Screen Grids And Ways To Skin A Cat

[rp]

“The circuit shown' uses a unique arrangement for obtaining a low impedance power-source for the screen grids”

I was looking through an old Acro catalog <http://www.clarisonus.com/Archives/Trans/Acro55.pdf> at the schematics and noticed this little oddity - a different way to drop screen voltage. It’s a choke input so I guess it regulates somewhat. Not sure why it’s needed in this case, maybe because of the more feeble metal 6L6s. People here are often looking to drop screen voltage so this might be something to try out. It’s an LC but there’s not much current on the screens and a small, cheap choke can be used so it doesn’t seem particularly excessive or impractical.

Don’t forget that 50K bleeder to protect the filter cap from cold start or tubes out over-voltage with the choke input. I’ve seen a .22/1kV film cap added before a choke input but that was for the whole amp not just the screens, so it may be fine as is. If anyone tries this, report back.

[tubeswell]

Another way is to have the plates and screens on completely separate secondary windings.

[Kagliostro]

May be I'm wrong ....

to me seems that we have a partition resistor voltage divider

the first resistence is rapresented by the inductor resistance, the second resistence is the 50K resistor, the e-cap is there to stabilize the voltage

[img:178:168]http://roboticsju.weebly.com/uploads/7/ ... 669146.png[/img]

Franco

[Kagliostro]

I was so curious about this thing that I asked to my friends at EL34world

if you are interested here is the thread, read replay #6
(there is a good explanation)

http://el34world.com/Forum/index.php?to ... 223042#new

Franco

[martin manning]

See attached Hammond rectifier configuration sheet. With modern silicon rectifiers using both sides of the HV secondary (upper right diagram showing FW choke input filter) would be easy and cheap. All R-C and L-C filters are voltage dividers, but since they have reactive elements the voltage division is frequency dependent. In that case you have L-R//C, with cut-off frequency ~12Hz.

[rp]

http://www.turneraudio.com.au/audiofilt ... page2.html

I was reading up looking at Williamsons and the Radio Craftsmen 500 orig and later 500A version (both triode not UL). Wondered why the later new improved version used an LC where the orig had a CLC. As a guitar amp sophomore builder I thought choke inputs were something obsolete from the paleolithic like trannies for phase splitters. Turns out the LC input / swinging choke gets a lot of respect from the hifi guys to this day. Turner gives the pros and cons and plenty of technical info.

FYI Here's the RC500 with its nice and 'primitive' supply. The 1100V PT is a mod the orig was a Stancor 1200V. The first model with a CLC had a 840V PT and a 5V4. Thinking the 500A supply might work well in a bass amp or if you find yourself with a nice but too high voltage PT you don't know what to do with.

[TUBEDUDE]

Using iron for interstage and phase splitting is not altogether extinct.

[rp]

Using iron for interstage and phase splitting is not altogether extinct.

Agreed, it's exotic.

Another way is to have the plates and screens on completely separate secondary windings.

Unless you don't have the windings The way in the Acro schematic is something you can try with a diode, a resistor, and a junk box choke.

[Kagliostro]

I had this problem in an amp I'm planning

I want to use a 1625 (same as 807, different heater voltage)

this tube don't like high G2 voltage and I haven't a PT that allow to

feed G2 with an adequate low voltage

A drop resistor from B+ is not adequate because of the oscillation on current demand and so on voltage feeded to G2

I asked and a friend give me one possible solution (that I inserted on my schematic)

read replay #12

http://el34world.com/Forum/index.php?to ... #msg217907

K

[rp]

BTW maybe off track but concerning 807s. I came across a post on DIYAudio forum that I'll likely never find again, but basically one of the poobahs there was saying that what applies to a tube, in this case the 807, at 800V doesn't also apply at 450V. So, at 800V you need to keep the 807 screens <300 but as the plate V comes down the screen V can come up w/o problems.

One of the Williamson styles has a paper from one of the 1950s USA E-mags with the American use of 807s over KT66s, pretty much straight up swap into the 1947 Williamson, article author said they had been running their prototype for 6 months and counting at 450V with screens close to plates with no problems whatsoever.

[Kagliostro]

Yes, you re right, also I've info that say near the same thing

Franco

[rp]

I hate hearsay, here's my references for the 807s, from the below database somewhere. I'd tell your friend that if he's running them at <450V to just go for it, 807s are pretty cheap.

http://dalmura.com.au/projects/Williamson.php

[Kagliostro]

Thanks for sharing

Only a thing, I don't want to use the tube triode conneced

but I'm not so skilled to say if this can be the difference

Franco