I Scored Last Night!
Moderators: pompeiisneaks, Colossal
Re: I Scored Last Night!
I've seen resistor networks that look like that.
Tom
Don't let that smoke out!
Don't let that smoke out!
Wow - What a MESS!
I bought this Sundown in South Carolina around 1998 for $125. Never used it all that much, but recently decided to give it a going over. CRIPES!
Now, I KNEW it was going to have issues. When I bought it, the PFY (Pimple-Faced Youth) went on and on about all the cool modes he'd had done on it, but I really didn't pay much attention. It had 2 6L6s, so I figured it was a Rebel 50. Based on that, I later took it to a (questionable) amp tech, and asked him to give it a good going over, and set it up for EL34s. He gave me a long song and dance about how the RMS control couldn't be implemented with EL34s, and at the time, not knowing any better, I took him at his word. Now that I understand how the whole thing works, I realize that he was simply full of $hit.
Anyway, along the way I decided to put it into a combo box with a 15" (originally a JBL E-130, now an Altec 418). With the chassis out, a glance revealed a plethora of components mounted by one leg to the board, taped to wires going hither and yon, but it worked, so I didn't look too closely . . . until today.
Working from right to left (looking from the rear), I've managed to remove the most heinous stuff, and, in keeping with it's long and sordid history, added a few of my own. (I never used the red channel, so I basically bypassed the first (common) stage, recognizing that the rest of that channel is nearly identical to a Marshall Plexi.
Then, I came to the power supply board. It was encased in some kind of rubber epoxy, and since I couldn't find any sign of a bias pot, I figured some genius must have buried it. Wrong. Since a picture is worth a thousand words, here's what I'm looking at (see attachment):
No pot, and, probing the juncture of the two 220K resistors reads -345V!
In any case, I have two brand new Gold Lion KT88s slated for the amp, but I'm not going to risk them until I know everything is ship shape.
Glad to have found some pics here of what it's supposed to look like!
Now, I KNEW it was going to have issues. When I bought it, the PFY (Pimple-Faced Youth) went on and on about all the cool modes he'd had done on it, but I really didn't pay much attention. It had 2 6L6s, so I figured it was a Rebel 50. Based on that, I later took it to a (questionable) amp tech, and asked him to give it a good going over, and set it up for EL34s. He gave me a long song and dance about how the RMS control couldn't be implemented with EL34s, and at the time, not knowing any better, I took him at his word. Now that I understand how the whole thing works, I realize that he was simply full of $hit.
Anyway, along the way I decided to put it into a combo box with a 15" (originally a JBL E-130, now an Altec 418). With the chassis out, a glance revealed a plethora of components mounted by one leg to the board, taped to wires going hither and yon, but it worked, so I didn't look too closely . . . until today.
Working from right to left (looking from the rear), I've managed to remove the most heinous stuff, and, in keeping with it's long and sordid history, added a few of my own. (I never used the red channel, so I basically bypassed the first (common) stage, recognizing that the rest of that channel is nearly identical to a Marshall Plexi.
Then, I came to the power supply board. It was encased in some kind of rubber epoxy, and since I couldn't find any sign of a bias pot, I figured some genius must have buried it. Wrong. Since a picture is worth a thousand words, here's what I'm looking at (see attachment):
No pot, and, probing the juncture of the two 220K resistors reads -345V!
In any case, I have two brand new Gold Lion KT88s slated for the amp, but I'm not going to risk them until I know everything is ship shape.
Glad to have found some pics here of what it's supposed to look like!
You do not have the required permissions to view the files attached to this post.
Re: I Scored Last Night!
At first glance it looks to me like someone jumpered that resistor that is half covered in goop to the wrong side of the diode.
That goop shit looks a bit weird too--is it conductive?
Dang, my language is really getting bad. I have to clean it up--classes start in two weeks--I don't want to make a bad impression until at least into the 2nd or 3rd week!
