bias and grid resistor power stage

[sergio]

Hi all , I have a question , in many schematics the grid resistor of the power tube augmented as far as 5k1 , why? what happen? In many schematics fender I see 1k5 for this.


The biasfeed resistor change often from 220k to 100k , most low value and more fine bias regulation?
Thanks all

[Ears]

Hi Sergio,

There may be a little confusion as to terminology in your question.
Pre-amp triodes often have Cathode resistors of 1k5 (or 820R).
Anode resitors (coupling resistors) often range from 100K to 220K in voltage amps.

I suspect that no replies to your post is because people (including me) don't quite understand the question.

[roberto]

Hi all,
Sergio talks about power tubes grid resistors, not preamp tube cathode resistors. Old Fender amps have 1k5 grid stoppers on power amp. That's ok for 6L6. EL34 needs higher value until 5k6 for grid stoppers. That's tipically due to the kind of power tubes you have. Soldano uses 2k2 on its 6L6 based amps and this rolls off a little the higher frequencies compared to Fender. But it depends on other stuff of the amp too (preamp, OT, etc..)

[sergio]

many schematics 4x6l6 use 5.1k resistor grid , it's ok? no is better 1.5k like original fender schematic?

[Andy Le Blanc]

the grid resistors on power tubes are there to reduce blocking distortion
occuring after the amp encounters a large trasients
they are grid current limiters
as opposed to a grid stopper in the pre amp
which are used to reduce noise and stabilize coupling
both can modify the "miller effect" and help voice the amp as well

[FUCHSAUDIO]

They can sometimes reduce the chances of oscillation.

[mhuss]

They can sometimes reduce the chances of oscillation.

+1, The Chinese KT-66 tubes in particular tend to sing like banshees without a 5k6 or higher connected right at the grid socket pin.

--mark

[sergio]

ok , thanks all , I'll use 5,1k like #124 (1984)

[krash]

I have gone as high as 10K. Depends on many factors. The voicing difference is not noticeable in my EL84 amps.

[sebastian]

Most amps use for the grid resistor in power amp,a "casual" value.This is due to the fact that some builders have a large quantity of "x value" resistors...
The resistor,as said,reduce blocking distortion of power tubes when phase inverter drive the tubes hard.In modern Marshall amps,like JCM2000 DSL the grid resistors for the power tubes are 220kohm !!!!!! Old schematics had 5.6k as standard,more modern schematics have 220k for grid resistors.All DSL100 that I seen in my life (about twenty..) have 220k as grid resistors. I don't know why,but I think to obtain a soft power tubes distortion when the amp is drive hard. (dsl/tsl has a PI powered at about 400 volt,puts out a lot of voltage...).The input capacitance of a power tube is very low,and the gain of the power tube is low;There is no big changes in tone changing the values of grid resistors,WHEN the amp IS NOT at full power.I can use 1k to 220 k and hear no big audible differences.

[krash]

If you are getting blocking distortion, you will hear a big difference when it goes away

Other than that, i don't really hear any difference going as high as 10K.

[Trace]

For what it's worth;


This is not meant to be confrontational so please do not take it as such. With that said, most hear a night-n-day difference when going from 1.5K to 10K, not to mention going from a value of 1K to 220K. This not only changes the tone/frequency response but also changes how the amp feels.


Again, for what it's worth
Trace

[krash]

really? night and day? maybe I should put in a dual-gang 25k pot in there on some amp and tweak it to see how it feels different.

I have not tried A/Bing them directly (that is, two identical amps, only difference being these R's). i build amps one at a time. last two 72 amps I have built were with 10K R's and one of them I started with 1.5K then switched to 10K and could not hear a significant difference after the switch. i'm pretty sensitive to this kind of thing, predictably.

[rfgordon]

I'm glad y'all brought this topic up. My latest build has, I think, blocking distortion. It's a sort of BF with verb into a pair of 6L6s and a pair of 6V6s, all cathode bias.

With either the gain past noon and you hit a hard note, there's this really farty, fizzy, sputtery distortion that comes on and fades and sputters out.

I've changed grid stoppers on the power tubes, going as high so far as 47k each, with no results. Does this blocking distortion just disappear when the right value is found?

It sounds similar to, but sputtery-er than, the parasitic in an old BF Pro Reverb I worked on last year. Without a scope, how to you tell the difference between blocking distortion and a parasitic?

[mhuss]

Measure the DC voltage on the grids while playing a chord at the problem level. Block distortion manifests itself as a change in the average DC level in the negative direction.

Note that the BF 'large coupling cap/large grid resistor' values tend to exacerbate blocking distortion.

--mark