Thanks for all the input guys
Katopan - I have tried to join 18 watt a few times but never seem to get a reply email with login credentials. Tried with a different email again yesterday
Structo - Yes I have a Hall fixed bias vvr on this amp (and a Hall cathode version on my similar combo build that has similar fizz issues)
My noobness will show here - on el84's plates are pin 7 and screens pin 9 right? if so I am at +4v testing as you describe on the head build and +10v on the combo. So that indicates plates are higher in my amps, correct? too bad that sounded like it might have been a good possibility and easy change to try
Firestorm - tried largish grid stoppers on the el84's - up to 100k with no diff on the fizzies. has 8.2k's on it now (on the board not sockets - I suppose I could move them or higher values to the sockets if you think that may help)
The tonestack runs are not particularly short due to how far the pots are from the spots on the board where the leads have to run, not so much due to any extra wire I have in there - I more or less just drop to the chassis and back up to the board with a pretty direct path - not a whole lot of slack - they are not twisted or shielded though. I could save an inch or so by just flying them straight to the turrets I guess? Either that or shift the whole board over an inch or 2 to shorten the runs??
If I twist tonestack wires would you twist the treble, middle and bass leads off the board and individually peel them out as you past the respective pots? I've not really seen pics of twisted tonestack wires in other builds.
question about power scaling
Moderators: pompeiisneaks, Colossal
Re: question about power scaling
I just found the thread with Andy's info:
http://ampgarage.com/forum/viewtopic.php?t=14770
Quite a while ago the 18Watt forum suffered a spambot attack. The forum was got up and running again but some parts of it are still not functional including the signup of new members. It's still pretty active but only people who were members before it was broken can login.
What screen resistors have you used? (Both supply chain and to the screen pins, separate or shared screen resistors?)
http://ampgarage.com/forum/viewtopic.php?t=14770
Quite a while ago the 18Watt forum suffered a spambot attack. The forum was got up and running again but some parts of it are still not functional including the signup of new members. It's still pretty active but only people who were members before it was broken can login.
What screen resistors have you used? (Both supply chain and to the screen pins, separate or shared screen resistors?)
Re: question about power scaling
So far I have not messed with screen resistors. Most any info I could find on fizzy 18 watters suggested higher grid stoppers, Ruby mod, conjunctive/zobel filters etc, but this thread is first mention I have heard about screen resistors, and/or the screen vs plate voltage thing
Below are layouts of that end of the board for both my head and combo. The head is a little unusual for an 18 watt build in that I have a 50/50 1st cap and 4h choke on it right now (I was messing around trying to tighten up the low end).
Combo is more traditional 32/32 can, and both have typical (I think) screen resistors - 100R
Head is fixed bias, Combo is cathode bias, neither layout shows the power scaling
Below are layouts of that end of the board for both my head and combo. The head is a little unusual for an 18 watt build in that I have a 50/50 1st cap and 4h choke on it right now (I was messing around trying to tighten up the low end).
Combo is more traditional 32/32 can, and both have typical (I think) screen resistors - 100R
Head is fixed bias, Combo is cathode bias, neither layout shows the power scaling
You do not have the required permissions to view the files attached to this post.
Re: question about power scaling
Am I wrong here.. If I measure screens to ground (pin 9) on my head I get 357v
If I measure plates (pin 7) to ground I get 349v - so my screen voltage is higher as described earlier right?
I had read the earlier comment to imply that a negative reading on my meter meant the plates were higher - or is the way they were being compared earlier talking about a different voltage relationship
If I measure plates (pin 7) to ground I get 349v - so my screen voltage is higher as described earlier right?
I had read the earlier comment to imply that a negative reading on my meter meant the plates were higher - or is the way they were being compared earlier talking about a different voltage relationship
Re: question about power scaling
I'd be interested to know if 560R or 1k separate resistors to each of the screens help your problem. Looks like you've got a shared 100R at the moment with the screens tied together.
My 18W didn't have any fizz issues. I've followed threads on 18Watt about builds that do and always been interested in what is the cause/difference. Like you I've never come across anything relating it to screen voltage before.
All depends which pin you put the red probe on and which pin you put the black. But yeah, it looks like your screens are higher than the plates by 8V. But earlier you said you measured 4V. I'd expect your combo with the standard 1k5 power chain resistor will have the screens comfortably below the plates.
My 18W didn't have any fizz issues. I've followed threads on 18Watt about builds that do and always been interested in what is the cause/difference. Like you I've never come across anything relating it to screen voltage before.