That goop shit looks a bit weird too--is it conductive?
Dang, my language is really getting bad. I have to clean it up--classes start in two weeks--I don't want to make a bad impression until at least into the 2nd or 3rd week!
Re: I Scored Last Night!
The amount of crazy stuff you see in amps that supposed pros worked on is depressing.
That goop almost looks like the aluminum caulking I've seen on metal roofs.
I would remove everything from that rectifier board except maybe the diodes and redo it.
That goop almost looks like the aluminum caulking I've seen on metal roofs.
I would remove everything from that rectifier board except maybe the diodes and redo it.
Tom
Don't let that smoke out!
Don't let that smoke out!
Cleaned Up Schematics
If anyone's interested, I've redrawn the stock schematic found all over the web using visio. I'll attach it in both visio and .pdf formats:
You do not have the required permissions to view the files attached to this post.
My Sundown Mods
Here are the changes I made to my Sundown. Basically, I bypassed the first preamp stage from the red channel, voicing the rest like a Marshall Plexi. Instead of the boost function, I used a DPDT push-pull to switch between the two Marshall channel configurations.
I revoiced the green channel to resemble a Dumble OD special (without the overdrive). The attachments show the schematic:
I revoiced the green channel to resemble a Dumble OD special (without the overdrive). The attachments show the schematic:
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Re: I Scored Last Night!
I just built a new one from scratch, reusing any components that matched the original spec. At some point I'll probably clean up that original circuit board, repopulate it, and swap it back.
[quote="Structo"]The amount of crazy stuff you see in amps that supposed pros worked on is depressing.
That goop almost looks like the aluminum caulking I've seen on metal roofs.
I would remove everything from that rectifier board except maybe the diodes and redo it.[/quote]
[quote="Structo"]The amount of crazy stuff you see in amps that supposed pros worked on is depressing.
That goop almost looks like the aluminum caulking I've seen on metal roofs.
I would remove everything from that rectifier board except maybe the diodes and redo it.[/quote]
You do not have the required permissions to view the files attached to this post.
Re: I Scored Last Night!
Crud appears to be non-conductive.
Looks to me as if they replaced the trim pot with a random resistor -- guess they wanted to implement a 'fixed bias' topology 0^0
[quote="Jana"]At first glance it looks to me like someone jumpered that resistor that is half covered in goop to the wrong side of the diode.
That goop shit looks a bit weird too--is it conductive?
Dang, my language is really getting bad. I have to clean it up--classes start in two weeks--I don't want to make a bad impression until at least into the 2nd or 3rd week![/quote]
Looks to me as if they replaced the trim pot with a random resistor -- guess they wanted to implement a 'fixed bias' topology 0^0
[quote="Jana"]At first glance it looks to me like someone jumpered that resistor that is half covered in goop to the wrong side of the diode.
That goop shit looks a bit weird too--is it conductive?
Dang, my language is really getting bad. I have to clean it up--classes start in two weeks--I don't want to make a bad impression until at least into the 2nd or 3rd week![/quote]
ECC823 in V4? We'll see!
Just ordered one of these. Basically, they're a reversed 12DW7, which in turn is a unbalanced dual triode -- 12AU7/12AX7. This will leave the last red-channel preamp stage unaffected, but will put the 12AU7 into the buffer position. While the AU7 has a lower gain factor (irrelevant in the buffer), it has a way higher current capacity. Have used this approach in the past in an original design and it worked very well.
Suggestions?
After recreating the mangled power supply board and installing it, everything seemed to work, except that there was an inordinate amount of hum - pretty much throughout the signal path, so I cleaned up the original power supply circuit board, moved all the components back onto that, and installed it.
At this point, my symptoms, shown in the attached .jpg, are as follows.:
Everything up to the input to the phase inverter works fine. Signal is clear. No hum whatsoever.
For testing purposes, I removed the bias supply and grounded the bias point, intercepting the signal just past the .02u capacitors. Here I have signal, and hum.
