'73 Super Lead - harsh distortion artifacts on low notes

[Ken Bob]

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.


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

 

[Tatzmann]

*post deleted, he already tried diff. speakercab.
Clips not working with my browser.

Ummph. OT maybe. I think we had such a problem here not too long ago, it was a 1987x and the problem was a couplingcap, if i remember correctly.

 

[Jon Snell]

Firstly fit the capacitors correctly or that will cause an issue, possibly as you describe and then set the bias to the correct current draw at idle.
Those anode load resistors are not fitted crrectly either. Who fitted them ... they shouldn't be so lazy. Do it right or leave it alone and take it to someone who will. Circled in blue.

 

[Ken Bob]

*post deleted, he already tried diff. speakercab.
Clips not working with my browser.

Ummph. OT maybe. I think we had such a problem here not too long ago, it was a 1987x and the problem was a couplingcap, if i remember correctly.

Thanks @Tatzmann, I'll take another scan through the forums for this. A coupling cap issue did cross my mind.

 

[Ken Bob]

Firstly fit the capacitors correctly or that will cause an issue, possibly as you describe and then set the bias to the correct current draw at idle.
Those anode load resistors are not fitted crrectly either. Who fitted them ... they shouldn't be so lazy. Do it right or leave it alone and take it to someone who will. Circled in blue.

Thanks for the reply, @Jon Snell. I agree about the super lazy solder jobs. I'm glad I wasn't the one who paid for that. Your opinion about them possibly causing the issue is definitely noted. I was under the impression that I had biased the amp for the correct idle draw, however.

 

[PelliX]

Firstly fit the capacitors correctly or that will cause an issue, possibly as you describe and then set the bias to the correct current draw at idle.
Those anode load resistors are not fitted crrectly either. Who fitted them ... they shouldn't be so lazy. Do it right or leave it alone and take it to someone who will. Circled in blue.

Totally agree that it's a bodge up, but are you suggesting that it could be causing some kind of parasitic resistance/inductance? Genuinely curious.

 

[mAx___]

Something you can try in the meantime is using some good contact cleaner in the input jacks as well as your guitar and speaker cables.
Also check the date of the ARS caps, it's usually printed along with the capacitance.

 

[Ken Bob]

Something you can try in the meantime is using some good contact cleaner in the input jacks as well as your guitar and speaker cables.
Also check the date of the ARS caps, it's usually printed along with the capacitance.

Thanks a lot for the suggestion, @mAx___ . I did check the date code on the filter caps - they're from 2021. And the jacks were just cleaned a couple of weeks ago. At this point, I'm kinda out of bright ideas, and pretty much at the point of doing the old chopstick tap inside while the amp's plugged in.

 

[Ken Bob]

Couple of further notes to any and all of the gurus here who are so familiar with the idiosyncrasies of these amps:

First, I have noticed that the PT of this Super Lead gets quite a bit hotter than my '79 2203. Running for about the same length of time - say, 45 mins - and same approximate level - say, 4 on the vol knob - the PT of the Super is up to like 138 deg F. whereas my 2203's PT only gets to about 113 degrees in the same length of time. (yes, I have double-checked the bias, and they're both at about 67% idle dissipation).

Secondly, both amps are plugged into the same power strip and same circuit in my studio. With one turned on at a time, and a multimeter plugged into an outlet on the same circuit, I notice that while playing, hard pick attacks on the 2203 barely cause a 0.1 VAC momentary voltage drop on the circuit, whereas hard attacks on the SL cause the circuit to momentarily drop 0.5 - 1.0 VAC. This obviously tells me the SL is drawing a lot more instantaneous current. So I'm wondering - is this normal???

Man, with the presence down to 0, mids down to 2, and the treble down to 4 I can almost dial out the harsh, ratty sound. Almost, but not completely.

 

[Matthews Guitars]

The elevated temperature of the PT may be a clue.

I was just playing my own '73 Superlead today, and as usual I ended up diming it. Sounded monstrous. Definitely yours has a ratty character to its overdrive that should not be there. Hoping it's not your output transformer...or the PT for that matter.

I can run the amp tomorrow for a good while and then use my thermal camera to scope out the temps of the tubes and transformers,
and offer that as a point of comparison. But I know my transformers run cool.

 

[ITburst]

I find this interesting. Just this past weekend I was playing my DSL20 and I thought I heard some sort of strange "fuzzy-raspy-low frequency" artifact on my low notes as the note decayed. It was not there on the clean channel and I really don't think I noticed it before.

