freestompboxes.org

I built a Firefly, and I thought the tone was a little dark and muddy. I then built a Powerman and had the same problem. Someone here suggested putting a much bigger capacitor on the Powerman's cathode, and it worked. The amp sounds wonderful now.

Now I want to do the same thing with the Firefly. Problem: I have no idea what the math is, regarding the capacitor and resistor values. I don't know what values to use in order to get which frequencies. Also, I understand that I can't mess with the resistor without doing other stuff to the circuit, and that would strain my tiny, primitive brain.

I saw a site suggesting the ratio between Fender and Marshall capacitors is about 25/.68, or about 37. The existing capacitor is 220pF. Can I hope that substituting a capacitor somewhere near 8nF will give me the desired result?

Alternatively, can I just use the same ratio of resistance to capacitance that I now have in the Powerman? In other words, can I keep the Firefly resistor constant and change the capacitance until the R/C ratio is the same as the Powerman's?

I love these little amps. You can get a great tone without making yourself deaf, and you can build them really cheaply and learn a great deal.

The New Steve H wrote: Someone here suggested putting a much bigger capacitor on the Powerman's cathode,

Which cathode? It has four... Helpful if you post schematics. I will do it for you:
http://www.harmonicappliances.com/power ... full_A.pdf

The existing capacitor is 220pF. Can I hope that substituting a capacitor somewhere near 8nF will give me the desired result?

I can only see one 220pF ca[p in the Firefly, and it isn't on a cathode... You need to be clearer.
http://ax84.com/media/ax84_m276.gif

Don't you have a bunch of caps in your junkbox that you can try on the fly?

Also, you are likely to get better help in the thread you already started on AX84, rather than on a stompbox site liek this.
http://ax84.com/bbs/dm.php?thread=443939

This just demonstrates how little I know about amps. I was looking at the wrong part of the circuit. I am FAIRLY sure I should be looking at C4, which is a 10uF capacitor. Correct me if I'm wrong; sorry about that.

I didn't know ax84 was a better place to post. I find that forum hard to read because of the strange way it displays threads, and it seems like it takes much longer to get answers.

I managed to parallel two 180uF caps and put them at C4. The amp seems to sound much clearer, although the higher I turn up the gain, the darker it sounds. Also, the boost channel is still dark; it seems to cancel any positive effect the new caps might be having.

I also have boost oscillation, once the knob goes past halfway. I've tightened everything up and looked for problems, but I haven't managed to get rid of it yet. I've read that grounding the output jack can cause this; maybe I should snip the wire.

Try a smaller value capacitor for C4. A .47, .68 or 1uF will sound much brighter than the 10uF or 180uF.
It's fine to use non-polarised capacitors such as polyester or metal film for cathode bypassing. Electro's are good too, just remember to check for correct polarity (+to cathode, - to ground).

Okay, now I'm more confused. I read that bigger caps in this position mean brighter tone, and I tried it and got the result I wanted, but now you're saying I went the wrong way?

The New Steve H wrote:Okay, now I'm more confused. I read that bigger caps in this position mean brighter tone, and I tried it and got the result I wanted, but now you're saying I went the wrong way?

A cathode-bypass capacitor (like C4) boosts frequencies. A small value boosts treble, a larger value boosts treble and mids, and as you make it larger (above about 3uF usually) you eventually boot everything. For example:

A quick and dirty formula is
C = 1/(2 * pi * R * f)
Where R is the cathode resistor (R8) and f is the lowest boost frequency.

An anode-bypass capacitor (like C12), on the other hand, cuts frequencies. A small one cuts treble, and if you make it large enough it will cut everything. It works exactly the same as the 'tone' cap in your guitar.

Well said *merlinb* a straight forward simple explanation.
Phil.

Thanks for the explanation. So I must have boosted everything?

The 180uF capacitors would have boosted all the frequencies from around 10 Hz and up by 35db - so it's really a straight gain boost. If you look at the curves starting at the 22uF bypass capacitor in merlinb's excellent explanation, and follow them across to the right, the most useful low frequency rolloffs are between 1uF and 47nF (.047).

It sounds like I did the wrong thing and got the right result. I can live with that.

Absolutely true. Anyhow, nothing new would happen if not for trial and error.

I wonder if you could maintain the boost, but tighten the sound a bit (keep your highs, but control the lows) by changing C5 to a value like .0047
Just a thought.

This stuff is fascinating. I'm really glad I made two small amps, because now I can rip them apart whenever I feel like it and try new components to back up what I'm learning.

I decided to build a 5f6a Bassman clone, and I got some schematics. I have been reading up on bypass capacitors and coupling capacitors and so on. It looks like the Bassman uses a 250uF bypass capacitor on the preamp, so I am wondering if that's a good value to try. I like the clear sound of the Bassman, and it's more or less what I was shooting at with the Powerman.

Here's something confusing, though. Merlinb's chart says anything over a few uF will boost everything. So why did Fender go with 250uF?

If I read these things correctly, Fender used .02uF coupling caps in the Bassman. The Powerman uses 22nF...wait, that's the same. Now my brain hurts.

The New Steve H wrote: Here's something confusing, though. Merlinb's chart says anything over a few uF will boost everything. So why did Fender go with 250uF?

Almost certainly because they bought a big box full of them for cheap. As long as it was 'big enough', that was all that mattered.

WHAT? YOU BLASPHEME LEO FENDER AND "THE VINTAGE TONE"?????

Too funny. Sounds totally plausible; I do the same thing when I need a component.

So I guess they were just looking for gain and didn't care too much about tailoring the frequencies.

The New Steve H wrote:WHAT? YOU BLASPHEME LEO FENDER AND "THE VINTAGE TONE"?????

Too funny. Sounds totally plausible; I do the same thing when I need a component.

Leo could have been a Yorkshireman (like me) he was so careful with money, it is probably right that he bought a big box of them because they were cheap.

Have a look on this link at his workshop at G&L. It has not been touched since he left it, the day before he died. His small parts bins were empty peanut tins.

http://www.bassesbyleo.com/forum/viewto ... =bins#p981

I worked with a guy who needed some TTL shift registers for a design he had done. It was cheaper to buy 100 because of the price break but he accidentally ordered 1000. Every circuit he designed after that had used them, whether it need a shift register or not!

Which can did he keep the tone in? I want to bid on it.

The New Steve H wrote:Which can did he keep the tone in? I want to bid on it.

All of them.