With the bias reconnected, and biased to -42V, I read 96mA/tube across 1-Ohm cathode resisters, which, based on the 402 V I'm reading at the plates, should be giving me about 39W/tube (KT88s).
My B+ is only 435. (Output from transformer is 411V)
The output is a loud hum, and the notes sound out of tune, as if there's a subharmonic present. I'm guessing that this is the result of 60 cycle hum mixed with my original signals? (Playing open strings, they are not uniformly lowered, so it's not an actual subharmonic of the signal.)
My guess is that somewhere in the phase inverter is the culprit? Odd because I didn't have this problem before, and haven't munged in that vicinity at all.
In any case, Pin 1 of the PI is 200V, Pin 6 is 184V, and the cathodes (3-8) read 31V.
Any suggestions would be greatly appreciated. At this point I'm at a loss for ideas.
At this point, my symptoms, shown in the attached .jpg, are as follows.:
Everything up to the input to the phase inverter works fine. Signal is clear. No hum whatsoever.
For testing purposes, I removed the bias supply and grounded the bias point, intercepting the signal just past the .02u capacitors. Here I have signal, and hum.
With the bias reconnected, and biased to -42V, I read 96mA/tube across 1-Ohm cathode resisters, which, based on the 402 V I'm reading at the plates, should be giving me about 39W/tube (KT88s).
My B+ is only 435. (Output from transformer is 411V)
The output is a loud hum, and the notes sound out of tune, as if there's a subharmonic present. I'm guessing that this is the result of 60 cycle hum mixed with my original signals? (Playing open strings, they are not uniformly lowered, so it's not an actual subharmonic of the signal.)
My guess is that somewhere in the phase inverter is the culprit? Odd because I didn't have this problem before, and haven't munged in that vicinity at all.
In any case, Pin 1 of the PI is 200V, Pin 6 is 184V, and the cathodes (3-8) read 31V.
Any suggestions would be greatly appreciated. At this point I'm at a loss for ideas.
You do not have the required permissions to view the files attached to this post.
Re: Suggestions?
Heh! Problem solved.
TWO bad capacitors. My guess is they're probably 30 years old, and the stress of desoldering, resoldering, etc probably pushed them over the edge. Might as well replace them all before closing it up, I suppse.
[quote="jbefumo"]After recreating the mangled power supply board and installing it, everything seemed to work, except that there was an inordinate amount of hum - pretty much throughout the signal path, so I cleaned up the original power supply circuit board, moved all the components back onto that, and installed it.
At this point, my symptoms, shown in the attached .jpg, are as follows.:
Everything up to the input to the phase inverter works fine. Signal is clear. No hum whatsoever.
For testing purposes, I removed the bias supply and grounded the bias point, intercepting the signal just past the .02u capacitors. Here I have signal, and hum.
With the bias reconnected, and biased to -42V, I read 96mA/tube across 1-Ohm cathode resisters, which, based on the 402 V I'm reading at the plates, should be giving me about 39W/tube (KT88s).
My B+ is only 435. (Output from transformer is 411V)
The output is a loud hum, and the notes sound out of tune, as if there's a subharmonic present. I'm guessing that this is the result of 60 cycle hum mixed with my original signals? (Playing open strings, they are not uniformly lowered, so it's not an actual subharmonic of the signal.)
My guess is that somewhere in the phase inverter is the culprit? Odd because I didn't have this problem before, and haven't munged in that vicinity at all.
In any case, Pin 1 of the PI is 200V, Pin 6 is 184V, and the cathodes (3-8) read 31V.
Any suggestions would be greatly appreciated. At this point I'm at a loss for ideas.[/quote]
TWO bad capacitors. My guess is they're probably 30 years old, and the stress of desoldering, resoldering, etc probably pushed them over the edge. Might as well replace them all before closing it up, I suppse.
