Biasing SE class A, fixed bias

[kg]

guys, dropping the g2 voltage will not change the plate dissipation...

if your intention is to keep the same plate current then all it will do is reduce your required -ve bias on g1.

if your intention is to lower plate current then reducing vg1 or vg2 will accomplish the same thing.

the problem is that we want to lower the plate dissipation, and to do that you either have to decrease Ip or decrease Vp. there is no other method.

i agree 100% that dissipation is a much more meaningful metric than absolute voltage in terms of tube life.

[Steve Conner]

Yes, but the point is that if you reduce the plate current by biasing g1 more negative, you have to accept asymmetrical clipping. If you do it with g2, then you can bias g1 halfway between cutoff and the onset of grid current, for symmetrical clipping.

[kg]

Yes, but the point is that if you reduce the plate current by biasing g1 more negative, you have to accept asymmetrical clipping. If you do it with g2, then you can bias g1 halfway between cutoff and the onset of grid current, for symmetrical clipping.

again, the idea of symmetrical clipping depends a lot on what's coming out of the driver stage.

it would be pretty simple to make sure that the plate load of the driver stage drops the same voltage (or slightly less) as the amount of negative bias on the output tube. that would limit your positive excursion just as well as running into output tube grid conduction, and would have the added benefit of not inducing grid blocking and bias shift.

i'm just saying that designing an ideal or close to ideal SE class A output stage with typical (ie cheap) receiving tubes demands low plate voltage, not just low vg2.

[Mike Sulzer]

In all the info on the net regarding biasing output tubes and don't believe
I've seen mentioned how to bias a tube if it class A and fixed bias. The
recommendation for push-pull is 70% of max dissipation. For class A it's
90%, but this presumes cathode bias which will limit the increase in
dissipation.

Paul P

Are you assuming that the dissipation in the tube (single-ended class A) goes up when applying an AC signal? It does not, assuming that you have achieved the correct matching ratio with your transformer. At a negative output peak the current should about double, but the voltage across the tube is small. The total dissipation in the tube should drop with applied signal when you average across both positive and negative signal excursions. But if the load impedance is too low, you could increase the dissipation in the tube. (Think of what would happen if you shorted across the transformer primary.)

[Paul P]

Are you assuming that the dissipation in the tube (single-ended class A)
goes up when applying an AC signal?

I does in my amp, at least that's what it looks like when current through
each tube goes up to 45ma and beyond when playing hard. This is for a
plate voltage of 384, so 17w dissipation. At idle the current is 27ma,
10w dissipation.

This 27ma current at idle is achieved with -32v bias.

Paul P

[Mike Sulzer]

I does in my amp, at least that's what it looks like when current through
each tube goes up to 45ma and beyond when playing hard. This is for a
plate voltage of 384, so 17w dissipation. At idle the current is 27ma,
10w dissipation.

This 27ma current at idle is achieved with -32v bias.

Paul P

But the power dissipation is the instantaneous current through the tube times the instantaneous voltage across the tube, averaged over time. When the current is high, the voltage is low and vice versa, so the dissipation may not be as high as you think. It may be that your load impedance is a bit low for the plate voltage you are using; that could explain why the current has gone up a bit more than you might expect even with hard overdrive.

[jbefumo]

I've been incrementally building an amp using a Gold Lion KT88 in single-ended configuration. For the preliminary experiments I used a (too) small 15-W Edcor OT, with a hefty cathode resistor to avoid damaging anything, but I have a larger 25-W rated OT arriving today, and will be experimenting with getting the bias to the best possible place, and would like to go with fixed bias, with an 'RMS' control derived from one of Dennis Kager's Sundown amps -- the circuit basically introduces a variable amount of cathode resistance, from zero to 250-Ohms. I'm shooting for 20W or so. Primary voltage is currently around 447V (mainly because that's the PT I had available.) Once I have everything sorted out I plan to have new PT and OT made by Mercury, so my questions are twofold: First, what current should I shoot for with the existing setup, and second, what would be ideal voltage when I spec the new PT? (I'll be opting for a somewhat heftier OT than the Edcor unit that's coming today.) Thanks.

