1968 vs 1969 Super Lead sound

[Tazin]

Did you ever try swapping the tubes between the two amps like you mentioned back on page 1?

[shakti]

Yep, tried swapping them many times back and forth to make sure I don't miss anything. Did that as late as today, both preamp tubes and output tubes. The 68 has tall base RFTs from the 70s and the 69 has Tesla from the early 70s. The Teslas do sound more solid and stout, but it doesn't change the early breakup of the 68.

[Tazin]

Well. I'll admit I'm a little stumped. I had my money on the filter caps being the main reason for the difference between the two amps but appearently it's more then just that in your case. I know from my own experiments that changing from a 16uF-16uF preamp filter arrangement to a 32uF-32uF changed the character of one of my amps considerably, so I could see going from a 32uF-32uF to a 50uF-50uF preamp filter yeilding similar results. I'm kinda surprised at the minimal results from changing two of the four Erie 100uF to F&T 50uF-50uF though....

[shakti]

I am a little stumped too. Admittedly I haven't done all these cap changes in one go - If I changed both the mains, screens and preamp filter caps with new 50+50 caps I suppose the cumulative change would be more pronounced, but I'm not convinced that it would change the early breakup which I suspect is coming from the preamp.

I am using NOS Iskras in both amps, which generally drift very little and should have very comparable readings. I haven't tried messing around with the .68uF mustard on V1 in the 68 (the 69 has a WIMA).

[danman]

I can't remember if it was mentioned earlier but do both amps have the same size bright cap over the volume pot? That wouldn't affect the feel of the amp but it could account for the difference in breakup between the two amps.

[shakti]

Yes, the 68 has a 5000pF bright cap but it is switchable. No bright cap on the 69. The 68 has always sounded/felt like this. It just doesn't have any clean headroom to speak of compared to the 69. The 69 does distort fairly early too like all Super Leads do, but the 68 is just very sort of warm and fuzzy sounding right from the start. You don't really notice as soon as it's above 5 or 6, which is where you typically play this kind of amp. From that point on, all you notice is how fat and fluid it sounds, but forget about getting much useful clean tones out of it...

[shakti]

Lastly, I tried two F&T 50+50 caps in place of the NOS Erie 100uF caps on the output tube screens. Again, no big difference. Perhaps, a tiny bit more punch with the new caps, but nothing to write home about.

Now, I suppose that replacing all the NOS filter caps with new production would amount to a bigger change in total, but I am not convinced it would do what I want. The more I listen, the more I isolate the "issue" to the very early breakup of this amp.

Now, this is where I need to decide, and I may want the help of you guys to decide. I can stop here, and leave the amp as it is. I've been happy with it for 3 or 4 years since I built it. It has a very, very cool sound. OTOH, I am beginning to think that *maybe* something isn't 100% right about it. Turned up high, it crunches like mad, and has this almost legato feel to it. But it plays almost as if there's some kind of built in sag on the attack of notes. All Super Leads I have played or owned, have had a much more firm attack to each note. Admittedly, none of them have been 68s...they were all 69-73 Super Leads. But as far as I can remember, all of them had the ability to get a clear, clean tone with some attack when you turn down the guitar volume. With this 68, it's almost as if the note "always" sags a tiny bit, and there's a little bit of distortion that nearly never goes away. I have to turn the guitar volume down to about 2 with humbuckers before it gets clean, and that is almost regardless of the amp volume setting (it is easier to get absolutely clean with lower amp volume settings, but it's always fairly rounded and with little attack). It sort of feels like it lacks power compared to my 69, and compresses a lot.

I'll see if I can capture on video what I mean, but it may be difficult to record it without good equipment. It's kind of subtle, as there's nothing else to indicate that anything is wrong, and the amp has always sounded like this. But I am thinking that maybe something isn't 100% right...could this be a case of parasitic oscillation? Could something be acting up in the phase inverter causing it to sag and distort too much? Unfortunately I'm not sure where to look anymore...

[Tazin]

Did you check all the signal caps (mustards, lemco's, RS, and Philips) for dc leakage prior to installation? Likewise, have you checked these caps lately since they've been installed in the amp for dc leakage?

[shakti]

No, I haven't. I have that cap tester that Carbia mentioned in the mail, and this is one thing I'll do when it arrives.

[shakti]

OK, let's get these amps sorted out before the outdoor season takes over completely.

Here are some clips to demonstrate the differences between the two amps. Tell me what you think. Caveat first; this is not a true A/B comparison as the 69 has now been set up with 6550s and a 12AT7 in the phase inverter, so it's bound to have more clean headroom. But the difference between these two amps was very similar even before that change, you'll just have to take my word for it. Nevertheless, to my ears it's becoming clear that something is not right with the 68. You don't necessarily notice at full blast, but you hear it once you try to play cleaner. I'm not sure if it's my ears and head playing tricks on me, and maybe it's just that I am now beginning to notice, but it is possible that the problem has become more pronounced lately. You can hear it very clearly in these clips though.

First clip, just some generic riffing with a Les Paul. Red light is the 68, green light is the 69. Same settings on both amps: volume one was up to about 8 IIRC, presence 5, bass 3, mids 10 and treble 6-7. Volume on the normal channel was off. Both amps played through a 1970 basketweave 1935B cab. Running through a Metroamp head switcher and then through an Alex' Attenuator at -9dB.



Second clip, some Mahavishnu style blasts and clean-up with a Les Paul. The 68 just never gets clean:



Last clip, with a Strat. The 68 is never clean, and there's a slightly nast, slighty microphonic ringing distortion. The pickups in this guitar are somewhat microphonic, but it's just much worse with the 68. Swapping over the preamp tubes between the amps does not change it.

[dukeamps]

@shakti

check to see if the OT is good, check the resistance between pin 3 of the output tube and the B+ center tap of the OT, they should read with in 10% of each other, if they read more then 10% the OT could be fried.

[shakti]

That's a good tip. Putting the two amps side by side like this only accentuates to me that something is not right with the 68. Could a partial short in the OT sound like this? The distortion that you hear in the last (Strat) clip is particularly bad, so whatever is acting up in this amp seems to have increased in severity lately.

Any other opinions?

[Carbia]

I don't know how you recorded the clips, but both sounds bad to me

[Tazin]

Yep, something is wrong with the '68. Could be a signal cap going bad. You could check the de-coupled side of the caps for the preamp and tonestack for the presence of dc voltage using a multi-meter. How's the lead dress on the '68?

[shakti]

I don't know how you recorded the clips, but both sounds bad to me

Really? The 69 sounds OK to me...remember it has 6550s and a 12AT7 PI tube. The recording leaves something to be desired though, it's just an iPhone into a cheap Fostex interface, so that puts its own tonal imprint on things. If you see any of my other videos you will probably get an idea of what the recording itself does to the sound.