Bias Circuit and Standby check

[Shadrock2]

I would appreciate comments and constructive criticism for this amp circuit that I have designed.
The amp part itself will be subject to adjustments and tweaking so I am mostly looking for an assessment of the bias circuit and the standby switch design witch might be a little unusual.

I really want to use the tube types shown since I have all NOS tubes.

My power transformer is a fully enclosed, potted and hermetically sealed unit I have wanted to use for a long time.
It even has an altitude rating! (10,000 ft)
It is rated @ 740V ct @ 90 ma. So I was concerned that the resulting B+ voltage will be to high for 6V6's while the current capability will be to low for higher rated output tubes. So I saw on this forum a reference to a bias circuit similar to the one I have shown which would effectively knock the operating voltage down to a more reasonable level. I am shooting for an operating B+ voltage no higher than 410V and preferably greater than 370V.

I may have tried to add to many whistles and bells. What do you guys think?
Will the standby setup work or is it undesirable to leave the output tubes in a cut off state?

[pdf64]

Some great ideas there; I've never used that back biasing thing so good luck with it.
The 'big cathode resistor' is my preferred standby of late; the power tubes still have a little current flow, they don't tend to totally cut off.
I would consider using a higher power resistor on the heater standby, preferably chassis mounted type.
For the LTP balance control, I prefer to wire it as a variable resistor and link the wiper and 'other' end, so that it can't go open circuit if the wiper goes dodgy, eg 100k fixed one side and 47k + 100k variable resistor on the other.

[Shadrock2]

Thanx pdf... I was a little worried about the standby. I couldn't find a commercial example that used that system.

About the PI adjustment... I think using a trim pot for balance is pretty dodgy; instead I plan on using a 4 section resistor array 4 X 2k to pick the best match.

[trem]

I am asking as a guy that has No Where Near the experience of most .......is 0.20 Ohms enough resistance.?
Doesn't that allow an awful lot of current through the filaments when the stand-by is on.?
Thank You

[uneumann]

I would appreciate comments and constructive criticism for this amp circuit that I have designed.
The amp part itself will be subject to adjustments and tweaking so I am mostly looking for an assessment of the bias circuit and the standby switch design witch might be a little unusual.

Nice ideas for standby/bias. Appreciate a follow up once you try it.
Just something to watch for..., although you are elevating the heaters, any impedance from cathode to ground is asking for hum problems.

[Shadrock2]

So... my heater circuit is drawing (0.6 + 0.6 + 0.45 + 0.3 + 0.3 + 0.15) = 2.4 amps. 2.4 * 0.2 = 0.48 volts that will be dropped by that resistor. (more or less) 6.3 volt heaters most of the time can work at +/- 10% of their rated voltage. So maybe in standby they are operating closer to the bottom of their rated range. Also I would've used a 0.3 ohm resistor but I happen to have a 0.2 ohm resistor. When I play, I always turn on the power and wait about 20 or 30 sec. before flipping on the standby. It might help limit the initial current surge through the cold heaters a little.

It is also possible it is much ado about nothing.

I am very much trying to use parts that I have inherited for this project.

[trem]

Another example of me thinking the opposite of what really happens.
I though that electrons move Slowly through a cold heater. If you keep them hot, they will pass electrons much faster.?
I have been wrong before, so I am use to it.
I have sen that resistor used across the HT stand-by switch to keep the filter caps partially charged, to limit a big current surge.

Yeah....sorry. I am confusing "Heater" current with "Direct Heated Cathode" current. I imagine the filament starts out with a pretty low DC resistance compared to when it is hot.....and the resistance rises Very Quickly.
That is a Nice Schematic drawing.
Thanks

[Enzo]

Conductors in general have lower resistance the colder they are, get them hot and the resistance rises. Tube heaters are like that, cold they have lower resistance than when hot. it isn't about how fast the current flows.