All depends which pin you put the red probe on and which pin you put the black. But yeah, it looks like your screens are higher than the plates by 8V. But earlier you said you measured 4V. I'd expect your combo with the standard 1k5 power chain resistor will have the screens comfortably below the plates.
Re: question about power scaling
So I tried 560R, and that still left plates lower than screens, tried a few slightly higher values but had to go up to 1.2k to get slightly positive. Still fizzy overtones. Went all the way to 2k to see if it would help which resulted in plate a few volts highers vs screens ... but, no dice on killing the swirlies
I was hopeful but it appears simply changing the screen resistor to drop the screen voltage below the plates is not the answer unfortunately.
I did also try messing with some resistors to ground from the screen pins as Andy described as his "quick bench fix" but apparently did not have the right values to make the voltage adjust below the plates as the few values I tried did not go far enough.
Would this voltage divider idea actually perform differently than a simple screen resistor?
I was hopeful but it appears simply changing the screen resistor to drop the screen voltage below the plates is not the answer unfortunately.
I did also try messing with some resistors to ground from the screen pins as Andy described as his "quick bench fix" but apparently did not have the right values to make the voltage adjust below the plates as the few values I tried did not go far enough.
Would this voltage divider idea actually perform differently than a simple screen resistor?
Re: question about power scaling
If you look at the specs of the EL84, it lists a VaMax of 300v and Vg2Max of 300v.
Also a VhkMax of 100v.
I think a lot of amps push those maximum ratings and as we know, sometimes current production tubes cannot take a lot of abuse.
Also a VhkMax of 100v.
I think a lot of amps push those maximum ratings and as we know, sometimes current production tubes cannot take a lot of abuse.
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: question about power scaling
I was aware of the 300v specs - what is vhkMax? I had mentioned earlier that I may try dropping my voltages significantly or even biasing class a if I cannot get satisfaction with what I am trying now (ha ha)
I've been playing with zener values in the Ruby clamp today - My bias voltage is about -14 v at idle and I tried progressively smaller zeners (smallest value I had was 10v) it seems to squash the fizzies as I scale down the vvr very well until the bias voltage reaches about 1 volt within the value of the zener.
i.e. as I scale down the vvr on the amp with a 10v zener in the Ruby clamp it does not start to show much in the way of buzzies at all until the bias voltage drops below -11v.
At -10v bias there is some pretty obvious but not super bad fizzies but as I scale the amp lower it gets progressively worse
Being a bit of a noob I had thought the zener would clamp the bias near the value of the zener but I'm not sure exactly how it is working since even with a 10v zener I see -14v bias at idle?? Maybe someone can better explain how it is working.
If I read Paul's buzz article correctly I am actually potentially clipping the signal before the el84 cutoff voltage by using a lower value zener - that said it seems to sound OK?? I don't have a scope nor really do I have the chops to understand what I am doing anyway (yet - I am learning quickly)
Would it be safe to put a low value zener in - say 5v? What in effect is that doing?
If a fairly low zener is safe in terms of the amp operationally, that may allow the amp to scale pretty low without fizzies if it continues as it has so far i.e. - killing the fizzies down to bias voltages near the zener value used
Any input would be appreciated
I've been playing with zener values in the Ruby clamp today - My bias voltage is about -14 v at idle and I tried progressively smaller zeners (smallest value I had was 10v) it seems to squash the fizzies as I scale down the vvr very well until the bias voltage reaches about 1 volt within the value of the zener.
i.e. as I scale down the vvr on the amp with a 10v zener in the Ruby clamp it does not start to show much in the way of buzzies at all until the bias voltage drops below -11v.
At -10v bias there is some pretty obvious but not super bad fizzies but as I scale the amp lower it gets progressively worse
Being a bit of a noob I had thought the zener would clamp the bias near the value of the zener but I'm not sure exactly how it is working since even with a 10v zener I see -14v bias at idle?? Maybe someone can better explain how it is working.
If I read Paul's buzz article correctly I am actually potentially clipping the signal before the el84 cutoff voltage by using a lower value zener - that said it seems to sound OK?? I don't have a scope nor really do I have the chops to understand what I am doing anyway (yet - I am learning quickly)
Would it be safe to put a low value zener in - say 5v? What in effect is that doing?
