Chris6870
Well-known member
Did you get that from GC by chance?
FourT6and2
Well-known member
I'm not a fan of PPIMVs. Just use an attenuator so you can get power tube saturation, with a stock Super Lead/Bass, if that's what you're after.
As far as pre- vs. post- tone stack masters go, it really depends on the rest of the circuit. For a stock Marshall circuit with cascaded gain stages, like a 2203/2204, a post-stack MV works fine. Even with 4 or 5 gain stages, it still works. For the kind of diode clipping mods I like, pre-stack works better. You can also run both. Or even run a PPIMV as well for three different "masters".
What I will do for this amp, will depend on what direction I decide to go in. If I add another 12AX7 and cascade 3, 4, or 5 stages, I'll run a post-stack master. I doubt I'll do diode clipping. I already have a pretty amazing 800 clone with that.
FourT6and2
Well-known member
Rock-N-Roll Vintage in Chicago.
Exo-metal
Well-known member
That particular 70 is well known. It went back a few times. I almost bought it myself at one point.
FourT6and2
Well-known member
I think I saw that one. I called up GC to ask and the guy was like... don't buy it lol.
Rocker68
Active member
Thanks for the reply . This is exactly how I'm thinking .
Following your decisions and progress .
psychodave
Well-known member
I wire in a ground plug and bypass the polarity switch.
Erock
Well-known member
schaweet man! 
Exo-metal
Well-known member
The best part about this thread is I will be plugging into this monster when it's done haha
SpiderWars
Well-known member
That V1a plate does seem low relative to the other voltages but I’m not looking at any voltage charts just going from memory. Maybe the V1a triode just runs hot or the plate or cathode resistor is off.
Will be interesting to see where you take it.
J
jasonP
Well-known member
Exciting man! Cool find!
FourT6and2
Well-known member
Small progress.
1. Replaced both 10K B+ droppers. The original carbon comps drifted up to 12K each. These both measure 9.98K.
2. Replaced a non-original carbon comp cathode resistor on the third stage with a period correct Iskra and tidied up the 0.68uF bypass cap leads.
3. Replaced the non-original 100K carbon comp NFB with an Iskra.
4. Removed death cap (not shown).
5. Replaced original Wima 0.1uF in PI with Philips/mustard. I know the Wima was original, but whatever haha
Waiting for ARS filter caps and BC/Philips bias caps to arrive.
Next step is to test that "hot shield" mod someone did and see why they did it and if it's needed.
1. Replaced both 10K B+ droppers. The original carbon comps drifted up to 12K each. These both measure 9.98K.
2. Replaced a non-original carbon comp cathode resistor on the third stage with a period correct Iskra and tidied up the 0.68uF bypass cap leads.
3. Replaced the non-original 100K carbon comp NFB with an Iskra.
4. Removed death cap (not shown).
5. Replaced original Wima 0.1uF in PI with Philips/mustard. I know the Wima was original, but whatever haha
Waiting for ARS filter caps and BC/Philips bias caps to arrive.
Next step is to test that "hot shield" mod someone did and see why they did it and if it's needed.
SpiderWars
Well-known member
Could the proximity of the treble peaker and the 2n2 have any effect on that area being so sensitive?
And would outer foil orientation make a difference?
And would outer foil orientation make a difference?
FourT6and2
Well-known member
It's possible. I'll have to test and see. That said, it's pretty common to see these components installed like this in vintage amps. Here's a similar 1970 SL a guy on the Marshall forum just posted. Built around the same time as mine and from the looks of the lead dress, possibly by the same assembler!
I've never tested that. But from what I've read, it miiiiiiight make a tiny difference but I doubt it's worth the trouble.
Exo-metal
Well-known member
Marshall's rule...
FourT6and2
Well-known member
So, something really interesting happened. I tidied up the power section wiring a bit and while I was at it, I decided to remove the shielded wire on V2a that was wired as a "hot shield", with the shield connected to the stage's plate. I know this supposedly does a couple of things: shields the grid with a signal (much) higher than ground potential and maaaaaybe interacts a bit with the miller capacitance of the tube to kill some high end. But the math on this, for a piece of wire that is maybe 3-4 inches long, doesn't really show any significant high-end cut.
I removed it and replaced with a normal piece of wire to see what would happen.