Clearly you have thought about all this. Just my opinion, but that half a volt drop in heater voltage is less than 8%. I am not sure that will get you much of anything. The fact that tube heaters start cold is not a problem for the tube. They all start cold, and they are designed to handle that with ease. Tubes wear out, their emission gets weak, elements get loose, but really, how many tube heaters have you seen burn themselves out? In my 60 years of doing this, I think I could count the burnt out heaters on my fingers. Oh I might have to use a toe or two, but no where near the full 21 I can count to. And I have no reason to believe any open heaters I did see were due to sold inrush. More likely due to internal shorting. I think it is a solution looking for a problem to have.

I don't know that I have ever seen a standby that left a trickle resistor in the cathode lead, but numerous amps have opened the cathode lead as a standby or in other cases just to disable a tube, as in a way to disable two of four as a half power switch. Other than the typical turning B+ off, there are numerous ways to standby. peavey tube amps have for decades turned off the screen supply to shut off the tubes. others have shorted the opposing grids together.

Have you determined what voltage the cathode drifts up to with that 62k? Consider bias and/or cutoff is a matter of voltage between grid and cathode, not voltage to ground necessarily. So if the cathode drifts up to 60 volts with -20v on the grid, that is the same as -80v on the grid alone. So talk about your cutoff. I am not sure what the advantage is leaving a half a milliamp or whatever through the power tubes, assuming they do not just cut themselves off as just described.

You have a -40v bias supply. You have 100k bias pots with parallels which we can just call 50k for discussion, and two of those in parallel. So lets call the whole thing 25k with 2k tails and headers. That makes your adjustment range 25k/29k x 40, which is about 34v. So bias will adjust from 37 down to about 3v. Do you really have use for 3v of bias? Perhaps a lower value for the pots and a much larger value for the tail resistor. The tail resistor essentially sets the lower limit to bias voltage. The whole thing is a simple voltage divider,so easy enough to calculate a new value for that lower resistor. Or if I have misunderstood something, then what is the design goal of that circuit?

[Shadrock2]

Thank you Enzo, this is exactly the kind of discussion I wanted to have.

" The fact that tube heaters start cold is not a problem for the tube. They all start cold, and they are designed to handle that with ease."
It is not to protect the tubes but to protect the power transformer. And that 0.2 ohm resistor will drop a lot more voltage initially when the current is higher. Also, I think I read somewhere on Merlin's site that a lower heater voltage can extend tube life.

"Have you determined what voltage the cathode drifts up to with that 62k? Consider bias and/or cutoff is a matter of voltage between grid and cathode, not voltage to ground necessarily."
You guessed what I had estimated almost perfectly. I figured about 0.5 ma per tube so 62K * .001 amp for 62V cathode to grid. I think I neglected the bias voltage though. But... I have elevated the heater supply I hope. I estimated that the heaters will run about 50V above ground. That seemed to lower hum in the other amp I built. Though it had a center tapped heater supply. The heater cathode rating for 6V6 is either 100V or 200V depending on how you interpret the spec sheet.
The trickle current is to help supply current for the back bias scheme which I got from Aiken's site.
"http://www.aikenamps.com/index.php/what-is-back-biasing"
The -40V is an estimate for the bias supply based on total current draw of B+. As long a milliamp flows the bias won't be less than -28V (zeners) and the preamp will also draw current. I figured about 25 ma per 6V6 + 7ma for the PI + about 3 - 4 more for the rest. Or about 60 ma total. 220 Ohm * .06 = 13V - 14V. So 13V + 28V = 41V.
Yeah I got ohms law down cold (lol). Not quite as confident of my current estimates though.
As for the 100K pots, I am building to some parts that I already have. My dad owned a TV repair shop in the 60's and he gave it up after everything became transistorized. But he had a box full (maybe 20) of these Ohmite type AB 100k pots with a screw driver slotted shaft and a locking nut. They would cost $20+ each today and seemed perfect for bias adjustment (except for being 100k). He also had hundreds of vacuum tubes though all the usual audio tubes were picked out years ago. Maybe half a bushel basket of 5U4's.
The 2K head and tail resistors are just guess's. Maybe I should increase the resistor to ground to 5 or 10k.

