Should I increase value of these EL84 grid resistors?

[Howlin' Mad Mac]

Simple enough design but is there going to be enough voltage difference using a 1K value for the EL 84 grids? Anticipated voltages are listed on the schematic & all values can be found on the parts list. Thanks.

HMM-18 Schematic.pdf
HMM-18 Parts List.pdf

[Chuck H]

From what I understand, these resistors aren't for dropping volts, they're for regulating and limiting current. A resistor in the B+ rail would be for dropping volts. Most agree that 1K is a good screen resistor value for your tubes.

[Dave H]

My EL84 amp has 1k screen resistors and has run for 10 years.

I think C8 should connect to the other end of R13 and the grid of V1a shoud connect to the pot wipers not C3

[Howlin' Mad Mac]

Ok but don't they do both? Anyway according to Duncan's PSU II designer the voltage at the reservoir will be about 286 volts & tapped at the 2nd capacitor about 284.5 volts. Is a 1.5 volt difference enough to get some decent amplification out of this little beasty or should there be more of a drop? Normally if I want a bigger drop I'd just calculate a larger resistor value except I'm not using a resistor but a 4H/50mA choke. If a larger plate to grid variance is desireable how would I best accomplish this? An additional resistor in line between the 2nd capacitor tap & where it feeds the grid resistors or would a resistor in parallel with the choke be better or a resistor in series with the choke but before tap be my best choice? Also if I'm to add a resistor, either in parallel or series with the choke, how do I calculate its value. I understand Ohm's law if I'm using 2 resistors (or 2 chokes or 2 capacitors for that matter) but not when in combination like a resistor & a choke. Please help... it's been 30 years since electronics school & I haven't used any of this stuff until lately & I've forgotten quite a lot (tho it does seem to come back pretty quickly fortunately.) Anyways thanks much & sorry if I'm a pest. Still figuring out who here has it going on & who is just full of it... but you impress me as the former as opposed to the later so I'd truly appreciate your good advice.

[Chuck H]

I had a little EL84 amp that was choke coupled at the screen node using 1k screen R's. It had significantly higher B+ than yours. The plates and screens were within two volts of each other. The amp is now with a friend, working fine and producing full power.

The resistance of the choke is likely under 200 ohms. So just ignore that. If you would feel better about a larger differentiation between the plates and screens just stick a 470R or 1k 5watt resistor between the plate node and the choke. It seems to me that any sag through the resistor that the circuit may suffer is lessened by placing the resistor behind the choke since the choke will be electronically attempting to smooth out the voltage via it's inductance. Though it may come out the same with the resistor ahead or behind the choke.

I don't know enough about it to say why the screens should be below the plates other than the plate should be more positive so it is the ultimate target for electrons. If the screen is MORE positive I suppose it could draw excessive current and take damage. But there are some 6L6 amps that actually do have the screens positive WRT the plates, so... I'd say your fine, really.

[Howlin' Mad Mac]

As for C8, yes, you've described the example on the Valve Wizard site & the capacitor would have a value of 100p typically. However, I took my example from the Brown Note 18 Watt Lite IIB schematic (see enclosed) & using their listed value of 0.1uF. As for the tone stack, yes I took a look & made necessary changes to my rendering (also enclosed) so that it now resembles the stack from a Fender 6G2. See anything else I may have overlooked?

HMM-18 Parts List.pdf
HMM-18 Schematic.pdf