JCM800 2204, no "chunk", thunk

[sonofmickel]

JCM800 2204 build:

The amp is LOUD. Sounds best half way up and more. I can dime all the pots and get no oscillations etc...amp is very quiet noise wise.
I have tried different NFB, 100k at 4ohm, 50k at 4 ohm, 50k at 8 ohm.

Low input is clean and sounds pretty good, a lot of attack on the high strings, bass notes sound a little weak(round and a bit mushy).
High input is basically the same, lots of attack on the high strings but a round, soft bass note attack.
The B+ 50+50 is a JJ cap, the screens 50+50 cap F+T cap, last two 40 caps are TUNG brand. All the caps are from my stash and between 5-10 years old(multi meter shows all caps to read correct farads, no dc measured at ground while in circuit).

I am missing the chunk, thunk, low end punch even at high volume. I can do the metal mid EQ scoop and it sounds ok just missing the fullness, punch.
I have tried other "real" Marshalls, JCM800 2203 and JCM 2205 though the same cabs and they have "it" even at low volumes.
The speaker cabs are a custom 2x12 loaded with Jensen Neo 100s and a Blankenship 1x12 with a Mesa Black Shadow 90 watt.

Power Transformer is a Weber W022798, Blue 125v, Red/White 660v, 6.3Vct @ 6A.
Output Transformer is Magnetic Components 40-18025, 50 watt 3.6k(new).

Filtering 50+50, {50, 50}, 40, 40
436v plates
4H fender style choke
433v screens, 15k 5 watt dropper
336v PI, 10k dropper
296v CF, 10k dropper
285v preamp

Preamp tubes are JJ ecc83s.
Power amp has Chinese EL34's (also tried with JJ KT77's, same basic lack of punch)
Biased at 40mv through 1ohm resistors. Bias is within 5mv on the tubes and low bias follows the tubes.

I actually seem to like the clean channel with pedals better(OCD, Bogner Blue, A.R.C., BlackBox OD2, Barber Small Fry-Gain Changer etc...). Still missing something on the bottom end, definition/woody chunk. Should I try to come up with better adjectives? Haunting mi...

Guitars used: Gibson 61 reissue, Homemade strat with SSH(H is a lil' 59).

Thanks for any insight.

[pdf64]

15k HT dropper for the screen grids?

[sonofmickel]

Should I replace the 15k dropper with the original value 10k?

It appears that "most" people prefer the lower B+ 2204, based on internet consensus.
The secondaries on the Weber PT only goes as low as 436v with diode rectification, I used the 15k dropper to get the power string down to the low voltage 2204 specs.

I do, however, have higher voltage secondaries that would get around 470v or so at the plates, the voltage string should be close to the MHuss schematic of 468, 457, 390, 348, 334.
https://mhuss.com/MyJCM/vchart.html

I tried the high B+ first and found that the gain characteristic was very strident, ice pick comes to mind.

How long does it usually take for a new amplifier to "burn in"?

[sonofmickel]

Maybe I should try some 6l6 or 6550 tubes?

I'll see what I have in the "stash".

[Steveo]

Hello, i have a similar build, but my plates are at around 484dcv and im using 6ca7’s. I have all 50uf-50uf filter caps, I’m using 10k droppers. I don't know what your using for your preamp plate, cathode resistors or cathode bypass caps. If normal 2204 values, in my novice thinking it might be your lower voltage levels attributing to less chunk. Hopefully a more experienced user chimes in.
I do occasionally use a variac, at 90vac the amp is less chunky but more jangly . At normal 120vac its definitely tighter and chunkier less compressed. A schematic of your amp can help identifying.

[Roe]

try to replace the jj 50+50 cap

[sonofmickel]

Thanks fellow electrocutioners.
I was thinking that the lower voltages would make the bottom end softer too. Then I thought about Dumbles and their low plate voltages/high plate resistors, that couldn't be it.
Then I though of Dumbles and the high filtering for B+, 100uf seemed to be plenty high(I still may try 100+100 as I do have a JJ dual 100 cap can).
So I made a pizza.

I then pulled some preamp tubes to swap out of a different/working amp.
VIOLA!
The 2nd preamp tube seemed to have been the culprit(cathode follower).
I swapped out another JJ ECC83s and a lot of low end definition came back to the amp.

What to do with under-performing preamp tubes?

I am finally happy with a JCM800 build.
It is still a rats nest compared to many on this site(you guys must have surgeons hands), but it is proof of concept.
I built this amp on the cheapest yellow tag-board with the bend up tabs, I like M.Fowlers builds with eyelet boards and may try sourcing a JCM800 board with enough room for a few board mounted filter caps. Or just make one myself.