NFB loop and cathode bias on blackface amps...

[bruno]

I seem to notice that blackface amps with the NFB loop removed get noticeable increase in volume, also they get noisier and brighter and more dynamic. I enjoy every outcome except for the noise and the extra brightness. The first, I believe can be improved with proper lead dressing, what would you guys do about the brightness?

I'm also thinking about doing an amp inspired on a blackface but cathode biasing it, any significant change in power stage sag or sound?

Lately I've been using a 1965 princeton non reverb and I really enjoy the fact that the amp almost doesn't distort and stays really defined and clear all the way up, although I think it lacks some volume. Is there any way I could use a long tail inverter for more gain, but keepiing the amp from flubbing out and distorting so much, like some (most) blackfaces do?

Thanks

[FYL]

What you describe is normal: feedback reduces gain and noise. It also flattens the response. Removing the NFB loop implies a new voicing, playing with filtering/coupling caps. It works for some amps, it doesn't for a lot of them, leading to a bloated sonic mess as soon as you play hard.

Re. the Princeton, max gain or clean level isn't limited by the PI or any preamp stage, but by the final 6V6GT PP. A variant using larger bottles such as 6L6GCs as well as a beefier/stiffer power supply will sound quite louder and more authoritative.

[Fresh_Start]

Brightness - try a "cut" control across the power tube grids maybe?  A small snubber cap across the plate of the PI driver? A resonance control in the NFB circuit? (put a 10K pot and a small cap, perhaps 470pf to start with, in series across the 2.7K NFB resistor)  Not sure that last part would work...

Someone with more experience can chime in on cathode bias and touch response.  Your effective plate-cathode voltage will drop unless you boost B+.  Seems like the voltage at the cathode would increase as you drive more current.  IMHO that might warm up a Blackface circuit.

Does a long-tailed PI really have more gain?  It would be a lot easier to try boosting the gain of the PI driver stage - or the input stage for that matter.  It's not Fender Blackface, but I really like the tone of a 220K plate resistor and 2k to 2.7K cathode resistor.  Seems warmer with more midrange.  Again just my opinion.

As far as distortion goes, there's a very recent thread about the PI in a 5E3.  Same PI, same potential solutions.  Seemed to be a consensus that a BIG grid stopper on the PI triode itself (IOW between the driver and the PI) could help.  Moving the PI's plate supply from node "B" up the rail to the un-used node might help too ("Stokes mod").

You've also got those big .1uf coupling caps on the power tube grids.  A lot of folks seem to prefer .047uf in that spot.

Last but not least, you can always diddle with tone stack values.  Try 470pf for the treble cap.  .068 for the bass cap.  .033 for the mid-range cap.  Duncan's tone stack calculator is great for getting an idea of the relative effects of these changes, but nothing beats experimenting with the actual amp/speaker/guitar IMHO.

Just a few thoughts to get the discussion started.

Chip

[sluckey]

You may want to consider adding a raw control to the tonestack. Very easy to try. Don't drill any holes unless you like it.

[tommytornado]

I set up my first 5E7 (tweed bandmaster) build with a cathode bias/fixed bias switch.  In that circuit the tonal differences were minimal.  I ended up taking it out and sticking with fixed bias. 

[bluesbear]

I did a long tail pair in a non-reverb Princeton a few years ago. It had far more gain. Most would prefer it but there wasn't enough clean headroom for me. Try lowering the values of the resistors in the B+ line. Also, add adjustable fixed bias. That may bring up the clean volume a little. If you go for 6L6's, you'll probably want to replace the OT with something a bit heftier.
Dave

[jjasilli]

Removing NFB in my SF Vibrochamp & Princeton resulted in "fizzyiness".  Probably parasitic oscillation:  an unpleasant distorted overtone riding along with lo notes.  For awhile I blamed the speakers, and went through a number of them.  There does seem to be a relationship between amp and speaker here.  Anyway restoring NFB fixed the problem.  If your set-up works with no NFB, then I think there's no reason to complicate the circuitry.  (Unless you want a presence control).

Per Sluckey's post, I too am a fan of the RAW control in these amps.  With the RAW control down, you can build the amp for max clean headroom.  With the RAW control up preamp gain increases, and you have the option for overdrive at lower vol levels; and the mid-boost is just right.

[phsyconoodler]

There is not much to be gained in a Princeton by removing the negative feedback but i like the brighter tone.
  I do however,like cathode biased AB73 builds.They don't have as much headroom and have a nice squishy feel.More organic is what I would like to think.But I do not like the tone at all with no negative feedback.It gets too trebly.I always end up hooking it back up in my builds.
  I think a cathode/fixed bias switch is a great idea in a 6L6 AB763.A deluxe I haven't tried it in yet but it would be cool for smaller rooms I bet.

[FYL]

I do however,like cathode biased AB73 builds.

Try a large bypass cap, say 470 µF and up. Or split cathodes with a pot, da Geezer-style.

[Fresh_Start]

I do however,like cathode biased AB73 builds.

Try a large bypass cap, say 470 µF and up. Or split cathodes with a pot, da Geezer-style.

How about a fixed/cathode bias switch coupled with da-Geezer style split cathodes pot?  IIRC he ultimately liked a switch over a pot, so maybe a rotary switch which goes fixed bias / hot cathode bias / mid cathode bias / quiet cathode bias?

Just a thought...

Chip

[phsyconoodler]

Sure why not? The tonal options are awesome.
  I just did the cathode/fixed bias switch and the low power switch on one of my V-Verb amps and I love it.Fixed bias when you want the punch,cathode biased when you want a little less with a bit of hair on the notes and 1/4 power for home,recording and small clubs.
  It's so versatile that it should be a standard feature.

[FYL]

How about a fixed/cathode bias switch coupled with da-Geezer style split cathodes pot?  IIRC he ultimately liked a switch over a pot, so maybe a rotary switch which goes fixed bias / hot cathode bias / mid cathode bias / quiet cathode bias?

Could work very nicely.

[bruno]

Is that the schematic where he would use split cathode and then remove the cap in parallel with one of the resistors, to attenuate the power stage?

[Fresh_Start]

Is that the schematic where he would use split cathode and then remove the cap in parallel with one of the resistors, to attenuate the power stage?

Yep.  Took me a minute to find it, but here's the "development thread".
http://www.el34world.com/Forum/index.php?topic=4401.msg37803#msg37803

I love this place!

Cheers,

Chip