[Enzo]

The bias doesn't require any current to speak of, so if you want to stick with 100k pots, fine, but just do some math and think about what range of bias voltage you want to work with. Ever have an amp where the volume jumps from nothing to too loud by the time you hit 2 on the knob? You wish that 0-2 was spread over the whole range of the pot to make it easier to dial in. That is what I am suggesting here. Set the range so that you have the entire range of the pot as useful, rather than all your adjustments are crammed down at one end.

Transformer? The transformer is just heavy gauge wire wrapped around iron, that moment of increased current won't bother it a bit.

Sure lower heater voltage CAN increase tube life, but 12AX7s or other preamp tubes can last decades on plain old full voltage. Power tubes wear out because we abuse them. I mean it sure won't hurt anything, it just seems unnecessary, to me.

yes, you elevated the heaters. That has the potential to lessen hum caused by heater-cathode interaction in the tubes. Like all other hum abatement techniques it only applies to that hum source. You applied it through the virtual center tap - the two resistors - which is fine, but in a case like that, all that is really needed is to get the cathodes up high enough, and a tap onto just one side of the heaters works just as well.

Your standby trickle is to ensure current for the bias? Your standby does not turn off the B+, so there are four triodes and a pentode in there that will be conducting all the time. That ought to be 4 or 5ma right there. So the trickle would not seem necessary to that end.

And I picked 60v out of thin air. Finding out what the cathode really would drift up to is an experiment you do and then tell us.

I am clearing out my shop, and just a night or two ago, I was looking at a few bins full of those nice Ohmite/AllenBradley pots with short slotted shaft and locking nut. I have a bunch in 10k 50k 100k. I haven't used them in years, and now that I look, you come up with them too. Small world.

[Dave H]

Shouldn't the negative terminals of the two 50u filter capacitors be connected to ground and not to the centre tap of the transformer?

[g1]

And that point can't be -40 either.

[Shadrock2]

Thanx again Enzo, I have tried to think this through but haven't thought of everything. I will use a bigger resistor to ground so that the pots operate in a more useful range.

And I suppose if the cathode got to 250V it would exceed the g1 to cathode rating and ruin the tube. That would be a problem. You don't think it could go that high do you? Or, do you think it better to just drop the leak resistor (make its value infinite?) and open switch for standby. And you are correct their are other places for B+ current to leak to ground.

Some of these ideas I got from reading Kevin O'Conners books. I can't say that I had a real good grasp of how the amp circuits worked until after I studied those books.


I studied Aikin's artical and believe I understand this back bias circuit. But I have never used it before. That article suggested connecting the main filter caps directly to the center tap in order that the zener diodes do not see the full initial charging current.

Yes G1 the X-former center tap CAN be -40V with respect to ground. Ground is just 1 point in the grand time voltage continuum. Think of it as a big cathode resistor for the whole amp. That is what is unique about the circuit. And it took me a while to wrap my head around the concept also.

[Dave H]

And I suppose if the cathode got to 250V it would exceed the g1 to cathode rating and ruin the tube.

I don't think it will ruin the tube. I've used a cathode disconnect standby switch for years without destroying any tubes. If you are worried about it put a 47k across the switch to limit the cathode voltage.

I think the circuit will work OK as it is on the schematic but the filter caps will have to withstand an extra 40V. Why use zener diodes + a 220R when you could use all zeners for a more stable bias voltage?

[g1]

Yes G1 the X-former center tap CAN be -40V with respect to ground.

Oops. I was so busy looking for a bias rectifier diode I missed the zeners. Interesting concept. Sort of the opposite of using zeners the other way to knock down the B+.

Edit: or not the opposite at all (duh). Thanks pdf64.