300B grid stopper resistor

[Deke609]

Someone posted about this in the past 6 months, but I can't find the thread. As that poster noted, PJ's design calls for a 215R grid stopper resistor on each 4 pin socket. But the manual and stock materials omit them. IIRC, PB's response to the previous poster was that there wasn't an easy way to implement the grid stopper, but that the owner could add one if desired.

@PJ or @PB: What are the potential benefits/drawbacks of including a 215R grid stopper on the 300Bs? Based on some quick reading I am guessing that it was intended to block rfi  and not to deal with grid current or attenuate high frequencies. Would rfi blocking come at the price of mucking a bit with the signal going into the 300B? What type of resistor should be used?

I ask b/c I've come to assume that PJ is pretty meticulous in his designs and that anything specified in the schematic is there for a purpose.

cheers and many thanks, Derek

 

[Paul Joppa]

It's there to reduce the chances that the 300B will oscillate, generating lots of radio-frequency electronic noise, w2asting power, and distorting the signal.

Except in severe cases, it is usually difficult to know whether oscillation is a problem; often it occurs only under certain signal conditions. Consequently, there is no good estimate of how common a problem it is.

Some people say that amps sound better without a grid stopper. Having experienced a severely oscillating 2A3 in an early prototype, I personally won't build an amp without them, but not everyone agrees.

 

[Deke609]

Many thanks PJ. I will add one and listen. If I hear nothing different or worse, I'll keep it.

What type of resistor construction do you recommend?

cheers and many thanks, Derek

 

[Paul Joppa]

The traditional recommendation is to use a carbon composite resistor, because it has very low inductance. But they are not very stable and are sometimes fragile. Metal film or carbon film, in values up to about 500 ohms, seem to work well and are more reliable. No precision is needed, 100 to 500 ohms is fine for a 2A3 or 300B. I'd use 1/4 or 1/2 watt, as close to the tube pin as practical.

 

[Deke609]

Perfect! Many thanks PJ.  I happen to have on hand some 1/2W Vishay Dale metal film resistors in the 100R to 150R range, so I'll use a couple of those.

cheers and thanks, Derek

 

[Thermioniclife]

Hello Folks,
I am expecting my BeePre kit this Wednesday. I have looked at the schematic and it shows 215 ohm resistors labeled Rstop
on the grids of the 300B and 6bq5 tubes And the parts list shows them as metal film type.
Are you fellows talking about grid leak resistors?

Lee

 

[Deke609]

Installed a 110R 1/2W Vishay Dale metal film resistor on each 300B grid pin socket, with body of the resistor right up against the socket (solder posts in my case b/c I'm using teflon sockets).

I detect no sonic penalty - except maybe a slight reduction in volume. Both of which make sense since before reaching the 110R grid stopper, the signal is already passing through 2 resistors (that look like Vishay Dale) in the BeeQuiet - so the grid stopper resistor adds a bit more signal voltage attenuation.

Cheap and easy insurance against oscillations - I like!

The addition also gave me chance to take a closer look at the circuit. My starting thought was "Right. I just need to find the wiring connecting the driver stage to output stage." And I found no such thing. A quick look at the schematic, and lo and behold it's a single stage and the El84s are just there for regulation. (which I guess explains why it can run 300Bs without the need for output transformers). Cool.

cheers, Derek

 

[Deke609]

I have looked at the schematic and it shows 215 ohm resistors labeled Rstop
on the grids of the 300B and 6bq5 tubes And the parts list shows them as metal film type.
Are you fellows talking about grid leak resistors?

Lee - I'll let PB or PJ deal with the nomenclature. If you look at the BOM you'll see that 4 X 215R resistors come with kit, but the schematic shows 6 (2 on each 6BQ5 and 1 on the grid of each the 300B), and the build instructions only provide for installing the 215Rs on the 6BQ5s.

cheers, Derek

 

[Paul Birkeland]

There are no grid leak resistors in the BeePre.

There was no convenient place to locate the grid stoppers, so we omitted them, but they certainly can be added if you'd like.