My modded JCM800 and Evil JJ's

[syscokid]

First post here!

I’m electronically challenged, so bear with me…

I have a 1982 JCM800 50 watt 2x12 combo (Model 4104), which was freshened-up by Don Butler, aka Toneman, over two years ago, with excellent results.

Recently, I became interested in the numerous mods available on the internet, and books, on this amp circuit, that supposedly can make this amp more fuller or thicker at less than ear-splitting volumes. Maybe a little more gain, too.

Four changes I have settled on:

1) The 470k resistor/470pf cap treble peaking circuit, wired between the preamp vol and the low input jack, was replaced with a 82k R. No cap. The .001uf Brite cap at the vol pot was already removed by the Toneman.

2) The 10k cathode R for V1, was replace with 4.7k R

3) The 470k R at the treble peaking circuit at V2, was replaced with a 220k R. I left the cap alone.

4) This next mod was done on my own: Originally, a 47k R was designed to be wired after the bias pot. The Toneman replaced this with two 47k’s in parallel (tested at 24.7k). Since I’m also on a mission to replace all the carbon comps with metal film, I used two 56k’s, which tested at 27.8k. I haven’t checked the bias yet.

Another Toneman mod I would like to note: A shielded wire, grounded at the vol pot, is used between the low input jack and the board.

Here’s what doesn’t make any sense to me (And the amp didn’t do this before I messed with it): With a JJ ECC83s in V1, and the vol pot set to zero, the amp pops and crackles, and becomes microphonic….nothing plugged into the inputs. But with the vol set fron .5 to 10, the amp works fine! I tried five other JJ’s, some with gold pins, and it’s the samething. However, when using a 12ax7 Sylvania, JAN Phillips, Ruby, Chinese, EH’s, Sovtek’s, GT’s (Sovtek), and even a GT ECC83, the amp behaved perfectly. BTW, the chimiest and quietest tubes were the Sylvania, Phillips, and Ruby.

Has the amp become unstable with the characteristics of the JJ’s?

What’s happening at mod #4?

Thanks and
Greg

[soma_hero]

I'd imagine lowering the resistor after the bias pot would decrease the bias voltage you could send to the power tubes.

[jaysg]

Since I’m also on a mission to replace all the carbon comps with metal film, I used two 56k’s, which tested at 27.8k. I haven’t checked the bias yet.

hmm...no carbon comps in mine. Did someone put some in, or do you mean carbon film?

The noise you're hearing with the volume knob down might be the input jacks bouncing, in which case they need their contact areas cleaned.

[Structo]

Or possibly a dirty socket.

Some tubes have bigger pins than others, could the JJ tubes possibly have smaller pins than what was in there for a long time, thus not making strong contact?

I would first make sure the filter caps are discharged then re-tension and clean the socket pins.

Clean and re-tension the shorting jacks as well.

Not sure why he paralleled the two resistors at the bias pot unless it was to increase the wattage of them.

Check your bias!

[syscokid]

Here's an image after the Toneman got thru with it. I know for sure that the R riding piggy- back on the other R is a carbon comp. I have been assuming all the other shinny brown R's are carbon comps, too. Or are those carbon film?
[IMG768]http://i443.photobucket.com/albums/qq15 ... G_0357.jpg[/img]

What is meant by "the shorting jacks"?

And I did retension the V1 socket before I started my mods on the amp. It just seems wierd that any JJ's that I put in there makes my amp go goofy at zero preamp volume. Any other brand of tube, and the amp works fine. I ordered a new socket from Metro anyways. Sockets for V2 and V3 were replaced last year.

[Structo]

Yep those sure do look like carbon comp resistors.

What are the shrink wrapped items?

[jaysg]

The piggyback resistor is carbon comp. The rest are carbon film.

Looks like Butler replace the diodes with FREDs. The shrink wrap is to protect the clumsy from the +/- 330Vac.

[soma_hero]

Your issue with the JJ tubes could be exactly what was suggested. Perhaps JJ tubes have smaller pins than all the others, and they just aren't making good contact. Either way, clean your input jack connections and tube sockets.

[Sonny ReVerb]

The piggyback resistor is carbon comp. The rest are carbon film.

So, the shiny, reddish brown resistors are carbon film? Any idea who makes them? My Z28 had one of those in the dropping string and I wondered what type it was.

[soma_hero]

Vishay/Dale, Xicon, KOA Speer, all make them. Check out Mouser or any other distributer.

Color isn't necessarily indicative of composition. For example, all the Metal Film's I've gotten from Xicon have been blue. All the metal films I've gotten from Vishay have been red/brown. You never know unless you're sure what was on the package.

[Sonny ReVerb]

I'm familiar with most of the common brands. The Xicon carbon films are usually tan, metal films are light blue, MO's are grayish white or blue, Dale RN65's are brown, Riken's are green, and Ohmite Little Demons and AB carbon comps are dull brown. I've just never bought any that were the shiny, reddish brown type. Sorry for the derail...

[Structo]

Yeah, I should have taken a closer look.......

I just glanced and saw the brown coloring.

[syscokid]

The piggyback resistor is carbon comp. The rest are carbon film.

Looks like Butler replace the diodes with FREDs. The shrink wrap is to protect the clumsy from the +/- 330Vac.

I thought the shinny ones were modern production carbon comps...

Don Butler is big advocate of using FRED's. I really can't say what difference, if any, the FRED's did to my amp because when Don serviced my amp, he also did a filter cap job, replaced most of the other caps in the signal chain with Sozos, new Mercury choke, and a Mercury OT.

[syscokid]

Or possibly a dirty socket.

Some tubes have bigger pins than others, could the JJ tubes possibly have smaller pins than what was in there for a long time, thus not making strong contact?

Since the sockets for V2 and V3 were replaced last year, I just got thru replacing the V1 socket, yesterday. Still the same problem with the JJ...any JJ's! Pins look identical to other 12ax7's. I'll repeat again: Any other brand of tubes...and the amp works and sounds excellent! This is OK though. I've never been a fan of JJ 12ax7's. But I do like their EL84's.

Check your bias!

Wow!.....the bias measured at 25ma's. Adjusted the bias to 40 (Well, one tube is at 40, the other is following at 37).

[billyz]

Just out of curiosity I measured the pin diameter on a few 12ax7's .

JJ 12ax7(box plates) ... .039"
Old GT 7025 ( Long flat plates )... .041"
RCA 7025................... .040"

Recently Fender has had some oversized tube sockets and they are causing some loose tubes and intermittant problems. Fender is sending out the Belton replacement sockets and they really tighten things up and solve the loose tube syndrome. The JJ's seem to work fine in the Belton sockets but they would fall right out of the Stock ( chinese ) Fender sockets.