SVT Screen and Plate Currents?

[Reidk]

Hi folks. I seem to remember a posting (maybe here, maybe not) in which someone listed figures for the actual screen and plate currents in the SVT (70s vintage, IIRC) at idle and at full tilt.

Does that ring a bell with anyone? If so, can you please either quote the numbers here, or direct me to where I might find the post in question? I've done lots of searching but to no avail.

Thanks a bunch!

Reid

[g1]

This is all I could come up with. It's for 6 power tubes, but not for SVT so voltages were different.
https://music-electronics-forum.com/...485#post751485

[Reidk]

Thank you for trying!

Unfortunately, I need the SVT numbers, so this request is still open, but I do appreciate the reply.

Reid

[Jon Snell]

Idle voltages are shown in the service manual BUT all valves vary in conduction so they must be used as a guide only.
These are shown at no signal conditions. Full power will be meaningless and there are too many variables to take into account for differences.

[Reidk]

I didn't ask about voltages (those are on all the schematics), but thanks anyway.

Reid

[Jon Snell]

I didn't ask about voltages (those are on all the schematics), but thanks anyway.

Reid

Each voltage point has a resistor to the HT feed with a stated voltage.
Ohms Law is all you need to work out the current draw by using the voltage source, take away the voltage measured, divided by the resistance.

Eg;
Pin 3 anode; measure voltage across R44. Ohms Law will give you the current.
Same for all points that you require.

[Reidk]

Thank you for being so helpful, but I'm not the e-newbie you seem to think I am.

I don't have a unit on which to take measurements. Even if I did, what I need is a list of the expected values. Some of those can be deduced from the voltages and resistances that appear on the schematic, but not all, and the schematic only specifies quiescent voltages, as you noted in your previous reply. I'm also looking for the numbers at full power output, which you dismissed as "meaningless", but they aren't meaningless to me.

Thanks again for trying to help. I'm sorry you wasted your time.

Reid

[nevetslab]

Well, using the range present in the Current SVT-CL's GRN and RED LED, the GRN's on threshold is 14.7mA, and the RED comes on at 29.4mA. 22-23mA idle current is what I've been using for the past 12 years. I haven't gone back to look at the original SVT documents to see what they call out. Does your SVT have plate resistors? 5 ohm/5W is typical. That would be 115mV across the plate resistor for 23 mA for nominal DC bias. Full power? 300W into 4 ohms is 34.64VRMS, and current total into 4 ohms is 8.66A (full cycle). Each half is contributing half the current to total load. Divide that by six tubes and you have 1.443A/2 or 0.722A per tube at full power. Is that right?

[Helmholtz]

300W into 4 ohms is 34.64VRMS, and current total into 4 ohms is 8.66A (full cycle). Each half is contributing half the current to total load. Divide that by six tubes and you have 1.443A/2 or 0.722A per tube at full power. Is that right?

Sorry that calc doesn't make sense. You seem to mix up primary and secondary current.
The OT steps down voltage but steps up the current, as pimary power must equal secondary power.

Tube current is pulsating DC, essentially a half-sine with a peak value of maybe 400mA.
A tube can only conduct in one direction, so plate and screen currents must be (varying) DC.
Average DC tube current can be found from the voltage drop across the cathode resistor at full power.

[g1]

Hi folks. I seem to remember a posting (maybe here, maybe not)

I think it must have been somewhere else.

[Reidk]

Sorry that calc doesn't make sense. You seem to mix up primary and secondary current.

Yes, 722ma per tube does seem a little hot. (Why don't we have a fire emoji?)

Thanks for the effort, nevetslab.

I think it must have been somewhere else.

That's quite probable. I did find specs for the PT, which at least give a clue about upper limits, thanks to the separate wingdings. In case anyone else finds this useful:

B+: 750ma (total, not per 6550 ) This supplies the preamp as well, of course.

Screen: 70ma

Bias: 25ma

Reid

PS Autocorrect added a "g" to "windings", but it made me laugh, so I kept it.

[g1]

There was a thread where nevetslab gave cathode currents at idle and up to 200W output: https://music-electronics-forum.com/...-ohm#post38950

[Reidk]

There was a thread where nevetslab gave cathode currents at idle and up to 200W output: https://music-electronics-forum.com/...-ohm#post38950

That's not what I was looking for, but it's definitely useful. Thanks!

Reid

[Helmholtz]

Don't forget that PT winding RMS current is 60% larger than rectified DC current with bridge rectifier.

[Reidk]

Don't forget that PT winding RMS current is 60% larger than rectified DC current with bridge rectifier.

I knew it was larger, but why 60%? That would seem to be different from the factor by which the voltage increases after rectification.

Thanks!

Reid