Bad KT77's
I am a dumb newbie, but this afternoon I read on the AA site
a long series of posts regarding the small pin sizes of this specfic JJ tube. This resulted in very interesting electrcal behavior and Mr. Mcshane (sp) had returned all but a small number of them to the factory.
I just returned a JJ 5AR4 to another vendor for the same reason.
So use a micrometer on the pins. Mine mic'ed .0865 and the old tube mic'ed .0935. The JJ wasd intermittent in the socket.
I am a dumb newbie, but this afternoon I read on the AA site
a long series of posts regarding the small pin sizes of this specfic JJ tube. This resulted in very interesting electrcal behavior and Mr. Mcshane (sp) had returned all but a small number of them to the factory.
I just returned a JJ 5AR4 to another vendor for the same reason.
So use a micrometer on the pins. Mine mic'ed .0865 and the old tube mic'ed .0935. The JJ wasd intermittent in the socket.
Victoria,
That could make sense... the grid on V3 is acting really screwy.
Was there any talk about the little type tubes having bad pins?
Gene,
Cut a lead on that mica... its bad... your amp will come to life. That explains why your treble pot is killing things... it's probably not the 0.1.
That could make sense... the grid on V3 is acting really screwy.
Was there any talk about the little type tubes having bad pins?
Gene,
Cut a lead on that mica... its bad... your amp will come to life. That explains why your treble pot is killing things... it's probably not the 0.1.
OK, Back,
I replaced that cap, No change. On these caps, when I measure them with my VOM, I do not seem to get a reading.
This is wild, My ear still hurts from day before yesterday running this the dimed for over an hour. The brother stops over, He runs it here about 20 minutes, its fine, heads home runs it 30-45 minutes at about 75% and it dies!
I am about ready to snip out the board and just reload it.
Check each piece on its way to the trash.
I have found however that the treble control is doing weird things, When reduced to zero, The amp distrorts terribly, when raied about 8 it cuts out entirely. I checked all those connections again, But nothing.
I think I am going to just sleep on it tonite and ponder cutting out the board.
Such a shame too, this was briefly the best sounding amp I have ever built.
Gene
I replaced that cap, No change. On these caps, when I measure them with my VOM, I do not seem to get a reading.
This is wild, My ear still hurts from day before yesterday running this the dimed for over an hour. The brother stops over, He runs it here about 20 minutes, its fine, heads home runs it 30-45 minutes at about 75% and it dies!
I am about ready to snip out the board and just reload it.
Check each piece on its way to the trash.
I have found however that the treble control is doing weird things, When reduced to zero, The amp distrorts terribly, when raied about 8 it cuts out entirely. I checked all those connections again, But nothing.
I think I am going to just sleep on it tonite and ponder cutting out the board.
Such a shame too, this was briefly the best sounding amp I have ever built.
Gene
poobah said:Victoria,
That could make sense... the grid on V3 is acting really screwy.
Was there any talk about the little type tubes having bad pins?
Gene,
Cut a lead on that mica... its bad... your amp will come to life. That explains why your treble pot is killing things... it's probably not the 0.1.
OK,, Thats an easy one, I'll run and do it,, Be Back In 15 Mins.
I read about that also, but only on the original KT77 sample shipment. These things fit the socket really tight as do the 12ax7's. I actually worry trying to remove the KT77's because they are so tight on the pins. I have gold plated 9 pin sockets on this amp, They seem to fit with equal force to the RCA tubes I have also tryd on this amp.
Gene
250 pf / mmfd at 500 Volts dipped Silver Mica These 500 Volt Silver Mica Capacitors made by "brand name" SIMIC. Simic Electronics is a world renowned producer of high quality ISO certified silver mica capacitors.
http://www.justradios.com/capacitors.html#product
Mid way down on the page.
I cut the lead rather than replace it. Instant horsepower!!
Sounds like CR@P without it, but I will replace it in the morning.
HOW ON EARTH did you arrive at that?
I would have replaced every single part to get this thing up n running. I would of never thought of that part as being the culprit.
Now, Trust replacing it with the same thing,
Or, I have 250pF @ 630V Axial Polystyrene Film Capacitors . The Little clear looking ones that look like foil inside.
Or Ceramic disc?
Gene
http://www.justradios.com/capacitors.html#product
Mid way down on the page.
I cut the lead rather than replace it. Instant horsepower!!
Sounds like CR@P without it, but I will replace it in the morning.
HOW ON EARTH did you arrive at that?
I would have replaced every single part to get this thing up n running. I would of never thought of that part as being the culprit.
Now, Trust replacing it with the same thing,
Or, I have 250pF @ 630V Axial Polystyrene Film Capacitors . The Little clear looking ones that look like foil inside.
