I have been using a pair of RCA 6L6GB's with the screen tied directly to the plate. I have recently seen some schematics where there is a 100 ohm resistor in between. One of these models I saw was using a switch to change between UL mode and Triode mode.
So my question is who has used a low value resistor between the plate and screen for triode mode and why?
I do not see anything about this resistor in the datasheets, RCA states G2 connected to plate for triode mode which is what I have done.
-bird
So my question is who has used a low value resistor between the plate and screen for triode mode and why?
I do not see anything about this resistor in the datasheets, RCA states G2 connected to plate for triode mode which is what I have done.
-bird
The resistor serves two purposes (1) suppression of screen induced oscillation, and (2) forces the screen below the plate voltage-wise.
I generally use 220-470 Ohms.
I generally use 220-470 Ohms.
Screen stopper. 6L6-oids have a tendency to make snivets when operated in Class AB where one 6L6-oid cuts off completely. That can cause a burst of oscillation of ~100KHz (depending on the OPT self resonance). For pseudotriode operation, ~100R will do nicely. 470R -- 1500R is useful when running as full pentodes.
I will add some 100 ohm resistors, thanks.
Is the oscillation only an issue in class AB, or does it just potentiate it? I only ask because I am running class A and have had no signs of oscillation.
Is the oscillation only an issue in class AB, or does it just potentiate it? I only ask because I am running class A and have had no signs of oscillation.
I use 100 ohm for 6L6GC and KT88. It prevents bursts of oscillation that sometimes happens when the amp is driven to clipping. Larger values can reduce power output slightly.
The EL34 wants a higher value in some cases. Original Mullard literature calls for 1000 ohms to produce the lowest distortion. I have found that 470 ohms is the best compromise on a wide selection of budget priced new production and used old tubes. If the amp will gee a lot of clipping, like a guitar amp, it should be a 2 watt resistor.
The EL34 wants a higher value in some cases. Original Mullard literature calls for 1000 ohms to produce the lowest distortion. I have found that 470 ohms is the best compromise on a wide selection of budget priced new production and used old tubes. If the amp will gee a lot of clipping, like a guitar amp, it should be a 2 watt resistor.
Hi, what kind of resistor is recommended? Carbon film or metal film or carbon composit? Does the resistor need to be high-qualitied (otherwise will sound be affected)? Could any one provide a product link? Thanks.
Running PL509/19 as Triodes in an OTL I made some years ago oscillated horribly.
A 270 ohm 2W carbon-film resistor in each screen to anode cured it completely.
A 270 ohm 2W carbon-film resistor in each screen to anode cured it completely.
Carbon comps are OK, but rather noisy when there's current flowing through 'em.
--Great for G1 circuits where there's practically no current but possibly could cause excess noise/hiss when used in a G2 supply where G2 can draw some comparatively large current to that found on a G1......
Only place I'll use 'em is G1 stoppers.
--Great for G1 circuits where there's practically no current but possibly could cause excess noise/hiss when used in a G2 supply where G2 can draw some comparatively large current to that found on a G1......
Only place I'll use 'em is G1 stoppers.