Dropping 280 volts of B+?

[Alexo]

Hi all,

I'm building a 100 watter with a pair of 6550's for a pal o' mine. To get 100W, I need a plate voltage around 560 and a screen voltage of 300. Screen current will vary from 1.5ma per to 15ma, depending on how hard the amp is working. This means that the 100K dropping resistor I'd need at idle will reduce the screen voltage by an obscene amount once the screens start pulling some current. But if I use a big enough filter cap for the screen supply, is this even an issue?

I'm planning on using a series of 8 5 watt 33 volt zeners to get the desired voltage without this silliness, but this in itself seems silly. I've considered a separate PT, but that adds cost and takes up space.

Any ideas for a better way here? Is there something obvious that I'm missing?

...I know 8 33 volt zeners is ridiculous, but I can only find 5 watt zeners!

[Ears]

Hi all,

I'm building a 100 watter with a pair of 6550's for a pal o' mine. To get 100W, I need a plate voltage around 560 and a screen voltage of 300. Screen current will vary from 1.5ma per to 15ma, depending on how hard the amp is working. This means that the 100K dropping resistor I'd need at idle will reduce the screen voltage by an obscene amount once the screens start pulling some current. But if I use a big enough filter cap for the screen supply, is this even an issue?

I'm planning on using a series of 8 5 watt 33 volt zeners to get the desired voltage without this silliness, but this in itself seems silly. I've considered a separate PT, but that adds cost and takes up space.

Any ideas for a better way here? Is there something obvious that I'm missing?

...I know 8 33 volt zeners is ridiculous, but I can only find 5 watt zeners!

I'm using KT88s tetrode connected at same plate voltage. When using a full wave rectifier after reasonable thought I rejected the zener regulation and decided to raise the screen voltage to 350V use a resistive divider and a choke. Without a separate transformer I figure you have to accept you are also building a personal heater. A completely separate screen supply will not allow the screens to track plates and bias supplies.
Another option is to use a bridge rectified supply with a voltage doubler. Then you can obtain a screen supply at half the plate value, which if I was to start again would be the way I'd go, but I have already spent my budget for PTs.

[Alexo]

Ah, that's the ticket!

[skyboltone]

There are probably several ways to do it but they all involve regulation. I think the screens need to flop around to get the best tone. So if the screens are at 560 when the plates are at idle it's no big deal. Select your dropping resistor based on a screen current of say 7 or 8 ma. This means that when the tube is working hard your screen voltage will be at your target. When at idle, the 560 won't hurt anything.

You could use a high voltage darlington transistor and make the source voltage up from a couple of 1/4 watt or larger cheap zeners. I've got one I'll give you along with a scheme that I used to use for regulating AM modulator amplifier screens. Five green stamps for postage oughta do it. It's a regulator though. I think it will sound like ass in a guitar amp application. We used it to produce a thick midrange punch on AM voice. 3.5khz bandwidth.

I would not use a doubler for the plate supply. Too squishy.

Several guys on here can help you wire a mosfet for the same effect as using the darlington. I think with the mosfet sourced off the plate supply, the screen voltage will track plate and bias voltages as well. This is a good thing. Ask UR12.

[skyboltone]

Double

[Ears]

<snip> When at idle, the 560 won't hurt anything.

KT88 allows 560V on G2 but not 6550.

You could use a high voltage darlington transistor and make the source voltage up from a couple of 1/4 watt or larger cheap zeners. I've got one I'll give you along with a scheme that I used to use for regulating AM modulator amplifier screens. Five green stamps for postage oughta do it. It's a regulator though. I think it will sound like ass in a guitar amp application. We used it to produce a thick midrange punch on AM voice. 3.5khz bandwidth.

I would not use a doubler for the plate supply. Too squishy.

Several guys on here can help you wire a mosfet for the same effect as using the darlington. I think with the mosfet sourced off the plate supply, the screen voltage will track plate and bias voltages as well. This is a good thing. Ask UR12.