If a fairly low zener is safe in terms of the amp operationally, that may allow the amp to scale pretty low without fizzies if it continues as it has so far i.e. - killing the fizzies down to bias voltages near the zener value used
Any input would be appreciated
Re: question about power scaling
The Ruby zener is clipping the negative part of the signal going into the power grid to discharge the PI to PA coupling cap each cycle (his doc has more detail but that's it in a nutshell). The lower you make the zener, the closer the clipping threshold is referenced to ground (or your fixed bias voltage in your case). Normally you don't hear the result of this clipping because it's effecting the signal going into the power grids below their cutoff. The side effect of using a zener that's too small is you'll start to clip in the Class A region before that side power valve has cutoff, and you'll see/hear that as another very nasty distortion artifact in the output.
Vhkmax is the max voltage rating cathode to heater. The 300V plate to cathode rating is largely ignored (within reason) and plenty of 18W amps at 340V odd B+ don't have a buzz issue.
I can try and dig up the variable Ruby mod I drew up and posted at 18Watt. After much discussion the guy I did it for didn't like the idea of having another pot to adjust. It is very much unproven so at your own risk. And it does have it's own pot and can't be integrated onto a VVR multipot arrangement. Means two knobs to adjust, potentially three if you then need to adjust the volume knob as well.
Oh, and another thing for you to test out. Check out your bias voltage at a low setting on the VVR and size your zener for that (or you can use a chain of 1N4007s adding up to the right voltage - just put them all the right way around, anode to fixed bias supply, cathode to power grid resistor). If you get no fizz then at that VVR setting it'll prove it's the fizz problem and not some other issue with your VVR installation.
Vhkmax is the max voltage rating cathode to heater. The 300V plate to cathode rating is largely ignored (within reason) and plenty of 18W amps at 340V odd B+ don't have a buzz issue.
I can try and dig up the variable Ruby mod I drew up and posted at 18Watt. After much discussion the guy I did it for didn't like the idea of having another pot to adjust. It is very much unproven so at your own risk. And it does have it's own pot and can't be integrated onto a VVR multipot arrangement. Means two knobs to adjust, potentially three if you then need to adjust the volume knob as well.
Oh, and another thing for you to test out. Check out your bias voltage at a low setting on the VVR and size your zener for that (or you can use a chain of 1N4007s adding up to the right voltage - just put them all the right way around, anode to fixed bias supply, cathode to power grid resistor). If you get no fizz then at that VVR setting it'll prove it's the fizz problem and not some other issue with your VVR installation.
Re: question about power scaling
Found something else too - http://www.ax84.com/bbs/dm.php?thread=443140
Bottom of the page, the idea that added plate resistors stop the cut-off side of the OT from ringing. Kinda related to snubber operation and sizing.
This amp has an example and they contribute to a smoother tone.
http://www.ax84.com/static/comanchero/C ... 110323.pdf
Might be worth a try instead of the Ruby fix. If it worked, then it should do so at any VVR setting.
Bottom of the page, the idea that added plate resistors stop the cut-off side of the OT from ringing. Kinda related to snubber operation and sizing.
This amp has an example and they contribute to a smoother tone.
http://www.ax84.com/static/comanchero/C ... 110323.pdf
Might be worth a try instead of the Ruby fix. If it worked, then it should do so at any VVR setting.
Re: question about power scaling
Intrigued by the plate resistor idea I soldered on some 100R resistors I had and disconnected ruby clamp - unfortunately there are fizzies, at least at that value (the Comanchero schem indicated 120R but I only had 100R on hand - figured that 100R would at least give me an indication I was going in the right direction)
Their schem indicates 280ish plate voltage, I'm more like 350v so perhaps I would need to size differently
Your comment on this being related to snubber operation/sizing, though I'm familiar with what a snubber is, I am not sure how you would try to size it - generally what are the guidelines to calculate an appropriate value?
Though I am finding this issue a bit frustrating has been another excellent exercise in terms of my learning more about tube amp operation and theory -
Thanks for all your help so far folks!
Their schem indicates 280ish plate voltage, I'm more like 350v so perhaps I would need to size differently
Your comment on this being related to snubber operation/sizing, though I'm familiar with what a snubber is, I am not sure how you would try to size it - generally what are the guidelines to calculate an appropriate value?
Though I am finding this issue a bit frustrating has been another excellent exercise in terms of my learning more about tube amp operation and theory -
Thanks for all your help so far folks!
Re: question about power scaling
Is the snubber or fizz cap the one across the PI plates?
If so, start with a 47pf and then try a 100pf, and listen to both.
