no screen resistors 6V6

[iknowjohnny]

Is there a reason fender used no screen resistors on 6v6 amps? Should i use them on a 18 watt build of mine that i converted to 6v6 from el84?

[paulster]

Because tubes were cheap, stout and expendable in the good old days.

These days they're not, so screen resistors are a good thing!

[iknowjohnny]

Thanks.

[Structo]

Yes it just helps the tubes. Keeps the potential a little lower than the plates so it adds longevity to the tubes.
Makes them happy!

[dave g]

Keeps the potential a little lower than the plates so it adds longevity to the tubes.

Not quite. The screens don't care if they're at the same potential as the plates. In fact, they typically work best when they are. The actual function of the screen resistors is to dissipate power that would normally be dissipated in the screen, as the screen current has a tendency to increase when the plate current increases.

[Andy Le Blanc]

if the voltages are lower, RCA typical 250/285, it's ok. It has a tone too.
You wont get the same watts in the end but it alright driven to a degree.
Old school keep it stupid simple, try pairing that screen dress, with a low
plate voltage and a fixed-paraphase inverter using zero-bias. I think there's
a car radio receiver in the schematic section of some of the older rca manuals
that used the inverter. low voltage

[Structo]

Keeps the potential a little lower than the plates so it adds longevity to the tubes.

Not quite. The screens don't care if they're at the same potential as the plates. In fact, they typically work best when they are. The actual function of the screen resistors is to dissipate power that would normally be dissipated in the screen, as the screen current has a tendency to increase when the plate current increases.

The screens might not care but the plates will.

[RJ Guitars]

Several of us built something I call the KT-66 Rockster amp which was basically a Trainwreck Rocket preamp into a set of Cathode biased KT-66's. As one fellow finished up his build he set out to document some of the voltages. Much to his surprise he discovered that the screen voltages were actually higher than the plate voltages. These had already proven to be great sounding amps and yet nobody else had noticed this little voltage issue.

Everything about this seemed wrong to me and since I had designed the amp and sold him most of the parts I really took the challenge to figure this out upon myself. After comparing notes and voltages with a few others we generally found similar results...

Ultimately with the help of the "Atomic City Audio Nerdz" (google them for info) and a few other gurus it all came to light that those plates are working pretty hard and drawing some serious current but the screens are not. Ohms law works and while the reality of it was a bit non-intuitive it turns out that this can and does happen and it does not mean that something is disastrously wrong.

There is a thread on it somewhere here on TAG. A great lesson for me to learn from and still one of the best sounding amps I've ever built.

If the tube can handle it, extra volts on the screen are not likely a problem - IMHO.

rj

[Structo]

So what did you atribute that to?

The tubes or the circuit?

[RJ Guitars]

So what did you atribute that to?

The tubes or the circuit?

Well it's likely the circuit and especially the ultimate B+ voltage we were using. I believe the big voltage drop on the plates was due to the big current draw and the big current draw was caused by the use of a lower voltage secondary / B+. With a lower plate voltage you can really crank the bias up.... and we did. I don't know exactly what that would do for tube life expectancy and where the operating point of the tubes actually turned out to be but the amps sounds great and I've had the same tubes in it for quite a while now.

In sorting out the problems, One of the gurus had me put in bigger screen resistors but I had to go to crazy values before the screen voltage dropped down significantly. He also had me switch the B+ nodes and still the B+ was higher on the screens. The crazy part was that several of us had built these amps and they sounded great.

it was my intuition that the bigger voltage on the plates would have electrons turning around before they got to the plate and being pulled back onto the screen... it didn't seem like it should work. But although there is some theory to uphold that, it doesn't really happen. Those electrons are really cruising when they pass the screen and their momentum takes them right to the plate just fine.

I don't think we would see this if we would have used a higher B+. I haven't done it yet but I really want to watch the plate voltage as I run the bias up and down on the cathode. This requires a variable cathode resistor on a cathode biased amp, which I happen to have included on that amp.

