Your VTVM and a handful of one ohm resistors?morcey2 wrote:If anyone out there has a setup that's easy to measure plate (or in this case, plate and screen) current under load and tell me what they see, I would really appreciate it. That way I know that I'm not just chasing ghosts.
Thanks,
Matt
What am I missing?
Moderators: pompeiisneaks, Colossal
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skyboltone
- Posts: 2287
- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: What am I missing?
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Re: What am I missing?
That's how I picked the one-ohm resistors I'm using. I can fax you a couple if it'd help. (I hope that came out right. It's probably too early in the morning to attempt humor.)skyboltone wrote:Your VTVM and a handful of one ohm resistors?morcey2 wrote:If anyone out there has a setup that's easy to measure plate (or in this case, plate and screen) current under load and tell me what they see, I would really appreciate it. That way I know that I'm not just chasing ghosts.
Thanks,
Matt
Andy Le Blanc wrote: are the plate leads from board to the socket pins ment to be twisted.....
ive had an amp where that was an issue......
Possibly. I just looked over my other amps and this is the only one that I twisted the PI plate leads on. I'll untwist them tonight and see what happens. I just figured, out of phase, same stage, fed from the same signal, twist them up. Maybe I was wrong. (I'm a guy. I'm not supposed to say that.)jjman wrote: Out of phase signal bucking jumping from one to the other on the twisted pair. And I'm watching bucking broncos on TV at the moment.
Do we have a bingo?
also forgot to mention. The first thing I did was pull the diodes on the output tubes thinking that may have caused it, just in case anyone was wondering.
Anyway. Thanks for the help. I'll change it after work and update everyone.
Matt
Re: What am I missing?
I untwisted the plate leads for the PI and nothing changed. I haven't untwisted the grid leads to the power tubes, but I'll try that next.
Maybe I should be concentrating on getting my oscilloscope fixed. I think the CRT is dead.
Matt
Maybe I should be concentrating on getting my oscilloscope fixed. I think the CRT is dead.
Matt
Re: What am I missing?
Here's what I've tried since our last little chat:
Re-built PI (C9,C10,R10,R11,R12,R13) using all new components. All resistors measured to confirm value. I lifted the Preamp board to make sure there weren't any dangling solder blobs or anything like that.
Changed R13 to 820, then to 680.
Untwisted power tube grid leads.
Untwisted OT plate leads.
Formatted amp drive and reinstalled.
Tried 6V6s, KT77s, TAD EL34Bs, JJ EL34s.
Bypassed the 220k grid leaks w/ a jumper, one at a time. This gave me some interesting results. Both sides behaved more like they're supposed to, but the current on the problem side only rose to about 70ma while the non-problem side rose to about 140ma with the same signal. I'll need to pull out my other amp and see if that's proper behavior. With a 10k tail and equal input coupling cap (C9) and ground cap(C10), the PI's going to be pretty unbalanced, but I don't know if it should be THAT unbalanced.
What else I found:
I think it might be the OT (hammond 1650H). I have been alternating between an attenuator connected to a pair of greenbacks and a dummy load. When the dummy load is in, I'm hearing a vibration/buzz inside the OT that's sympathetic with the signal going into the amp. The more buzzing I hear, the lower the current drops. If that's the cause though, I wouldn't expect the behavior to change when swapping PI plate connections or grid connections.
I chopped the ultralinear taps really short and shrink-wrapped them. I thought that they might be arcing or something, but they look fine when I pulled up the board. All tranny leads go through grommets, so there shouldn't be any arcing to the chassis from worn leads.
If I don't find something else, I'll probably replace the OT with a known-good one. Although this one should still be fine as it's brand new.
Matt
Re-built PI (C9,C10,R10,R11,R12,R13) using all new components. All resistors measured to confirm value. I lifted the Preamp board to make sure there weren't any dangling solder blobs or anything like that.
Changed R13 to 820, then to 680.
Untwisted power tube grid leads.
Untwisted OT plate leads.
Formatted amp drive and reinstalled.
Tried 6V6s, KT77s, TAD EL34Bs, JJ EL34s.
Bypassed the 220k grid leaks w/ a jumper, one at a time. This gave me some interesting results. Both sides behaved more like they're supposed to, but the current on the problem side only rose to about 70ma while the non-problem side rose to about 140ma with the same signal. I'll need to pull out my other amp and see if that's proper behavior. With a 10k tail and equal input coupling cap (C9) and ground cap(C10), the PI's going to be pretty unbalanced, but I don't know if it should be THAT unbalanced.
What else I found:
I think it might be the OT (hammond 1650H). I have been alternating between an attenuator connected to a pair of greenbacks and a dummy load. When the dummy load is in, I'm hearing a vibration/buzz inside the OT that's sympathetic with the signal going into the amp. The more buzzing I hear, the lower the current drops. If that's the cause though, I wouldn't expect the behavior to change when swapping PI plate connections or grid connections.