I was only playing for a few minutes and got called away, so I never had the opportunity to really look into it. The amp is rather new with no modifications at all. What a strange coincidence.

 

[pgmlml]

I dont hear nothing strange. I think that's power tube distortion but with the bias too low...

 

[Nicolas S]

The bias resistor for the cathode follower is 10k instead of 820 ohms if i see correctly

 

[Pete Farrington]

I have noticed that the PT of this Super Lead gets quite a bit hotter than my '79 2203. Running for about the same length of time - say, 45 mins - and same approximate level - say, 4 on the vol knob

That’s nowhere near an apples to apples comparison. The 2203 needs its master volume turned up to max. Then adjust the tones and volume controls of the 2 amps to get them as close as possible.
FYI due to the big difference in bright cap etc values, and pots being cheap with poor tolerance, setting the controls to the same numbers aint going to do it, just close your eyes and use your ears, or better still get someone else to adjust to controls to your instructions, with you looking the other way.
But the 2203 master MUST stay on max for the mains or power supply current comparitive readings to be valid.

 

[Nicolas S]

On your first photo it's the one witch is lifted and you can read under it that it shoulb be 820 ohms, and they put a 10k, it will change the bias of the V2A cathode and the character of the distortion of the preamp

 

[Ken Bob]

That’s nowhere near an apples to apples comparison. The 2203 needs its master volume turned up to max. Then adjust the tones and volume controls of the 2 amps to get them as close as possible.
FYI due to the big difference in bright cap etc values, and pots being cheap with poor tolerance, setting the controls to the same numbers aint going to do it, just close your eyes and use your ears, or better still get someone else to adjust to controls to your instructions, with you looking the other way.
But the 2203 master MUST stay on max for the mains or power supply current comparitive readings to be valid.

@Pete Farrington, you're absolutely right. An excellent reality check - I definitely overlooked this and should've known better. I'll redo a proper comparison tonight. Thank you sir!

 

[dommarsh]

On your first photo it's the one witch is lifted and you can read under it that it shoulb be 820 ohms, and they put a 10k, it will change the bias of the V2A cathode and the character of the distortion of the preamp

HI folks, I had a 1970 Marshall 1987 Lead with a similar harsh distortion in for service. I found as mentioned by Calvin that the first stage of V2 had a 10K cathode resistor, instead of a 820 or 1k resistor. That affects the triode bias and throws the triode into the wrong operating range. An honest mistake from the factory I suppose, as at first glance it may be hard to distinguish red and orange. In my case the resistor was hiding under the cap, so it wasn't obvious at all. So the amp was a dog from the factory, and had been passed around from owner to owner (or dumped lol). I've heard about this error a few times, so it happened. Check the value of that resistor over the 820 label!

 

[Ken Bob]

@Calvinr and @dommarsh, this is a great heads-up about the V2A bias resistor. I sincerely hope the resolution here is something so simple - but correct me if I'm wrong - I'm seeing brown-black-red-gold (1k), not 10k, no?? 1k is pretty close to 820 ohm...

 

[Matthews Guitars]

I checked my '73 Superlead this morning. Turned it on and let it run for an hour (quiet, not playing) and that was in full operation, not standby. Neither transformer even reached 100 degrees F in an 80 degree room. They both reached about 95 degrees as per my thermal camera.

That amp is in perfect health and sounds right. So it may be assumed that it's a good example to use as a point of reference.

If I biased it a bit hotter it would of course draw more current and run a bit warmer, but as I recall it's biased right in the middle of the recommended range right now. I see no reason to mess with it.

 

[Ken Bob]

I checked my '73 Superlead this morning. Turned it on and let it run for an hour (quiet, not playing) and that was in full operation, not standby. Neither transformer even reached 100 degrees F in an 80 degree room. They both reached about 95 degrees as per my thermal camera.

That amp is in perfect health and sounds right. So it may be assumed that it's a good example to use as a point of reference.

If I biased it a bit hotter it would of course draw more current and run a bit warmer, but as I recall it's biased right in the middle of the recommended range right now. I see no reason to mess with it.

@Matthews Guitars, thank you for checking! That was kind and helpful of you. FWIW, I have read the whole gamut regarding the normal operating temp of SL PT's... Some say some run hot, some don't, some say it's bad, some say totally normal. You know how the internet is - you can find info to back up any opinion you want...