Happy Sunday.
I just decided to run an amp colder while looking for a notch.
I was biased at 75mA but now I'm at 30mA and the amp sounds fine.
12AV5GA trioded with 320V B+...
I am still running 6P45S with 120mA but I might reduce them to 60mA for the hell of it.
What are you all using as bias set points?
I just decided to run an amp colder while looking for a notch.
I was biased at 75mA but now I'm at 30mA and the amp sounds fine.
12AV5GA trioded with 320V B+...
I am still running 6P45S with 120mA but I might reduce them to 60mA for the hell of it.
What are you all using as bias set points?
Looking for a notch means you're running a PP amp, yes? Biasing is all about what your goal for the amp is... what its limits for driver swing are... OPT specs... I'm not sure what the point of your question would be other than a poll.
No point other than a poll, and yes, it's PP.
The tubes were dissipating 24W before, 9.6W now.
The driver can swing over 250Vpp... One day, I'll see if I can bias it as class B and still get good performance.
The tubes were dissipating 24W before, 9.6W now.
The driver can swing over 250Vpp... One day, I'll see if I can bias it as class B and still get good performance.
Last edited:
Why would you need a driver to swing more than the bias voltage of the outputs? You can only take the grid to 0v.
Headroom? gNFB loop to power?
Also a pair of 6P45S as triodes with 320V B+ requires almost 200Vpp to get to 0V on the grid (PP)...
Also a pair of 6P45S as triodes with 320V B+ requires almost 200Vpp to get to 0V on the grid (PP)...
I once had a big, multi-chassis PP 300B amplifier monstrosity running.
It had a giant tube regulator for the B+. I could vary the B+ voltage up or down 60V from about 360V to 420V.
The audio amp chassis were big and open, so I could swap different cathode bias resistors in and out.
The power transformer was a giant toroid, 720VCT 360mA, if I remember correctly.
I was able to do extended taste tests, with the B+ higher or lower, plate current higher or lower.
The OPTs were UTC LS63 (10k ohm p-p primary), using the 6k ohm p-p primary taps.
If I remember correctly, I found that I preferred the sound of the output stages with about 350V plate-cathode, about 65V cathode-ground, and plate current of about 85mA for each 300B. That had the 300Bs dissipating about 30W each.
I also tried it with the B+ unregulated (shorted out the regulator circuit) so I could try it with 400V plate-cathode, 85V cathode-ground, about 70mA plate current.
Now, maybe it was the 'sound' of an unregulated B+ vs. a regulated B+, but I preferred the sound of that amp with the lower plate voltage/higher plate current configuration.
For years I ran a push-pull 2A3 amp (which I really need to get running again). I ran some taste tests on that one too. That amp uses pair of extra-super-dooper, 1990s vintage Tango XE-45-5 OPTs (5k ohms p-p, 45W). I tried the 2A3s with plate-cathode voltage at 270V and cathode-ground voltage of about 50V (plate current of about 50mA), and then I tried plate-cathode voltage at 245V and cathode-ground voltage at 45V (plate current of about 65mA each). Once again, I preferred the lower voltage/higher current operating point.
Both of these were push-pull triode amps with no negative feedback applied. That 1990s DHT thing, ya know? Things may be different for amps with negative feedback, etc.
But it looks like I prefer my push-pull output stages biased deep in class A, if possible.
It had a giant tube regulator for the B+. I could vary the B+ voltage up or down 60V from about 360V to 420V.
The audio amp chassis were big and open, so I could swap different cathode bias resistors in and out.
The power transformer was a giant toroid, 720VCT 360mA, if I remember correctly.
I was able to do extended taste tests, with the B+ higher or lower, plate current higher or lower.
The OPTs were UTC LS63 (10k ohm p-p primary), using the 6k ohm p-p primary taps.
If I remember correctly, I found that I preferred the sound of the output stages with about 350V plate-cathode, about 65V cathode-ground, and plate current of about 85mA for each 300B. That had the 300Bs dissipating about 30W each.
I also tried it with the B+ unregulated (shorted out the regulator circuit) so I could try it with 400V plate-cathode, 85V cathode-ground, about 70mA plate current.
