A new clip to try my theory

[Ralle]

Ralle,
All I have to say is Very Interesting! That sounds alot like what I mentioned too! I guess you could call it a bass-treble pot and it does have a .0022uf cap and it really doesn't have to be a mustard either a CDE seems to work fine there but what ever kind you prefer.
The pot type and value I'm not sure about but maybe a 1 meg audio and I recommend a CTS pot with push-pull switch that way you can pull out to activate or push in to bypass. Also, you can add a 1/4" switching jack to the pot and now you have a pre-amp out activated with knob pulled out.
If it's possible that this may be the "secret" part it has been done probably long before your friend found out about it. Sorry, not trying to hurt any feelings here but this mod has been around since the mid 80's when Jose' installed it in my amp and maybe before that. So, if you ask me it's no secret at all, really.

He also installed the 2203 type mv on the same 4 holer amp and he recommended not running the power amp up full and to back off on the mv. Keep in mind I have an extra 12ax7 for extra high gain. Now something special seems to happen with the NFB presence control. I have 100k NFB and .1uf on the presence pot. The amp seems to be gaining more balls as you turn the presence up, not quite how the presence control usually works. Pretty cool! I need to play around with this some more. All this time and I guess I never realized all the possibilities of this amp.

No feelings hurt my friend... I know that's not the secret... what I'm talking about is not this pot arrangement on the backside... it's a STOCK none typical part...

[Tazin]

Ralle,
I know this may be a stupid question since you usually run your amp full throttle, but what amp setting did you use on this audio clip?

[Ralle]

I had full tilt at everything except the bass... that one was off... and vol II was at 2 aclock...

[rgorke]

I had full tilt at everything except the bass... that one was off... and vol II was at 2 aclock...

How much attenuation with the PB?

[Ralle]

It's recorded at home, so at full attenuation ( not the line out )...

[jnew]

I think I have it. And for further clarification, that part is stock but it's value is not. Nor is it typical in that position. And yes, it has been talked about and covered. If it's what I'm thinking, it's something very easy to have read about and yet discard as something too minor or minuscule. Sound about right Ralle?

[Ralle]

I think I have it. And for further clarification, that part is stock but it's value is not. Nor is it typical in that position. And yes, it has been talked about and covered. If it's what I'm thinking, it's something very easy to have read about and yet discard as something too minor or minuscule. Sound about right Ralle?

That depends on what you mean by not typical in that possition...

[jnew]

Well if for example, If I was talking about a resistor in a certain place on the board, I could say that a resistor there is a stock part. It's value in that position is not stock for a plexi circuit (in any era for that matter) thus making it non typical. Again this is just an example. It's not a resistor that I suspect as the secret.

[Ralle]

Every part in the amp is where they're suppose to be... it's the values that differs from the typical values...
Otherwise we're talking about a different circut or schem... wich it's not... Yes that's correct.

[jnew]

Yep. I'm with you. I think I know what it is and will be trying it when I get back onshore. I've ordered that part last week and should have it by then.

[Tazin]

It sounds like it's something simple along the lines of 2.7K/0.68uF on v1b along with a 10nF-12nF coupling cap on the bright channel.

[neikeel]

It sounds like it's something simple along the lines of 2.7K/0.68uF on v1b along with a 10nF-12nF coupling cap on the bright channel.

Didn't add that much when I tried that a while back !

[N3m0 7h3 Fi5h]

http://www.pleximods.com/evhspec.html" onclick="window.open(this.href);return false;


From the Plexi Palace:

(1) By Mark Cameron from pictures taken in the early 80′S.
There ARE small tone altering and/or gain altering mods.
I do have pic’s so I CAN physically see that the amp has a split cathode arrangement.
…the cap on v2(330uf) cathode.
…its a 2.7k/.68 and the resistor looks like the stock part (but I don’t think it is) but the cap is one of those yellow square type from the 70′ Marshall’s. From the circuit card It looks like it came stock with this arrangement. The post that is pressed in looks stock too which tells me more than anyone could. In the pics the serial # is 12301.

(2) ****** 1980.
To try to wrap up is ED thing…
His head was a 67/68 and he had the first stage valve with BOTH cathodes tied together sharing the same 820 ohm resistor that was bypassed with a 330uF blue cap…
His treble cap was a round shaped ceramic that was a 250pF that said MURATA … With 56K on the tone circuit feed..

One of the 470K mixer resistors was bypassed with a round hollow tubular MURATA cap 500pF… If memory serves me right on the value, or it was a 330pF..I will check my notes….

The real kicker, his phase-inverter “get-rid-of-the-FIZZIES” cap was a 100pF instead of the normal 47pF ….and this my friends is how the “brown sound” with that added compression happens..

Oh, BTW those 820 ohm resistors were carbon-comp and drifted in value up about 1.1K and make the amp much more gainy and warmer, since these re-bias the 12AX7 valves in a bit more non-linear region..

I am pretty sure the feedback resistor was a 47K …I will have to check my notes…

The filter cap in the center of the board was a dual 16uF grey RS cap…

The screen filtering was 2 DALY 32uF light-blue caps in series… The voltage doubler were 2 100uF DALY royal-blue caps…

Rear cap on top of chassis was a royal-blue HUNTS 32uF or 16uF…need to check notes..

The value of the coupling cap between V1 and V2a….022uF.

At least that was what it was in 1980…

(3) Plaap (a friend of Peter Van Wheelden who restored Eddie’s amp.)
Edwards amp’s internal measurements were as follows:
1.The first 820 ohm resistor (carbon type) measured a little over 1K. It was bypassed with a blue 330uF resistor. can

2.His treble cap was a 250pF Murata flat ceramic one. The cap across his 470K was a Murata hollow round 330pF. cap

3.His second stage 820 ohm (which also measured a little over 1K, was also bypassed with the exact same type 330uF blue coloured cap that was on the first cathode resistor.

4.The filtering caps for the middle of the board were grey coloured RS caps that had dual 16uF values.

5.The screens were 2 blue caps in series (DALY 32uF’s).

6.The voltage doubler were two big blue caps (100uF DALY’s). The one on the outside was a blue Daly – and was a single 32uF.

7.The feedback resistor was a 47K of unknown origin.

8.His power transformer was the smaller one of that era. His OPT was also the smaller one with 1.5″ stack.

The amp was either left stock into a load resistor or, a big Ohmite (or other) power resistor was placed somewhere in the circuit to cut the power of only the output stage meaning Sylvania 6CA7′s were the only valves to hold up to this.

If this is true – doesn’t this kinda dispel the whole ‘special’ 67 slp 100 myth …as almost all amps of that year had pretty much identical configurations (with the small exception of the 330uf cap on the second preamp stage?

Dankuwell ! (dutch for many thanks)
Plaap

[Tazin]

Didn't add that much when I tried that a while back !

Perhaps it's the combination of having the Normal channel volume pot turned up to 2 o'clock in conjunction with the 2.7K/0.68uF on v1b and the 10nF-12nF coupling capacitor on the bright channel?

[Ralle]

I hope you're not staring your selfs blind on this added part... it just hit me; it might not have been in the amp during the demo days... the more I play with that additional 500pF ( again, I'm not sure it's this part that's the added part ), the more I think it sounds like the first album... more of the raw edges than in the demos...

I have no clue when it's supposed to have been istalled... and without it, sounds more like the After Zero demos...