JCM 800 Micro 1W build

jtcnj said:
Thanks for the info!

I had not been watching for red plating but think I would have noticed it (I know better but forgot).
The updated layout / schematic uses a 10k for the NFB resistor which is what I used. I wont sweat the presence for now.
I just swapped the 470Ω I used for the 12au7 cathode resistor (what i have on hand instead of the listed 510Ω) to 1k.

This brought my plate voltage up to 391v and 16.6v at the cathode.
That brings me to 16.6/1k x 391 = 6.49w total / 2 = 3.25w per triode which is more in range.

I went with the updated circuit MV; I think it is pre PI.

I only played it briefly and didnt get a chance to dial it in. If anything it did not sound as good, but it could be my day to day perception of what I hear.

@syscokid - my plate voltages were always the same when I put a meter to them; so I figure the wall voltage was what I just measured at 121.9v ac.
Click to expand...
I wasn't even aware that the micro design had been updated, so mine is based on the old schematic. I did make a few changes to get a little more pre-amp dirt, 18k V1a grid stopper instead of 68k, so the guitar signal hits the front of the amp a bit harder, 4k7 V1b cathode resistor instead of 10k, so a bit more grit there. Using a pre PI master volume as well as the type 3 post PI master volume is handy as I can control how hard the PI is overdriven & how hard out 12AU7 output tube is overdriven.
I should check the 12AU7's quiescent current to see if the cathode resistor's value needs increasing for ideal bias.
syscokid said:
That's a lot of NFB! Do you plan to try with less NFB? 22K? 47K? I vote for: Let the one watter run wild... :dood:
Click to expand...
It's not really a lot of NFB (the NFB series resistor value is mis-leading). Using the equation to determine output power (off the OT secondary) of voltage squared divided by impedance = power in watts, we could look at say, a 2 X EL34 amp driven to 50 watts out,,, this requires a voltage of 20VAC at the 8 ohm tap (20 X 20 = 400 ÷ 8 = 50 watts).
Now let's look at the 12AU7 driven to 1 watt output, again using the 8 ohm tap, it takes only 2.83VAC to give a smidge over 1 watt power output (2.83 X 2.83 = 8.0089 ÷ 8 = 1.0011125 watts).
So that's 20 volts on the 8 ohm tap vs 2.83 volts on the 8 ohm tap.
I'd have to hit the text books when I get home to find the equation needed to compare the amount of NFB taken (db) between the two, but we can see from the voltage that, even though the micro has a much smaller value NFB series resistor, it's most probably still taking quite a bit less NFB than a 50 watt Marshall with a 47k/8 ohm tap NFB arrangement. Cheers
 
syscokid said:
I wish I understood this... :eek:
Click to expand...
Me too.
I quoted from robrobinette's site.
I understand basic concepts of tube operation and circuit theory / signal flow but not enough of the nuts and bolts beyond ohms law / power / some of the impedance concepts.

Decades ago I had a lot of the engineering maths and circuit theory down pat, but much was forgotten as I never completed my degree or used that knowledge on the regular to ingrain it.

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Made up the mounting brackets yesterday; the sheet metal nippers worked great to square up the holes for the capture nuts.
They are cut from old filing cabinet dividers.

Not proud of these atrocious mig welds.
the tack welds I did first looked ok, the additional beads, ouch.
I realized afterwards my stick out was too close.

I was a bit hurried and didnt make any practice beads first; my machine was setup from the last beads I was running on pieces cut from this same material, a month or so ago.

I could not see the work well either, ughh.
But, the penetration is enough these are not coming off.

My ground clamp was right next to this; I was a little leary of sending current through the chassis if it would feed back into the circuit components.
But I figured the least resistant path would be to the ground clamp.
No ill effects.

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syscokid said:
That's a lot of NFB! Do you plan to try with less NFB? 22K? 47K? I vote for: Let the one watter run wild... :dood:
Click to expand...
NFB is a waste of feedback.
Don't even bother to connect NFB and save wire.
I'm all in favor of positive feedback.
When I sell something, I always look forward to positive feedback. And when I wire an amp for mischief, I always include positive feedback.

Tone controls are also a waste of space.
The second thing I do is bypass the controls. Why bother controlling? Controlling spoils the sound.
In fact the more parts you take out, the better it sounds.
You've been missing the power and the glory for ever and ever, amen..

Then the things that bother me the most:
Hiss caused by resistors in the signal path.
Buzzing hum caused by filaments magnetic field entering the signal path.
Unwanted oscillation of the high frequency kind. You could say this is a layout issue.

And I get to work on those problems each one having it's own particular solution.
At the end most of the UN-wanted noises eliminated; I can finally stand to play the thing...
 
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Amp Mad Scientist said:
Then the things that bother me the most:
Click to expand...
.................................................................................we are gonna need another forum to list all the things that bother me the most ..............................................

loves me some Marshalls
 
So, I am Finally building the cabs for this and the 6G2.
the 6G2 cab is on the bottom in the middle pic. Hard to see but it is actually about 1-1/2" taller to allow a little more room above the rectifier and 6v6gt power tubes / heat concerns.
The JCM800 Micro is all 12A_7 tubes.

I'm probably gong to add the Lo input (I omitted) to the JCM800 and will have to make a new faceplate.
The Hi input triode is cascaded with the Lo input triode; the Lo input shunts out the Hi input triode via the Lo input jack Switch terminal.
Its not like the early Fender Hi and Lo where the both go to the same triode, and Lo just dumps more signal to ground.

Both cabs are glued up; next I'll be sanding away (and filling) some of the sins of my shoddy box joint skills.

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I have an old Craftsman small router table that was real wobbly the one time I tried it in,,,,,, maybe 1991.
I do have 3/8" and 1/2" roundover bits and plan to round the outer edges freehand.

I will add filler to get a fairly smooth run for the guide bearing to run against, then sand out the wah wahs before covering.
 
2 of 2 - for Brown Princeton 6G2.
Not great, but not bad; good enough for my DIY use.
In a hurry, I did not realize I had covered the wrong side of the lower vented front panel; the other side is painted flat black.
It doesnt show much and the reddish-brown of the fir plywood isnt bad either.

Most of the pine board stock for these came from the demo of my old 1972 kitchen cabinets.

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Here are both (almost) finished; warts included.
I need to label the rear jacks. not making a faceplate.

I realized quickly playing the 6G2 the controls are labeled incorrectly.
I started the prototype drawings for these in 2019.
Coming back to it I didnt verify them.
MASTER -VOLUME -TONE should be VOLUME - TONE -MASTER!

I will paint the front of the 6G2 face plate, apply the corrected decal, and clear coat.
While it is off I will paint the top edge of the chassis, as you can see some red in the photo.
Eventually, I may just make another faceplate, I have enough acrylic sheet stock.

The JCM is correct.

I had some trouble with the waterslide decals, mainly the edge folding over on itself and things got worse from there trying to fix it, and couldnt get the wrinkles to smooth out.

But overall, I am happy with my builds, warts and all.

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