Bias cap for BF Fender amps

[aclempoppi]

Always replaced the 25/50vdc caps with 50/100vdc. The wall voltage was my concern. Is this an unfounded concern ? The capacitance change was just a filtering guess, on my part. This would only apply to 2X6L6 power tube models; Super Reverb, Bandmaster, etc... Any comments would be great ! Art

[Cliff Schecht]

25uF 50V is sort of a standard value for Fender in conjunction with the 250 Ohm bias resistor. It helps to shape the response of that power stage for that "classic" Fender sound (clean and dirty). The bias voltage on any two-6L6 Fender I don't think is ever above 40V and usually closer to 30V for the typical 350-450V B+ amp.

So to answer your question, this modification is not necessary unless you know the cathode voltage will be sitting higher than 40V. The 25uF value is already considered fully bypassed (rolloff of 25 Hz) and so going slightly bigger shouldn't change the sound too much, it may be slightly noticeable under some circumstances. Going WAY bigger in value will make the output seem stiffer, more like a fixed bias amp as you go up higher in value (larger value means the cathode voltage will move around less).

[FYL]

25uF 50V is sort of a standard value for Fender in conjunction with the 250 Ohm bias resistor.

Mo' like BF for Blackface, fixed bias etc. The 25/50 cap is the bias filtering cap, and yes, it should be uprated - 63V min, I use 100V. Using higher capacitance modifies the circuit time constants, leading to a stiffer but slower bias supply.

[FUCHSAUDIO]

Many Fenders used 50-V bias caps, which was dangerous. I have found many running 55 to 60 volts (especially with modern tubes), that were heating up or failed. A 100 @ 100 or 50@100UF are cheap, and safer to use for long term health.

[dobbhill]

To the original poster:

I think we need some clarification:
Output tube cathode bias, output tube fixed bias supply circuit, or preamp tube cathode bias??
Which caps are you asking about??
D

[Cliff Schecht]

25uF 50V is sort of a standard value for Fender in conjunction with the 250 Ohm bias resistor.

Mo' like BF for Blackface, fixed bias etc. The 25/50 cap is the bias filtering cap, and yes, it should be uprated - 63V min, I use 100V. Using higher capacitance modifies the circuit time constants, leading to a stiffer but slower bias supply.

Yeah I should have stated that more specifically towards most cathode biased 6L6 amps. There isn't a set higher limit to what can go there voltage-wise (minus obvious size constraints) so it's really just a matter of margin. How much margin is enough? If you're running tantalum, I go for at least 25% margin on any PEAK voltage. Anybody who has seen a tantalum explode or has actually been hit by molten tantalum knows why to do this. Electrolytics can tolerate a bit more abuse before they pop though, it's not uncommon to see ecaps rated somewhat close to the max value they'll see, assuming that this is the PEAK that they will see. Ecaps can also handle over-voltage stresses pretty well ("pretty well" means this varies from brand to brand) but it's always better to be safe than sorry. The difference between a 50V and 63V cap is so minimal that use of underrated caps in a circuit is a great indicator of a budget-minded manufacturer (that's putting it nicely ).

[Structo]

100uF/ 100v is a very common value used in fixed bias (6L6) amps.

[tubeswell]

If your bias supply is from a ~40V tap, then 47uF and more is fine because you are not concerned about the time it takes for the cap to charge up. As to the voltage rating, it depends how you configure the bias supply voltage divider. 50V would be under the target if you were rectifying the bare 40V (40 x 1.4142 = 56.6), but maybe not if you drop the voltage supply with a divider first.

However, if your bias is from the HT supply (like a vintage BFPR) then you want a cap that charges up much faster, (because its dependent on the same supply as your B+, so it has to be ready to go when your B+ is) so go with 8-10uF. You can get a steadier supply if you use 2 of them in a Pii filter. The voltage rating should be a minimum of 10 or so above whatever voltage is going to hit them.

[aclempoppi]

Thanks for all the responces ! Should have been more clear on the application. The amps are '63-'67, fixed bias supplied off a dedicated PT secondary tap. And yeah, it's just a filter cap for a circuit that would see about negative 55 volts at a max setting, but usually a lower opertating level. The power tubes recieve this bias supply and the heater voltage prior to the standby switching the B+ supply. These amps are tube or solid state rectified, depending on the model. I believe both of my concerns have been covered. Thanks Again ! Art