I have read this forum pretty long time as guest, now registered an account too.
I am seeking ideas for new guitar amp that I am designing now. I just love slightly wicked and original things, copying old ideas is just not for me. Last amp was small home practice/recording amp that used NOS CK6485 output tubes and 6J1P-EV-s for first gain stage and triode-strapped cathodyne PI. I used slightly modified Route 66 tone stack and the amplifier sounded amazing. The amp I am designing now uses 6P15P-ER output pentodes with fixed screen supply, 6N3P (or 2C51) Schmidt phase inverter and EF86 gain stage. I probably use similar tone stack as I used in my previous amp, but maybe I play more with component values.
I thought yesterday about power amp biasing and got an (probably stupid) idea to use some fixed grid bias with cathode resistor. It should be connect both worlds - sharp and precise fixed bias sound with slightly added compression that gives the cathode bias. I am actually not aware any industrial amplifiers that use that kind of "hybrid bias", but I don't see any reason why it couldn't work.
I'd appreciate any ideas and criticism about that idea
billyz wrote:Yep, Fender did it too on the worst sounding amps of the late 60's.
If not done judiciously, it makes the amp very hi-fi. Cathode resistance (larger than, say, 1-10R current-sense levels) works to prevent the power tube grids from ever being driven positive wrt the cathode so there is a significant reduction in harmonic distortion. It could work in an amp where all the tonal character comes from the preamp, but in that case the output may as well be solid-state.
rnx wrote:It should be connect both worlds - sharp and precise fixed bias sound with slightly added compression that gives the cathode bias. I am actually not aware any industrial amplifiers that use that kind of "hybrid bias", but I don't see any reason why it couldn't work.
Any reason why one should expect the outcome to be the best of both worlds rather than a blending of the unfavorable attributes?
Firestorm wrote:Cathode resistance (larger than, say, 1-10R current-sense levels) works to prevent the power tube grids from ever being driven positive wrt the cathode so there is a significant reduction in harmonic distortion.
Well, of course I thought that it wasn't new idea.
I haven't looked any amps that you mentioned here, actually. When I said no industrial amps used it, I meant none of classic and most common designs I am familiar ever used this kind of biasing.
As I thought - stupid idea and I probably will go fixed bias path.
I've done it in a couple of amps I've built. I just worked on a stereo Magnatone that was set up like that along with being ultralinear. No lack of harmonics in any of the amps mentioned. I say try it. You can always go back to one single bias method.
Dave
billyz wrote:Yep, Fender did it too on the worst sounding amps of the late 60's.
I'm gonna go out on a limb here ...
The reason the revisions Fender made in 1968 were not so well received is not solely because of the 100 Ohm cathode resistor on the outputs.
These were added on top of a few other "refinements" that were aimed toward stability.
There were just too many snubbing capacitors.
The resistors were separate (per tube) to help with balancing the tubes in conjunction with the bias balance adjustment.
Done differently, with either a single or separate cathode resistors, this could be made to work better than what Fender did (and undid within 2 years).
billyz wrote:Yep, Fender did it too on the worst sounding amps of the late 60's.
I'm gonna go out on a limb here ...
The reason the revisions Fender made in 1968 were not so well received is not solely because of the 100 Ohm cathode resistor on the outputs.
These were added on top of a few other "refinements" that were aimed toward stability.
There were just too many snubbing capacitors.
The resistors were separate (per tube) to help with balancing the tubes in conjunction with the bias balance adjustment.
Done differently, with either a single or separate cathode resistors, this could be made to work better than what Fender did (and undid within 2 years).
rd
Sorry if I implied that the reason those models of Fender amps sounded poorly, was entirely due to the circuit in question. There were many other reasons , lead dress, component changes, Voltage differences, as well as other notorious post CBS circuit changes. And Cabinet construction etc. The extra snubbing caps, which were needed due to poor lead dress, were also a small part of the final poor sound.
rdjones wrote:I love those little amps ! Sold a few when they came out. I'm surprised they didn't catch on, or sell better than they did.
Very cool, fairly unique circuit with DC first filament, separate bias trims, post inverter master volume, transformer balanced line out.
I suspect that most of the output tube bias is set by the 270 ohm cathode resistors.
rd
...built-in attenuator, speaker line-out (in addition to the balanced line-out), and a headphone out, 100k grid stoppers on the power tubes for some more unique features. And with ~500 made, it has some cachet, too!
