Purpose of the Resistor between pin 5 and 6 on the EL34s

[mesherm]

The resistors are grid stoppers. They are there to prevent any RF oscillations associated with the input capacitance (see Miller Effect).
In an ST70 it was common to wire the resistors between pins 5 and 6 because pin 6 of an EL34 is not connected and it works best if the resistor is as close to the tube pins as possible. Pin 5 is the control grid so the connections from the VTA should go to pin 6, through the resistor to pin 5.
The value is not really critical and the 2ks should work just fine.

[erichayes]

Hi All,

Mark, every amplifier--tube or solid state--is an oscillator waiting to happen. In the output section of a power amp, we're feeding the grids of the output tubes with a fairly healthy signal voltage. If we're not careful with how that voltage is applied to the grids, we can have us a real jim-dandy of an oscillator.

An oscillator needs a tuned, or "tank" circuit between the grid and plate in order for it to operate. The higher the efficiency, or "Q" of the tank, the more likely the oscillator will oscillate. Since what we want is no oscillation, we need to louse up whatever Q we might have created in wiring the amp.

The easiest way to do this (other than proper wiring techniques, of course) is to put a "stopper" resistor in series with the lead going to the control grid of the output tube to introduce resistance into the tank circuit. By putting the resistor as close to the grid as possible, i.e. between pins 6 and 5, we maximize its effectiveness.

The value of the stopper isn't critical. I use 1000Ω ½ watt Xicon carbon composition in my amps--the consensus of opinion seems to be that carbon comps sound better--but I've seen values ranging from 100Ω to 10K. Since, yes, they are in the signal path, they will affect the sound of the amp; experiment with both value and type to see what extent, if any, it affects your listening pleasure.

[mesherm]

Mark, regarding your gain, what speaker tap are you taking the negative feedback from. If your using Roy Mottram's VTA board, it assumes you are using the 16 ohm tap for feedback. If you are taking the feedback from the 8 or 4 ohm tap then the feedback resistor and capacitor values need to be changed. Also are you using 12AT7s?

[mesherm]

your 51K series resistor followed by a 20k pot to ground sounds like a reasonable thing to try. Even if the 20k pot were turned to 0 the output of the preamp would still see a 51k input and should have no problem driving into it. I am not familiar with the Foreplay I but I am assuming it uses a cathode follower output which even if shorted should not damage the preamp.

[EWBrown]

I remember an old maxim from the Army Signal School in Fort GOrdon, GA:

Amplifiers Oscillate, and Oscillators Don't.

The school taught very effective, if somewhat unorthodox, trouble-shooting procedures, which they drummed into our heads.

The EL84 grid stoppers can be as high as 10K, but 1 to 2 K is best, use carbon composition resistors, if possible.

It's also a good idea to use 100 ohm to 1K screen grid (G2) stoppers between the UL taps on the OPTs and the screen grids. Not absolutely necessary, but it can help reduce any further instabilities.

/ed B in NH

[WA4SWJ]

Screen grid "stoppers" (100 ohm, 2 watt) eliminated an output tube arcing problem I had with my ST-70. I'm using solid state rectification and "stoppers" are important with solid state rectifiers. I've used them in all the amps I've built. No more problems.

See:

http://www.audioxpress.com/magsdirx/ax/ ... ie2544.pdf

Regards,