You can take a 47pf and try it across the V2 plates, or just about anywhere from signal to ground to try and tame the fizzies.
If so, start with a 47pf and then try a 100pf, and listen to both.
You can take a 47pf and try it across the V2 plates, or just about anywhere from signal to ground to try and tame the fizzies.
Tom
Don't let that smoke out!
Don't let that smoke out!
Re: question about power scaling
Katopan - I did some reading and I would imagine you had designed something similar to the link below for a variable zener circuit?
http://www.brighthub.com/engineering/el ... 86446.aspx
Got me thinking - it would probably be possible to mod the power scaling setup to use a 3 gang pot where the 3rd pot would regulate the zener if the right values were used for resistors, etc for the new 3rd leg off the vvr
A bit of a stretch for my ability but I would think it might be doable
That said I think there MUST be a way to design the swirly fizz out of my amp short of an exotic variable Ruby mod band-aid??
Interestingly - I fired up my Gibson Goldtone GA-30 today and noticed it gets pretty bad swirly fizz when I really crank it - its 4 x el84.... was checking out the schem thinking I might put a Ruby Clamp on it to see if that cures it on that amp too - ha ha.
I guess even the pros like Gibson (Trace Elliot) can't always engineer around things like this sometimes
http://www.brighthub.com/engineering/el ... 86446.aspx
Got me thinking - it would probably be possible to mod the power scaling setup to use a 3 gang pot where the 3rd pot would regulate the zener if the right values were used for resistors, etc for the new 3rd leg off the vvr
A bit of a stretch for my ability but I would think it might be doable
That said I think there MUST be a way to design the swirly fizz out of my amp short of an exotic variable Ruby mod band-aid??
Interestingly - I fired up my Gibson Goldtone GA-30 today and noticed it gets pretty bad swirly fizz when I really crank it - its 4 x el84.... was checking out the schem thinking I might put a Ruby Clamp on it to see if that cures it on that amp too - ha ha.
I guess even the pros like Gibson (Trace Elliot) can't always engineer around things like this sometimes
Re: question about power scaling
Damn, I don't have it on my home computer. It's on my work computer so I'll post on Mon morning. It's pretty simple, but you can't match pot values with what the VVR uses to use a 3 gang pot (even if you can find one). But you could temporarily wire it up just to see if it works.
I'd love to see an amp with and one without this issue side by side on my bench. Ruby worked out a fix, but didn't really get to the bottom of it.
I'd love to see an amp with and one without this issue side by side on my bench. Ruby worked out a fix, but didn't really get to the bottom of it.
Re: question about power scaling
So my local electronics place had a variety of zeners in stock and I was able to pick some up in spite of it being Saturday. I tried a few different values and found that with a 3 volt/5 watt zener in the Ruby clamp I can dial down the vvr all the way with no fizzy, swirly artifacts like in my original clip.
This vvr has a 330k resistor in it to avoid scratchy pot issues, so it does not dial ALL the way down but none the less it dials down to pretty low voltage
Interestingly it works well through the whole vvr sweep where I thought it might exhibit distortion or some other odd behavior as Katopan speculated a few posts back..
Maybe a variable zener is not required after all???
To my ears, clean or dimed it sounds great through the whole sweep of the vvr - but my ears are not a scope so perhaps there is some odd stuff going on that I just do not hear
Perhaps some of you enlightened folks might chime in on whether having such a low value zener in the Ruby clamp is harmful in any way to the amp other than perhaps causing unusual voltage swings as Katopan speculated. If not, I may have solved my problem as I am very happy with how it sounds and I can now dial down the vvr without issue
I will try and record some with and without the clamp clips in the next day or 2 for future reference - its pretty effective
This vvr has a 330k resistor in it to avoid scratchy pot issues, so it does not dial ALL the way down but none the less it dials down to pretty low voltage
Interestingly it works well through the whole vvr sweep where I thought it might exhibit distortion or some other odd behavior as Katopan speculated a few posts back..
Maybe a variable zener is not required after all???
To my ears, clean or dimed it sounds great through the whole sweep of the vvr - but my ears are not a scope so perhaps there is some odd stuff going on that I just do not hear
Perhaps some of you enlightened folks might chime in on whether having such a low value zener in the Ruby clamp is harmful in any way to the amp other than perhaps causing unusual voltage swings as Katopan speculated. If not, I may have solved my problem as I am very happy with how it sounds and I can now dial down the vvr without issue
I will try and record some with and without the clamp clips in the next day or 2 for future reference - its pretty effective