It was quite interesting.

rj

[Richie]

As one fellow finished up his build he set out to document some of the voltages. Much to his surprise he discovered that the screen voltages were actually higher than the plate voltages.

did you add in for the cathode voltage? is there a schem of this amp somewhere?

Some marshall amps didn't use screen resistors either.

[RJ Guitars]

As one fellow finished up his build he set out to document some of the voltages. Much to his surprise he discovered that the screen voltages were actually higher than the plate voltages.

did you add in for the cathode voltage? is there a schem of this amp somewhere?

Some marshall amps didn't use screen resistors either.

Say more about adding in the cathode voltage... and why we would do that? Both the plate and the screen voltages were measured relative to ground. This was a cathode biased amp if that matters. Sounds like there is still something for me to learn here.

The schematic -- https://tubeamparchive.com/download/file.php?id=7748

thanks,

rj

[rhinson]

i'm not sure what the op was talking about with fender not adding in screen resistors-----i think if you look at any fender schematic (tweed, blackface or silverface) you'll always see something in there dropping the screens-----except on a few in the "low powered twin" era where they used the "brute force" power section with a cap/choke/cap first sections and tied the screens to the ct.

in cathode biased amps you subtract the cathode voltage from the overall plate voltage, since this difference is what the plates "see". this means you can run a cathode biased amp a bit "hotter" than you think you can, all things being equal. ie---you have 2-6l6's at 425v and cathode biased you have 32v across the cathode resistor. this means the tubes "see" 393v---so you may be able to run them a bit hotter since the "real" plate voltage is 393v. rh

[Cliff Schecht]

Several of us built something I call the KT-66 Rockster amp which was basically a Trainwreck Rocket preamp into a set of Cathode biased KT-66's. As one fellow finished up his build he set out to document some of the voltages. Much to his surprise he discovered that the screen voltages were actually higher than the plate voltages. These had already proven to be great sounding amps and yet nobody else had noticed this little voltage issue.

Everything about this seemed wrong to me and since I had designed the amp and sold him most of the parts I really took the challenge to figure this out upon myself. After comparing notes and voltages with a few others we generally found similar results...

Ultimately with the help of the "Atomic City Audio Nerdz" (google them for info) and a few other gurus it all came to light that those plates are working pretty hard and drawing some serious current but the screens are not. Ohms law works and while the reality of it was a bit non-intuitive it turns out that this can and does happen and it does not mean that something is disastrously wrong.

There is a thread on it somewhere here on TAG. A great lesson for me to learn from and still one of the best sounding amps I've ever built.

If the tube can handle it, extra volts on the screen are not likely a problem - IMHO.

rj

I believe in having the screen at a higher potential than the plate you are generating some internal negative feedback in the power tubes. While doing this slightly reduces the gain, I would think that the bandwidth would increase and make for a more pronounced power stage (more mids/highs) that is easier to overdrive, especially operating at a lower voltage. Does this at least sort of describe the sound of the amp (more specifically, the power stage)?

[rhinson]

the screens being higher than the plates in a "normal" type power section like most of the ones we build (ie--has a choke then cap for the separate screen supply) is totally due to: a) how the output trannie is wound realative to b)the the power tubes you're using with it. if you have a trannie with a low dcr on each pri leg (say an express <50ohms) and you're using 6l6's/6v6's (neither of which draw much screen current) you would probably have them at about equal. if you're using an a470 (maybe >190 on one leg and 210 on the other) or a deluxe reverb trannie (maybe 300 or 400ohms on each leg) you'll have the screens above the plates, because of the voltage drop across the realtively higher winding resistance of the trannie due the the plates current draw. with real el34's this hardly ever occurs because el34's draw much more screen current than the other tubes we use---can be as much as 8 or 9ma at idle (compared to only 2ma with 6l6/6v6 etc.). rh