'73 Super Lead - harsh distortion artifacts on low notes

[Ken Bob]

Hey guys -

Just to follow up on this great discussion, because many of you have been so helpful... I did end up doing as I said, and replaced the crazy 1.0uF factory presence cap - -

First, I put in a new 0.68uF cap - the stock value for this year SL. Honestly, I couldn't tell much difference, if any. So then I continued on and put a new 0.1uF cap in as several of you suggested. Now I can tell a difference for sure. It's not an overwhelming change, and the character of the amp hasn't changed (I guess a good thing). But the presence control now works exactly as I'd expect it to. Instead of a freaking crazy distortion knob with the 1.0 and 0.68 caps, now it's definitely a presence control. Adds the bite and sizzle, I guess I would say. I still find certain chords a bit ratty in the lows, but it's not terrible, and overall the amp sounds awesome. I also took the opportunity to measure the pot (6.0k) and the resistor (4k8).



I think I'm satisfied, and I'll keep it as is - for now, anyway. I made another audio recording with the new 0.1uF cap. Same exact everything as the earlier clips. I start out with the presence knob on 0, and then at around 1:16 you can hear the increase in hiss as I roll the knob up to 10:



Thanks again to all of you for being so cool and helpful!!

 

[jagrenet]

T

Hey guys -

Just to follow up on this great discussion, because many of you have been so helpful... I did end up doing as I said, and replaced the crazy 1.0uF factory presence cap - -

First, I put in a new 0.68uF cap - the stock value for this year SL. Honestly, I couldn't tell much difference, if any. So then I continued on and put a new 0.1uF cap in as several of you suggested. Now I can tell a difference for sure. It's not an overwhelming change, and the character of the amp hasn't changed (I guess a good thing). But the presence control now works exactly as I'd expect it to. Instead of a freaking crazy distortion knob with the 1.0 and 0.68 caps, now it's definitely a presence control. Adds the bite and sizzle, I guess I would say. I still find certain chords a bit ratty in the lows, but it's not terrible, and overall the amp sounds awesome. I also took the opportunity to measure the pot (6.0k) and the resistor (4k8).

View attachment 137697

I think I'm satisfied, and I'll keep it as is - for now, anyway. I made another audio recording with the new 0.1uF cap. Same exact everything as the earlier clips. I start out with the presence knob on 0, and then at around 1:16 you can hear the increase in hiss as I roll the knob up to 10:



Thanks again to all of you for being so cool and helpful!!

That thing sounds great now. I wish I could get my '75 1959SLP to sound that good. I have been trying ..........

 

[Ken Bob]

T

That thing sounds great now. I wish I could get my '75 1959SLP to sound that good. I have been trying ..........

@jagrenet thanks man! I'm always here if you ever need to compare internals or settings

 

[jagrenet]

@jagrenet thanks man! I'm always here if you ever need to compare internals or settings

I would definitely be interested in that - for sure !!! We should probably take our discussion off of this thread though.

 

[Pete Farrington]

I made another audio recording

It sounds pretty darn good, I’d be smiling with that tone, and especially so if I could play that well
Well done for chasing this down.

 

[Ken Bob]

It sounds pretty darn good, I’d be smiling with that tone, and especially so if I could play that well
Well done for chasing this down.

@Pete Farrington much appreciated!

 

[Mastershon]

Great job everyone helping to narrow down the issue! It seems as though the 4k8 resistor should be removed also. From what I understand, the resistor was only used with 25k pots to form a voltage divider and essentially create a 5k resistance. No resistor is used when using a 5k pot. This would mirror the most commonly used presence circuit in Superleads i.e. 5k pot with .1uf cap only. Thoughts?

 

[neikeel]

Great job everyone helping to narrow down the issue! It seems as though the 4k8 resistor should be removed also. From what I understand, the resistor was only used with 25k pots to form a voltage divider and essentially create a 5k resistance. No resistor is used when using a 5k pot. This would mirror the most commonly used presence circuit in Superleads i.e. 5k pot with .1uf cap only. Thoughts?