[quote="jbefumo"]After recreating the mangled power supply board and installing it, everything seemed to work, except that there was an inordinate amount of hum - pretty much throughout the signal path, so I cleaned up the original power supply circuit board, moved all the components back onto that, and installed it.
At this point, my symptoms, shown in the attached .jpg, are as follows.:
Everything up to the input to the phase inverter works fine. Signal is clear. No hum whatsoever.
For testing purposes, I removed the bias supply and grounded the bias point, intercepting the signal just past the .02u capacitors. Here I have signal, and hum.
With the bias reconnected, and biased to -42V, I read 96mA/tube across 1-Ohm cathode resisters, which, based on the 402 V I'm reading at the plates, should be giving me about 39W/tube (KT88s).
My B+ is only 435. (Output from transformer is 411V)
The output is a loud hum, and the notes sound out of tune, as if there's a subharmonic present. I'm guessing that this is the result of 60 cycle hum mixed with my original signals? (Playing open strings, they are not uniformly lowered, so it's not an actual subharmonic of the signal.)
My guess is that somewhere in the phase inverter is the culprit? Odd because I didn't have this problem before, and haven't munged in that vicinity at all.
In any case, Pin 1 of the PI is 200V, Pin 6 is 184V, and the cathodes (3-8) read 31V.
Any suggestions would be greatly appreciated. At this point I'm at a loss for ideas.[/quote]
Now, about that reverb level ...
So, to synopsize. . . I revoiced the green channel slightly, configuring the two stages like those from one of the Dumble OD Special schematics out there.
I bypassed the first gain stage on the red channel, configuring the remaining two (plus buffer), to parallel the respective stages in a Plexi Marshall. I gain the gain push/pull to switch the biasing circuit between the configurations of the two Marshall channels.
Replaced all the tubes as follows:
12AX7 (Green Channel Stage 2 & reverb recovery),
12AU7 (Reverb driver)
5751 (first gain stages for each channel),
ECC823 (Red channel gain stage 2 & Buffer),
12AT7 (Phase Inverter).
2 X Genalex Gold Lion KT88 (reissue).
I've only messed with it for a half hour, but am quite happy with the results. Will try to record some samples shortly.
Now, question . . .
I know that there's some general consensus that the reverb level changes when switching channels, and I do notice that effect, but for the life of me, I can't identify the source in the circuitry, unless it's a result of the buffer reacting differently to the reverb driver following it?
I've heard mention that there's a fix for this, but haven't found any details yet.
Joe
I bypassed the first gain stage on the red channel, configuring the remaining two (plus buffer), to parallel the respective stages in a Plexi Marshall. I gain the gain push/pull to switch the biasing circuit between the configurations of the two Marshall channels.
Replaced all the tubes as follows:
12AX7 (Green Channel Stage 2 & reverb recovery),
12AU7 (Reverb driver)
5751 (first gain stages for each channel),
ECC823 (Red channel gain stage 2 & Buffer),
12AT7 (Phase Inverter).
2 X Genalex Gold Lion KT88 (reissue).
I've only messed with it for a half hour, but am quite happy with the results. Will try to record some samples shortly.
Now, question . . .
I know that there's some general consensus that the reverb level changes when switching channels, and I do notice that effect, but for the life of me, I can't identify the source in the circuitry, unless it's a result of the buffer reacting differently to the reverb driver following it?
I've heard mention that there's a fix for this, but haven't found any details yet.
Joe
Re: I Scored Last Night!
[quote="martin manning"]Do you know what the seven-legged thing on the back of the pot is? Some kind of pre-packaged filter network?[/quote]
If you examine it closely, you'll see that it's the Baxandall tone control for the 'green' channel - it's in the schematic. I've copped this layout in my own amps, even mounting it on a single dual concentric pot where panel space was at a premium.
If you examine it closely, you'll see that it's the Baxandall tone control for the 'green' channel - it's in the schematic. I've copped this layout in my own amps, even mounting it on a single dual concentric pot where panel space was at a premium.