1968 vs 1969 Super Lead sound

[shakti]

As we all know, the 68 and 69 Super Leads are almost identical in circuit. The differences being only the amount of filtering and the amount of negative feedback (fixed 47k/8ohm on the 68, 47k on the speaker jack on the 69 so it is variable), and possibly the voltage (said to be generally a little lower like 460ish for a 68 and around 490 for a 69). Beyond that, the only difference I can think of is the orientation of the output transformer, which was rotated for 69, apparently to reduce hum.

I've built a clone of both amps using just about the same parts. That is, the 68 clone has mustards, whereas the 69 has mostly WIMA TFF caps. Both of them use M e r r e n transformers throughout. I have new ARS filter caps in the 69, and NOS Erie caps in the 68 except phase inverter which is a F&T 50+50. CTS pots in the 68 and Alpha pots on the 69.

The 68 has always sounded much rounder, softer and almost sluggish by comparison with the 69, which is much more aggressive, tighter, bolder and in-your-face. The 69 is louder much faster, but overall output volume is about the same. The 68 feels gainier overall though.

I didn't like the stock filtering specs of the 68, 16uF on the screens was just way too low for me, so I put in NOS Erie 100uF caps instead, so I have almost identical filtering in the two amps (i.e. 50uF mains, 50uF screens, 100uF PI) except preamp which is 32+32 in the 68 and 50+50 in the 69.

This brings me on to my question. I have tried to make things exactly equal in the two amps; I have set them both at 490V plate voltage, subbed in a little extra preamp filtering in the 68, put in a variable feedback pot in the 68 so it goes from 47k@16 ohm and lowers the negative feedback from there. I've swapped tubes back and forth and biased them the same.

No matter how much I "correct" the 68, it is just much more sluggish than the 69. It's not a bad thing, in fact I think many would prefer it, and if going for a lot of gain, VH style, it is preferable. However, when I attempt to do Mahavishnu-style machine gun blasts, I like the much tighter, faster response of the 69. As I am going to use the 69 build for a "West Coast" type late period Hendrix amp with 6550s or similar, I really want to cover all the EL34 Super Lead 68-69 ground with the other amp.

For the record, I have a tried a couple of original 69-70 Super Leads, and they all behave much more like my 69 build; tight, aggressive sounding, fast attack, and not as much perceived gain as my 68 build. I have however never tried an original 68.

Has anyone experienced the same? Is there something inherent to the 68 design that makes it "rounder" and gainier sounding than the 69? The only differences I can think of, layout-wise, is the orientation of the OT. Could that influence how the OT saturates? What about the rectifier? I have a self-contained bridge rectifier in the 69, and 4 individual diodes in the 68 in the classic "X" formation. No snubber caps in either amp. Would the rectifier influence power amp impedance?
Is what I am hearing/feeling due to using NOS Eries in the 68 and new caps in the 69? The Eries all reformed very nicely and measured at least 100uF. I don't have bleeder resistors in the 68, but do have them in the 69 though.

I am fumbling in the dark here, and would really like to hear if anyone else has experienced the same; that these two amps sound/feel quite noticeably different even when the circuits are all but identical?
I should try the 68 preamp with the 69 power stage and vice versa to further pinpoint where it is coming from, but I forgot how to do that...

[neikeel]

I never feel that my 68 is sluggish, yes it is softer and smoother than later metal panel amps that I have had but I have always put that down to the 16uF screens filtering and 48uF PI filtering. The 5nF bright cap does not make it harsh or spiky either.
Only difference from stock is that I used UF5408 diodes as that is what I had available, liked what they did so kept them, infact did the same on my son's SB rebuild.
I very much doubt that it is the OT orientation.
It may well be a combination of diodes and filter caps (big single 100uF 350v rated caps vs smaller 50uF caps in series.

The way to swap pre and power amps is to disconnect the middle wire of the treble pot (wiper) on each. Use a length of shielded wire grounded at each respective pot (out) with shrink wrapped shield and solder across.
Things to check are your neutral and live polarities to your PT and I would use a croc clip between each chassis to make sure shared ground.

If you wire both amps up you can ABY toggle between the two.

