Early beam tetrode output stages and screen voltages

[Rhodesplyr]

When beam tetrodes like the 6V6 and 6L6 were first used for push-pull audio output stages in the late 1940s, designers often tied the screens directly to the center tap of the output transformer, running them at full B+ voltage, slightly higher than the tubes' plate voltages due to output transformer DCR. (Hammond and Leslie both did this. Leslie abandoned the practice fairly quickly; Hammond, as usual, took longer to change.)

I was wondering how this idea of running screen grids directly from the B+ supply became so widespread and what the supposed advantage was. I've heard that it tended to make amps unstable.

Just curious...

David

[Jazz P Bass]

I do not know if this applies, but weren't most of the first push-pull amps cathode biased?
And yes, no screen resistor.

[Enzo]

It was simple, and it worked.

[jrfrond]

In addition, the lower B+ voltages and resulting lower power output du jour, together with good, sturdy tubes didn't make screen current much of an issue.

With current balls-to-the-wall, SOA-be-damned guitar amp power stages and Russian (or worse, Chinese) tubes, I wouldn't dream of designs like that, unless tube pyrotechnics is a new field you are looking to explore. :O As I stated in another thread, I am writing my own rules on screen current limiting by DOUBLING traditional screen resistor values to increase tube life, and it has been working so far.

[Steve Conner]

Also, back in my great-granddad's day, people didn't overdrive amps. The screen current rises drastically under overdrive, whence John's tube pyrotechnics.

[pdf64]

'screen current limiting by DOUBLING traditional screen resistor values to increase tube life'
I've found that can reduce power output a little and give a 'looser' feel high power clean tones (cue missile launch from St Petersburgh!). Maybe because of the local negative feedback that increases due to the characteristic that Steve mentions, which can also be used for self split push pull outputs, as noted in
http://music-electronics-forum.com/t24022/
Have you tried adding resistance, eg in series with the choke, to the screen supply node. To my thinking, 470 ohms there is worth 1k more on each tube screen grid, thereby lowering the screen grid node voltage under overdrive (=heavy current draw from screen grids), but without increasing the local NFB that raising the value of the individual screen grid resistors does.
Hope that makes sense! Pete.

[Rhodesplyr]

To Jazz P Bass: Hammond tended to use fixed bias in its early amps, often deriving the negative grid voltage via the 250 Ohm field coil resistance of one of the speakers, connected between the main audio ground and the power transformer center tap.

To John: Yes, in the amps I'm thinking about, you do find lower B+ voltages (280V) and conservative operating points, e.g., four 6V6s running at a total of 10 Watts each plate + screen dissipation for 20 Watts output. Also, in these amps, power supply energy storage was minimal (6uF oil blocks), and tube rectifiers and chokes acted as current limiters. Hammond used choke-input supplies and often used two chokes in series.

But Leslie in their "Series III" version of their 31H amplifiers started dropping the screen voltage below plate voltage and quietly recommended to people servicing the first two versions that they update those amps to the third version for better stability and reliability. Leslie used capacitor-input filters and larger filter capacitors (30uF). Would that have made full B+ screen voltages more likely to cause trouble?

To Enzo: I'm just playing circuit archaeologist here. Sometimes I like to try to figure out why designers did things a certain way, especially when they later abandoned that approach.

[riz]

I recently added a screen resistor to a Champ, and I watched the tube in the dark before and after. EH 6v6. BIG difference! The OD tone was better, too.

[jrfrond]

Doubling the SG resistors in a typical guitar amp design has nearly no measurable or even audible effect on output level. Sure, you can REALLY jack up the values to "soften" it a bit and approach the triode feel, but I am just talking about limiting current in amps that are used in a rehearsal studio (actually, "tortured" is the verb I should have used). I also employ R3000 FR diodes on each side of the OT primary to squash transformer-eating HV transients, which helps the tubes a bit as well.

What I am getting at here is that we all should start rethinking those "traditional" SG resistor values, foregoing them for higher values to work better with the new generation of tubes. Or, you can just parlay the old way into extra tubes sales. That is up to you, but I sleep a bit better at night knowing that I am doing my best to increase reliability for the client.

[mooreamps]

What I am getting at here is that we all should start rethinking those "traditional" SG resistor values, foregoing them for higher values to work better with the new generation of tubes.

We already have.. Where have you been ?????

-g

[redelephant]

+1 on the screen resistors. There is a great read we all should study: 'Getting the Most Out of Vacuum Tubes' which talks about tube life and designing for reliable tube operation, (dwnloadable from P.Millet's website)
According to that book having no series resistors on the screens is regarded as poor engineering and even fixed bias is regarded as risky business.
I think in the old days the designers needed the most watts per dollar, while today we have mike'd cabs and high power PA systems. So today we can afford the luxury of designing for max tube life. But then again, if you want to build clones true to the original, I guess you're stuck with replacing tubes every few years.

[Steve Conner]

Pete: It all boils down to how much resistance you put before the screen supply filter capacitor, as opposed to after it. Resistance after it reduces the gm and peak current capability of the tube, giving reduced power output and earlier clipping.

Resistance before it still allows high peak power. The gm isn't reduced because the screen is decoupled at AC, but the limited screen current squashes the average power, with an audible time constant like a compressor. This may or may not be desirable from a tone point of view. Because screen current increases non-linearly, the effect might be minimal until the tubes are heavily overdriven. Some old Bogen PA amps were wired that way, with a capacitor after the screen dropper.

If you go down the "MOSFET Follies" route, there's no reason not to run the screens from a MOSFET dropper, which can have a current limiter built into it. That would take the non-linear idea further, it would do absolutely nothing until screen current reached a dangerous level, then it would squash the signal. I think if you did this and also slugged it with a capacitor, you would have a very nice circuit that sounded as "period" as possible without actually endangering anything.

Of course the purists insist on the period accurate screen voltages and resistances, which soon destroy the tubes when subjected to the abuse of the post-Hendrix era. And if you go down the MOSFET follies route the logical conclusion is an entirely solid-state amp.

[jrfrond]

We already have.. Where have you been ?????

-g

I've been right here!

Not really Gary. Only SOME manufacturers have caught on. The bulk of production amps and a fair amount of customs I've seen a sticking steadfastly to the 470 ohm and 1K or 1.5K resistor values for beam tetrodes and power pentodes, respectively, suggesting to me that too many designers have either buried their heads in the sand, or just aren't privy to the shortcomings of new tubes.

[Rhodesplyr]

Pete: It all boils down to how much resistance you put before the screen supply filter capacitor, as opposed to after it. Resistance after it reduces the gm and peak current capability of the tube, giving reduced power output and earlier clipping.

Steve, you may just have inadvertently answered a question I've had for a long time. In their third version of their first amp designs, Leslie dropped voltage to the screens--and used NO filter capacitor for this leg of the power supply. Though they probably exist, I don't think I've ever seen another amp that had no filter capacitor for the screen supply, but I guess that would help limit current. Leslie also added a fuse in series with the dropping resistor.



That suggests to me that they might have been doing several things at once to try to limit excessive screen current. The speaker's field coil drops voltage through the same resistor, providing shunt regulation.

David

[imaradiostar]

Hey John, I think you have a good point. If you don't mind my asking, what values do you prefer for commonly available modern tubes? (as in what do you use for a 6l6, what do you use for an EL34, etc)

I've found that I need to keep the screen resistors smaller on beam power tubes for they don't make much power. On true pentodes it seems like I can go quite a bit larger and still keep decent power output. I like Steve's idea of limiting the screen current. It would be nice to be able to tune the screen current limit to prevent the screens from lighting up like a light bulb filament!

jamie