Fender Bassman 100 cap question

[Erik Aa]

Hi. I am planning to build a Bassman 100 and putting together a parts list.

My question: what are the values of the bypass caps in the cathode bias circuit for the first tube?

In the schematics I have found online it is hard to read…it seems to be 5 50v for the bass circuit and 25 25v for the normal circuit.

Does this mean .5 uF and .25 uF?

Thanks!

[sluckey]

My question: what are the values of the bypass caps in the cathode bias circuit for the first tube?

In the schematics I have found online it is hard to read…it seems to be 5 50v for the bass circuit and 25 25v for the normal circuit.

Does this mean .5 uF and .25 uF?

No.

It means exactly what it says... 5µF @ 50V and 25µF @ 25V.

[JPK]

I think that people who are seasoned in this kind of hobby/work already know that if there's no unit, it's uF. Because from a schematic I downloaded and attached, I can see why someone would ask this (who is not as seasoned in guitar amp schematics, like me for example). There are no units shown on the two capacitors in question. The one I attached could use a note that says all capacitor values are in uF unless otherwise noted. On the one I attached it's assumed. But from this thread I'll know, so the note is not needed.

[Erik Aa]

Thanks guys! Much appreciated.

[sluckey]

None of the caps on that schematic have a units designation, ***EXCEPT*** the few pF caps such as found on the tone stack treble caps or bright switch cap. Why clutter up the schematic with all those µF designations? Everyone knows they're all µF unless otherwise stated. Well, at least they did a long time ago when that schematic was drafted. Things were much easier back then. Today we even have nF designations. Now that's just crazy! Why would you say 100nF when you really mean .1µF? Makes us old farts have to do a head conversion every time!      

[Erik Aa]

Isn’t it a bit strange that the bypass cap for the bass channel is 5 uF while the bypass cap for the normal channel is 25 uF? Doesn’t a higher value bypass cap increase bass response?

[sluckey]

Nothing about this stuff seems strange to me any more.   

[PRR]

It's not a one-way street. Too much bass may be flubby or farty.

I note another difference just before the mix network, which may be less bass on Normal.

Or it may have been a bad day in the drawing room.....

[Erik Aa]

One more question. In the Bassman 100 schematic there are two 20 uF electrolytic caps at 525 volts. Is it ok to use 500 volt caps here instead? 525 volt caps are not stocked by the store I have bought from in the past. Thanks!

[sluckey]

In the Bassman 100 schematic there are two 20 uF electrolytic caps at 525 volts. Is it ok to use 500 volt caps here instead?

I count three. Maybe OK. Be sure to measure voltage on each cap if/when you build this, especially the screen node cap.

[Erik Aa]

Sorry, yes three. What is the danger or potential problem with respect to the screens with a 500v cap? Too high or too low voltage? Can perhaps fine tune this with the screen resistors? Thanks again!

[sluckey]

Got nothing to do with the screens! The screen node cap is just downstream from the plate node which means it sees about the same voltage as the plate node. The only danger is if the actual voltage on the screen node cap is higher than the cap rating. The danger is damage to the cap.

I still think you will be OK if you choose a proper PT. Back at the time that schematic was drawn Fender was putting 20µF/525V rated caps in many of their amps, even when a 450V rating would be sufficient. Leo probably bought a train load of those 20µF/525v caps at the time. Buying in bulk cuts production cost.

[Erik Aa]

Thanks a lot Sluckey. I will check this carefully. These caps are not easy to find in Norway…

[sluckey]

If you choose a PT with 330VAC secondary you will be fine using 500VDC caps.

[Erik Aa]

My PT has 334 VAC on the secondary. Is this close enough you think?

[sluckey]

My PT has 334 VAC on the secondary. Is this close enough you think?

Yes. 334VAC can only make 472VDC with that Bassman 100 rectifier.