Plugged the amp in holy crap... so much buzzzz! And the tone of the amp was significantly brighter. Almost like a bright cap had been installed (and this amp has no bright cap). At first I was confused and thought I mis-wired something or was getting some weird interference somewhere. I looked at everything I could think of and nothing was getting rid of the noise or fixing the overly harsh/bright sound of the amp.
So I took a stab in the dark and put the "hot shield" back in.
Plugged the amp in and... bingo. Glorious, harmonically rich Super Lead sound was back. And the noise/buzz was significantly reduced.
This makes no sense to me because there shouldn't be enough capacitance from this one wire to affect anything. But, here we are...
I'm puzzled.
I removed it and replaced with a normal piece of wire to see what would happen.
Plugged the amp in holy crap... so much buzzzz! And the tone of the amp was significantly brighter. Almost like a bright cap had been installed (and this amp has no bright cap). At first I was confused and thought I mis-wired something or was getting some weird interference somewhere. I looked at everything I could think of and nothing was getting rid of the noise or fixing the overly harsh/bright sound of the amp.
So I took a stab in the dark and put the "hot shield" back in.
Plugged the amp in and... bingo. Glorious, harmonically rich Super Lead sound was back. And the noise/buzz was significantly reduced.
This makes no sense to me because there shouldn't be enough capacitance from this one wire to affect anything. But, here we are...
I'm puzzled.
glpg80
Well-known member
One of the pots are bad. I bet one of the volume pots or presence pot have lost their reference to ground. Is it noisy with both low and hi inputs? What if you jumper the channels, still the same issue? Maybe a dirty jack? You can disconnect and check the pots with an ohm meter and see if it’s jumping around as you turn it.
V1A looked low in voltage earlier. What do the cathode and anode resistors measure?
V1A looked low in voltage earlier. What do the cathode and anode resistors measure?
FourT6and2
Well-known member
Without the hot shield, the amp has a buzz without anything plugged into the inputs. This is with both volumes at zero. As I raise the normal channel volume, the buzz suddenly disappears once the control is up around 7 on the dial. If I continue to raise normal channel volume, the buzz comes back by 7.5. Bright channel volume doesn't behave this way. Buzz is present, just gets slightly louder and brighter in timbre as I raise bright channel volume. Treble control has no effect on the noise. Mid control increase signal gain overall a bit, so the buzz gets louder. Bass has zero effect on the noise. Presence has zero effect on the noise.
I put the hot shield back in and the buzz all but disappeared again.
Tapping on V1b coupling cap, treble peaking cap, and either 470K mixing resistors yielded tons of noise/pop before removing hot shield. After I put the hot shield BACK in, it's much quieter, but still quite sensitive.
The actual tone of the amp is amazing with the hot shield in. I'm guessing it's adding 300pF+ of capacitance and killing some highs. I'll check all the pots the best I can, possibly a bad coupler on V1b or bad filter cap. But I have new filter caps coming.
Jumping the channels has no effect on the buzzing. I don't want to disconnect the pots just yet as all the solder joints are original and I don't want to molest the amp too much at this point.
Cathode and anode resistors on V1a/b and V2a/b all measure spot on. After replacing the two 10K B+ droppers, V1a came up to 158v. Still quite low for this sort of amp IMO.
H
harddriver
Well-known member
I agree with glpg80 about the bright channel pot possibly being bad. My bright channel pot went bad in my 68 NO build and contributed to alot of noise and even RF and other issues when turned above 6. I guess the carbon tracks were worn out in the pot in that region of the pot travel, I always have played the amp at 7 since it was built, If I dime the bad pot it would make all kinds of RF type noise, I thought I had a bad V1 or V2 12ax7 tube but it was the pot. I also had to replace the bright channel volume pot in my 1972 Marshall SL a couple years back, since this is the most used channel the pots' see alot of use.
I replaced the pot and the amp was quiet again with no extraneous noises, the new ARS filter caps also helped the amp quiet down some even though the F&T's still tested within spec capacitance and ESR wise.
You should not need that shielded wire with a stock 1959 Marshall so something is going on and if that is an original pot to the amp then more than likely it's past it's prime and worth checking it or replacing it.
Here's how I wired in the new grounded plug and removed the entire polarity switch in my 72 recently. I also did the Larry grounding scheme in the 72 and it seemed to help reduce noise down to a low white noise level.
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J
jasonP
Well-known member
I wonder if there is something going on with the 50/50uf can cap, filter cap in that stage or if the 500pf mica has shorted...