5E7 Screen Grid Resistors?

[JohnLochner]

Hello,
Quick question about screen grid resistors...I have noticed that most (maybe all?) higher powered Fender amps starting with the Brown series, around 1960 or so, have one 470-1W screen grid resistor on each power tube.

I believe I understand the function of these resistors to help prolong the life of the tubes when the amp is run full bore. Something to the effect of, the screen starts acting like a second plate under heavy output operation, thus placing an amazing amount of stress on the tube.

My question is this: the 5E7 Bandmaster does not have 470ohm screen grid resistors. Is there a reason for this, or was it simply lack of knowledge on Fender's part in the earlier days?

I am building a 5E7 circuit, and would like to include these two resistors if there is a reason for them in the 5E7 circuit.

Thank you in advance for any information.

Best,
John

[famp69wv@yahoo.com]

John

I'm certainly no expert but I'll give it a go.

Fender didn't use screen resistors on a number of different models. Chronologically speaking the 5E_ class, if you will, appears to be the last overall class of Fender production amps that didn't use them. FWIW I think this was the case for a number of reasons including, but not limited to, the following;

1) Lower overall voltages tended to operate the screens in a more tolerable, if not tube safety conscious, manner.
2) In the late 1950s Tubes were plentiful and the general population had no reason to suspect that they wouldn't continue to be amnufactured and improved upon.
3) From a manufacturing standpoint it was cheaper not to use them.
4) No one had convinced Leo Fender that they should put them in until the 5E_A or the 5F_ series

Whatever the reasons were the lessons learned over the years probably shouldn't be ignored. As you have noted screen resistors are an important safety precaution designed to limit the amount of screen current and guard against possible tube failure resulting from too much screen dissipation and have an effect on overall tube life as well.

FWIW I used to use the absolute minimum value (i.e. 470 ohms like the old Fenders) but I have become a fan of the larger values/sizes such as 1K/5W in recent years even though I generally use lower power rail voltages than the 5E7 (for instance).

Perhaps someone else will step in with more theory than I can provide but; IMHO, they should be in there.

[Ray Ivers]

John,

Just to add to the excellent points Philip made... here's a quick & dirty DC-only analysis of circuit operation in the 5E7 PA:

The screens are directly connected to the same B+ supply as the OT center tap - which means that while the plates operate with an effective 'plate resistance' of the OT winding resistance, the screens do not - so most (if not all) of the DC idle current will flow thru the screens, not the plates. 6L6G screens were rated for 2.5 watts dissipation, and I think it's safe to assume this was being far exceeded in the 5E7 circuit.

I worked on a Marshall 50 watt once that had no screen resistors (schematic on page 200 of the Doyle Marshall book), and it would burn the orange paint on Groove Tube EL34's black within a few hours; the screens just got hotter and hotter over time, even with a fairly cold bias point (and the DC choke resistance, something the 5E7 circuit lacks). I never had this problem with any other 50 watt Marshall I worked on, and adding 1K screen resistors eliminated it completely.

FWIW, I'd definitely add screen resistors to your 5E7 - the 470 ohm resistors you mentioned should be fine.

Ray

[JohnLochner]

As always on this board, you guys rock (no pun intended).

I appreciate your information and experience. Hopefully one day I can return the favor to the next guy (or gal)...

Thanks again,
John

[riscado]

would they be benefitial in a 5E3?

[famp69wv@yahoo.com]

The same safety concerns certainly exist for the 5E3 just as they do for the 5E7. Perhaps even more so. The 5E7 is designed around 6L6s I believe whereas the 5E3 is designed around 6V6s.

Current production 6V6s (and even most NOS that I've encountered) don't seem to tolerate 400+ volts at the plate very well over time and the screens are even less tolerant. The lone exception I've any experience with is the JJ6V6 which appears to stand up to much of what a 6L6 can. I built a couple 5E3s with the 'basic Fender' 470 ohm 1-2 watt screen resistors and have felt much more comfortable with 380 volts on the screen resistors because of the current limiting feature. I've gotten pretty decent tube life too although I don't have one without them to compare.

Some feel there is a tonal difference with them in there and I agree there can be a perception of 'darker' tone, however, I've found this doesn't occur until well over 1K in my designs & I've been able to compensate by 'brightening' the design in the preamp stages. You could experiment with an on/on DPDT by running the supply to the center lugs (both), the 470 ohm resistors from each bottom lug to each top lug and wire from there to the screen grids. That way you could switch them in & out & decide if you hear a tonal change. I bet it would be very slight at that value.

[Bruce / Mission Amps]

I am aware of NO saftey issues in a 5E3 with respect to the screen voltage.
If Philip is building 5E3 type amps with 380 volt screen voltage, and it makes him feel better, that's one thing but a 5E3 runs the screen voltage at around 310vdc (+ or - 10 to 15 volts) with respect to ground and 20v less then that with respect to the cathodes....the tubes are as happy as could be.
IMHO, a separate screen resistor on the 6V6 socket of a 5E3 is really unecessary.

[famp69wv@yahoo.com]

My bad. I assumed everyone was getting 380v like I was and that is a little high for my comfort level with a 6V6. I've really got to get out more.

Thanks for the reality check Bruce.