martin manning wrote:
...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!
I bought one of those when they first came out. It was a perfect grab and go amp. I'm still kicking myself for selling it. At least I sold it to a friend who promised I have first dibs on it if he ever decides to sell it.
Only reason I sold it was it didn't have quite enough headroom to compete with the drummer I was playing with at the time.
I had a chance to buy one (4001) cheap a few years ago and I regret passing it by. It has been on my "build one someday" list for a long time--never seem to do it.
I always figured the reason Fender tried that setup in the late sixties was so that even if you completely lost your bias supply you wouldn't melt your power tubes.
If you believe in coincidence you're not looking close enough-Joe leaphorn
Nothing wrong with a mixed bias approach, But...
The cathode resistor will introduce degenerative feedback into the circuit.
You'll have to re-evaluate other sources of feedback in the power side.
Including inverter type ..
I repaired a Divided by 13 FTR 37 a while back that had this kind of bias set up. It was a pretty cool amp but it didn't really knock my socks off. I wouldn't pin that all on the bias circuit though.
This thread looks abandoned for several years - but was very useful for me. I acquired a ‘69 Bandmaster head recently as a project amp, all original and of course needing some work. I was scratching my head on the bias circuit(s) as it appeared to have both fixed grid bias and a small amount of cathode bias at the same time ( ??). It indeed does and this thread explained it right down to the two cathode resistors, odd even for a cathode bias circuit - each pwr tube having its own, as explained here, to retain balance adjustment. This one has 150=ohm vs. the 100 ohm mentioned in thread but within the range of some adjustment they were going for at the time. I don’t have things to a point yet to evaluate tone, but guessing I’ll ditch the hybrid bias and maybe put in a switch to select either fixed OR cathode biasing. Who knows what problems they were trying to work around with the unconventional bias - but hey it’s a project amp. Thanks forum! I immediately joined after it helped me solve this riddle.
I will say something logical, but when I tried this method I've exactly found what expected: it depends on the preamp circuit.
I like most high gain amps to be stiffer in the power amp, while cathode bias for "warmer" sounds (if "warmer" explains the kind of cleaner sound I've in mind).
Combining fixed and cathode bias give an intermediate feeling (depending on the % of one and the other) that worked for me on clean and crunch sounds, but not so much on lead sounds.
Just my experience. As I say, if something doesn't work, it's just because you still haven't found how to make it work.
The last two amps I built used combination bias. The setup was recommended by Gingertube.
The cathode resistor is approx 15% of what would normally be used for cathode bias. No bypass cap. The cathode feedback is the only feedback I used in the power section. Fixed bias provides the rest of the bias.
I like sound/feel/smoothness of the combo bias better than cathode or fixed - subjective / preference...
With that configuration you drastically reduce the 3rd harmonic of the poweramp (even ones are reduced being a PP).
Just to better explain: you need a single common resistor to ground that is equal to 15% of the value of the resistor that each tube would need to bias where it is.
EG: a EL34 with -43 V and 30 mA would need 1.5 kOhm on each cathode. The common resistor will then be 220 Ohm.
This will give 13V on the cathodes and so the fixed bias will only be -30V to get the same bias point.
I have a friend who is the original owner of a Marshall 4001/Studio 15. Fantastic sounding little amp! Its got that Marshall, mid range growl with that Fender "clarity" provided by the 6V6 tubes. I've played through it a bunch! I think what made them not sell was the lack of reverb and/or effects loop. Made it pretty hard to compete with the Deluxe Reverb and Princeton Reverb, as well as the Rivera designed Super Champ of that era! Of course, Mrshall has never been well known for good implementation of reverb in their amps.
Mixed bias? An interesting concept! I've always kinda wondered at what was going on there, when looking at the 4001 schematic! IIRC, the fixed bias is not adjustable.
Over time I have worked on a great number of Fender amps with the partial Cathode bias output stage and overall they sound better to me then the UL output stage amps, by far!
I think it's the UL era amps that people ment to say sounded bad, but where never correct on the amps birth date!
The partial Cathode based output stage sounds very sweet, very much the way a 1960s Ampeg Jet model and such sounds with there full Cathode bias.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
IIRC, the fixed bias is not adjustable.