Cathode Bias bypass cap quesiton

[Doug H]

Been foolnig around with an old Gibson Crestline, my current gigging amp. I switched it to SS rectifier, more than I bargained for, well over 300 on the plates now. I've knocked the screens down with some diodes, NOS tubes are holding up. Runs a couple 6aq5s and it's got enough volume to hang with a live kit, maybe even a loud one, it's enough for my purposes.

Lately I've been reading up trying to get more punch at louder volumes, more of a solid presence in the low end. I've come across a lot of discussion about the bypass cap value, making it higher, like 250u to 1000u tightens up the bottom. Similarly so does doing away with the cap in the first place, which is the first experiment I have done. First impressions the amp does seem more full bodied down low, more of a sense of volume. But it also seems to have lost some of it's edge or sizzle. It usually get's slammed with a si fuzz face these days so that's not an immediate problem. Still I plan on testing out

individual resistors for each power tube bypassed
individual resistors for each power tube unbypassed
different bias levels

question, without the bypass cap, should I be expecting to be able to bias the power tubes hotter or should I be more wary of running them too hot? I've already got them way out of spec, so I"m just looking for some rules of thumb here.

I run it through a 10" scuminco blue, It's become a real beast . IN retrospect I couldhave chosen a lighter speaker lol, but it's not too bad for a grab and go jam amp..

[Doug H]

Just to follow up, squeeze a quick post in here, I've tried the non bypass cap shared cathode. Really full and smooth, missing some edge though, not what I was going for with this amp, so that's nixed. I have sense split the cathodes each with their own 470uf bypass cap. I'm thinking this is how it stays for now. At some point maybe I'll try with separate unbypassed cause maybe something about not being shared will make it edgier, but I've read that you lose significant power this way so It'll have to go on a switch. I'm digging it so far the way it is now anyhow.

[HTH]

trying to get more punch at louder volumes, more of a solid presence in the low end.

I'd be looking at a high sensitivity ceramic speaker for what you describe above, plus changing to fixed bias could be worth a shot. Good call on the change to a SS recto too. Could also try the Weber copper caps too - it means you can easily go back and forth between a valve recto and SS.

Are the filter caps original to the amp? - worth a cap job I'd say.

[Doug H]

Caps shoudl be good, they've been changed in recent history.

Ya, I'm looking into the loudest 10 inch speaker I can find, eright now it looks like the red fang or maybe the red fang.

Ive been down the rabbit hole and back since I posted this. I ended up adding a new filtering node to the front end, putting separate bias resistors on the PTs and adding 470 screen resistors and easing up on the dropping zeners between the plates and screens, wow that was all she wrote, big mistake. I had the output caps jumped to around .09 which I undid cause ai thought I ight be pushing too much bass thorugh the little OT, which is likely true, then I upped the cap to the pi from 430pf to something rational to make up for it, the new preamp filter cap was 33, had to go out of my way to get it, by that time the amp was done.

I think the config would have sounded fine were I running a couple 6v6s in spec, but with almost 300 on theplates of the 6aq5s and the screen not too far behind it had all gone to pot.

First problem was lifeless screens. 470ohm instead of choke and then separate 470 ohm screen resistors on the screen node was too much. Undid that in a hurry, added a 5v zener back into the chain, went with a 470ohm r after that leading to the screen node and then just on 50ohm resistor on a single screen to help with occilation. The screens are well below the plates, don't really need current limiters from what I've read, might put another 50 ohm on the other side, but ran without any for ever so...

Actually befere I did that I undid the sparate bias resistors and a lot of the roundness on the bottom end came back and some harshness went away. Again, I think this had to be undone cause I was pushing the tubes so hard. I think I can bias hotter again now, before I was having to run it fairly cold. I haven't cheked the numbers since I went back to shred though.

Then I changed my 13k new dropping resistor for the new preamp node to 4.6k to hopefully get a bit more headroom and brightness back in the preamp.

I liked the look of the 022 black caps coming from the pi and didn't want to jump them, so I put the preamp output cap to 022 to keep as much bottom end off the preamp as I could, and instead of adding a bright cap to the volume I jumped the input to wiper with a resistor to keep excessive series resistence off the 2nd stage grid at lower volumes. I dropped the new preamp filter cap down to 15uf while I was at it.

whew, my amp is back .

I'm still missing the deeper end I used to have with the larger pi caps, I could probably get away with some 047s, but I really don't want to yank those sprague black beautys lol, I'm hoping the Fang will kick out some more bottom.

Talk about a learning cyrvue, I'm finally getting to know power tubes, well sort of...

[Doug H]

oh, ya, I keep thinking about going with fixed bias, but I'd like to keep it simple and preserve some of that bluesy tone. I have thought about hanging a fixed bassman style tonestack bwtween the two preamp stages. Ironically that'd be a lot like the little package this amp came with that was the first thing to go when I started "improving" the tone lol.