A New 12XXX for Ted B.

[Ted B]

I've owned several plexi era Marshalls over the years, including a 10XXX JTM45/100 and a '68 SLP w/caps 'upstairs'. However, I don't recall ever owning a 12XXX, so I decided to throw one together.

I'm not trying to make an aesthetic reproduction of a museum piece, so no pink wire and such (sorry). That's been done to death, so I wanted something different. I started with a Metro chassis, Marstran's iron (-80 PT, -132 OT, 3H), and some cues from Friedman's 12361 observations just to make it more interesting.

Notes and observations:

- Ch 2 is cascaded into Ch 1, a la 1959RR. I feel this would be more usable without the 5N bright cap on Ch 1.

- Now I remember why I never cared much for the 5N Ch1 bright cap. I might go smaller to make lower volumes more usable and less tinny.

- The PI filtering is a single 32uF. I tacked in an additional 16uF while playing (don't try this at home), but didn't notice any real difference.

- NFB is 100k and 4 ohm tap

- 330uF fat cap on V2. I have a 0.68UF on there as well so I can A/B.

- Sozo vintage caps

- amp can be rebiased without removing chassis

Comments, suggestions, questions, criticism, overlooked boo-boos all welcomed.


Sorry for the mediocre photo quality:

[vanhalen5150]

Where did you get those black circuit boards?

I see you split the red heater wires up on both sides of the tube sockets. Any reason? Most guys take them up on one side so you get more winds and less exposed single wire. However, I have a pretty expensive "boutique amp" from famous plexi builder that has just a few turns between sockets and the "hover" actually above the sockets, not in the chassis back edge.

Looks like a dandy!

[Ted B]

The black board material I obtained from Watt's Audio, and had them finished by Brian at Reproduction Turret Boards. I much prefer the clean, modern look of black to the old school stuff, but that's just me.

As for why I wired the heaters that way, well, I honestly don't think I have an answer for you. I've always done it that way, and if I had a specific reason why, I can't remember at this point.

Anyway, there is just a teeny bit of 60-cycle hum audible with volumes at zero, but this isn't the reason. I'll have to hunt that down, but the grounding scheme is more or less what you guys call "Larry grounding".

[vanhalen5150]

I think that hum is from the laydown PT regardless of what you do with the grounding. Nature of the beast.

[Ted B]

You're probably correct. That makes me feel better.

[vanhalen5150]

You're probably correct. That makes me feel better.

I came across that same question when I built mine and everybody said the same thing. Thats why later Marshalls went with a standup PT. However, there are those that will also argue it changed the magnetic field in and around the chassis, affecting the OT and power tubes for a different sound.

I also believe in Sasquatch, but have never talked to him myself.

[vanhalen5150]

I often wonder if slapping on a bell cover inside the chassis would help?

Perhaps its more the coil orientation rather than the exposed coils.

[Strat78]

Awesome, nice to see a new build here. It's a thing of beauty and I got some good ideas from your work. Where did you get those little red external contact things for biasing the power tubes? Do you have some close up shots of the biasing set up? Also, can you check the plate voltage externally? Although I can pretty much feel how this amp sounds just by looking at it, some clips would be fun!

[Ted B]

Where did you get those little red external contact things for biasing the power tubes?

Here you go. First item on this page:
http://www.muellerelectric.com/banana_jacks.html

They are available in red and black.

Do you have some close up shots of the biasing set up? Also, can you check the plate voltage externally? Although I can pretty much feel how this amp sounds just by looking at it, some clips would be fun!

I don't have a good means to record sound clips at the present - haven't replaced much of what I lost in Katrina's flooding. I see an SM57 in my future.

I do have a means of checking the plate voltage externally (see military jack on back panel), but it isn't yet connected. I have some external supporting hardware in the works.

Closeups of the bias arrangement follow below. Bias is adjusted via an external 10-turn trim pot w/locking nut.

[Beavis]

Very nice

[vanhalen5150]

Thats a nice mode for the bias.

