bias caps: Sprague Atom or Xicon?

[mike_mccue]

I just but a couple of each brand. 100uf 100v.

which would you use?

do you even think it matters?

thanks for any opinions,

mike

[jrfrond]

I use Xicon caps freely, not only in my amps, but in repairs as well, and have never had any issues, so it's your choice. No, it doesn't make a difference, and 100/100V is what I usually use for a bias cap. Change the bias rectifier too. THAT is the component that, if it goes south, can REALLY cause you problems, as in power tube meltdown via complete loss of bias. Don't even think about it, just do it. One 1N4007 diode, and you are good to go.

[mike_mccue]

ok then


thanks

[lowell]

just chiming in here... isn't 100uf going to sound VERY "bassy" as a bypass cap on your standard triode stage??

[mike_mccue]

the 100uf 100v selection is too replace the two capacitors used in bias and bias adjust circuit on the power stage of a Fender Silverface push-pull amp.

Historically they ranged from apprx 50-80uf at 70-100v.

best regards,
mike

[Enzo]

Lowell, he is not talking about cathode bypass caps. he is talking about the filter caps in the negative voltage supply for fixed bias. These filters are not in the signal path, and negligible current is drawn from the supply, so I don't think it much matters which cap you use there.

[pdf64]

Lowell, see link

http://66.102.9.104/search?q=cache:U...lnk&cd=1&gl=uk

This shows that for a 12ax7, 100k plate, 1k5 cathode ie standard setup, 22uF cathode bypass is -3dB at 9.3Hz. It doesn't actually show exactly what a 100uF bypass would give, but it will be 2 octaves down from that, ie about 2.2Hz.
Clearly either are octaves below the frequency range of the the instrument or speaker systems, so using a 22uF or 100uF bypass cap is very unlikely to have any perceptible tonal difference. Go too low though and you will increase the suseptibility of the circuit to motorboating oscillation via the power supply. Peter

[lowell]

pdf64 that's good info... yeah guys I just reread the initial post and don't know how I came to the conclusion that this was about bypass caps. brains're weird. heh heh

[mike_mccue]

I don't know why you'd be confused...

I only mispelled bought (but)

and didn't sufficiently describe which/what bias I was speaking of.

In any event, thanks for the observation and suggestion.

best,
mike