60s Marshall Presence Circuits

[Liam]

Was repairing my Marshall BBRI (Model #1962, 1989 vintage) the other day, and pulled out the schematics, looking at the presence control. (Dry joint on the back of the presence pot as it turned out).

But then comes a design question.

On the BB reissue the presence control is a 4k7 pot wired as variable resistor in series with a 0.1 uF cap. However on the original 1962 combo, as with the JTM45 and JTM45 reissue, the pot is wired as a fixed resistor, with the wiper varying how much of the resistor the cap is across.

As far as I can see from the schems, the presence control effectively grounds part of the negative feedback from one of the taps on the OT (8 or 16 ohm, varies by the looks of it). This seems to mean that in the case of my BBRI, the presence control will have a big effect on the overall gain of the amp, rather than just acting as a tone control. In the circuits where the DC ground resistance is fixed, the presence control shouldn't have too big an effect on the gain, but will short out some of the NFB at higher frequencies (dependent on the pot's wiper position). Right?

3 questions. Am I barking up the right tree? Why do both designs exist? Is there any reason why I shouldn't just change it back to the 1960s style presence control?

(Just had a thought. Is this to do with log/lin pot availability or something?)

Any thoughts gratefully received.

[Enzo]

Which model exactly has the series pot? I have two different JTM45 drawings handy, the JTM45-100 and the JTM45 reissue (2245) and they both have the cap to the wiper and ground. The pot is the connection to ground for the PI. Looking up 1962 I get the same circuit, and looking for Blues Breaker comes up with the 1962 reissue as well.

If the drawing were wrong and omitted the ground connection at the bottom of the pot, then the tail of the PI would have no ground reference.

OK, I found it. 1962 reissue. In either case, the pot serves to roll off the highs at the point of the pot. That is where the NFB returns. The NFB comes from teh 16 ohm tap. That doesn't switch, the speaker switches taps, but NFB stays on 16 ohms.

I don't know. is the amp wired like that schematic? A quick scan and I don't see that arrangement on other models. The PI still has a DC ground path through the OT secondary and the 27k feedback resistor.

[Liam]

Enzo, how lovely to hear from you!

Yep, definitely wired as per the schematic. One of the cap leads shorts two of the pot connections, the other one goes to ground.

2203s were wired like that too.

I'm going to have to try it both ways round. Will report back before the week's out!

Liam

[kg]

liam, can you post up or link to the schemos in question? i think i'm following your description but i want to be sure.

[Liam]

Hi Ken.

Here's the original circuit. It has a 5k pot wired as a fixed resistor to ground from the NFB application point on the PI. There is a 0.1 uF cap going from the wiper to ground. This seems like it should just have an effect on how much of the high frequency content of the NFB is shunted to ground.

Early Marshall 45 watt amps

Then this is the reissue schematic. There is a 4k7 wired as a variable resistor, in series with a 0.1 uF capacitor. Why?

Marshall Bluesbreaker reissue schematic

Haven't had the chance to play about with this yet, but it's high on the priority list! I'd love to hear your thoughts on it.

Thanks

Liam

[kg]

well one thing that pops out at me is something that enzo hinted at.. follow the tail current path.

in the jtm45 schemo, there will never be more than 5k between the bottom of the self-biased LTP arrangement.

in the bb ri, that's not the case, in fact there will never be less than 27k.

that's a difference of around 22k.

assuming a couple mA per section, say 3, that means the LTP will be pulling 6mA total. across the 22k difference that amounts to some 60vdc difference that subtracts from the total voltage across the LTP differential pair.

since the whole LTP is stacked on this longer tail resistance, the pair will be more "constant current" which will improve balance and CMRR. not to say that it will sound better... hell, this is geetah amp business after all, and basically anything goes.

it will also reduce the max undistorted signal from the LTP since there will be less effective B+ available. you will likely clip at the LTP output sooner in the BB RI case. however, this can also be a "good thing" as it reduces the tendency of the output stage to go into grid conduction and create blocking distortion.

a secondary difference is in the expected feedback factor at various frequencies. in the jtm45 case, that 5k shunt resistor doesn't have any reactance to it, so it reduces the feedback factor (ie reduces the NFB and increases gain) across the board. when the presence control is turned up, the shunt cap decreases the nfb at hf even more, in addition to that broadband reduction by the resistor. when the presence is at a minimum, the shunt cap is completely out of circuit.

in the bb ri case, that shunt cap is ALWAYS in circuit and ALWAYS providing a frequency dependent reduction in feedback factor. the reduction in NFB is proportional to frequency, ie higher frequency = higher attenuation. it will provide less HF attenuation when the presence is turned down, but there will always be some.

those are the differences that popped out at me in the first few minutes of looking it over. in practice, there will be some effective difference in terms of how they achieve the HF presence boost, ie moving the breakpoint vs altering the amount of attenuation.

hth
ken

[Liam]

Thanks Ken, that's an interesting appraisal. For some reason I hadn't thought about the effect on the operating point of the PI, just had NFB on the brain I guess.

Hopefully I'll get a chance to try it both ways round this week, and will post a highly subjective opinion on it!

Liam

[John G]

Hi all,
There is a possibility that the BB reissue schem is in error.
Has anyone actually sighted this arangment ??
John G

[Liam]

Yes, my 1989 BB reissue is definitely wired as per the 1989 schematic, although possibly not for long.

Liam

[John G]

Liam,
Thanks for that, as KG has suggested and ohm meter from the end of the tail 10k to zero volts (ground) should measure approx 22k. If it measures 4k7 or there abouts them there is another resistor present. or if not then as you imply just rewire the pot and cap to the convention and away you go.
Thanks for the information, I will log it away for when I get a BB in for repair.
John G

[John G]

Liam,
I have just looked through my Marshall schems and have found an "issue 4" of that drawing and shows the presence pot connected to grnd, with the cap of the wiper to grnd.
John G

[kg]

Liam,
I have just looked through my Marshall schems and have found an "issue 4" of that drawing and shows the presence pot connected to grnd, with the cap of the wiper to grnd.
John G

that's very interesting....

[drewl]

The early amps (JTM45) were bassman copies with alot (27k) of feedback.
When they changed the circuit to the jtm50, they reduced the NFB and changed the presence circuit.

Why? who knows. I've seen lots of funny stuff in Marshalls over the years.