I have one of those external single bias plugs that goes over the tube sockets, but having that external adjustment pot would be a huge time saver.

[Ted B]

True. If you think about it, there seems to be little reason to install bias jacks on the outside of the chassis unless the bias control is also on the outside. Otherwise, it remains necessary to remove the chassis to rebias. The primary goal here was to eliminate that hassle.

In this case, the 56k (47k in 100w schematics) resistor to ground serves as a short flying lead to the bias pot wiper, one of the other ends which is grounded, and said pot is installed directly across from that part of the board.

...

Presently, I'm comparing several different brands of preamp tubes in V1 and V3. V2 will remain a JJ or Chinese type, as those seem to be more durable in that demanding position.


Two questions for anyone reading this:

(1) Has anyone audibly compared the 32X32uF screen filtering to 50X50uF?

(2) Has anyone made a real time comparison by lifting the V2 fat cathode cap while leaving the 0.68uF in place?

I'd be curious to know your observations.

[Strat78]

(1) Has anyone audibly compared the 32X32uF screen filtering to 50X50uF?

(2) Has anyone made a real time comparison by lifting the V2 fat cathode cap while leaving the 0.68uF in place?

I'd be curious to know your observations.

The answer to question #1 is, your amp would be noticeably stiffer with less squish.

As far as question #2 goes, I've got a 25uf and 250uf fat cap connected up to the grounds on V2 to do this test as we speak. Perhaps I can get a A/B/C clip up tomorrow.

[Ted B]

The answer to question #1 is, your amp would be noticeably stiffer with less squish.

Fair enough. I'll play with it a bit more and try to decide where my preference lies. It still has a touch of an 'organic' feel at this point, which is just enough to let you know it's an old circuit. I'm thinking the stiffer feel may be a little bit too much 'polyester', but I'll ponder it.

As far as question #2 goes, I've got a 25uf and 250uf fat cap connected up to the grounds on V2 to do this test as we speak. Perhaps I can get a A/B/C clip up tomorrow.

Well, today I answered a few of my own questions, including this one, and really got the amp smoking (figuratively).

For starters, I never liked that 0.005uF bright cap across the Ch 1 volume pot, and I just proved that I still don't. I swapped in the old SL value of 100pf, and now the amp has a much greater range of sound. It's a little too dark with no bright cap, but the 100pF lets just enough high frequency content through at lower volumes to give classic rock tones that are just skipped over with the larger bright cap. I'm not sure why Marshall ever got away from the smaller bright cap. I imagine if I had significant high frequency hearing loss, it wouldn't matter so much, but that large bright cap just fatigues my ears.

I A/B'd the amp in real time with and without the V2 cathode 330uF fat cap, leaving only the 0.68uF in place. The difference is subtle, but noticeable. Clipping the fat cap in results in a boost to lower frequencies that gives the sound a bit more balls. It's fine either way, but I prefer the fat cap in place. It's about half as much difference as pressing the Loud contour switch on an audio receiver. I should note however that I conducted this part of the test with the 100pF bright cap in place. I suspect that with a 0.005uF bright cap, the effect may not be as noticeable - except perhaps at or close to full volume.

[Strat78]

The answer to question #1 is, your amp would be noticeably stiffer with less squish.

Fair enough. I'll play with it a bit more and try to decide where my preference lies. It still has a touch of an 'organic' feel at this point, which is just enough to let you know it's an old circuit. I'm thinking the stiffer feel may be a little bit too much 'polyester', but I'll ponder it.

I'm no expert with this stuff, but upping the bias tap or PI filter cap might be an easier place to start instead of the screens if you want a little more head room. I guess it depends on what you are going for. If you have a single 32up for the PI, A/B a dual 32/32 and see what that does. Interesting about the bright cap, with it, my amp seems to jump in at around 3 on the volume. Still, with it in, when you get to 9 or 10, the magic presents itself in spades, more so than when the bright cap is not there.