Sovtek 6550 datasheet question

[Randall]

With regards to biasing, I see this is a 42 watt max diss. tube, how does the 6.6 watts of screen diss. figure into this? Is it subtracted from plate diss.? I am looking at a tube running at 52mA at 461v for 24w, how does it compare?

https://www.cedist.com/sites/default...0we-sovtek.pdf

[Dave H]

It should be fine 24W is running the tube at 57% of its 42W max dissipation.
I'm pretty sure you don't have to subtract the 6.6W of screen dissipation.

[Helmholtz]

As always limiting values are independent and have to be met separately - unless otherwise noted. No problem to run the tubes colder. You might check for crossover distortion though if you care.

The original 6550 is/was a 35W plate dissipation tube.

[nsubulysses]

If you are measuring cathode current then you are measuring plate current and screen current

If you are measuring plate current, well then you are just measuring plate current.

6.6W screen dissipation is the maximum value of screen current that the manufacturer recommends before you might risk having a problem. You will not reach levels like this until you crank the amp really loud and start going into power tube distortion territory.

Screen current draw is usually not factored into bias too much because it is only 5-10% or so of total current draw at idle.

[J M Fahey]

until you crank the amp really loud

Now WHO would do THAT????

[nsubulysses]

Recently I observed screen grids to see if they glow when amp is cranked. An amp I have has some screen grid glow so I wanted to see if it's normal. It has "conservative" 460v B+, 3H 80 ohm choke, 1K screen grid, 6CA7 power tube, 1.7k OT primary. Same amp but 485V B+, 500 ohm resistor in place of choke, 6CA7, and 1.7K OT primary still has screen grid glow but less than 460V version. 460V version sounds better, I guesss because the screens are getting punished more, damn. Both are 100W 4x6CA7 amp.

Amps where screen grids glow when amp is cranked to rock territory:
Marshall JCM800 (1K screen grid, EL34)
Ceriatone marshall copy, JMP? (445v B+, 3H 80 ohm choke, 1K screen grid, EL34)
Orange OR120 (~500v B+, 1K screen grid, choke, el34)
Soldano avenger ~500v B+, choke, 6l6, 470 ohm Screen grid)
Ampeg v4 (530v B+, 500 ohm resistor in place of choke, screen grid resistor raised to 1K, 6l6, OT primary 2.9K )
Rivera knucklehead (~490v B+ {guessing B+, forgot}, choke, 1K screen grid, EL34)
Sound city 120 (460v B+, 100 ohm resistor in place of choke, EL34)
Orange OR80 (don't remember the specifics, but the screen grids glow)


Amps where screen grid doesn't glow:
MArshall JCM800 (2.2k screen grid {someone modded this from 1K or 1.5k?}, choke, EL34)
Sunn model t (530v B+, choke, KT88, 1.5k screen grid)

In my experience I always want to raise screen grid from 1K to 1.5k for less glow but it just sounds worse. Do you literally just have to abuse the tubes for good sound ? Is this rock n roll?

Sorry for the aside but this is why screen grid dissipation is not a consideration when biasing (except for a couple traynors as G1 notes). Screen grid max dissipation is only approached/exceeded during rock n roll. Straddle the line of melting the screen and it seems to sound pretty damn good.

[Chuck H]

I've had to do some funny things to mitigate screen over-dissipation in a couple of builds. This is not a problem when designing a whole amp, but it sure can be with an existing design that has a particular sound. The issue, as I've seen it, is that for most amps the OT presents a relatively high impedance to HF. So now you start clipping the amp and there's all manner of HF peaking and spikes happening. Where does that dissipate? Well, the screens, if they present a lower impedance. Which they often do. The solution, as you've found, is to increase screen circuit impedance, even if that means doing it with resistance. Some amps with larger chokes (more henries) may not do this as badly? I haven't tested this. I wonder if the much discussed "conjunctive filter" couldn't help by reducing HF impedance at the OT end.?. I do know that I use such a circuit in one of my designs that did over-dissipate the screens before the filter was added, but doesn't now. But other changes were made to the design at the same time so I haven't verified this. Reducing grid drive a little can greatly reduce peaking and spiking, but for obvious reasons this isn't a good solution much of the time with classic designs as it changes the overall gain and tone.

