I am converting a Heathkit A7E amp for guitar and am considering clipping out the high-frequency damping capacitor - a 200MMF (pF) capacitor that's tied from the input of the phase splitter to ground.
The general idea is to give the amp a more "open" sound, which is similar to converting a silver-face Fender to black-face. As I understand it, the problem would be risking high frequency oscillations.
Another option would be to replace the capacitor with a lower value, which would push the roll-off frequency higher.
I'm also wondering if it's possible to replace the capacitor with a small inductor in series. I'm looking at the 250mH Hammond RF heavy duty hash choke as a possibility, although it's only rated for 2.5 mA.
The big question is tone, since it's now a guitar amp.
Any comments/suggestions/ideas?
Here's a schematic
http://www.naturdoctor.com/Chapters/Amps/heathkita7e.gif
The general idea is to give the amp a more "open" sound, which is similar to converting a silver-face Fender to black-face. As I understand it, the problem would be risking high frequency oscillations.
Another option would be to replace the capacitor with a lower value, which would push the roll-off frequency higher.
I'm also wondering if it's possible to replace the capacitor with a small inductor in series. I'm looking at the 250mH Hammond RF heavy duty hash choke as a possibility, although it's only rated for 2.5 mA.
The big question is tone, since it's now a guitar amp.
Any comments/suggestions/ideas?
Here's a schematic
http://www.naturdoctor.com/Chapters/Amps/heathkita7e.gif
Hi
The problem of taking out the capacitor , is that the amp can become instable and with bursts of oscilation...
But if fyou have a scoop , take it out , or replace with a small cap and inspect for stability...
Cheers
The problem of taking out the capacitor , is that the amp can become instable and with bursts of oscilation...
But if fyou have a scoop , take it out , or replace with a small cap and inspect for stability...
Cheers
You can try it. It could easily be the case that *some* examples of the amp, perhaps wired especially poorly exhibited some oscillation without the cap, so Heath put it in as insurance. A scope will tell...
As for other suggestions, for guitar you'll probably need the gain of input #1. You'll want to remove the filter network between the first two tubes. It looks like it might be a RIAA network for phono or something. Anyway, it presents a serious high-pass (low-cut) filter. You'll probably have to dime the bass control if you leave it there.
Just clip out the 0.0035, 0.01 caps and the 22k resistor. If you want even more gain then you can connect the grid of the second 12SL7 to the top of the 470k resistor. But just clipping out the extra parts would be the easy first step.
You should also replace the 22k resistor at the input with something larger. 1M should work fine. 22k will load down the guitar pickup and squash the tone.
Into the right speaker it will probably make a pretty decent guitar amp. I like 'SL7's in guitar amps. Incidentally, you could probably drop a 12SL7 right into the 12SN7 driver socket without changing anything, just to see... Might want to lower 150k and 100k resistor values in have if you like it with the 12SL7.
-- Dave
As for other suggestions, for guitar you'll probably need the gain of input #1. You'll want to remove the filter network between the first two tubes. It looks like it might be a RIAA network for phono or something. Anyway, it presents a serious high-pass (low-cut) filter. You'll probably have to dime the bass control if you leave it there.
Just clip out the 0.0035, 0.01 caps and the 22k resistor. If you want even more gain then you can connect the grid of the second 12SL7 to the top of the 470k resistor. But just clipping out the extra parts would be the easy first step.
You should also replace the 22k resistor at the input with something larger. 1M should work fine. 22k will load down the guitar pickup and squash the tone.
Into the right speaker it will probably make a pretty decent guitar amp. I like 'SL7's in guitar amps. Incidentally, you could probably drop a 12SL7 right into the 12SN7 driver socket without changing anything, just to see... Might want to lower 150k and 100k resistor values in have if you like it with the 12SL7.
-- Dave
Dave Cigna said:
Must be a little lexdysic. Should have said low-pass, dime the treble control...
- Status
- Not open for further replies.