M
Mojojojo
Guest
Well, tolex on the corners a little ragged, but otherwise in good cosmetic shape. Handle a little rusty, one knob missing cover.
This amp may actually be a '72 due to the chassis sticker on it, but I will need to run the transformer codes. Anybody know where I can look the codes up, or how to decode them?
First thing I did was open it up and took a look around. Except for the master volume mod (more on this later) installed in the lower right jack hole (original jack taped up on the inside) and new LCR caps, everything appears stock. This is a point-to-point wired amp. I understand that midway during '73 was the transition period.
It came with Svetlana EL34s and the first thing I noticed was no 1K screen grid resistors, but even the old Plexi schematics didn't have 'em, but then again, the old tubes were probably more robust in the screen grid area. I didn't think too much about this, but more on this in a bit... hehehe.
Preamp tubes were some British one, but not a Mullard. Can't quite make out the name. Sylvania and an RCA.
So I fired it up and biased it. The guy before me had it set so cold, I'm surprised any sound came out. After checking the plate voltatge (460) and setting the bias to around 34mA (conservative), I plugged in the Goldtop and... This thing sounds friggin' great already.
So I crank it up and as I'm playing one of the EL34s flashes and the HT fuse pops. I immediately suspected that this amp was originally equipped with 6550s and didn't need the 1K screen grid resistors, and I suspected the tube just shorted. Out came the soldering iron and I installed the 1K, 5W screen grid resistors from pins 4 to 6 and wired the screen supply to pins 6 of the tubes. In went a known good set of EL34s and a new HT fuse, rebiased and no problems since, and I played this thing cranked for quite a while.
As for the master, whoever did this used black PVC like goop to coat where they tapped into the PI caps. This doesn't appear to be the RICH mod as it seems to use two additional caps and uses a 1M ganged pot instead of 500K. This will soon be rectumfried.
Where do I get the 500K dual pot from?
AC/DC - no problem. Page - no problem. Crank up the volume and Michael Schenker is no problem. Plenty of gain. Clean tones.... well... obviously not as good as others, but what these things do, no body else quite seems to get it.
I'm going to walk through the amp with my meter and the schematic. Any favorite mods would be most appreciated! I already unsoldered one lead of the channel 1 bright cap across the volume pot. What about the 500pF cap across the 470K resistor on channel 1?
I'll see if I can get some pics up by the weekend.
This amp may actually be a '72 due to the chassis sticker on it, but I will need to run the transformer codes. Anybody know where I can look the codes up, or how to decode them?
First thing I did was open it up and took a look around. Except for the master volume mod (more on this later) installed in the lower right jack hole (original jack taped up on the inside) and new LCR caps, everything appears stock. This is a point-to-point wired amp. I understand that midway during '73 was the transition period.
It came with Svetlana EL34s and the first thing I noticed was no 1K screen grid resistors, but even the old Plexi schematics didn't have 'em, but then again, the old tubes were probably more robust in the screen grid area. I didn't think too much about this, but more on this in a bit... hehehe.
Preamp tubes were some British one, but not a Mullard. Can't quite make out the name. Sylvania and an RCA.
So I fired it up and biased it. The guy before me had it set so cold, I'm surprised any sound came out. After checking the plate voltatge (460) and setting the bias to around 34mA (conservative), I plugged in the Goldtop and... This thing sounds friggin' great already.
So I crank it up and as I'm playing one of the EL34s flashes and the HT fuse pops. I immediately suspected that this amp was originally equipped with 6550s and didn't need the 1K screen grid resistors, and I suspected the tube just shorted. Out came the soldering iron and I installed the 1K, 5W screen grid resistors from pins 4 to 6 and wired the screen supply to pins 6 of the tubes. In went a known good set of EL34s and a new HT fuse, rebiased and no problems since, and I played this thing cranked for quite a while.
As for the master, whoever did this used black PVC like goop to coat where they tapped into the PI caps. This doesn't appear to be the RICH mod as it seems to use two additional caps and uses a 1M ganged pot instead of 500K. This will soon be rectumfried.
Where do I get the 500K dual pot from?AC/DC - no problem. Page - no problem. Crank up the volume and Michael Schenker is no problem. Plenty of gain. Clean tones.... well... obviously not as good as others, but what these things do, no body else quite seems to get it.
I'm going to walk through the amp with my meter and the schematic. Any favorite mods would be most appreciated! I already unsoldered one lead of the channel 1 bright cap across the volume pot. What about the 500pF cap across the 470K resistor on channel 1?
I'll see if I can get some pics up by the weekend.
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