CF Amp Mods

I'd been thinking about what mods would benefit our little CF amplifiers. I had a discussion with Mark pertaining to my pre build and mentioned modding the CF amps. He pointed out several places in the schematic that a better part might improve the overall sound. So not being able to leave well enough alone, I'm going to put in some Obbligato caps and Mills or PRP resistors in the signal path. I picked the Obbligatos because they made my Fortes come alive and 'it's all about the tone, man'. Their sound has been called 'analogue' in the cap shootouts and I really like what they do for the Klipsch speakers.

Good sounding, reliable NOS tubes are getting harder to find as time goes on. I found matched quads for $450. IFAIAC that's too much money to spend on something older than I am especially when you don't know how long it's really going to last. SO... I think going with a good strong new tube that's priced right and readily available and rolling in a new high quality cap is a better option....at least for me.

I've noticed in the reading I've been doing over the past several months caps are discussed like tubes. The authors use the same language and terms when describing what they lend or don't lend to the sound. Keeping that in mind I developed a 'modding plan' for my CF amps. I've read where some modders have replaced just the resistors and that alone made huge differences.

Better parts and all huh?

I welcome discussion....what do y'all think?

ray

I think obligatto's are the ticket,,or any good pio cap-- which caps and values are you speaking about. A picture would help greatly.

I think obligatto's are the ticket,,or any good pio cap-- which caps and values are you speaking about. A picture would help greatly.

I think I'll replace the .1uf in front of the el84s and the 100uf in what I assume is the negative feedback network, along with all of the resistors. I hear good things about the PRP resistors. Parts Connection says they were purpose built for hi-end audio. I thought I'd give them a try.

ray

Ray,, do you know the voltage ratings on those two caps?

The .1's are the pair of coupling caps. They pass the AC signal to the output tube stage. The voltage needs to exceed the DC levels present. So it should be comfortably above the driver stage supply.

The existing are those small brown 600v caps. A small step up, in the same series of caps, is to go to the orange drop 716P series. The 716's, differ from the 715 more common ones as they have gold leads. Not bad sounding at all for econo caps.

The physical size of the caps there are something to watch. Remember all components should be physically mounted and not just dangling and in danger of having leads move and short out to something else.

The 100uf are just above where the feedback comes from the outputs. But that's not it's function. They are cathode bypass caps. It ties across that resistor that's known as the cathode resistor that provides the tube it's bias voltage. In the cathode it's a DC function and can be a electrolytic. Usually to get a desired value, it is. This position does have an influence on the amps sonic character. This was a great place for Blackgates in the past. High quality is called for in the cathode. It's a pretty low DC voltage there. The schematic shows it as a 16v. You can vary that upwards and the capacitance value can also and often is pushed up.

The next cap position that I'd look at is the quality of the cap shown as a 220uf in that bias circuit next to those zener diodes. That again is doing the same things as in the cathode circuit of the input stage.

The last cap I'd look at would the last one in the power supply stage. It is in the main B+ current loop and will impart some sonic character. I'm not a huge bypass cap believer but that could be an option at that point. Or do it right and put in a HQ electrolytic or oil cap.

Mark

Mark,, what are your thoughts on using a .047 in place of the .1?

I found some Bleckgate 100 UF's,,, in the VX? series-- like the one pictured,, different value than the one pictured. (100 uf/16 volt),, bout 20 bucks a pop.

Mark,, what are your thoughts on using a .047 in place of the .1?

You could George. Won't harm. You will raise, at that stage of the amp, the low frequency cutoff some. So that can slightly lighten the load on the output stage. The reason I say at that stage is that the inductance of the output iron may not pass all that low either. It should be a balancing act that you expect the designer to have taken into account with parts selection.

Inversely, if you lower too much the cutoff then you can have an unstable amp. That's no so good of a thing.

With regards to the Blackgates. Look for the sites that sold those and see if they still show a chart as to the intended use of the varies series, the letters, and make sure they sound appropriate. But BG's are getting hard enough to find that you can't be too picky.

There are other good caps, but BG's were truly a unique construction. It seems crazy that somebody else hasn't picked up the rights to that patented design.

Mark

http://www.carvermanuals.com/bob/Carver ... lifier.jpg

George, looking at the schematic above I plan to change the .1uf 600v caps and all of the resistors between them and the input. Mark suggested changing the 100uf 16v cap also. I thought an F&T electrolytic cap?

Alex, do you have a complete kit you'd like to sell me? or a chassis and a BOM?

ray

Allright guys,, I'm going in. Pics when I finish the procedure.

Allright guys,, I'm going in. Pics when I finish the procedure.

damn Quicksdraw my parts aren't even here yet and you're starting your mods. I'm very jealous:-)

what follows should be taken with a boulder size grain of salt. I may try this because I don't know any better, if I were you I'd seek expert advice which is something I can't give you. What you're see hear is the culmination of some reading I've done and as with all info on the 'net may be spurious(bullshit), I'd follow VERY carefully.

I could be very wrong here but lowering a value might not be the right thing to do. Mark will know WAY more than I do but 'I' would go with a higher value cap if I was going to change the value. My understanding is raising the caps' value extends bass response(allowing lower frequencies to pass)....up to a point. After you reach that point I just don't know. I HAVE heard the signal may distort though.

ray

Not lowering the value-- I'm raising the uf to 180 (electrolytic),,,and keeping the .1uf (but changing the cap)

I am gonna guess.......no, hell no, don't even think about it......no.....not even.....ok, I am just guessing.

I am gonna guess.......no, hell no, don't even think about it......no.....not even.....ok, I am just guessing.

Shit Angelo, that's what i said:-)

ray

Listening to The Ramones and Black Sabbath's self titled LP on 180gm vinyl on my CF amps. Kewlness!!!

anyone got a schematic of the latest version of the CF amp? If so,,can you pm it to me.