode, if only for kicks, not because I'm expecting wonders.

[martin manning]

I read somewhere that too large a cap(s) on a fixed bias supply will delay the bias voltage on the power tubes on boot and for a few seconds you have no bias voltage. I could see this being true with ss rectification but you probably need to be using like 10,0000mF caps on the bias. Anyone verify this?

A few seconds more for 100u vs. 25u. Usually the bias supply is powered in standby mode so there is plenty of time for it to stabilize while the power tube filaments are heating up regardless of whether the rectifier is vacuum or solid-state, or if you use the standby switch or not.

[pops]

Amps with tube rectifiers need a smaller cap after the rectifier than solid state rectifiers where it doesn't make a difference.

I am not sure I follow what you are saying.
This is before the Tube Rectifier, and it is a SS Rectifier.
But I might be misunderstanding your advice.
I am looking at this schem.....
thanks

Sorry it was late and i missed the word 'bias' duh!!!!!!!

[C Moore]

I read somewhere that too large a cap(s) on a fixed bias supply will delay the bias voltage on the power tubes on boot and for a few seconds you have no bias voltage. I could see this being true with ss rectification but you probably need to be using like 10,0000mF caps on the bias. Anyone verify this?

A few seconds more for 100u vs. 25u. Usually the bias supply is powered in standby mode so there is plenty of time for it to stabilize while the power tube filaments are heating up regardless of whether the rectifier is vacuum or solid-state, or if you use the standby switch or not.

I am having a hard time digesting this...

You are saying it is actually Seconds...not micro seconds.? I would not have thought there was that much difference between 25 and 100. I am not arguing...just stating my ignorance/surprise.

And.....are you saying that (even if there is no SB Switch) by the time the cathodes are fully conducting, the neg bias has stabilized....or are you saying the opposite.?
Sorry
Thank You