Even with Mosfet and Zeners you still have to dissipate about 18 watts so I figure that you don't gain much over a divider, don't just use a series dropping resistor though. As you say (?) you probably end up sounding too Hi Fi. My reading indicates that doublers should work OK and became quite popular for tube power amps in 60's through to end of golden age. We had a great line of KT88 amps manufatured here in NZ in 70s by Holden that used doublers - actually ran about 750V on plates and something outrageous on the screens. They sounded great and old units are still sought after.

[Ears]

[img718]http://ozvalveamps.elands.com/holden/sl ... 200wka.gif[/img]

Holden 200W

[Tavda3172]

Hi Guys,
Mouser stocks 1W zeners in 100v (part# 78-1N4764A). They also have an 82v zener in the same line. Now you only have 3 zeners in series creating a composite 3W 282v zener.
As skyboltone said above, you can use a MOSFET with these zeners to create a voltage clamp. It won't necessarily be a regulator, just a voltage clamp, meaning it only keeps the voltage from being too high... but as voltages go lower (like when the amp sags) the output voltage will follow it down. I think this is what was meant by tracking above. It is a pretty cheap/simple circuit and the power MOSFET will handle most of the dissipation. N-channel MOSFETs are cheap and more reliable than the resistor method. Good luck!
-Kevin

[skyboltone]

<snip> When at idle, the 560 won't hurt anything.

KT88 allows 560V on G2 but not 6550.



Even with Mosfet and Zeners you still have to dissipate about 18 watts so I figure that you don't gain much over a divider, don't just use a series dropping resistor though. As you say (?) you probably end up sounding too Hi Fi. My reading indicates that doublers should work OK and became quite popular for tube power amps in 60's through to end of golden age. We had a great line of KT88 amps manufatured here in NZ in 70s by Holden that used doublers - actually ran about 750V on plates and something outrageous on the screens. They sounded great and old units are still sought after.

I think we agree here. I was just presenting something other than a string of five watt zeners. The point we are making is that regulation of screens is not really a very good idea in a guitar amp. Ditto on the divider. To be truthful I hadn't thought of it that way but it would be a better solution than a single dropping resistor. Then they don't soar at idle.

As far as the voltage doubler scheme it depends on the transformer doesn't it? I mean, every high power application I've seen using a doubler just seems inadequate. If the tranny will push the current it shouldn't be a problem.

[Ears]

.I think we agree here. I was just presenting something other than a string of five watt zeners. The point we are making is that regulation of screens is not really a very good idea in a guitar amp. Ditto on the divider. To be truthful I hadn't thought of it that way but it would be a better solution than a single dropping resistor. Then they don't soar at idle.

As far as the voltage doubler scheme it depends on the transformer doesn't it? I mean, every high power application I've seen using a doubler just seems inadequate. If the tranny will push the current it shouldn't be a problem.

Yes I agree re the transformer and add its also the capacitance used. My comment on the divider is based on RCA manual recommendations, pg 87. Maybe guitar amps are different, I'm happy to be re-educated!

[tubetek]

I recently posted this in response to a similar question...HTH

[Ears]

Thanks for this, that's cool. What was the tube compliment?
Where did you post it? was there any further discussion?

[Alexo]

Wow, thanks guys. Quite an interesting discussion here. I failed to mention that this is a bass amp, so maybe regulation won't be so evil?

For some reason, voltage doublers and mosfets make me nervous.... but that is probably the most efficient route.

I think I'll spend some time doing some load lines today and see if I can't get away with lowering the OT's primary Z a little to go easier on the screens and let them sit at a higher voltage. Good point about letting them idle a little higher btw...

[Alexo]

Wow, thanks guys. Quite an interesting discussion here. I failed to mention that this is a bass amp, so maybe regulation won't be so evil?

For some reason, voltage doublers and mosfets make me nervous.... but that is probably the most efficient route.

I think I'll spend some time doing some load lines today and see if I can't get away with lowering the OT's primary Z a little to go easier on the screens and let them sit at a higher voltage. Good point about letting them idle a little higher btw...

[skyboltone]

Yes I agree re the transformer and add its also the capacitance used. My comment on the divider is based on RCA manual recommendations, pg 87. Maybe guitar amps are different, I'm happy to be re-educated!

My 50 watt 6L6 amp pushes out 110 watts in full feed back mode. Just plan for that in the calcs.