So, no clever solution here. Just observations on the matter.

[Helmholtz]

I can't think of a way the screens could notice plate voltage spikes as there is no connection to the OT (except in UL designs).

Generally screen current and dissisipation stay low as long as (instantaneous) plate voltage is higher or not much lower than screen voltage. But at high plate current plate voltage drops to the saturation voltage of only 50V to 75V while screen voltage stays high. Clipping increases the average time of low plate voltage and excessive screen current.

[Chuck H]

I can't think of a way the screens could notice plate voltage spikes as there is no connection to the OT (except in UL designs).

Perhaps through the tubes own internal resistance? What I know is that increasing screen circuit resistance can often solve the problem. Since resistance can be analogous to impedance when no relevant inductance is present, and the particular circumstances under which the problem seems to manifest always includes HF peaking and spikes I came to my own (albeit) non engineer conclusions on the matter. Which is really the only course to be taken since there's not a lot of "engineer" tested and observed documents on how to intentionally clip power tubes.

[Helmholtz]

So now you start clipping the amp and there's all manner of HF peaking and spikes happening. Where does that dissipate?

It depends if the spikes/ringing also show in secondary voltage or not. If yes, the energy contained will be dissipated by the load (speaker) and the active primary plate circuit.
If no, the spikes are produced by leakage (uncoupled) inductance caused by current interruption (flyback effect). They typically can be seen at the plate of the tube in cut-off. Cut-off means zero plate and screen currents.
The only way to dissipate the energy I see is by the DCR of the respective primary half.

[Chuck H]

Well please forgive my interpretation if it's entirely wrong then. But I do know that increasing screen grid circuit resistance/impedance and reducing grid drive to minimize peaking and spikes seems to solve the problem of over-dissipation in the screens. Since both can have a detrimental affect on the tone of many existing amplifier designs I think the subject of the thread was about how one might solve the problem without the drawbacks. Because of my (possibly misinterpreted and therefor misguided) experience I thought that increasing screen circuit impedance (and possibly decreasing plate impedance if possible) may be a place to start.

[Old Tele man]

Excerpted from F. E. Terman[2]

“1. To the extent that mu (µ) is constant and that Eq. (6-10) is true, the plate resistance (rp) depends on the plate current (Ip),

Eq. 6-10: Ip = K· (Vg+Vp/mu)3/2

and is inversely proportional to Ip^1/3." [emphasis mine]

[g1]

In my experience I always want to raise screen grid from 1K to 1.5k for less glow but it just sounds worse. Do you literally just have to abuse the tubes for good sound ? Is this rock n roll?

I think screen supply node 'stiffness' has a big part in this (screen glow and it's tonal impact).
Choke or dropper resistor, dropper resistor value, capacitance at screen node, screen grid resistor value, are all factors in this.
Measuring the idle screen voltage may not tell you much about how much voltage is at the screens at full sag.

However, if you've found the tone is always poorer when the screens don't glow, I don't think there is much you can do.

[Chuck H]

I think screen supply node 'stiffness' has a big part in this (screen glow and it's tonal impact).
Choke or dropper resistor, dropper resistor value, capacitance at screen node, screen grid resistor value, are all factors in this.
Measuring the idle screen voltage may not tell you much about how much voltage is at the screens at full sag.

However, if you've found the tone is always poorer when the screens don't glow, I don't think there is much you can do.

And this is especially unfortunate because some vintage designs were already abusing the screens with less fragile tubes than what is available now. As a result I've had to buy several sets of tubes on two projects to get a single matched set that didn't fail. On that note I've been using Ruby tested/branded el34's. But they tend to be mechanically loose and microphonic so they aren't a good choice for combo amps.

Also... I still think, and will until an experience of my own or someone here proves otherwise, that increasing the HF impedance of any choke feeding the screen circuit may help without detriment to an amps fundamental sound.