Rocket screen grid resistor values?

[Mark]

I decided to revisit this topic. I was seeing people say how they roughly doubled the 100 ohm to 220 ohms or even went as high as 1K ohm in order to minimise excessive screen grid dissipation.

I wondered if there was real value in doubling the value of the screen grid resistors or whether is was a small component in the overall impedance of the screen grid.

These figures are a static test with no signal applied but it shows the screen grid dissipation isn't greatly effected by the use of a 1K resistor versus the 100 ohm resistor. I have included mains and rail voltages to account for variations in voltages taken, though I wasn't constantly monitoring the mains voltage. The meter used to take measurements was my 80's vintage Fluke 77.

I will have to build the amp up again and see what happens when a signal is applied.

Any thoughts?

EDIT: The Links that reference where all this is coming from.

http://ampgarage.com/forum/viewtopic.ph ... n&start=15
http://ampgarage.com/forum/viewtopic.ph ... n&start=30

[martin manning]

http://ampgarage.com/forum/viewtopic.ph ... 056#292056
This shows the effect of a screen resistor on the anode and screen current characteristics of a 6V6. The largest effect occurs when the anode voltage is pulled down relative to the screen, which is the case when the drive signal is high.

[Roe]

I prefer 100R individual screen resistors in series with an 420 ohm choke or 470R shared screen resistor (on a quad of el84s). For a pair of el84s, you can use a 910R shared resistor instead

[Mark]

Thanks for your replies. I'm slowly working on the amp and I will be able to see what is happening to the screen grid then.

I'll make time to read your post Martin.

[gktamps]

Hi Mark,

I've tried 100, 470 and 1k screen grid resistors, and am now regularly using 470, following earlier recommendations in the TAG schematic and layout documents. Drops the screen voltage down about 3v below the plates, but doesn't choke tone too much. I didn't like the effects of using the 1k resistors.

Cheers,

Greg

[Mark]

Hi Greg

Love your build, did you put the resistors on a switch to allow quick A/B-ing of components?

I'm also thinking of getting a TC Ditto to give me a consist guitar signal when testing different component types and values.

I have been looking at Richard Kuehnel site too, though there really isn't enough info in the article.

http://www.ampbooks.com/home/classic-ci ... power-amp/

[gktamps]

Hi Mark,
Sorry for the slow reply; catching up after being away for work.
Thanks for the kind words on my last build.
I just soldered and desoldered each set, no switch.
Good luck with your build!
Greg

[Mark]

Yesterday I put the 100 ohm and 1K ohm resistors on a 4PDT switch. The idea being to find out if there really is any audible difference between thee two values.

As far as testing goes, I used my 82 1957 reissue Strat, and a R0 Lespaul.

I didn't use Celestion speakers I used Kendrick Blackframes in a Fender Bassman cab as they aren't as loud as Celestion speakers and the bass and mids are more balanced than the Celestions.

In short there is a difference and it is debatable as to which one sounds better. Unfortunately I liked the 100 ohms better than the 1K ohm screen grid resistors.

I heard a difference in the mids, when I stood above the speaker box I didn't hear any difference at all, when I stepped back from the cab the difference was noticeable.

I think the 1K's were a little thinner sounding and a bit "clearer" sounding than the 100 ohm's. If you were to use the amp on the verge of break up the 1K resistor might be better. To my ear the 100 ohm resistors sounded more natural with both guitars.

Try them both, neither are horrible sounding, they are just different.

[Gaz]

Man, you were just participating in another thread on the same topic, why the new one?

[Mark]

Check out the dates Gaz, mine started on the 4th of Aug while Hans started on the 19th of Sept.

Hans' topic is regarding 100 ohm screen grid resistor in Rocket amp's versus 1K ohm resistors in Liverpool amps. My thread is purely about 100 ohm resistors in the Rocket amp.

I contributed to Han's thread as I had been doing A/B testing between 100 ohm and 1K ohm screen grid resistors and thought I could add something to his thread which might assist him.

BTW, I'm happy to talk about Rocket builds, I have quite few posts here that address findings and issues. I gave it a run with the Les Paul this afternoon, it goes hard and cleans up nicely.

[matt h]

(deleted)

[Gaz]

Check out the dates Gaz, mine started on the 4th of Aug while Hans started on the 19th of Sept.

Hans' topic is regarding 100 ohm screen grid resistor in Rocket amp's versus 1K ohm resistors in Liverpool amps. My thread is purely about 100 ohm resistors in the Rocket amp.

I contributed to Han's thread as I had been doing A/B testing between 100 ohm and 1K ohm screen grid resistors and thought I could add something to his thread which might assist him.

BTW, I'm happy to talk about Rocket builds, I have quite few posts here that address findings and issues. I gave it a run with the Les Paul this afternoon, it goes hard and cleans up nicely.

Gotcha, I missed that. I am just interested in this topic and dislike when a thread splits off for no reason. Sorry about that!

[Mark]

No problem Gaz, I usually provide links when it looks like splitting off.

[hans-jörg]

Sorry, wrong thread.

[hans-jörg]

OK, GUYS, IM in the wrong thread !