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FDP Forum / Amp Mods, Repairs, and Projects / On Fender SF Champs , does anyone know why the screen is higher than plate


LA , Calif

I try my best
Nov 13th, 2016 03:29 PM   Edit   Profile  

I edit to say. I've spent days trying to find info I can understand on this .I can build an amp off the schematic and have build about tube amps and replaced caps and tube sockets because they were old in three small fender amps. All I need if possible is something put into simple terms. I've really tried to look at tube charts and graph's and my eye's glaze over. I understand what not to do so not to burn anything up or kill myself.
never mind i figured it out.
I get lost when it comes to 6V6 cutoff or finding the center bias of the tube. I built amps the sole reason that they cost so much to buy . Also I do enjoy doing it. The most I ever spent was on a Fender 1994 blues Deluxe and I traded 4 small amps in to get that.

I'm not there when it comes to all the tube amp vocabulary. I had a 470 ohm 3 watt as a 6V6 screen resistor , can't remember why I put one in so I took it out. There was no voltage drop at idle and I never checked to see how much current the screen pulled when I plugged in and played. I don't quite understand why the screen is 9 VDC above the Plate. From what I've read over the years is Champs are biased to hot and not to exceed 14 watts IPD. I know I don't like the sound at all biased at 17 watts and above which two champs I had were with the 470 ohm cathode resister. I have a 680 ohm in it now and it sounds fine , since the 6V6 is about 12.66 watts now . Could the reason be that since the 6V6 is drawing less current the B+ increases and this is why the screen on the 6V6 is 9 volts higher than the plate? And also the reason the 12ax7 plates are up to 238 and 240 volts? My main concern is since I removed the 470 ohm screen resistor which no SF Champ ever had. Can I burn the JJ6V6S screens? I know I never checked the screen voltage on my 73 SF Champ even after I biased it to where I am now and used a JJ 6V6S in it.

Just a simple should I worry . I built this thing in 07 and just can't remember when I did what and why other than the bias. I never really crank it up more than 4 on the volume. I have but not for long it's loud in an apt. It doesn't break up till maybe 7 on the Volume, bass and treble maxed with a strat .Jensen c10R .

(This message was last edited by catnineblue at 08:37 PM, Nov 17th, 2016)

Contributing Member

USA / Virginia

Nov 22nd, 2016 07:20 AM   Edit   Profile  

When you ask technical questions it's a good idea to state exactly which Champ circuit you are asking about. "73 SF Champ" doesn't really tell me which circuit, so I"m going to answer for Champ AA764.

This amp doesn't have a screen resistor in it because there is alreay a 1K ohm resistor between the screen and the power supply. If you want to add a screen resistor in I don't think it will hurt any thing because under normal conditions the screen draws little current, so there will hardly be any voltage drop across a screen resistor.

Regarding the screen voltage, it doesn't hurt the amp if the screen is slightly higher than the plate. The plate has a huge surface area compared to the screen, so most of the electrons are going to be attracted to the plate even if the screen has a higher voltage on it. The main issue with the screen is the screen current. The screen is a very delicate structure, so it can't handle much current.

The purpose of screen resistors is not to drop voltage, they are in there in the event of a screen short. But like I said for this specific circuit in the event of a screen short that 1K resistor will be the current limiter.

I hope this helps.


LA , Calif

I try my best
Nov 22nd, 2016 12:38 PM   Edit   Profile  

Thank's ECS-3.

I did a bit of searching and realized why the plate voltage was less. All to do with the ma draw of the plate through a lower DC ohm load of the OT primary . The screen does not have a high ma draw and the 1k screen is larger than the OT DC primary.

I pulled the 12ax7 and yes it is a AA764 copy I built, pulling the 12ax7 as you know removes the ma draw so all I had was the screen draw @idle and got a 1 volt drop across the 989 measured 1K screen resistor which is 1 ma. I did finally realize and remember the 1k was not there to drop voltage yet as you said current. One would need much more than 1k ohm to drop the screen voltage , probably more like 3k or even more. That would change the sound and feel of the amp.

FDP Forum / Amp Mods, Repairs, and Projects / On Fender SF Champs , does anyone know why the screen is higher than plate

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