Yup!

 

[Pete Farrington]

It seems as though the 4k8 resistor should be removed also

If that was done without reconfiguring the presence control, the amp wouldn’t work, at least properly, as the LTP V3 circuit would have lost its intended DC path to 0V common.

 

[Mastershon]

Thanks Pete. I don’t know how his is configured. I believe the attached to be the most common configuration…

 

[Ken Bob]

If I may, @Mastershon, the 2005 drawing you attached for the 1959HW isn't quite the configuration of my '73 SL. The presence configuration in your drawing is the older style, with the presence cap in parallel with the pot to ground. My amp has the cap in series to ground, which is a bit later style. I believe the switch to the even later style 25k pot came shortly after my amp.

I was actually reading these threads just last week which cover this subject quite thoroughly:

Earliest Use Of .68uf + 47k On Presence Pot?

It seems like it started in ~73? What's the effect of this change compared to the .1uf? Thanks!

www.marshallforum.com

Can't Hear Presence Control. '73 Circuit Help (Really Probably Solved This TIme!)

6/16/23: Solved. Had the LarMar wires going to the wrong power tubes. Switched them and presence adjustment came back! That's all that it was. Quite the oversight on my part but I sure learned a lot. Ok you've seen I've recently got this '73 50 watt. I've done a couple things to it but nothing...

www.marshallforum.com

 

[mickeydg5]

Thanks Pete. I don’t know how his is configured. I believe the attached to be the most common configuration…

His is like this one except he now has a .1uF instead of the .68uF capacitor.
It reacts just like the one you mentioned except no worries about potentiometer failure and less chance of possible DC noise.

 

[Mastershon]

His is like this one except he now has a .1uF instead of the .68uF capacitor.
It reacts just like the one you mentioned except no worries about potentiometer failure and less chance of possible DC noise.

View attachment 137978

Thanks for the input guys/gals. It’s an interesting subject for sure. How about this ‘generic’ one on Rob Robinettes site… this would block DC also would it not?

 

[mickeydg5]

Thanks for the input guys/gals. It’s an interesting subject for sure. How about this ‘generic’ one on Rob Robinettes site… this would block DC also would it not?

Yes it does and actually I was going to mention this setup in the other post because I would rather the PRESENCE potentiometer on the ground side of the capacitor.
But I did not.
I would not call it generic. To me, there are just different ways of going about things, different designs.
Notice in that article and diagram you mentioned he is placing the PRESENCE control before the feedback resistor, on the transformer side and not the PI side.

 

[Pete Farrington]

How about this ‘generic’ one on Rob Robinettes site…

If I understand the drawing correctly, that’s a bad idea. Adding a significant capactive loading in parallel to the strongly inductive load of a speaker cab will create a highly resonant load, which will tend to reduce the margin of stability of the power amp.
The result being that the amp may tend to cease to function normally and oscillate at high presence settings.
I’m not aware of any competently designed amp that has such a presence control arrangement?
Apologies if I’ve got the wrong end of the stick here.

 

[MullardFillmore]

Hello again Marshall-mates!

Posting here again regarding the '73 UK-spec Super Lead I recently acquired. The amp was, prior to my ownership, restored to the original circuit, as best as I can tell, and not all that much was done aside from new V1 bypass caps, and a few minor components here and there. It works great, and is a monstrous, aggressive beast. The only issue I seem to be having is I'm noticing a harsh, ratty kind of distortion artifact on low notes, which increases/decreases with guitar volume. I've owned quite a few Marshalls over the years, but haven't heard this exact thing before. It seems to be present on both I and II channels, but obviously is way more apparent on the I channel. The amp cleans up just fine with guitar vol rollback (clean being relative, as the amp is super gainy and hairy past "2"). Wondering if any of you super-knowledgeable gurus can help diagnose what's going on here?

