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Screen Grid Voltage

2/11/1998 7:25 AM
Dave Eames	Screen Grid Voltage I have a pair of Bogen PA amps that I am reworking for guitar amp apllication. I have several years experience working on musical instrument tube amplifiers but I am puzzled why the Screen Grid voltage (355) on these amps is more than 100 volts less that the Plate voltage(460). The amp uses a pair of 6L6GC's for outputs and they are cathode biased at around 28ma. The amp operates fine. Any insight into this design would be appreciated.

2/11/1998 7:25 AM

Dave Eames

Screen Grid Voltage

I have a pair of Bogen PA amps that I am reworking for guitar amp apllication. I have several years experience working on musical instrument tube amplifiers but I am puzzled why the Screen Grid voltage (355) on these amps is more than 100 volts less that the Plate voltage(460). The amp uses a pair of 6L6GC's for outputs and they are cathode biased at around 28ma. The amp operates fine. Any insight into this design would be appreciated.

2/11/1998 10:21 AM
Doc	Probably conservative design for long, trouble-free life as a reliable PA amplifier.

2/12/1998 3:10 AM
stephen	Dave, It's an example of conservative amp design (the opposite pole from Leo Fender trying to squeeze 20 watts out of a a pair of 6V6's) The Bogen amps are old-time PA amps -they were built to last...and to run for years with minimum attention. If you are converting to guitar amps there's the more subtle point,that the same tubes may produce a slightly different tone when run with a lower screen voltage. Don't feel that this will 'spoil' your amps for guitar use -I've tried it both ways and I'm still not sure which I prefer. Particularly if you intend to use modern tubes and push them hard sometimes, consider that the lower screen volts will be kinder on slightly out-of- -line grid assemblies. BTW, I'm interested in old tube PA amps, and in NZ we don't have anything remotely like the choice which you guys have in the US. If you have a schematic for your amps (or finally bite the bullet and hand-draw one) then I would be grateful for a copy. I'll happily pay your postage, or trade tou for some old Brit and NZ amp schems. Hey...let us know how the conversion progresses. . - Stephen

2/12/1998 3:10 AM

stephen

Dave,
It's an example of conservative amp
design (the opposite pole from Leo Fender
trying to squeeze 20 watts out of a
a pair of 6V6's) The Bogen amps are old-time PA amps -they were built to last...and
to run for years with minimum attention.

If you are converting to guitar amps
there's the more subtle point,that
the same tubes may produce a slightly
different tone when run with a lower screen
voltage. Don't feel that this will
'spoil' your amps for guitar use -I've
tried it both ways and I'm still not
sure which I prefer.

Particularly if you intend to use
modern tubes and push them hard sometimes, consider that the lower screen volts
will be kinder on slightly out-of-
-line grid assemblies.

BTW, I'm interested in old tube PA
amps, and in NZ we don't have anything remotely like the choice which you
guys have in the US. If you have a
schematic for your amps (or finally
bite the bullet and hand-draw one)
then I would be grateful for a copy.
I'll happily pay your postage, or trade tou for some old Brit and NZ amp schems.
Hey...let us know how the conversion
progresses. .
- Stephen

2/12/1998 7:38 AM
Dave Eames	Stephen, Thanks so much for your input. I have two of these Bogens(model CH33) The only difference between the two is that one is set up for dual rectification with (2) 5Y3GT's and the other is using a single 5U4. I will be getting around to drawing up a schematic, so as soon as I do, I will get you a copy. The amps sound pretty darn good the way they are right now, so my intention is to just modify the Tone circuits to a more Fender/Marshall configuration and re-cap. Again thanks!

2/12/1998 7:38 AM

Dave Eames

Stephen,

Thanks so much for your input. I have two of these Bogens(model CH33) The only difference between the two is that one is set up for dual rectification with (2) 5Y3GT's and the other is using a single 5U4. I will be getting around to drawing up a schematic, so as soon as I do, I will get you a copy. The amps sound pretty darn good the way they are right now, so my intention is to just modify the Tone circuits to a more Fender/Marshall configuration and re-cap. Again thanks!

