68 plexi

[neikeel]

Is the bright ch coupler .022? Are mixer and tone stack caps 500 or 560pf? What is your plate voltage? Seriously, this is just pure awesomeness!

looks to be .0022 to me.

Is the difference between 500pf and 560pf audible? Just curious.

My earlier amp is 0.0022uF so very likely in this one too. Mine has RS 500pF in tone stack and mixers (same as the 68 SL). Both my 67s (45/100 and JTM50) are 256 and 556. Lemco on the 100w and RS the 50.

The ones in pleximasters amp both look like Lemcos. His normal channel (V1a) bypass cap looks to have been replaced. I too would like to know what values they are.

Normally I would not expect to hear much difference between 500pF and 560pF. Early amps (KT66 fitted) seem to have 560pF, my 67s have 556, the 68s 500pF and we see later amps with 470pF, this is only one observervation, wonder if this stacks up generally.
Suspect the real secret in the amp discussed is in the OT and how many winds it acquired whilst the operator was asleep. Sadly there is no other way of finding out other than stripping it and counting, so Pleximaster in the interest of science and your fellow forumites do the honest thing and send it to Brian Wallace for a teardown

[julkke]

I do hear a difference. 560 in both sounds better.

[pleximaster]

Sorry if I missed but are you attenuating? That's basically VH club days tone your having there bro. Another killer example here that you just have to dime a 68 plexi. Where is your normal channel volume set at?

It is straight into the cab, however it sounds ok with an attenuator I have hotplates and an ultimate. I prefer a 16 ohm hotplate to this head.

Plexi

[Tazin]

It is a very nice sounding amp; that's for sure. These late 11Series 50w amps tend to run a bit different filtering which maybe contributing to the overall sound.... The Mains filter looks like a single HUNTS 50uF/500v verse the other varients (i.e. 40uF+40uF, 32uF+32uF, 50uF+50uF). The other top mounted filter can; MARCON 40uF+40uF/450v; it pretty much a "staple" in these amps at that position. I can't tell for sure but the pre-amp filter on the tag board looks like the usual HUNTS 32uF+32uF.... Then again, it could also be the less popular HUNTS 20uF+20uF since both filter caps look the same. If it is a 20uF+20uF then that would also change the tone of the amp.

The tone stack cap and mixer cap look like LEMCO 556pF just like Neil outlined. The rest of the circuit componets look like the usual for this time frame. I'm with Neil in that the hidden mojo is probably contained in the OT. One other thing that maybe a different is where the voilet NFB wire is connected. I can't tell from the pictures but it looks like it's connected at the Selector compared to say the speaker output jack. If it is connected to a dedicated tap then that would also effect the tone of the amp.

[EddyInChicago]

Just noticed the two 2-watt CC resistors in the second photo being 8.2K BOTH (next to the bias circuit). Shouldn't one be a 10K? Lower resistance there will send a higher voltage to the preamp, I believe.

[neikeel]

Just noticed the two 2-watt CC resistors in the second photo being 8.2K BOTH (next to the bias circuit). Shouldn't one be a 10K? Lower resistance there will send a higher voltage to the preamp, I believe.

Very variable in Marshalls this period. Mine has two 8k2 Iskra there (both measure in tolerance), the 100watter has one of each. It is possible that the ccs have drifted up but strangely the big ccs on my similar era amps are still within tolerance (unlike the ones in an AC30 I restored where 50% were out of whack!). Funny my amp has two 50/50 cans for screens/PI and mains and dual 32uF preamp, I always think of the Marcons being earlier. The bias caps in mine are marcons and test in spec.

[pleximaster]

You guys have asked a few questions. I´ll try to answer them by taking more closeups tomorrow.

What value is the v1b cathode resistor? 820ohm or 2.7k?
Is the bright ch coupler .022?
Are mixer and tone stack caps 500 or 560pf?
What is your plate voltage?
His normal channel (V1a) bypass cap looks to have been replaced

This is not my personal favourite of my amps I have a 1969 100 plexi that eats this one for breakfast. I´ll try to record it as well but it is so lound and you neeed two cabs. Perhaps I put some attenuation on it to tame it. It is however less brown and more “cream with sugar” AFD meets EVH!

plexi

[julkke]

You guys have asked a few questions. I´ll try to answer them by taking more closeups tomorrow.