PT, OT, and choke are original. Power tubes are new, and I biased them myself: @120 VAC, plate volts 499, 34 mA. Tube sockets were cleaned and tensioned a couple weeks ago. Filter caps are newer-ish ARS, though not sure exact age. No hum or other weird noises. No caps are swelled or leaking. Heaters are 6.31 VAC. Swapping preamp tubes doesn't change the issue. I've tried different speaker cabs with no change. Rolling back the Presence does lessen the issue somewhat, but does not eliminate it. NFB appears to be the stock 100k/4-ohm, and the presence cap appears to be an unusual factory-original 1.0mF (asked about this in a previous post). Bright cap is a non-original 1000pf (though I kinda like it). While scanning the forums here for any related info, I did come across a prior discussion regarding oscillations that recommended watching the idle current while rolling the knobs up with no input to the amp - I tried this this morning using a bias probe, and there is no change in the idle current whether everything is on 10 or 0. At this point, I'm kind of out of ideas.

View attachment 136469View attachment 136470
Here's a couple recorded clips. Excuse my rubbish noodling - I'm just trying to get various sounds out of the amp. But both clips capture the harsh distortion thing happening in the last 40 seconds of each clip. It's very apparent in the room - most especially on the low notes and chords. Hope it comes across.




Sincere thanks to one and all for any helpful insight! So many knowledgeable folks here, and I always learn a lot from you!!

-Ken

What's up with that grey wire on the first tube socket? Looks like a second wire going to the plate? Also check that 820 resistor, I can't make out what that is.

 

[mickeydg5]

If I understand the drawing correctly, that’s a bad idea. Adding a significant capactive loading in parallel to the strongly inductive load of a speaker cab will create a highly resonant load, which will tend to reduce the margin of stability of the power amp.
The result being that the amp may tend to cease to function normally and oscillate at high presence settings.
I’m not aware of any competently designed amp that has such a presence control arrangement?
Apologies if I’ve got the wrong end of the stick here.

Pete, did you notice I mentioned that in a way. Robinette wrote "Placing the cap on the output transformer side of the NFB resistor will give best control authority."

That arrangement would not only provide a variable low pass circuit for the PI negative feedback but also act as a variable low pass to the output as well. You know, sort of like a 'TREBLE" control to the speaker cabinet as seen in Hi-Fi speaker cabinets or something.

To add: ..........until it blows up.
The loading @Pete Farrington mentioned above would burn up the "generic" potentiometer Robinette indicated when dialed at lower resistances (turned down).

 

[matttornado]

This may sound silly but I have mistaken not so good string vibration from fretted notes for weird amp noises. Does it do it when you play open strings?

 

[Ken Bob]

This may sound silly but I have mistaken not so good string vibration from fretted notes for weird amp noises. Does it do it when you play open strings?

Not silly, @matttornado. I think I know what you mean. I've had that issue at times with other guitars and amps in the past. This Les Paul from the clips was actually just refretted and set up two months ago And I'm definitely not getting the same occasional low end ratty thing from any of my other amps. It's definitely something about this Super Lead. I love it, but that's in there, for whatever reason. (and yeah, you can hear it a little bit in just a plain low E power chord) Definitely reduced with the smaller presence cap though, for sure.

 

[Ken Bob]

What's up with that grey wire on the first tube socket? Looks like a second wire going to the plate? Also check that 820 resistor, I can't make out what that is.

@dtier mentioned this earlier in the thread - hot shield:

There appears to be a hot shield on V1b, likely left over from a past 2203 type master volume conversion. That's not typical for a Superlead. I've heard some claims that it affects tone but haven't verified myself. It amounts to the out of phase plate signal wrapped around the grid wire. It's not likely to be causing the symptom but could be affecting tone on the treble channel.

As for the 820 resistor - I assume you mean the one all the way at the left edge of the board? If so, that one checks out.