In Search of: picture of amp with the odd presence circuit

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In Search of: picture of amp with the odd presence circuit

Postby dai h. » Fri Apr 10, 2009 12:30 am

(Out of my own curiosity,) has anyone seen a picture of one somewhere or own one? I'm referring to the one with the 4.7k in parallel with the presence cap. The one where the resistor is on the wiper so that when the presence is up on 10, less voltage is fed back giving more gain than the other more common presence circuit configurations (instead of 100k and 4.7k as on the more common, it would be something around 100k and 2.5k). I was just wondering since I don't recall seeing a chassis pic of one with the circuit (even though it is in one of the factory schematics). Personally I like it since it's more gainy, and I suppose you could set an amp this way to get a sort of idealized "gain monster" early 70s spec with high B+, split cathodes, linear taper volume (plus whatever else could be thought of).
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Re: In Search of: picture of amp with the odd presence circuit

Postby Brian Wallace » Fri Apr 10, 2009 11:55 am

This is an interesting subject. I'm going to have to search through all my folders now because I don't think I have seen that set up before.
Remember....You can fix it with a little extra energy and a positive attitude. ;)
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Re: In Search of: picture of amp with the odd presence circuit

Postby dai h. » Sat Apr 11, 2009 9:48 am

hey Brian I started looking through my own pics(again) and didn't get completely through (went shopping for parts) all my pics but couldn't find one (some were too fuzzy, not enough resolution so possibly missed). I'll try to continue again tomorrow.
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Re: In Search of: picture of amp with the odd presence circuit

Postby neikeel » Sat Apr 11, 2009 11:56 am

I confess I had one :oops:

But I replaced the 25k pot with a 5k and used the 0.68uF mustard on V2 replacing it in the presence circuit with a 0.1uF. The 4k7 is lying in the 'odd' resistors tray.

It was on my early '77 JMP 1987 (I always thought it was a 76 'H' Suffix toggle but trannies and inspection say March 77). Now I have the late 60's spec 50 watters (Lead and Organ) it is time to put it back to stock with the 100k NFB etc.
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Re: In Search of: picture of amp with the odd presence circuit

Postby dai h. » Sat Apr 11, 2009 4:40 pm

thanks Neil. I think you are talking about a different one however, since you mention a 25k pot. The one I'm referring to has a 5k pot which (when presence is maxed) ends up in parallel with a 4.7k fixed resistor. The one that used to be in your amp sounds like the feedback network/voltage divider off the speaker out had 4.7k only as the 'tail resistor' which was fixed as the presence was turned, while the one in question changes as the presence knob is turned(so the presence cap effect of grounding out the frequencies it effects and increasing gain at those frequencies takes place as well as some gain increase from the change in the 'tail resistor'). It's pretty noticeable, and I'd imagine it'd be even more so on one of those amps with 100k off 4 ohm tap (in my guinea pig 50W it's 47k off 8 ohm tap IIRC).
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Re: In Search of: picture of amp with the odd presence circuit

Postby neikeel » Sun Apr 12, 2009 4:35 pm

Ah yes I have seen those schematics and wondered what the difference in behaviour might be, might be time to try it!
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Re: In Search of: picture of amp with the odd presence circuit

Postby dai h. » Sun Apr 12, 2009 7:25 pm

if you want something more gainy, definitely try it! :twisted:
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Re: In Search of: picture of amp with the odd presence circuit

Postby neikeel » Thu Oct 15, 2009 6:10 pm

To resurrect this thread as I get round to restoring the amp:

I pulled the pot of this amp during the resto and it is indeed a 5k pot (platic shaft).

IIRC it was wired like this:http://www.drtube.com/schematics/marshall/1959mk2u.gif

It is going in with a 4ohm/100k presence arrangement so I expect it to be very different to my other 50watters (we'll see how close it gets to my 2204/1987 2 in1 :wink: )
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Re: In Search of: picture of amp with the odd presence circuit

Postby dai h. » Fri Oct 16, 2009 1:06 pm

is this the '77 above? Got me excited for a moment, but I take it this can't be 100% confirmed that it was wired that way stock?

(BTW I think I might have it backwards above since 100k is less fdbk., so there is less fdbk. voltage to shunt to ground, so maybe I should say in addition to (the extra gain with the 100k) are some of the things that theoretically (that is if somehow in one amp or a number of early 70s leads, there was higher B+(more for 50Ws I suppose), less fdbk., odd presence, class 2 ceramic, higher off spec 5k pot cap, linear taper 1M volume)could make for something a bit distinctive and more gainy compared to the earlier 1959SLP type split cathode cct..)
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