capacitor choice for cathode to ground cap in fender champ

[buildafriend]

hi,

i'm recapping an early 70's silverface champ and I'm curious what you more experienced guys might do in a certain situation here. there are 2 1uF mallory caps in parallel coming from the cathode of v1 to ground that are accompanied by a 1.5k resistor also in parallel to ground. They are the vintage polarized mallory caps that came with the amp. if I was made of money I would just order up proper replacements but I have some non polarized ceramic caps on hand that are both 1uF. I also have some cheapie 1uF electrolytic caps that are from a radio shack electrolytic pack, but im not sure how I feel about ramming those into my beloved vintage little tube amp. What would you guys do? Maybe order a single 3.3uF cap? just use the ceramic caps I have? Use the cheapies? Order some nos 1uF caps? I assume all would work.. but what choice and why.

here is a link to the schematic. they are coming from the second half of the 12ax7 cathode.
http://www.geocities.jp/ec1957st/champ_aa764_schem.gif

thanks regardless

 

[critterkllr]

1uF cathode bypass caps? Does it sound really bright and thin to you? I'm not familiar with the Silverface schematics, but I know Blackface uses 22 or 25uf on pin 3 and 2 uF on pin 8. I would stick with those values at 25v. Or both 25uf. Electrolytics are fine.

It sounds like somebody modded it to sound more like a Gibson amp.

You should also do more research on cathode bypass caps and their affect on tone. Here's a pretty popular graph to show you what they're doing.

 

[ln76d]

Original have 2uF but i saw that some people modify them by using higher capacitance. Here's an example:
http://scottyaudio.net/wp-content/uploads/2011/01/champ_aa764_schem.gif
Use two 1uF film caps (or one film and one electrolityc) or go higher

 

[buildafriend]

1uF cathode bypass caps? Does it sound really bright and thin to you? I'm not familiar with the Silverface schematics, but I know Blackface uses 22 or 25uf on pin 3 and 2 uF on pin 8. I would stick with those values at 25v. Or both 25uf. Electrolytics are fine.

It sounds like somebody modded it to sound more like a Gibson amp.

You should also do more research on cathode bypass caps and their affect on tone. Here's a pretty popular graph to show you what they're doing.

Hi thanks for your response.

It doesn't sound super bright or thin.. but the bass knob only seems to work between 1 and 3 and from 3 up it pretty much stays the same and sometimes I'm getting little low end dropouts while i'm playing. I'm gonna check the wiring, the jack, and change the pots in my guitar before I blame the short little bass dropouts on the amp. the treble on the amp seems to work right.

I replaced the cathode bypass of the first triode with a 25uF 25v sprague cap, and the same with cathode of the 6v6.

I guess I'll try replacing the second cathode bypass with a 22uF(went with a 10uF working fine) and see what happens. I'm pretty sure I have a good brand one of those laying around.

I'm hoping that recapping helps with the bass knob response.

Thanks for the chart! very useful.

Original have 2uF but i saw that some people modify them by using higher capacitance. Here's an example:
http://scottyaudio.net/wp-content/uploads/2011/01/champ_aa764_schem.gif
Use two 1uF film caps (or one film and one electrolityc) or go higher

Thanks for the help!

I think a 22uF is in order now.
Edit* I went with a 10uF. It seems to work well.

That schematic makes me wonder what removing the negative feedback would do. I've never tried doing that but I've read about lots of people choosing between negative feedback or no negative feedback. I don't know how to do that mod. this might be a good opportunity to learn it.

 

[critterkllr]

Go ahead and give the negative feedback mod a shot. It's hard to describe, but it will make it more like a tweed. You'll gain volume, treble and it will be a lot more dynamic. The edge of breakup is also a lot more gradual and natural without the negative feedback. I prefer it for more old school raunchy lead tones, but like the feedback loop for cleans and more compressed/smooth sounds.

 

[buildafriend]

Go ahead and give the negative feedback mod a shot. It's hard to describe, but it will make it more like a tweed. You'll gain volume, treble and it will be a lot more dynamic. The edge of breakup is also a lot more gradual and natural without the negative feedback. I prefer it for more old school raunchy lead tones, but like the feedback loop for cleans and more compressed/smooth sounds.

that sounds fun. I like breakup, it's why I bought the amp. To break up a lot a low volume.

 

[ln76d]

You can make negative feedback mod switchable with shorting switch

 

[CJ]

cap selection for cathode bypass does not affect sound much,

coupling cap choice can make a slight difference,

some amps have no bypass caps at all,

 

[critterkllr]

cap selection for cathode bypass does not affect sound much,

coupling cap choice can make a slight difference,

some amps have no bypass caps at all,

At higher cap values the low end is only changed VERY slightly. It makes a world of difference on some amps though. I have a Gibson GA-5 that came stock with 20uF and .1uF cathode bypass caps. It was horrible. Swapping that .1uF cap made the biggest difference in the amp. I guess they were trying to make Les Pauls sound more like Teles?

I do think that anything from 4.7uF and higher is pretty subtle. Especially since the speaker seems to roll most of that off anyway.

Switchable cathode bypass caps and a switchable negative feedback loop were both great mods. Switching the caps out, with negative feedback off gives it a more raw Tweed feel. Switching them in with negative feedback off gets me more in the Marshall 18watt range. Having both caps and negative feedback in is more in the Blackface range.

I'd highly recommend these things for a studio amp. Unless we're dealing with something of higher value or collectibility.