What value is the v1b cathode resistor? 820ohm or 2.7k?
Is the bright ch coupler .022?
Are mixer and tone stack caps 500 or 560pf?
What is your plate voltage?
His normal channel (V1a) bypass cap looks to have been replaced

This is not my personal favourite of my amps I have a 1969 100 plexi that eats this one for breakfast. I´ll try to record it as well but it is so lound and you neeed two cabs. Perhaps I put some attenuation on it to tame it. It is however less brown and more “cream with sugar” AFD meets EVH!

plexi

Sounds like higher filtering. Can't wait to hear it!

[Tazin]

Just noticed the two 2-watt CC resistors in the second photo being 8.2K BOTH (next to the bias circuit). Shouldn't one be a 10K? Lower resistance there will send a higher voltage to the preamp, I believe.

Very variable in Marshalls this period. Mine has two 8k2 Iskra there (both measure in tolerance), the 100watter has one of each. It is possible that the ccs have drifted up but strangely the big ccs on my similar era amps are still within tolerance (unlike the ones in an AC30 I restored where 50% were out of whack!). Funny my amp has two 50/50 cans for screens/PI and mains and dual 32uF preamp, I always think of the Marcons being earlier. The bias caps in mine are marcons and test in spec.

In the case of the 100w amps (12series) you do tend to see the MARCON 40uF+40uF/450v can coming into play before the the HUNTS 50uF+50uF/450v for the PI filter. With the 11series 50w amps things seem a bit different. Around the 11150 mark you see a short run of amps using the MARCON 40uF+40uF/450v cans for both the mains and screens&PI filtering. Then shortly after that there appears to be a run of amps (11200 thru 11500 or so) that utilize the HUNTS 50uF+50uF/450v cans for both the mains and screens&PI filtering. Following that, you tend to see the HUNTS 50uF/500v (for the mains) and the MARCON 40uF+40uF/450v (screens&PI) arrangement on amps (11550 thru 11800).

Now, anyone who has tried to make sense of the tail end of the plexi era amps knows that the info I mentioned above is just a generalization....Trying to bring some order to chaos.

[pleximaster]

Sounds like higher filtering. Can't wait to hear it!

Yes it has all cans on top!

I have a 100 watt 68 as well with only 1 cap on top and its a bit "browner" but I dont think filtering alone does it. My 67 Blackflag is not brown at all but very loose sound.


plexi

[herbvis]

Awesome amp tone! I gotta say I am jealous of that tv yellow d/c jr Nice guitar collection!

[Tone Slinger]

Definatly a ABOVE average 50 watter that was 'spared' the death many a Marshall from that vintage has seen.

It is certainly GREAT to know that there are CLEAN and GREAT sounding examples of 'WHAT IT IS ALL ABOUT' still out there.

I bet you could end up selling that particular head for a RIDICULOUSLy high price.

Here again, lightning doesnt strike the (exact) same place twice. Technology cant simply replicate certain things.

[pleximaster]

http://i139.photobucket.com/albums/q282 ... ld-110.jpg" onclick="window.open(this.href);return false;
http://i139.photobucket.com/albums/q282 ... ld-111.jpg" onclick="window.open(this.href);return false;
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Here are some mor pictures for you!

I tried to do a similar recodring with my 69 100watter but its so damn loud. I need to use two cabs and I couldnt fit them both in my closet/cuboard so I had one cab out in the room with me and it was feedbacking hard. Since we accidently entered the brown talk again I tried to push it hard. This one has a nice round sound between 6-8 and then between 8-10 it shits itself into Eddie alnd. The sound turned out harder then I´d hoped its hard to find the sweet spot... I did one clipp and tried to move the mic a few centimeters and reshoot but then I had a tube failior and fuse burn... I dont have any Sylvania spare tubes at the moment. I don´t know if I should post that clip now its not that good...

plexi

http://i139.photobucket.com/albums/q282 ... ld-109.jpg" onclick="window.open(this.href);return false;

[julkke]

Thank you for the pics! That amp is indeed very close to what one dutch dude used to record some pretty kewl songs.

Sweet collection btw!

[pleximaster]

Did the pictures answer your questions?

plexi