[Tazin]

My guess is that the sound difference between a stock '68 and '69 would mainly fall upon the filtering difference. A secondary difference would be from different bridge rectifiers, and lastly a minor difference in PT voltage....If any at all.
BTW, the '68 negative feedback setup is 47K from the speaker jacks just like the '69.

[shakti]

I never feel that my 68 is sluggish, yes it is softer and smoother than later metal panel amps that I have had but I have always put that down to the 16uF screens filtering and 48uF PI filtering. The 5nF bright cap does not make it harsh or spiky either.
Only difference from stock is that I used UF5408 diodes as that is what I had available, liked what they did so kept them, infact did the same on my son's SB rebuild.
I very much doubt that it is the OT orientation.
It may well be a combination of diodes and filter caps (big single 100uF 350v rated caps vs smaller 50uF caps in series.

The way to swap pre and power amps is to disconnect the middle wire of the treble pot (wiper) on each. Use a length of shielded wire grounded at each respective pot (out) with shrink wrapped shield and solder across.
Things to check are your neutral and live polarities to your PT and I would use a croc clip between each chassis to make sure shared ground.

If you wire both amps up you can ABY toggle between the two.

I do have UF5408 diodes in the 68 build, and as mentioned, I have a self-contained bridge rectifier in the 69 (bought from Valvestorm). I don't know the specs of the bridge rectifier, and I don't know if it is fast recovery like the UFs...perhaps I am hearing "too much smoothness" from the UFs? I sort of doubt it though, in the other amps where I've tried fast recovery diodes I have preferred them over the regular 1N diodes...less high frequency "hash", cleaner sound with less clutter. If anything, the 68 has less of a clean sound to it...it's like it's lower in volume on low volume settings, but the dirt still sets in really fast.

Funny thing is - I compared my 69 build with a real 69/70 (I wrote about it in another thread) and went on and on about the small differences between original and clone. Yet these two clones are much farther apart in sound and feel than the 69 clone vs original, even if the circuits are virtually identical.

Keep in mind that I have upped the filtering in the 68, so I have 50uF screens. I love, love, love the chugging feel it has and don't necessarily want to alter or lose that, but it's just that sometimes I prefer that fast, tight, slightly more aggressive yet cleaner sound/attack of the 69.

I'll see if I can get to doing an A/B test swapping pre- and power amps and see what that does. I also put in a dual voltage switch in the 68. The M e r r e n T2562 clone has dual HV taps, one set for roughly 460V and another for around 490V. If it does indeed have something to do with the bridge rectifier, then it would be easy to implement dual rectifiers - one for the 460V tap and the other for the 490V tap.

[shakti]

My guess is that the sound difference between a stock '68 and '69 would mainly fall upon the filtering difference. A secondary difference would be from different bridge rectifiers, and lastly a minor difference in PT voltage....If any at all.
BTW, the '68 negative feedback setup is 47K from the speaker jacks just like the '69.

Just a clarification; when I talk about a "68" amp I am talking about typical 12 series specs...which were not consistent, I know, there are even 12 series amps with all filter caps on top of the chassis IIRC, but typically when people think of 68/12-series specs it's the one with one cap on top, 2x100uF for mains and 2x32uF for screens. PI filtering varied from 48uF to 100.
I always thought the 12-series, at least the early ones, had 47k on the 8 ohm tap though?

And comparing stock amps I would probably agree that filtering and possibly voltage would be the main differences, but in my case I strayed from stock specs to make my 68 more like a 69, yet it still sounds quite different.

It does seem like you think perhaps the bridge rectifier has something to do with it though?

[Roe]

I own a 68 and a 69 and have serviced various original 69-72 superleads. My 68 is more similar to a 69 than yours is. But I'm using 40uf on the screens and 100uf on the mains and a marstran PT which gives me 490v at the plates. I changed the PT on my 69 from the 450ma Metro 1203-80 which gave me 473v to a Classictone t2562 which gives me 465v at the plates. The classictone sags much more than the metro, since it is rated at 290ma instead of 450ma. The stiffer powersupply was just too hard on the el34s (it was easy to get redplating). My 68 is actually louder than my 69 now, perhaps because of higher voltage and stiffer el34s

[Tazin]

Once again I'm just guessing here, but maybe some of the difference is coming from the Wima TFF caps verse the mustards? Even through their both film/foil types doesn't mean that they'll sound the same. The same basic logic could also be applied to the filter caps....You may have the same microfards in a particular spot on the circuit but the difference between a new production electrolytic verse an NOS electrolytic might be yielding different results. A lot of people claim that new production F&T caps tend to make an amp tighter, brighter, and more "punchy" sounding compare to old production caps that are still in good shape.
The UF5408 is a 3amp 1000PIV diode compare to the 25amp 1000PIV self-contained bridge rectifier. The performance difference between these two should equate to some kinda sonic difference I would think.
The 'real' 12-series amps (serial number(s) SL/12xxx with split cathode V1) had the 47K resistor from the speaker jacks....Minus one exception.

[VelvetGeorge]

I'd say your clones are very accurately showing the differences the original '68 vs '69 amps. NF and filtering are the most significant factors, of course.

Does your '68 PT have a dual HT secondary? Does your '69 have a single?

george

[shakti]

I can always rely on you, Tazin, to get the finer details right! Thanks!

Yes, the difference may in fact be due to the filter caps (NOS vs new production). But in that case, I suppose it must be mainly the mains filter caps. I put in a switching pot in the 68, wiring up an extra filter cap where I could use the switch to connect or disconnect the ground, so I could do immediate A/B comparison. I used a 50+50 cap and subbed in extra filtering in various spots throughout the amp. The difference was miniscule when adding extra filtering anywhere in the preamp including phase inverter, but also when adding extra filtering (going from 50 to 100) on the screens. I was only able to test this in 460V mode as I didn't want to risk blowing the cap (rated 500V and screens were close to 500V before screen resistors and sag under load).

The bridge rectifier is indeed interesting though. I should get another self-contained bridge rectifier and try that in the 68 to see what that yields.

As for WIMAs vs mustards, I highly doubt those would account for such a (relatively) dramatic difference in feel and attack. The 69/70 SL I compared my 69 with had mustards vs the WIMAs in my 69, and the original was still much closer to my 69 than my 68 with mustards running "69 specs".

[shakti]

I'd say your clones are very accurately showing the differences the original '68 vs '69 amps. NF and filtering are the most significant factors, of course.

Does your '68 PT have a dual HT secondary? Does your '69 have a single?

george

But as I've said, I have upped the filtering in the 68 to 69 specs, and it's still much softer and less immediate sounding. It sounds very fat right from the start and gets this very cool almost sagging, gainy quality running flat out, so I really like it for what it does. Yet I am at a loss to explain why it behaves so different from my 69 when the specs are virtually identical, including NFB and filtering.

Both amps have identical transformers, both with dual tap PTs (460 and 490V).

[Tazin]

What about the other componets in each amp; resistor type, ceramic disc or mica caps, cathode bypass cap types? Basically just trying to see how even the playing field is between the two amps.

[VelvetGeorge]

Jumper the '68 preamp to the '69 power section and vice versa. Try to isolate where the difference in attack is coming from, then focus on those sections.
I like to measure NF in db's to compare originals to replicas. That may shine some light.

george

[VelvetGeorge]

Is there a 10k 1W carbon comp in the B+ line of the '68, but not in the '69?

george

[shakti]

They both have all Iskra resistors. Same exact B+ dropping line. NOS RS silver micas in both, identical ratings IIRC (i.e. 560pF). Cathode bypass cap for V1 is BC on both amps IIRC. Mustard .68s in the 68, and WIMA MK in the 69.

I just thought of one thing; I need to check if they both use the same choke. The 68 may have been a Marstran, whereas I'm almost certain the 69 has a M e r r e n. They may both be M e r r e n, but I can't remember. That could of course account for a difference, although they do have the same specs AFAIK.

[shakti]

Jumper the '68 preamp to the '69 power section and vice versa. Try to isolate where the difference in attack is coming from, then focus on those sections.
I like to measure NF in db's to compare originals to replicas. That may shine some light.

george

Yes, this is what I intend to do when I get time. First up is to complete my 18w build though.

I don't really know how to measure NFB, but in the 68 I have a variable setup (250k pot in series with the 47k, going to the speaker jack), so using just one cab I get from 47k@16ohm and can lower the feedback from there. It doesn't really change any of the fundamental difference in "immediacy" and attack.