1968 vs 1969 Super Lead sound

neikeel · Post by **neikeel** » Tue May 24, 2016 4:16 pm

What do you see when you compare the two outputs from the PI (i.e. the grids of the output tubes)?

shakti · Post by **shakti** » Tue May 24, 2016 4:27 pm

I didn't compare those two side by side this time, but when you look at the video, they would *appear* to be identical only OOP with each other. As you can see in the video they are both shifted compared to the input signal.

rgorke · Post by **rgorke** » Tue May 24, 2016 11:34 pm

So there appears to be something entering the signal early on, right? Is that what it is showing? Even in the first stage or does it just show up in the PI?

Where does that type of interference come from? Lead dress? Rectifier as you mentioned? I think I found an issue in how my CT from the OT was routed. It may have been causing some noise.

I need to get a scope...

shakti · Post by **shakti** » Mon May 30, 2016 8:14 am

I am happy to report that the issue has been fixed!

And now, the million dollar question; what was it?

The honest answer is that I am not 100% sure...but it may have been a number of things. I worked my way backwards through the amp. Cleaned up the OT wiring - didn't make a difference. Rebuilt the phase inverter and put back the mustards I had there from the start, as the caps had been verified to *not* be the culprit. Again, no difference.

Then I rebuilt the tone stack, again putting back in the mustards I had there from the start. I removed the .68 on V2a for testing just to keep things as simple as possible. Resoldered all the socket connections incl the 100k across V2 tube socket as well as all V3 socket connections. Replaced all the pots incl the presence pots with Alpha pots from another amp (I had CTS in the 68). I also soldered in an F&T 50+50 in place of the Erie 32+32 in the preamp. Voila! No more fizz, nicer EQ, more punch and clean-up.

Sooo, what was it? I then went backwards, putting in the CTS pots, one by one. To be honest, each and every one of them seemed to do *something* with the sound that I didn't like. The bass pot seemed the worst, and it measures a measly 820-830k. I replaced it with another CTS which measured almost exactly 1M. Even then, putting the CTS pots back in did seem to make the EQ spikier and harsher, with slightly less punch (the treble, middle and presence pots all measured fine BTW). Putting the Erie preamp cap back in gives just a *tiny* bit less ooomph, but it's very subtle. All in all, I am a little bit confused, because putting the original parts back in doesn't mess up the sound of the amp like it was before. But the tone stack pots was a significant change, and I will be replacing them with Alphas just because I like the sound better.

So I am guessing it was a number of unfortunate things; a set of tone stack pots with a very low reading bass pot messing up the EQ, making it sound a little wooly and with a nasty spike which *may* have caused some kind of oscillation. Couple that with maybe an ever so slightly dodgy solder joint? I really don't know. None of the solder joints looked bad, there was no crackling or anything that I could elicit with a chopstick.

It was an interesting find, BTW. In my previous A/B comparisons with original Marshalls I have found the pots to be perhaps the most significant difference between a clone and an original. Putting the original pots into my clone amp made them sound virtually indistinguishable. Particularly the bass pot seems to be sensitive- Originals tend to read much higher, often 1.3-1.4M, and in my experience that leads to tighter, faster bass and almost "drier" but much more concise sound.

In any case, A/Bing my 68 and 69 now, they sound very similar with a Les Paul even when the 69 has 6550s and a 12AT7 phase inverter. A Strat shows off the difference in breakup a little better though, so the 69 really shines for that big late period Hendrix sound, but with humbuckers they exhibit very similar punch and clean-up now.

Tazin · Post by **Tazin** » Mon May 30, 2016 11:06 am

You've definitely been on quite a jouney tracking down the problem with the '68....Hopefully the issue stays resolved. Interesting description about the CTS and Alpha pots. Equally interesting is your observations reguarding the CTS pots with low measured values and there effects on the overall sound of the amp.

shakti · Post by **shakti** » Mon May 30, 2016 11:29 am

Indeed a bit of a journey. The upside is the 68 is looking (and sounding) better than ever, and I have a nce oscilloscope and a cheap but handy cap tester here on the bench. Both very useful tools.

