Screen Resistors and Voltages - 5E7

[Klugetone]

Need some opinions on adding screen resistors to my 5E7 clone. As it is, the plates are driven off the OT primary fed with the B+ line. The screens are fed directly with B+. No screen resistors at all on this version. It is a hair splitting issue I know but the Screen voltages are slightly higher (+3 or 4 volts) than plate at idle due to the slight drop from resistance in the OT winding. Would getting the screens slightly lower than the plates at idle tend to be of any substantial benefit or more bother. Resistors would offer some protection of course. The amp sounds great right now with the power tubes pulling about 40 mA each.

By the way, B+ is around 430 vdc.

Anyone have thoughts on this? Should screen resistors be considered as part of any PP design? This circuit has been around for a long time and is certainly proven but were the screen resistors considered an unnecessary expense? Fender and others took all kinds of liberties with established theory of the day so was wondering if this was one of them.

Thanks!

Dave

[loudthud]

Screen resistors protect the screens when the amp is driven to clipping. At clipping, the plate voltage is very low and the screen current shoots up, it can be almost as high as the plate current. Screen resistors cause the screen voltage to drop when the plates clip. Don't worry about the screens being slightly higher than the plates at idle, 95% of the cathode current still goes to the plates.

[Klugetone]

Thank You! It has always been a question as to why some amps use screen resistors and others don't. Fenders in particular come to mind. Even models that are very similar to the 5E7 such as the 5F6 Bassman use them. They do also have slightly different HV arrangements. The Bandmaster is a bit more stripped down - probably to meet a price point as much as any other reason.

Anyway, I am changing out the ceramic octal sockets in the amp and generally making the wiring a bit more tidy in that area and thought about adding the resistors while I had it apart.

Thanks again!

Dave

[stokes]

"Fender and others took all kinds of liberties with established theory of the day so was wondering if this was one of them"."Established theory" is intended to get absolute efficiency in a circuit.It is more critical in a hi-fi situation where you want sound reproduction with no distortion whatsoever.Guitar amps are better sounding when they "color" the signal rather than reproducing it clean.I'm sure there were many reasons parts are eliminated in guitar design,could be just to meet a "price point" originally,but it could be because,thru trial and error,they found it was both cost effective and still produced an acceptable output from the amp.

[mooreamps]

for that this is worth. I've found different values of SGR mostly affects the touch response of the power tubes. thus, I use different values depending on what type of power tubes I'm using....


-g

[Klugetone]

Thank you all for the input on this!

for that this is worth. I've found different values of SGR mostly affects the touch response of the power tubes. thus, I use different values depending on what type of power tubes I'm using....
-g

Looking at the interactive voltage and current effects on the power tube elements, this makes sense. I'm wondering, in your experience, what range of SGR values you have found to give more or less response?

Thanks again!

Dave

[Wombaticus]

"Fender and others took all kinds of liberties with established theory of the day so was wondering if this was one of them"."Established theory" is intended to get absolute efficiency in a circuit.It is more critical in a hi-fi situation where you want sound reproduction with no distortion whatsoever.Guitar amps are better sounding when they "color" the signal rather than reproducing it clean.I'm sure there were many reasons parts are eliminated in guitar design,could be just to meet a "price point" originally,but it could be because,thru trial and error,they found it was both cost effective and still produced an acceptable output from the amp.

I don't think the "adding screen stops to old circuits" debate is going to be resolved anytime soon, but I think it's telling that Fender themselves, when they reissued the Tweed Deluxe as the "'57 Deluxe", went ahead and added 470R screen stops to their own hoary 5E3 circuit:

http://support.fender.com/schematics...UN%2007%29.pdf

Did a couple of other interesting little modernization changes too, like adding silicon rectifier diodes in series with the 5Y3GT.

[GregS]

I had this dilema when I recently built a copy of a Marshall 2061x. The schematic had no screen resistors but everything I'd read said you should have them. I ended up leaving them out to have the circuit to spec. So far there haven't been any problems after about 8 months of use although certain tubes were prone to redplating in it.

As I understand tubes manufactured in the 50's, 60's were more ruggedly made than modern tubes and could stand more abuse. For this reason screen resistors would have been less critical for amps made in this time.

Greg

[mooreamps]

Thank you all for the input on this!



Looking at the interactive voltage and current effects on the power tube elements, this makes sense. I'm wondering, in your experience, what range of SGR values you have found to give more or less response?

Thanks again!

Dave

Let's see. A 5E7 is the Fender Bandmaster using 6L6G. For this tube I use 1 K ; 3 watt ; flameproof. Secondly, I read some of these comments like people are still referring to screen grid current as a constant ; instead of as a variable. I have a couple power tubes in my stash, almost no measurable screen current from them. I can only assume when these tubes were made the wires on the screen were set-up in perfect alignment with the wires on the control grid. But when used in such a harsh environment like gigging on the road, the amps get thrown in back of some truck while they are still hot, it's going to knock the screen wires out-of-shadow with the wires on the control grid, and that's where the screen current comes from...

Vacuum tubes are not electrical devices. They are mechanical devices ; "with electrical properties".

-g

[fyl]

As I understand tubes manufactured in the 50's, 60's were more ruggedly made than modern tubes and could stand more abuse. For this reason screen resistors would have been less critical for amps made in this time.

Tubes were better, amps were used at low to mid levels, playing styles were less aggressive. Screen R's a re *must* today if you fancy reliability, 470R/3W for small medium slope tubes, at least 1K/3W for larger bottles and high slope tubes. Double this if the amp is going to be played 100% in crunch, OD or disto.

[Klugetone]

To All: Thank You! Exactly the kind of info I was hoping to learn about. I know theoretically what is going on with screens, currents, voltage and the plates who love them but it is another thing entirely when you have a resource like this with so many people with great, real world experience.

Thanks again

Dave