Grid stopper on PI input...

yankeerob · 10-11-2009, 07:43 AM

As some of you know - we build our own versions of tweed Deluxes and Pros... something we've come across - thanks to JAM and his directing us to the Valve Wizard site - is adding a 220K grid stopper on the PI input - according to the paper on cathodyne or 'split-load' PI's you can use anything from 100K to 1M - I just plucked 220K out of the air and tried it and hey presto - even clipping and a couple more watts before clipping occurs...

This will no doubt be a prefence thing and subjective - I thought I'd introduce the thread (and apologise if it's been brought up before) to see what anyone else thinks...

NTBluesGuitar · 10-11-2009, 08:29 AM

Well, now, that IS interesting.

Were you having issues with the clipping? I haven't built one that had a problem with it yet, but it's good to know this is out there.

yankeerob · 10-11-2009, 10:05 AM

I'm glad to oblige in the 'now that is interesting' dept for someone...

Well - we'd had a long standing problem with 5F6A derived amps in that unless you stuck strictly to a 12AY7 in V1 the likelyhood of grid clamping in the output stage increased greatly - and in a hurry - especially if you were trying to marry the HR or Blues Dv output stage with a 5F6A front end although it'd occured in a JTM-45 clone we had in for repair... we've since come up with our own solution which is here somewhere... but the funny thing is that it looks almost the same on a 'scope... where as you increase the volume control the bottom half of the wave appears to clip before the top... the fact of the matter is that two different things were occuring... in the case of the 5F6A amp the whole signal was being pushed down by a rise in grid current whereas in the 5E3/5E5 it's what is being described under 'Avoiding unpleasant overdrive tones'...

I was intrigued because there was a kind of vague similarity in the sine waves but unlike the 5F6A - the 5E3/5E5 would - as you increase the vol control - first appear to clip at the bottom of the wave and then eventually just clip top and bottom whereas the 5F6A would just go doo-lally and clamp... but we looked closer at the wave on the 5E5 and sure enough it's exactly what Merlin describes as 'frequency doubling' and sure enough a grid stopper does the trick... you'll have to experiment with the value and we haven't had time to complete the exercise but I just thought it'd be worth sharing to see what anyone else might have found...

yankeerob · 10-14-2009, 04:01 AM

Well - we tried it in this (tweed 5E5 type 212 combo)

and it really didn't work - haven't tried a 100K or lower but just taking it out and going straight in works a lot better - thought it was worth a try and to see if anyone else had any experience with it - though there was a coupla clean watts to be gained (about 2.5 or so) the overall response was lacking in top so we ditched it for now

Cygnus X1 · 10-14-2009, 12:29 PM

Well, it WAS interesting.

10-11-2009, 07:43 AM	#1
yankeerob Forum Member Join Date: May 2007 Location: Amongst the Pennine mills... Posts: 1,107	Grid stopper on PI input... As some of you know - we build our own versions of tweed Deluxes and Pros... something we've come across - thanks to JAM and his directing us to the Valve Wizard site - is adding a 220K grid stopper on the PI input - according to the paper on cathodyne or 'split-load' PI's you can use anything from 100K to 1M - I just plucked 220K out of the air and tried it and hey presto - even clipping and a couple more watts before clipping occurs... This will no doubt be a prefence thing and subjective - I thought I'd introduce the thread (and apologise if it's been brought up before) to see what anyone else thinks... __________________ If I could find a road to get away it wouldn't be too soon....... Shipwreck Moon.......

10-11-2009, 08:29 AM	#2
NTBluesGuitar Forum Member Join Date: Apr 2006 Location: North Texas Posts: 5,254	Re: Grid stopper on PI input... Well, now, that IS interesting. Were you having issues with the clipping? I haven't built one that had a problem with it yet, but it's good to know this is out there. __________________ "Exit the cowboy and enter the mama's boy." -Tom Hoffman

10-11-2009, 10:05 AM	#3
yankeerob Forum Member Join Date: May 2007 Location: Amongst the Pennine mills... Posts: 1,107	Re: Grid stopper on PI input... I'm glad to oblige in the 'now that is interesting' dept for someone... Well - we'd had a long standing problem with 5F6A derived amps in that unless you stuck strictly to a 12AY7 in V1 the likelyhood of grid clamping in the output stage increased greatly - and in a hurry - especially if you were trying to marry the HR or Blues Dv output stage with a 5F6A front end although it'd occured in a JTM-45 clone we had in for repair... we've since come up with our own solution which is here somewhere... but the funny thing is that it looks almost the same on a 'scope... where as you increase the volume control the bottom half of the wave appears to clip before the top... the fact of the matter is that two different things were occuring... in the case of the 5F6A amp the whole signal was being pushed down by a rise in grid current whereas in the 5E3/5E5 it's what is being described under 'Avoiding unpleasant overdrive tones'... I was intrigued because there was a kind of vague similarity in the sine waves but unlike the 5F6A - the 5E3/5E5 would - as you increase the vol control - first appear to clip at the bottom of the wave and then eventually just clip top and bottom whereas the 5F6A would just go doo-lally and clamp... but we looked closer at the wave on the 5E5 and sure enough it's exactly what Merlin describes as 'frequency doubling' and sure enough a grid stopper does the trick... you'll have to experiment with the value and we haven't had time to complete the exercise but I just thought it'd be worth sharing to see what anyone else might have found... __________________ If I could find a road to get away it wouldn't be too soon....... Shipwreck Moon.......

10-14-2009, 04:01 AM	#4
yankeerob Forum Member Join Date: May 2007 Location: Amongst the Pennine mills... Posts: 1,107	Re: Grid stopper on PI input... Well - we tried it in this (tweed 5E5 type 212 combo) and it really didn't work - haven't tried a 100K or lower but just taking it out and going straight in works a lot better - thought it was worth a try and to see if anyone else had any experience with it - though there was a coupla clean watts to be gained (about 2.5 or so) the overall response was lacking in top so we ditched it for now __________________ If I could find a road to get away it wouldn't be too soon....... Shipwreck Moon.......

10-14-2009, 12:29 PM	#5
Cygnus X1 Forum Member Join Date: Aug 2007 Location: Sunny South Carolina Posts: 2,506	Re: Grid stopper on PI input... Well, it WAS interesting. __________________ From now on, ending a sentence with a preposition is something up with which I will not put.-Winston Churchill.

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