Or Ceramic disc?
Gene
Use whatever you got... it was probably just a fluke.
How did I arrive at that? We knew some POSITIVE DC was getting into the grid of V3 because the grid was higher than the cathode. All of your tone circuit connects to ground. Except the 0.1 & the 220 pF... they are connected to the 182 volts at the cathode of V2. When I realized that the problem was changing with the tone control... is was pretty clear.
DC Current was flowing through the 200 pF through the pot then to the grid of V3.
I'm not counting mind you... but that's 2 beers now.
Assuming you can do the 32 steps one more time... good night!
P.S. ISO doesn't mean that people don't make garbage... it just means they have a well documented procedure for doing so.
How did I arrive at that? We knew some POSITIVE DC was getting into the grid of V3 because the grid was higher than the cathode. All of your tone circuit connects to ground. Except the 0.1 & the 220 pF... they are connected to the 182 volts at the cathode of V2. When I realized that the problem was changing with the tone control... is was pretty clear.
DC Current was flowing through the 200 pF through the pot then to the grid of V3.
I'm not counting mind you... but that's 2 beers now.
Assuming you can do the 32 steps one more time... good night!
P.S. ISO doesn't mean that people don't make garbage... it just means they have a well documented procedure for doing so.
2 for you 2 for me,, I think 3 sounds better, No sense letting the last 2 get warm 
Hey, Could being hit with a rather aggressive Overdrive pedal induced this?
I know for sure he was using this old Hughes Kettner tube overdrive pedal at the time. Its a very rare unit they only produced a few hundred of.
Just a thought.
Thank you so much for spending your evening here helping me sort this thing out.
Gene
Hey, Could being hit with a rather aggressive Overdrive pedal induced this?
I know for sure he was using this old Hughes Kettner tube overdrive pedal at the time. Its a very rare unit they only produced a few hundred of.
Just a thought.
Thank you so much for spending your evening here helping me sort this thing out.
Gene
Poobah 
I got up about 6AM CST,
Went downstairs and replaced that mica cap. Fired up the amp, and clearly it was indeed that part. I put in a 630V 250pf Axial Polystyrene Film Cap. I just could not get myself to trust another one of those dipped mica's from that purchase.
The one I have now looks the top one in the picture.
I do not detect any audible difference in using this type cap, Though I have no idea which would be concidered better.
Later(after everyone been awake a bit) I will run it hard and see how it really holds up.
Gene
Went downstairs and replaced that mica cap. Fired up the amp, and clearly it was indeed that part. I put in a 630V 250pf Axial Polystyrene Film Cap. I just could not get myself to trust another one of those dipped mica's from that purchase.
The one I have now looks the top one in the picture.
An externally hosted image should be here but it was not working when we last tested it.
I do not detect any audible difference in using this type cap, Though I have no idea which would be concidered better.
Later(after everyone been awake a bit) I will run it hard and see how it really holds up.
Gene
SY said:
Ut Oh, That I do not need,
I'll swap it out later today if I have something suitable, What would be the better way to go SY?
I most likely will need to order a bunch of whatever. I have 20 of those Silver Mica's and 20 of the 630V Polystyrene.
I will be building several more of these amps before July, Both guitarists in his band will be playing these out on the 21&22 of July. Plus a couple backups.
Gene
Edit:
Oh BTW, I have run it near Dimed for 30 minutes, It actually sounds a good bit better than as it was originally built. That had to of been a borderline bad part right from the beginning.
Hey Gene,
You really can use one of the micas you already have.
Very often what distuingusihes "quality" in electronic parts is not how one behaves against an another... bur rather does 1 of a 1000 fail or does 1 of a 1,000,000 fail.
I would agree with SY though I have no direct experience with those caps at high voltage. Styrenes are tender at best... hard enough just to solder them in. There claim to fame is data acquisistion and sampling circuits... low level filers etc...
Just run that mica and be sure is doesn't get warm. If it does. we'll take some measurements and do it the hard way. You don't have to worry about sonic stuff... you want that cap to screw with the sound... it's a tone control.
You really can use one of the micas you already have.
Very often what distuingusihes "quality" in electronic parts is not how one behaves against an another... bur rather does 1 of a 1000 fail or does 1 of a 1,000,000 fail.
I would agree with SY though I have no direct experience with those caps at high voltage. Styrenes are tender at best... hard enough just to solder them in. There claim to fame is data acquisistion and sampling circuits... low level filers etc...
Just run that mica and be sure is doesn't get warm. If it does. we'll take some measurements and do it the hard way. You don't have to worry about sonic stuff... you want that cap to screw with the sound... it's a tone control.
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