I chopped the ultralinear taps really short and shrink-wrapped them. I thought that they might be arcing or something, but they look fine when I pulled up the board. All tranny leads go through grommets, so there shouldn't be any arcing to the chassis from worn leads.
If I don't find something else, I'll probably replace the OT with a known-good one. Although this one should still be fine as it's brand new.
Matt
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Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
Re: What am I missing?
another thought.... the 1600 series hammond has a specified hookup
for its secondary connections..... theres two windings...
is it correct for load and is the connection properly phased?.....
for its secondary connections..... theres two windings...
is it correct for load and is the connection properly phased?.....
lazymaryamps
Re: What am I missing?
Andy,
I checked that and they're correct.
I think I'm chasing phantoms.
I swapped the OT for a OT18PP from musicalpowersupplies and put a pair of 6V6's in and had the same behavior. I wonder if it's just the way that the preamp is clipping.
With the volume at or below 2, it behaves as I would expect it. With a signal, both power tube jump from ~26mA to ~60mA. That's with the 6V6s. As I turn up the volume, the power tube fed from R17/C14 drops to about 15mA, 6mA of which is screen current.
If the PI is being fed a signal that is heavily clipped on one side and much less so on the other, I wonder if this is what would happen.
My only hope right now is to find a working o-scope that I can borrow. I've got a friend checking on one for saturday.
Matt.
I checked that and they're correct.
I think I'm chasing phantoms.
I swapped the OT for a OT18PP from musicalpowersupplies and put a pair of 6V6's in and had the same behavior. I wonder if it's just the way that the preamp is clipping.
With the volume at or below 2, it behaves as I would expect it. With a signal, both power tube jump from ~26mA to ~60mA. That's with the 6V6s. As I turn up the volume, the power tube fed from R17/C14 drops to about 15mA, 6mA of which is screen current.
If the PI is being fed a signal that is heavily clipped on one side and much less so on the other, I wonder if this is what would happen.
My only hope right now is to find a working o-scope that I can borrow. I've got a friend checking on one for saturday.
Matt.
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Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
Re: What am I missing?
its a heck of a way to tour the power side of an amp........
theres some freeware\shareware O-scope programs you can download
while your waiting...... usually a wave that is half clipped is a sign of a
design issue..... the operating point of the stage is to far one way or the
other into the non-linear portions of the plate curve...... the power side responds
symetricaly up to a point then demonstrates a severe imbalance....
and it deos it consistantly now matter what you do even though every thing
checks out........ die-in to know........
theres some freeware\shareware O-scope programs you can download
while your waiting...... usually a wave that is half clipped is a sign of a
design issue..... the operating point of the stage is to far one way or the
other into the non-linear portions of the plate curve...... the power side responds
symetricaly up to a point then demonstrates a severe imbalance....
and it deos it consistantly now matter what you do even though every thing
checks out........ die-in to know........
lazymaryamps
Re: What am I missing?
Ok. So I put a bid in on Ebay for a scope that's being sold locally. I pull out my old one that isn't working and fire it up for once more for old times sake.
The $@%#@!%!@$#!#ing thing is working now!
Yes that's good, but I really don't need another scope. Ok, this probably shows that I do.
Hopefully I'll have more info very shortly.
Matt
The $@%#@!%!@$#!#ing thing is working now!
Yes that's good, but I really don't need another scope. Ok, this probably shows that I do.
Hopefully I'll have more info very shortly.
Matt
Re: What am I missing?
Now that the o-scope is working, I see what is causing the behavior I'm seeing on the power tubes. (60mv P-P 1khz sine wave fed to input.)
Asymmetric square wave.
From about 2.5 to 8 on the volume control, the PI is putting a significantly less than symmetric square wave. The top half of the wave is about twice as wide as the bottom half of the wave. Going over the schematic and each of stages w/ the scope, I don't think this will be a problem. Once the volume is almost maxed, it turns to a symmetrical square wave and both sides even out.
I added grid stoppers (22k) the the second and third stages and that got rid of a weird buzz I was getting right after hitting the strings.
Now all I've got to do is throw the real OT back in and wire it back up. The one that I threw in for testing is a PP18OT from musicalpowersupplies.com. It sounds real good. I've got 2 of them and they're going into a Plexi6V6 and a BFDR-type circuit.
Thanks for the help.
Matt.
Asymmetric square wave.
From about 2.5 to 8 on the volume control, the PI is putting a significantly less than symmetric square wave. The top half of the wave is about twice as wide as the bottom half of the wave. Going over the schematic and each of stages w/ the scope, I don't think this will be a problem. Once the volume is almost maxed, it turns to a symmetrical square wave and both sides even out.
I added grid stoppers (22k) the the second and third stages and that got rid of a weird buzz I was getting right after hitting the strings.
Now all I've got to do is throw the real OT back in and wire it back up. The one that I threw in for testing is a PP18OT from musicalpowersupplies.com. It sounds real good. I've got 2 of them and they're going into a Plexi6V6 and a BFDR-type circuit.
Thanks for the help.
Matt.