Garnet Rebel II with mystery caps??? help

[LustandFury]

I have a Garnet Rebel II that seems to have two extra caps not on the schematic. They appear original, but in bad shape and need replacing, they also seem to be in a strange section of the preamp. I may have it wrong. Any help clearing this up would be greatly appreciated! Below is a photo of the actual amp and where it appears the extra 100 pF caps are. The one circled in green is the only such cap I can find on the schematic. Thanks fo any help in advance!

http://jsygta.bay.livefilestore.com/y1pkmyIZhRfDj1vdUMkiue9-2i1eq1xloSUH5ypfS1E7sdtzbk5V6rFTdSqPRh56ltgrtShgwi jNJI/RebelCap.jpg

http://jsygta.bay.livefilestore.com/y1pmY2vTaMKwPmkOUmQevlubrYZDwH2ia2XAtSgjhDC0FlHOEu Mvfw3RraEg4N9dqF7CjGiP59zo50/garnet_Rebelcaps.jpg

http://cid-6a8571572944cee8.skydrive.live.com/embedgrid.aspx/Public/garnet%7C_Rebelcaps.jpg

[LustandFury]

No ideas?

[WaltC]

looks like they're placed to bleed off high end signal (too much treble? too bright?) or to limit oscillation (maybe a problem with the "lead dress" forced by the point-to-point wiring?). Relatively easy to disconnect them (one at a time, one end only until you decide what you want to do with them) and see what you get.

If you need them in there and they're leaking, then they'd be easy to replace them.

Is there something wrong with the way the amp sounds?

[LustandFury]

Thanks for the answer I had the exact same response on another amp forum. It's hard to tell from the picture but a lot of these old caps are cracked or have chunks missing out of them. The amp has started to make some loud popping noises when I first turn it on regardless of anything being plugged in or not. It runs fine after a few minutes but has a little hum starting to come through so I'm going to replace the filter caps as well. So I'm in the midst of replacing everything that looks funky to see if it solves that problem.

[donnyjaguar]

Agreed. Post-production band-aid to stabilize the amplifier. It could be due to the NFB circuit in the final stage being prone to oscillation. Show us that picture too.

[LustandFury]

Ahh the plot thickens...haha
Here is the schematic for the output section. What do you think?
http://jsygta.bay.livefilestore.com/y1p-4JprDm6HGpb_XVEM_Ex4lXoYYNjdP5kbI5wAcjogfoj-TNySz8H4iNsQVTsHKvLs7LUXY1ZIe8/garnet_Feedback.jpg

[LustandFury]

Any ideas about the NFB loop based on the schematic?

[donnyjaguar]

Just as I thought. The NFB will undergo a phase change to PFB at ultrasonic frequencies. This is caused by the way the NFB is applied vs the slight phase difference in the inverting and non-inverting signals to the output tubes. Note the presence of yet another capacitor to ground on the plate of the driver tube second section. The capacitors in the preamp section are there to remove any ultrasonics and reduce the likliehood of excitation.

I would just wire that 12AU7 as per standard paraphase design and remove the capacitors in the preamp. If it still breaks into oscillation I can address that at the time with a better solution.

There are other solutions too.

DJ

[LustandFury]

Donny I clipped the caps last night and they were definitely there for a reason. With both caps clipped the amps volume was almost nothing and it did not sound well at all. I reattached the second cap (near the blue 25/25 cap) and the volume came back quite a bit. Then I reattached the final cap and it seems things are close to normal.
What do you mean by a Standard paraphase design? How would that differ from what is there now? In my limited understanding the Rebel already has a Paraphase inverter now...Thanks for all the help so far!

[RedMan]

Nothing wrong with the split load inverter there. In fact there's really nothing wrong with that circuit at all. Why do you want to change it? Every amp is prone to oscillation from lead dress and phase reversal at higher frequencies.

[LustandFury]

I don't really want to change it. I'm just starting to toy with fixing my own amps and the caps in this amp were cracking and losing chuncks of material and I noticed the amp starting to make loud popping noises. So I decided to change some of the old capacitors. Like I said earlier the amp I have had caps not shown in the schematic...and then I was given advice on changing the PI. I'm just getting as much info as I can and trying to learn about it all. I've been reading about PI and the paraphase inverter is and older design so I thought maybe changing it to a Long Tailed Pair might be a good mod. But I really liked the amp before all this and since it is vintage Canadian and the original maker has died I really don't have my heart set on changing anything just getting it to sound as good as possible!

[LustandFury]

Also As I see it this is not a Split Load Inverter...now I'm getting confused...

[donnyjaguar]

You have to decide between keeping it stock or modifying it for more stability. IMO, if you are replacing components with new you are in essence bastardizing it anyway so what does it matter if its modified? Kinda of like rebuilding a Ford Model T to stock or shoving in a Chevy V8. Plenty of examples of each out there and I don't need to tell you which will perform better. Now if it was a '32 Ford then I'd say keep it stock. ;)

[LustandFury]

Well if a part goes bad and needs replacing that's much different then rewiring an amp in my opinion. I'd still like a clarification of the PI. Is it not a paraphase design that the amp uses now?

[donnyjaguar]

The good thing about wiring modifications is they can be reversed if desired. The PI is not a conventional paraphase, which is a differential amplifier. Its more like a modified Williamson style.

[bolero]

if you can wire that thing up like the older rebel amps it will sound really good, give you some nice grind when it's cranked up

I have a rebel II & it is pretty tame sounding by comparison