JCM 800 Mod question

[steve_e_nixx]

Hi,

I just opened up my 50 watt JCM 800 and found an odd looking mod. Someone put what looks like two sets of resistor/cap combos in series at C6 and pin 7 of the first preamp tube. The r/c combos are 4k7/25 uf and 4k7/.001 uf(orange drop 102j). There is also some other junk floating around between one of the inputs and the first tube stage. Can anyone fill me in on what someone was trying to do here? I know this mod boosts gain a lot, but Ive never seen a series mod like this.

Here are some shots (i hope!):








Thanks!

[cbarrow7625]

You forgot to mention that they also put a shunt around the original 10K cathode resistor. They basically replaced the existing 10K resistor with the two 4.7k in series (plus caps). Basically the same value for the resistor.

The 25uF cap bypasses half of the 10k cathode resistance, giving a mild gain boost to the entire frequency range. The .001uF cap around the other resistor works with the 25uF cap to fully bypass the cathode cap but only at very high frequencies. Should give some screaming top end distortion.

The value of .001uF seems really small to me. I would expect that to only affect the very tippy top end of the high frequencies. Something more along the lines of 0.1uF - 0.68uF is usually found in that position with mods like this.

The idea of using the 25uF cap to boost the entire frequency range mildly is a nice touch. It should help keep the tone sounding full & not too thin. Try experimenting with different caps where the .001 is (just put them in parallel around it) to see if you can get a sound you like (unless you like it as-is).

Chris

[steve_e_nixx]

Chris,

thanks for the walk through of the mod. It really is a good sounding mod with this amp, so I wont mess with it, but I will keep it in mind for future work.

-S-

[Bruce / Mission Amps]

.001uF across a 4700 ohm resistor has a -3dB point of about 34KHz... sorry, but in my opinion, that is completely useless.
The 22uF cap across the other 4700 ohm resistor is a huge difference and the -3dB point is about 1.5Hz!!!
That means full amplification and AC bypass from near DC to max Freq of the tube circuit.
The stacked 4700 ohm resistor with the .001uF cap is applying a little NFB though and that could help.

[Rick Erickson]

At least they did a nice clean job on it...

[Rick Erickson]

It looks to me like someone was trying to select which network to use. If you jumper across one then only the other is in circuit. This is one way to select by ear when modifying an amp. Whoever did it should have tidied up his work before selling it though. I pull stuff like this out of amps on a regular basis.

RE

[Gtr_tech]

Yea....its pretty messy work. Not the way I'd go about a gain boost for an 800 either. A 2.7k partially bypassed with a .68u/1k with a 470p to gnd after the coupling cap works well on the second stage.

[Tubis]

.001uF across a 4700 ohm resistor has a -3dB point of about 34KHz... sorry, but in my opinion, that is completely useless.

Nope, it doesnt work that way in the catode bypass. If you have a single 4k7 cathode and an 100k anod there will be a +3dB at 3.5kHz and a -3dB at 8.5kHz.
If you have two serial 4k7 and only one is shunted you have more of a boost at +3dB at 6.5kHz.
One have to se how much boost there will be and not the resistense difference in the resistor vs the cap. This is not a passive filter.
You cant use the formula youre using for the bypass cap

[Tubis]

The 22uf is part wasted. You can put the 10nF over the entire 10k resistor. It makes marginal differense. It basicly provide a boost of treble with the lower 3db point at 3.5kHz and the upper at 8.5kHz. The basic trebleboost will be at +10dB.

Sorry is the 10k still connected parallell to it all? If so the boost will only be +8dB.

[Tubis]

I love to tweek. I would take all that shit away and put a 220nF over the original 10k resistor.

Thene i would put a 470pF-1nF from the plate to the cathode. Thats very easy to do straight on the tube socket. You will get a much midfater tone thats realy crunchy.

[Robert M. Martinelli]

Thene i would put a 470pF-1nF from the plate to the cathode. Thats very easy to do straight on the tube socket. You will get a much midfater tone thats realy crunchy.

Yeah, you' re right; that' s the way they did it on AC30s, the brilliant channel is coupled to the next stage with a very small cap to enhance highs.

BTW, very messy job, looks like the guy who did this had more to do with plumbing than with electronics - he should be locked away and I would volunteer to swallow the key!

[cbarrow7625]

Tubis,

What formula are you using to determine that the two frequency breakpoints will be @ 3.5kHz & 8.5kHz for this configuration? I haven't sat down to work it out but those two frequencies don't necessarily "feel" right to me. With a split cathode those frequencies can be tricky to work out. I'm hoping you can teach me something.

BTW, I tried out a similar version of this setup in an amp I have just because I thought it was so unusual (see my original post above) and I quite liked it. The large cap provided a warm gain boost across all frequencies & the small cap provided some top end harmonic "sparkle" (that I like and can always use more of). I like a smooth distortion with good upper harmonic content. I was surprised a little, but I liked the results of this setup (actually a variant of this setup, but inspired by it none-the-less).

Chris

[dai h.]

just a passing thought. Maybe a lin10k pot with the different values tied to the wiper. Interesting?

[Tubis]

Tubis,

What formula are you using to determine that the two frequency breakpoints will be @ 3.5kHz & 8.5kHz for this configuration? I haven't sat down to work it out but those two frequencies don't necessarily "feel" right to me. With a split cathode those frequencies can be tricky to work out. I'm hoping you can teach me something.

BTW, I tried out a similar version of this setup in an amp I have just because I thought it was so unusual (see my original post above) and I quite liked it. The large cap provided a warm gain boost across all frequencies & the small cap provided some top end harmonic "sparkle" (that I like and can always use more of). I like a smooth distortion with good upper harmonic content. I was surprised a little, but I liked the results of this setup (actually a variant of this setup, but inspired by it none-the-less).

Chris

The calculations become so complex so i havent bothered learning them. I let the computer do the calculation for me. Get at good spice simulation program like multisim and you can reach new higts in your building. To get a complete overview of the circuit with only calculation is stoneage in the computer era.
When youre tweeking with the help of a spice simulator ju can deside the right value for a component by the push of a button.

[Bruce / Mission Amps]

Nope, it doesnt work that way in the catode bypass. If you have a single 4k7 cathode and an 100k anod there will be a +3dB at 3.5kHz and a -3dB at 8.5kHz.
If you have two serial 4k7 and only one is shunted you have more of a boost at +3dB at 6.5kHz.
One have to se how much boost there will be and not the resistense difference in the resistor vs the cap. This is not a passive filter.
You cant use the formula youre using for the bypass cap

I don't know what formula you are using so it is hard to comment but if you assume that the cathode impedance of a 12AX7 with a 100K plate resistor is around 1600 ohms and that Zed is in parallel with a 4700 cathode biasing resistor, which is bypassed by a tiny .001uF cap), ... then my math figures the -3dB point of that circuit is about 49KHz.
Show me what I'm doing wrong there.