I was surprised at the effect of the pots myself. It really doesn't look like any of them were faulty - they all measure fine (except the low bass pot), but I don't like the sound of them vs Alphas. However, this is at a corksniffing level - they don't sound "wrong", only *something* is better with the Alphas. Could be just a particular balance between the values - I suppose the overall value of the pot as well as the taper pays into how the wole tonal blance falls into pace. But it could also be something in the composition of the pot. I really don't know, but I do know that from now on pots will be an area to look into for finetuning the sound. So many variables though...as in; what is the ideal value of the treble pot, vis a vis the middle pot etc. All my tests indicate that a high rading bass pot is beneficial though. Removes some clutter and keeps tings tighter.

What's also interesting is that entering different values into the Duncan tonestack calculator yields fairly small effects, so there must be something more going on other than just the theoretical gain for different frequencies.

edit: I suppose it's possible with a partly/intermittent malfunction in a pot causing the fizz and breakup. In which case the problem should return. I will keep off celebrating until everything has been put permanently back into place with a new set of pots.

shakti · Post by **shakti** » Mon May 30, 2016 11:34 am

Oh, and also, for the purpose of learning for others; in my videos with the oscilloscope I went on about the phase shift between the stages. I have learned that this is perfectly normal. The breakup I saw may or may not be...it would be interesting to show oscilloscope plots now that it has been fixed!

Tazin · Post by **Tazin** » Mon May 30, 2016 12:00 pm

The only thing I'm not crazy about regarding Alpha pots is how quickly they become scratchy when subjected to varying environmental conditions. They have better mfg tolerance compared to standard CTS pots (new ones) which is one thing I like about Alpha's. In my observations when comparing Alpha's to '60s CTS/AB pots I found the CTS/AB's to sound "smoother" for lack of a better definition.

shakti · Post by **shakti** » Mon May 30, 2016 12:37 pm

My own experience with older pots says that the older pots tend to sound more "interesting", as in there is more "mojo" going on in the transition from clean to dirty. At least in the case of the 1M volume pots. In my 69 I am currently using a pair of old (AB?) 1M salvaged from a 1970 Super PA. They really did "something" cool to the sound - tightened up the bottom end and gave it a more gradual ("smooth"?) transition when turning up the volume. With new Alphas, the amps tend to sound a little sterile and clinical until you get up to at least 6 or 7, then the harmonics start coming in. With the old pots the "magic" seems to start happening a little earlier, so even if the amp is still clean and controlled sounding, there are harmonics happening and a little bit of "something" that's missing with the Alphas.

But at least in this case, the Alphas were clearly preferable (to my ears anyway) to new CTS pots.

And the Alphas may become scratchy (haven't really noticed yet though), but they are dirt cheap and easy to replace as long as you don't use a ground buss.

shakti · Post by **shakti** » Mon May 30, 2016 12:40 pm

And one more thing; between all the New CTS pots I have had, hardly a single one measured up to its rated value. Almost always just a little bit lower, but within 10%. This has often been the case with Alphas too, but I do at least find *some* that read a touch higher, like 1.1M.

I've said it before; someone should accurately clone the performance and sound of the older pots...

neikeel · Post by **neikeel** » Mon May 30, 2016 4:20 pm

Thank heavens for that!
It is interesting that something that should be so fundamental (accurate reading new pots should be so important). Thankfully I too have a bundle of used CTS pots from a master PA for future use and will use the highest reading ones!
Oh and did you sort out the 18w?

shakti · Post by **shakti** » Mon May 30, 2016 5:43 pm

The 18w magically sorted itself out. Either it was one of the caps just bedding in and ridding itself of moisture. Or it could have been that the rectifier tube was vibrating in the socket. I was missing one tube retainer, and after I seated it very firmly and also adjusted the chassis slightly to reduce cabinet vibrations I no longer had any problems. But I have not been able to reproduce it by tapping/gently wiggling it.

As for the 68 - I did not put all the original CTS pots back in all at once, only the bass pot first and that muddied things right up again but not with the fizz. So something else must have caused that, or the pots have an intermittent problem. I can only liken the sound of all those CTS pots together as almost like a cocked wah sound. I think you can hear it in one of the videos where I do the Mahavishnu licks. It has an unpleasant spike and some frequencies seem to be very subdued. I think this is part of what I was hearing and having a hard time of putting into words - it just felt like a lack of punch particularly in the bass. But that fizzy, crackly thing could have been something else.

uiovbged332 · Post by **uiovbged332** » Wed Jul 06, 2022 6:34 pm

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1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound

Re: 1968 vs 1969 Super Lead sound