Now, maybe it was the 'sound' of an unregulated B+ vs. a regulated B+, but I preferred the sound of that amp with the lower plate voltage/higher plate current configuration.
For years I ran a push-pull 2A3 amp (which I really need to get running again). I ran some taste tests on that one too. That amp uses pair of extra-super-dooper, 1990s vintage Tango XE-45-5 OPTs (5k ohms p-p, 45W). I tried the 2A3s with plate-cathode voltage at 270V and cathode-ground voltage of about 50V (plate current of about 50mA), and then I tried plate-cathode voltage at 245V and cathode-ground voltage at 45V (plate current of about 65mA each). Once again, I preferred the lower voltage/higher current operating point.
Both of these were push-pull triode amps with no negative feedback applied. That 1990s DHT thing, ya know? Things may be different for amps with negative feedback, etc.
But it looks like I prefer my push-pull output stages biased deep in class A, if possible.
I think without NF lower plate voltage higher plate current = lower output impedance = less boomy bass. Maybe that's what you are hearing. The 6550 amps with around 20dB of NF work fine with 35ma per tube @ 430V. Certainly cool is good for maximizing lifetime and reliability.
For me, it wasn't so much lower Zout = tighter bass (better damping), it was more of the sound being 'richer' and 'fuller' at lower voltage/higher current as opposed to sounding more 'dry' and 'thin' with higher voltage/low current operation.
But this was with push-pull DHTs with no NFB, and now we're into subjective stuff, so it's really just my opinion for the poll.
But this was with push-pull DHTs with no NFB, and now we're into subjective stuff, so it's really just my opinion for the poll.
Nearly half a century ago, i bought a McIntosh MC240, used but fresh from the McIntosh clinic, with a brand-new set of RCA tubes, fired it up, and was... horrified! The sound was harsh and gritty, very much like the solid-state lo-fi stuff from the 60's.
I opened it up, found a bunch of things I don't like, but the major culprit was a standing current of about 26mA: lots of cross-over distortion, just like the SS stuff. I set the Iq to 66mA, yeah, the other extreme, but that's only 28W of dissipation for 35W tubes. They actually lasted 30 years of not too heavy use, and the sound is quite enjoyable, after I changed the other stuff, such as removing the positive FB, DC coupling the input stage, beefing up the power supply, replacing the bumble bees with Swiss made MKP's, etc.
I retubed it 12 years ago with some genuine Winged-C tubes from St-Pete, and turned down the Iq to 55mA. The sound was altered lightly, but I can't say if it's the tubes or the Iq. Probably both. It's still enjoyable.
I opened it up, found a bunch of things I don't like, but the major culprit was a standing current of about 26mA: lots of cross-over distortion, just like the SS stuff. I set the Iq to 66mA, yeah, the other extreme, but that's only 28W of dissipation for 35W tubes. They actually lasted 30 years of not too heavy use, and the sound is quite enjoyable, after I changed the other stuff, such as removing the positive FB, DC coupling the input stage, beefing up the power supply, replacing the bumble bees with Swiss made MKP's, etc.
I retubed it 12 years ago with some genuine Winged-C tubes from St-Pete, and turned down the Iq to 55mA. The sound was altered lightly, but I can't say if it's the tubes or the Iq. Probably both. It's still enjoyable.
That's what theory predicts. When you stay in Class A, there is no crossover distortion, and tube mismatch doesn't matter (as long as DC is balanced in output transformer). Higher current = more linear operating point.
The only thing that cancels in a PP OPT is the B+ ripple since it splits and goes 2 directions, makng the flux out of phase. All other music signals are in phase and additive.
Not true. See the link below.
http://www.valvewizard.co.uk/pp.html
As stated in the second paragraph:
"A perfectly balanced push-pull stage will cancel all even harmonic distortion and sum odd harmonic distortion generated within the power stage."
http://www.valvewizard.co.uk/pp.html
As stated in the second paragraph:
"A perfectly balanced push-pull stage will cancel all even harmonic distortion and sum odd harmonic distortion generated within the power stage."
A statement with no explanation. If the signal current through the OPT is in phase in each half, including the harmonics, how can they cancel?
Last edited: