1968 vs 1969 Super Lead sound

[Roe]

There are also reports of a few original plexis with 220uf main caps

[shakti]

Yes, I will try to turn every stone here. I'll try to compare the NOS Erie preamp cap with a modern production one too. Subbing in more filtering there doesn't change it much, but I suppose the NOS cap could influence the sound as long as it's in the circuit.

[neikeel]

Yes, I will try to turn every stone here. I'll try to compare the NOS Erie preamp cap with a modern production one too. Subbing in more filtering there doesn't change it much, but I suppose the NOS cap could influence the sound as long as it's in the circuit.

I think you should look at the ESR differences between two 100uF 350v cans in series and the series parallel arrangement of the later amp, as I mentioned above I am sre it has an effect.

On clones I have built they all sound louder and tighter with new caps, even when using original iron, compared with good reformed old or NOS caps.

[Carbia]

A higher ESR makes sound softer and more compressed. Lower ESR makes the amp tight and full of headroom.

Many people like old high ESR caps... but they are more prone to fail because high ESR means that the cap runs hotter.

We're costumed to hear old caps nowadays, but for sure that the amp in the 1960's sounds tighter than now.

[shakti]

You guys are really talking sense here now. I've never learnt to measure ESR, IIRC you need a good meter to do that? My meter is kind of cheap...my brother in law can get Fluke meters for a decent price so maybe I need to upgrade.

I suppose I could set something up to switch between a "68" type mains filter with the NOS Eries, and a "69" setting with two new 50+50 caps for increased punch. Would have to add the extra filter caps on the inside wall of the chassis, but that would at least be minimally invasive and just require the mounting holes for the cap holders.

[Carbia]

Here you have a cheap capacitor tester that measures ESR, leakage and capacitance:
http://www.ebay.es/itm/MK-328-MOS-TR-LC ... SwHmhV8AuO

[shakti]

A higher ESR makes sound softer and more compressed. Lower ESR makes the amp tight and full of headroom.

Many people like old high ESR caps... but they are more prone to fail because high ESR means that the cap runs hotter.

We're costumed to hear old caps nowadays, but for sure that the amp in the 1960's sounds tighter than now.

Thanks for the tip on the tester, Carbia!

Just to be clear - are you saying the caps used back in the day (RS, Erie, Daly etc) were higher ESR than current production (F&T, ARS etc) even when they were new in the 60s and 70s, or are you saying that all older caps will increase in ESR over time, even when they sit unused/NOS and are reformed slowly? The Erie caps I used have 1975 date codes, were all NOS and were reformed slowly with good results using Larry's 100k resistor method (2w 100k resistor directly after rectifier before first filter caps, measuring voltage drop until it goes below 2V). However, I did not measure ESR before installation or after.

Is it possible that one of the reasons Marshall went to the setup with two 50+50 cans in parallell/series rather than two 100uF caps in series, is that it may yield a different total ESR? I don't know if it does, but perhaps smaller caps in series and then in parallell yields less ESR, just like neikeel said? It has always been assumed that it was just done to cut costs and stock only 50+50 caps, but maybe that wasn't the case?

[Tazin]

With the introduction of the six filter caps mounted topside on the 100 watt chassis Marshall used a combination of filter cap values.
Filter cap arrays...Any of the following could be paired up.
Mains & Screens:
HUNTS 100uF/350v
HUNTS 50uF+50uF/350v
HUNTS/ERIE 50uF+50uF/450v
HUNTS 200uF/350v
Marcon 100uF/350v
Dubilier 100uF/450v
Dubilier 50uF+50uF/450v

Phase Inverter & Preamp:
HUNTS 50uF/450v
HUNTS 50uF+50uF/450v
HUNTS/ERIE 32uF+32uF/450v
RS 32uF+32uF/450v
Marcon 40uF+40uF/450v
Dubilier 50uF+50uF/450v

I think this chaos stemmed from a couple reasons. One is that there was a decent number of take overs and mergers in the capacitor industry in England throughout the 60's which might have affected products brought to market. Two, RS componets was supplying Marshall with whatever they could get there hands on. As 1970 approached things calmed down to the familar 50uF+50uF/450v caps in all positions.
I haven't done the reseach but I would guess that the 60's caps would have had a higher ESR and lower ripple current rating compare to modern day caps.

[Roe]

With the introduction of the six filter caps mounted topside on the 100 watt chassis Marshall used a combination of filter cap values.
Filter cap arrays...Any of the following could be paired up.
Mains & Screens:
HUNTS 100uF/350v
HUNTS 50uF+50uF/350v
HUNTS/ERIE 50uF+50uF/450v
HUNTS 200uF/350v
Marcon 100uF/350v
Dubilier 100uF/450v
Dubilier 50uF+50uF/450v

Phase Inverter & Preamp:
HUNTS 50uF/450v
HUNTS 50uF+50uF/450v
HUNTS/ERIE 32uF+32uF/450v
RS 32uF+32uF/450v
Marcon 40uF+40uF/450v
Dubilier 50uF+50uF/450v

I think this chaos stemmed from a couple reasons. One is that there was a decent number of take overs and mergers in the capacitor industry in England throughout the 60's which might have affected products brought to market. Two, RS componets was supplying Marshall with whatever they could get there hands on. As 1970 approached things calmed down to the familar 50uF+50uF/450v caps in all positions.
I haven't done the reseach but I would guess that the 60's caps would have had a higher ESR and lower ripple current rating compare to modern day caps.

thanks for confiming the 200uf caps (220uf was probably an anachronism)

[Carbia]

Just to be clear - are you saying the caps used back in the day (RS, Erie, Daly etc) were higher ESR than current production (F&T, ARS etc) even when they were new in the 60s and 70s, or are you saying that all older caps will increase in ESR over time, even when they sit unused/NOS and are reformed slowly? The Erie caps I used have 1975 date codes, were all NOS and were reformed slowly with good results using Larry's 100k resistor method (2w 100k resistor directly after rectifier before first filter caps, measuring voltage drop until it goes below 2V). However, I did not measure ESR before installation or after.

Is it possible that one of the reasons Marshall went to the setup with two 50+50 cans in parallell/series rather than two 100uF caps in series, is that it may yield a different total ESR? I don't know if it does, but perhaps smaller caps in series and then in parallell yields less ESR, just like neikeel said? It has always been assumed that it was just done to cut costs and stock only 50+50 caps, but maybe that wasn't the case?

I guess (not checked) than even being NEW, the Eries, Dalys... had higher ESR than modern caps, but, even being reformed well, the ESR nowadays is higher (the ESR use to increase with the use).
I've checked old reformed caps and the ESR was notoriously higher than in new caps.

In addition, wiring caps in parallel reduces ESR to half (ESR is the resistance of the cap, so think about it like wiring two resistors in parallel)

[shakti]

I've checked old reformed caps and the ESR was notoriously higher than in new caps.

In addition, wiring caps in parallel reduces ESR to half (ESR is the resistance of the cap, so think about it like wiring two resistors in parallel)

This is starting to look clearer and clearer... So in my 68, I have old (and great-sounding, I might add) caps that are wired in series on the mains and series on the screens, presumably adding up to a lot of ESR. While in my 69 I have new caps wired series/parallell on both mains and screens, with (presumably) much lower ESR. Possibly this is enough to result in a very audible difference.

I'll get that capacitor tester and do some measurements before I start hacking away, but if the readings and preliminary test confirm this as being the reason for the two amps sounding/feeling so different, then I will probably turn this into an "all-in-one" switchable monster. If I use a separate rectifier for each HV tap (460 and 490), I can use the typical 3PDT standby switch to do all the switching; one pole for mains (460 with old 100uF caps in series or 490 with new 50+50 caps in series/parallell), one pole for bias switching, and one pole for screens (low voltage 68 mode with old 100uF caps in series, or high voltage 69 mode with new 50+50 caps in series/parallell).

[shakti]

PS; yes I do know that caps will go up in ESR over time when used, but I'm still curious if that's the case with NOS caps being put to their first use now?

[shakti]

OK, some more experimentation; I swapped the two NOS Erie 100uF 350V caps in the mains filter of the 68, to a pair of F&T 50+50 caps. *Maybe* there is a small difference, a fraction tighter...but it is not enough. And the tone is less interesting for lack of a better word...kind of duller/rounder. Not the blistering, fast attack I am looking for.

I need to look elsewhere. Could look at the screens filter as well of course, but I suppose it could be anywhere in the amp...

[danman]

Maybe the slight variance between the two handwound OT's would be enough to cause the sound differences. That along with slight differences in all of the components (coupling caps, filtering) between the two amps could add up to cause the sound difference. It's an interesting discussion and I hope you get to the bottom of it.

[shakti]

I'm still in the dark, I must admit.

I tried replacing the preamp filter cap (NOS Erie 32+32uF axial) with a new F&T 50+50 radial cap. Out of all the changes I've done, this was probably the most significant I would say. It did make the amp feel a touch faster, but it was not a true A/B comparison, so it's hard to say how big the difference was.

I have not yet tried to swap the filter caps on the screens. However, I did start out with two BC 33uFs in series in the 68 amp, found it to be too squishy and swapped those for two NOS Erie 100uFs in series. That made a very positive difference in making the amp tighter and faster sounding, yet still sounding very fat and musical.

However, I have pinpointed another important difference between the two amps, so the difference I am hearing may actually be two different issues (which may be related). The 68 amp simply does not clean up as well as the 69...it seems to overload very easily, so even with the guitar volume dialed way back, there's a little bit of a slightly fuzzy distortion still. It doesn't sound like unnatural distortion if you will, it's more like the amp's designed to overdrive very early. If you play it turned up well and aiming for a distorted sound, then you never would notice - the amp has a phenomenally fat, grinding sound, nasally like we want it. But it just doesn't clean up like the 69 does. The other "issue" or difference, is the one I have already talked about, how the 68 feels more sluggish and almost sags by comparison with the 69, which is tighter and faster altogether.

I did try to put the .68uF in V2a on a switch, but that doesn't really change the fundamental differences between the amps.

So in summary, of the things I've tried so far, increasing the preamp filtering to 50+50 had perhaps the biggest effect, but the amp was still overdriving very easily. I've measured idle voltages which are all very comparable between the two amps. I should measure under load to see if the 68 sags much more than the 69.

One minor difference between the amps; I have the secondary wires on the OT routed away from the primaries on the 69, so they actually run to the impedance switch between V6 and V7. On the 69 I have them routed along with the primaries "down the middle" between V5 and V6. Since the OT is already much closer to the PT on the 68, is there any way the transformers could be interacting more on the 68, inducing more sag in the 68?? Long shot, I know....