Micro JCM800 EF80 build with some 3d printed chassis parts

[ievans]

I finished my first scratch build of RobRob's JCM800 Micro EF80 (I put together a Ceriatone 18W EF86 kit with a pre-assembled board a while ago). It's great! Love hitting that high-gain crunch at reasonable volume.

I used a square-front Princeton Reverb chassis from zachmdhunter on eBay, along with an Antek torroidal PT, a Musical Power Supplies OT, and an inexpensive choke from AES.



Here's my layout:


I 3d printed some parts for this build that might be useful for other people doing similar builds. Instead of using RobRob's layout where the first output tube is in the fourth preamp socket, I put a blank there, and designed two octal-to-noval adapters so the output tubes would be in line.


For the third octal socket, I made a blank with a curved opening for the PT primaries.


I also didn't use a cap can (more on that later) and instead designed a blank with another curved opening in the middle for the PT secondary and choke wires.


Finally, I designed a blank for the PT opening with a hole for the Antek transformer mount.


I'm going to post the STL files for all these to Printables so other people can use them.

 

[ievans]

Here's some pics of the 3d printed parts in the amp.

The octal-to-noval adapters. I shock mounted the sockets with some silicon o-rings I use for my espresso machine.


From underneath.


The Antek mount from underneath.


The cap can blank with the "grommet" for the PT secondaries/choke wires:


You can see what I did instead of a cap can, mounting some Nichicon radial electrolytic caps to some tag boards. I saw Lyle at Psionic do this on an old Princeton Reverb. Seemed like a good solution, as I think these are better caps that will last longer, and can be replaced individually.

I used plastic nuts to mount the tag boards so they wouldn't ground on the chassis, and instead of using a power amp grounding point like in the RobRob layout ran a wire to the end of the preamp ground bus so everything grounds at the input jack.

You can also see that I added a 2w Vishay 180k bleeder resistor.

 

[String Tree]

I hope it sounds as nice as it looks!

 

[joulupukki]

That’s some clever building! How’s it sound and how’s the noise floor/hum/etc?

 

[ievans]

No hum at all, there is some hiss as you increase the volume. Interestingly, there's different levels of hiss when setting the different V1B cathode "cold clipper" values in the RobRob 3-way on/off/on switch. The least amount of hiss is the 39k Soldano setting. Not quite sure what to make of that. I followed Rob's recommendations on using metal film resistors for parts of the input to reduce hiss.



I don't have a shielded bottom for the amp right now as I haven't made the enclosure, just haven't decided on whether to make it a head or a combo. Which is why I haven't done the face plates yet. I wouldn't think that the hiss would be affected by that, though.

I still need to get some parts for the pilot light, which will be an LED, possibly with some fiber optic cable to diffuse it? We'll see.

The sound of the amp is great. Really impressive. I like the drive channel on my 18W, which is a 1974 circuit, but this is just on another level. You can dime the amp and get just boatloads of crunch, and when I use my JohnH attenuator you can talk to a baby over it.

 

[chas.wahl]

Here's some pics of the 3d printed parts in the amp.

The octal-to-noval adapters. I shock mounted the sockets with some silicon o-rings I use for my espresso machine.

Great ideas here. What type of plastic is used for the 3-D printing, and do you think it will hold up to the heat environment?

 

[ievans]

Great ideas here. What type of plastic is used for the 3-D printing, and do you think it will hold up to the heat environment?

I used PETG, which is good up to 80-85 Celsius. That's the same temp as PVC wire insulation, which is used in amps without issue, so I figure it's adequate. I probably wouldn't use PLA, which is weaker and less heat resistant. ABS has a higher heat rating, but is harder to print I guess (I haven't tried).

I'm not any expert in 3d printing, but it was pretty cool to just take some caliper measurements, model up a part in Tinkercad (free) and print out the part in 30-60 minutes on my relatively inexpensive 3d printer.

 

[ievans]

Here's the voltages on the amp, fwiw:

V1:

1	186.5
2	0
3	4.56
4	6.4 a/c
5	6.4 a/c
6	223.5
7	0
8	2.68
9	-

V2:

1	210.2
2	0
3	4.56
4	6.4 a/c
5	6.4 a/c
6	260.3
7	210.3
8	210.5
9	-

V3:

1	179.3
2	18.5
3	29.1
4	6.4 a/c
5	6.4 a/c
6	167.5
7	19.2
8	29.1
9	-

V4:

1	4.98
2	0.6
3	4.98
4	6.4 a/c
5	6.4 a/c
6	0.5
7	279.8
8	277.0
9	4.98

v5:

1	4.98
2	1.2
3	4.98
4	6.4 a/c
5	6.4 a/c
6	0.5
7	279.8
8	276.6
9	4.97

B+: 279.8
B+2: 278.0
B+3: 270.3
B+4: 260.1
(forgot to measure B+5)

wall: 122 a/c

 

[ievans]

Just posted the STL files for the parts on Printables. Adapt/remix away.

 

[chas.wahl]

I think this is a new dimension in DIY amp-dom. Thanks for the post and the photos!