2/12/1998 11:09 AM
Bruce	Stephen, I wonder what you sense is the tonal difference are between the two" tube sounds" with lower and higher screen voltages. Cleaner, brighter, rounder, etc. I am of the group that likes to keep the screens about 3-5 volts or so below the anode voltage. I have a couple (country/western players) cutomers that are looking for a much cleaner sound then what a normal 30-50 watt tube amp will do when played loud but they also hate transistor amps! You think droppping the screen voltage down a bunch will clean them up a bit and still allow the tube to have a strong output? Curious... Bruce

2/12/1998 11:09 AM

Bruce

Stephen,
I wonder what you sense is the tonal difference are between the two" tube sounds" with lower and higher screen voltages.
Cleaner, brighter, rounder, etc.
I am of the group that likes to keep the screens about
3-5 volts or so below the anode voltage.
I have a couple (country/western players) cutomers that are looking for a much cleaner sound then what a normal 30-50 watt tube amp will do when played loud but they also hate transistor amps!
You think droppping the screen voltage down a bunch will clean them up a bit and still allow the tube to have a strong output?
Curious...

Bruce

2/12/1998 12:35 PM
J Fletcher	I ran a test on a homebrew 40 watt amp.Originally it had 2 Sovtek 5881's with 425 volts on the plates and screens.I dropped the screen voltage by (if I remember correctly)75 volts by using zeners in series with the screen dropping resistors.The sound of the amp didn't seem to change,however I had to rebias and change to a different tap on the output transformer to get it back to full power.The reflected impedance had to be doubled as I recall.By the way,I play C+W,and find I need about 75 watts from a tube amp,to get the nice clean tone that's required with a moderately loud band.

2/12/1998 12:35 PM

J Fletcher

I ran a test on a homebrew 40 watt amp.Originally it had 2 Sovtek 5881's with 425 volts on the plates and screens.I dropped the screen voltage by (if I remember correctly)75 volts by using zeners in series with the screen dropping resistors.The sound of the amp didn't seem to change,however I had to rebias and change to a different tap on the output transformer to get it back to full power.The reflected impedance had to be doubled as I recall.By the way,I play C+W,and find I need about 75 watts from a tube amp,to get the nice clean tone that's required with a moderately loud band.

2/14/1998 11:22 AM
Bruce	Yes, I agree. I think these guys both need about twice the 35-50 watts they are currently using. But that isn't what they have so I thought maybe I could help clean up the power a bit, I did try the zener in the screen circuit last year and also did NOT notice a big difference in the clean tone. But I did get the experience needed to convince me that it is OK to drop the screen voltage quite a bit lower then I once had thought was needed for that wide open tone. All I had was a big bunch of 36v/5 watt zeners though and that is what I used. I forgot to try a change on the OT load though, darn it! Also, I did try yanking all 4 of the 6L6s out of an old Twin Reverb and replaced them with 6V6GTAs after inserting a pair of the 36v/ watt zeners in series with the screen voltage. That and a bunch of 1K/5watt resistors that Kevin O' is recommending now. That zener/1K mod dropped the screens down to about 400 volts and the amp ran fine for hours and hours like that with 460 volts on the plates of the 6V6GTAs. It sounded pretty good too. Any other OPINIONS? Bruce.

2/14/1998 11:22 AM

Bruce

Yes, I agree.
I think these guys both need about twice the 35-50 watts they are currently using.
But that isn't what they have so I thought maybe I could help clean up the power a bit,
I did try the zener in the screen circuit last year and also did NOT notice a big difference in the clean tone.
But I did get the experience needed to convince me that it is OK to drop the screen voltage quite a bit lower then I once had thought was needed for that wide open tone.
All I had was a big bunch of 36v/5 watt zeners though and that is what I used. I forgot to try a change on the OT load though, darn it!
Also, I did try yanking all 4 of the 6L6s out of an old Twin Reverb and replaced them with 6V6GTAs after inserting
a pair of the 36v/ watt zeners in series with the screen voltage. That and a bunch of 1K/5watt resistors that Kevin O' is recommending now.
That zener/1K mod dropped the screens down to about 400 volts and the amp ran fine for hours and hours like that with 460 volts on the plates of the 6V6GTAs. It sounded pretty good too.

Any other OPINIONS?
Bruce.

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