1987 to 2204 conversion Issues.

[Lummox]

I built a 1987 clone (with y'alls assistance) a few months back. It sounded really good cranked, but I needed more gain at lower volumes, so I added a PPIMV, and it was much better... But still not quite my cuppa tea.
I was thinking, for fun maybe I could convert it to 2204 specs.
It looks like a pretty simple conversion... Any common pitfalls I should look out for ?

[Stevem]

Note that the master volume addition just let you get more preamp distortion than output distortion into the mix, but you still only had the same built in stock level of preamp gain going on!

You might want to go with the JCM800 deal for more gain than you had before, as these to me sound hotter, yet more focused!

[xtian]

I built a series of three 1987 heads. The first had a series/parallel switch for the preamp stages (parallel is 1987 stock, series is more like JCM 800). Works very nicely.

https://tubeamparchive.com/viewtopic.php?t=19042

[Lummox]

Ah, thanks. Lots of good readin' there.

[Big Al]

Lummox, before you change the topology of your 1987 circuit, there may be something you can try. In his book and on his website, Merlin Blencowe has suggested "bootstrapping" the buffer stage to the cathode follower for a gain increase. I think the stock buffer stage has a gain around 50-60 before it goes to the CF and the tone controls. Bootstrapping should get it up around 85+.

The circuit and description is at the bottom of this linked page: http://www.valvewizard.co.uk/dccf.html
And he goes into more detail in his preamp book.

I haven't tried it yet. If my '74 JMP50 needs a boost, then I might. Two resistors and a cap. Just figured if your heating up the soldering iron anyway, might be worth a shot.

Good luck!

-Allan

[Lummox]

That looks really interesting, Al. I think I might try that and report back. I actually went ahead and converted it to stock 2204 specs from a schematic I had from an early 80's JCM800, and then for a bit more gain, I swapped out the 10k resistor on V1a for an 820 ohm resistor with an .022uf cathode bypass cap. Now it's a bit overly gainy, so the bootstrapping will be the next thing I try.

[Smokebreak]

For V1a cathode, try 5k, or 2K7, with .68u or 1u. That will cut down on gain from the way you have it, but more than stock JCM800

[Lummox]

I tried a 2k7 with a .68uf, and didn't love it, but I'll keep playing around with other combinations.
I'm chasing a holy grail tone of a 1971 Marshall 1987 that I owned throughout the 80's and 90's. That amp was supposed to have come from Leslie West, who had modded it back in the 70's
My memory is a bit sketchy, but I seem to recall that each of the 3 gain stages had cathode bypass caps, with one of them being a .68uf, and the other two being .25uf.
I don't remember the order, it was either 25-68-25, or 25-25-68. I don't remember what value resistors were in there either.
I think the plate resistors might have been changed to 220k's as well, and there was a high pass filter between two of the gain stages.

I sure wish I had never sold that old girl,I sure do miss her.

[Lummox]

So, I ended up trying the bootstrapping deal, but didn't really notice too much of a difference, so I took that out, and using clip leads, tried a whole mess of combinations.
The
one I'm digging the most is a 5.6k/.22uf combo on v1a, the stock 2.7k/.68uf on v1b, and the stock 820ohm on v2a.
Tons of gain, but with a nice clarity, and touch sensitivity. Cleans up beautifully by rolling back the volume knob.
I didn't touch the stock 100k plate resistors, is that a bad idea ? I know they work in conjunction with the cathode resistors to bias the tubes, but I wasn't sure if it was a big enough difference on v1a to justify having to adjust the plates.

[Lummox]

So after all that, something isn't quite right... It's much hummier than it was, and the voltages seem pretty well off the mark. I haven't had time this week to do much troubleshooting, Here's what I'm getting :

- Pin 1 - Pin 2 -Pin 3 - Pin 4 - Pin 5 - Pin 6 - Pin 7 - Pin8 - Pin 9

V1 - 245.6 - 2.90 - 3.11vac - 208.8 - 0 - 1.89 - 3.11vac

V2 - 154.6 - - 1.00 - 3.11 vac - 285.1 - 153.7 - 154.0 - 3.11vac

V3 - 211.4 - 24.3 - 39.8 - 3.12vac - 215.1 - 26.5 - 39.8

V4,V5 - 438.2 - -41.32 - 436.0