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plexi reissue questions 1987x

9/18/1999 6:30 AM
Ted Matsumura
plexi reissue questions 1987x
I opened up my 1987x to adjust the bias and try some different EL-34s, and I wanted to describe my circuit board to see if others have similar, as it doesn't quite match the description in the pittman tube book.  
Opening up the chassis, turning upside down, knobs facing me, to left of bias pot I have R28-56K, R27-10K (with long leads, rework??), OR wire, R26-220K, R25-220K, GR wire, C22-just a wire, cap must be on other side, R24-10K.  
To the right of the bias pot is R29-220K.  
using wall AC (121 VAC), and a bias probe, I set the bias a little low tonight -38ma, and tried the stock tubes and a stash of old el-34s left over from my Dynaco ST-70 days. With many of the tubes I would notice with the lights off and playing, that the filaments would brighten and soften as I played, and the blue gas amount would vary. Only one old tube got real red, and I imagine it was shorted. Is this common with this Class A/B design. The tone broke up more with some of the older tubes I had, but seemed most stable with the Sylvania 6CA7/EL34's which I reverted to.  
I'm keeping this guy out of it's chassis for awhile to experiment with. Anyone out there have favorite tubes or bias settings for this amp? I was mostly playing through the treble input, with channels jumpered, at various settings.
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9/18/1999 12:50 PM

Let me dig my old (1987x reissue) PCB out of a box later today.  
I'll post a reply later today/early tommorow.  
Stay tuned.....  
9/18/1999 5:57 PM
Ted Matsumura

Thanks Mook,  
Here's more data --  
Even though it's a 1987x 50W, PCB has printed:  
"1959SLP-60-00-2345 94v-0"  
Sticker on side of chassis says model:  
1987X U  
spec: USA  
another sticker: test I. PLIMSAUL  
PT is a Drake 1202-324, OT is 784-131, choke 1471-263  
All 95 date codes  
9/19/1999 12:47 PM

My PCB doesn't have those writtings....I have:  
Marshall ST1 iss. 202  
(x) Lead Units  
(s) 100W Units  
My inspector was "Sheena". On the same sticker it says "1987x USA".  
The "x" and "s" (from above) because this PCB is used for the 100W and 50W versions.  
My trannies are Drake as well with a date of 1993.  
Hope this helps....  
9/19/1999 4:18 PM
Ted Matsumura

Thanks Mook,  
sounds like they changed pcb layout between '93 and '95 reissues.  
9/18/1999 3:05 PM

My first impression is that you are actually seeing the screens glow red and reder!  
I know you know your tubes so are sure it was the filaments that would get brighter?  
Does this have the 5w 1K screen resistors on the tube sockets Ted?  
9/18/1999 6:11 PM
Ted Matsumura

Hi Bruce,  
I don't know a lot about the EL34's. Here's more of what's happing that seems odd, even though the sound is pretty good:  
The 6CA7/EL34 closest to the middle (preamp tubes) seems to have different behavior than the tube closest to the edge (farthest away from preamp tubes).  
Both tubes have two vertical rods that go the length of the tube. These light up red in both tubes when power is on, and standby is on. The tubes look the same, some blue gas, nothing looks too hot here.  
Then putting standby off, tubes still look the same, but as I start playing, the tube closest to preamp tubes has the 2nd vertical rod lighting up red, not overly or cherry, just red so there are now two vertical red bars in this tube only as I play, if I quit playing, 2nd vertical rod in this tube goes out and both tubes look the same.  
I have swapped tubes, and behavior stays with position, so it is apparently not the tubes. Both outputs are biased at -38ma. and both have the 5W 1K screen resistors.  
My feelings here, which could probably use some expert clarification, are that since this happens across several pairs of tubes, it is not a tube issue, but an amp setting (disregarding that cherry red tube I described and think is shorted).  
The sound is good, and I'm thinking that this amp is just class a/b, where idle is < 70% of plate load, where all the other amps I've recently been fooling with are class a.  
I wonder if biasing to -45ma would keep the tubes looking stable, and still sounding good?  
Does anyone have a Marshall (plexi type) that whether idling or playing hard, the tubes glow the same? If so what tubes and bias / plate voltage is it setup for?  
I can kind of see why someone thought going to a 6550 would result in a more stable tube setup, regardless of sound. However, I want to solve this EL-34 quirkiness, if in fact it is quirky. If I recall from tinkering with the ST-70s, those tubes when biased high used to dance to the input as well.  
I think I'm spoiled with these inefficient, but consistent tube behavior of class A amps, but I don't necessarily want to setup or mod this amp for class A if I can get good sounds out of its class a/b design. I think I'll move the bias to -42 and then -46 and see what stability may emerge.  
Thanks for the ideas! Keep 'em coming!  

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