Dropping screens voltage

[EFK]

Are there any 'tricks' to lowering screens voltage a bit other than using resistors? i.e., something w/ a zener maybe, or something else? I have an amp running 6793 tubes, plates are at about 345 w/ 122VAC at the wall and the screen are through a 1K shared resistor for @ 335 idling. Of course the datasheet says 330 max for screens. What I'd really like to do is increase the plate voltage a bit for more headroom/punch etc. (5Y3 rectifier to maybe a 5V4) but obviously the screens are going to jump up also. I can find scant info as to just how robust the 6793 screens are. Some tubes are often run way over datasheet values, to no apparent ill effect. As pricy as these tubes are, I don't want to risk it. I know I can increase screen resistance inline, but then it kind of defeats the purpose I'm going for as everything is going to compress to a greater extent. So all in all, I'd like to increase the plate voltage @ 30-40V but keep the screens where they are. Any neat ideas?

[Chuck H]

Check this out:

http://music-electronics-forum.com/t19103/

[pdf64]

Along the lines of zeners, if you'd rather keep it old school, Leslie used OC3 for that function
http://www.captain-foldback.com/Lesl...matics/147.GIF
Pete

[EFK]

I knew someone was going to bring up a regulator tube! Unfortunately there is no way I could cram one of those into this chassis, nor frankly do I want to go cutting up the amp for an extra socket etc. Would be interesting for a scratch build though - don't ever remember seeing one in a guitar amp.

Bruce Collins posted an interesting little schematic in post #3 of the Dual rail thread. So I can essentially use the secondary to power two different rectifiers? For example, I like to mount diodes on unused rectifier terminals for protective purposes, so I could use the 5Y3 to power the screens and the rest of the amp, nothing changed really, but also tap off those diodes post-rectification to run the plates at a higher voltage (as if the amp were SS rectified)? I always go a little overkill and use 3A diodes for this anyway. I'd just need to add an extra filter cap for the plates, right? Will this maintain the same load on the transformer, or will I see the screens jump up a bit because the 5Y3 is not being loaded as heavily?

Going back to the idea of a zener, I wonder if there is a way to use one or a string to 'bleed off' excess voltage off an increased screen supply, kind of how they can be used to drop the entire B+ off the PT center tap. I don't fully understand the whole zener thing though, so maybe this is not possible.

[jmaf]

This is what I've done in the past, so far no returns.

[pdf64]

A suitably rated zener could be subbed in for the OC3, in the Leslie arrangement for the screen supply. That's probably your simplest option.

[EFK]

This does seem simple but I'd like to understand how it is working. So the zener actually goes inline to the screen supply, before the cap as per jmaf's schematic above? I don't understand exactly what the 50V zener is doing there - is it somehow "eating up" 50 V? I have a good supply of different 5W zeners, so what would I use to drop say 20V? And, does this avoid the sag or compression issues inherent with using a big resistor inline? Appreciate the help guys!

[EFK]

Wait I see - it's making use of the breakdown voltage, correct? So if I stick say a 20 volt zener in there with the cathode toward the supply, it will drop somewhere around 20V across it? I guess my question then is, is this preferable to a resistor whose value is chosen to drop 20V? I ask this because I realize the compression effect large screen resistance is very real.

[jmaf]

Wait I see - it's making use of the breakdown voltage, correct? So if I stick say a 20 volt zener in there with the cathode toward the supply, it will drop somewhere around 20V across it? I guess my question then is, is this preferable to a resistor whose value is chosen to drop 20V? I ask this because I realize the compression effect large screen resistance is very real.

Yeah. It's just like a diode drop, except for zeners it's reverse biased using the avalanche effect... The Zener will drop its rated voltage. Voltage'll still be hard, the zener has close to zero resistance after it breaks down, no sag/compression like with a resistor.

Suppose a 50 volt Zener, 5 mA current on the screen, that's 1/4 Watt... I've used a 1 Watt, but 5 watts please me even more (cost is your limit, higher watt = better). If one zener you have doesn't do, chain them up.

[jmaf]

Edit: Correct, cathode points to power supply.

[log1982]

Hi guys. It's not sufficient to size your zener by only considering the quiescent grid current. You must must consider the full-signal grid current. Seems to me that a 5 watt zener is OK for two tubes, but a 1 watt zener is not good.
Joe.

[sand]

This is what I've done in the past, so far no returns.

[ATTACH=CONFIG]15970[/ATTACH]

Is there a need for a a heatsink? I've seen configurations dropping B+ from center tap that require a heatsink (using a mosfet, diode and resistors).

An amp I have has PV 470 and SV 460. I have one of those kits with a mosfet for lowering B+, but I was thinking that maybe I could get away with lowering only SV (so that I can use any type of EL34s).

Would it be a problem if PV remained at 470 and I lowered SV to 430? Thanks.

[JC1]

Want to lower the sreen voltage using zener diode (s).

I have a question...I want to lower the screen voltage going to my JCM800 4210 50w combo. I replaced the PT with a higher voltage with KT90's but the tranny isn't providing enough current to properly drive them (what a waste) so I'm planning on switching back to EL34's or Kt77's. I have NOS ones.

The PT with the KT90's has @ 520v on the plates. I'd like to just lower the screen voltage using zeners to 450v and leave the plate voltage alone. I'm thinking the plate voltage might even raise a little higher. It's a Hammond 274Bx series, 375-0-375, 201ma.

http://www.hammondmfg.com/pdf/EDB274BX.pdf

I'm assuming since the screens draw about 5ma each for a 10ma total, that's a 1/2 watt. Could I use just 1 zener rated 5w with a 50-60v rating in series coming from the B+ node to the 1k screen resistors junction, cathode side facing the B+ node, or would I need to chain a bunch of lower voltage zeners together?

Do I leave the 1k resistors on the screens?

[pdf64]

Screen current is proportional to plate current. Just as plate current will increase with signal (in a class AB amp), so will the screen current.
Also consider tube failure modes, eg screen may short to other elements within the tube, resulting in significant fault current; do you want the screen supply to be robust enough to cope with those currents without being damaged?
Pete

[JC1]

Screen current is proportional to plate current. Just as plate current will increase with signal (in a class AB amp), so will the screen current.
Also consider tube failure modes, eg screen may short to other elements within the tube, resulting in significant fault current; do you want the screen supply to be robust enough to cope with those currents without being damaged?


Pete

Right, I thought about that last after I posted this. Are you're saying that lower screen voltages to plate voltage without using an ultra linear TX to accomplish this would damage the tubes?

Or, with screen current raising with plate, thought considerably less of course, more current = more wattage = more robust zeners?

Ok, getting back to first base, a lot of people will place their zeners on the CT of the PT which of course drops the B+ porportionally to the zeners, but, why not just leave the plate voltage higher as long as the tubes are rated to except that voltage & just lower the screens to their intolerance range? The only way to do that of course with a conventional guitar amp non ultra linear PT is to lower the screens with either a large B+ voltage dropping resistor in series with the choke, thus effecting the whole preamp section too, or the zener diode trick in series from the screen B+ node to the screen resistors.

Hasn't anyone else tried this? Am I missing something?

Plan B would be the B+ dropping resistor in series with the choke & Plan C just lower the whole B+, plate included, with a zener to CT.