Joe

[Dave H]

That's a blast from the past. The thread is five years old. It would help if we knew the primary impedance of your OT.

[jbefumo]

5k

[imaradiostar]

I didn't see mention of regulated or otherwise lowered screen voltage. I have a strong suspicion that you're going to need that to make it play nice and distort in a pretty way without cooking the tube.

Jamie

[Bruce / Mission Amps]

OK you didn't say if your B+ of 447vdc was while the PT and B+ rail is loaded or unloaded.
Assuming it was loaded and the B+ rail voltage being 447vdc... it is relatively easy for me to see that your Hi-V AC secondary voltage is around 335vac to 360vac.
So, you could simply use a 5Hy 200ma choke right after the rectifier and then install your first filter capacitor. Maybe around 22uF to 47uF @ 450v or so.
A choke input supply will lower your final B+ of 447vdc by around 20% to 30% (maybe more) and the power supply circuit will be able to source more current to the power tube for a nice Class A amp at quite reasonable voltages.
Quick guess.... 360vac to the rectifier, X .9 factor (a choke input filter) = 324vdc .... and LOTS of DC current available for the Class A KT88.

[jbefumo]

Thanks all!

Okay, that was unloaded -- haven't put the new OT in yet. 2 500u X 500V caps with a 5H inductor inbetween Right now I have a 1K X 5W resistor on the screen, and a hefty 3-way switch for pentode/tetrode-UL-triode connection -- mainly for my own edification in hearing the difference -- probably won't have that in the final version. I'll post the schematic as soon as I have a moment to bring it up to date and clean it up some.

OK you didn't say if your B+ of 447vdc was while the PT and B+ rail is loaded or unloaded.
Assuming it was loaded and the B+ rail voltage being 447vdc... it is relatively easy for me to see that your Hi-V AC secondary voltage is around 335vac to 360vac.
So, you could simply use a 5Hy 200ma choke right after the rectifier and then install your first filter capacitor. Maybe around 22uF to 47uF @ 450v or so.
A choke input supply will lower your final B+ of 447vdc by around 20% to 30% (maybe more) and the power supply circuit will be able to source more current to the power tube for a nice Class A amp at quite reasonable voltages.
Quick guess.... 360vac to the rectifier, X .9 factor (a choke input filter) = 324vdc .... and LOTS of DC current available for the Class A KT88.

[jbefumo]

This is what I have in the power section at the moment. Have to update the rest of the circuit to reflect recent changes.

Joe

[Dave H]

OK you didn't say if your B+ of 447vdc was while the PT and B+ rail is loaded or unloaded.
Assuming it was loaded and the B+ rail voltage being 447vdc... it is relatively easy for me to see that your Hi-V AC secondary voltage is around 335vac to 360vac.
So, you could simply use a 5Hy 200ma choke right after the rectifier and then install your first filter capacitor. Maybe around 22uF to 47uF @ 450v or so.
A choke input supply will lower your final B+ of 447vdc by around 20% to 30% (maybe more) and the power supply circuit will be able to source more current to the power tube for a nice Class A amp at quite reasonable voltages.
Quick guess.... 360vac to the rectifier, X .9 factor (a choke input filter) = 324vdc .... and LOTS of DC current available for the Class A KT88.

Why do you want to reduce the voltage Bruce? He has a 5k OT. He's not going to get much power into 5k with only 324V.

[Bruce / Mission Amps]

Why do you want to reduce the voltage Bruce? He has a 5k OT. He's not going to get much power into 5k with only 324V.

Yeah, 5K is probably too high for a SE KT88 anyhow but I assumed from his previous post he was not happy with the PT he had at 447vdc and was wanting to buy something else.... presumably with a lower B+ so he could run the SE KT88 in class A at high current.
Still, with a little nudging, experimentally using a small value uF@500v cap at the front end of the choke input filter... you can "massages these things to get the B+ up a little.
I'd be looking for something around 375vdc and 2K5 OT which could be done with his 5K OT and half the speaker load on the next higher tap.
And there always is the trick of using voltage bucking on the primary winding with an out of phase low voltage tranny.

Added.... I meant running something like an 8 ohm load on the 16 ohm tap.....