JCM800 Micro EF80 Build

pelayostyle

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Decided to create a thread of my JCM800 EF80 build for posterity. I used Rob's V3.8a schematic/layout. Parts used in this build that I had laying around were the Antek AS-05T200 and Triad C8X choke. Chassis was a custom order from Seaside Chassis Design in 14" x 7" x 2" aluminum and I highly recommend Terry's work. I opted for the nicer MusicalPowerSupplies OT5PP output transformer.

The build went together without any major problems. I initially forgot to ground the inputs to chassis because I used Cliff jacks, which are plastic, but that was the only issue. The amp plays into a Warehouse 12" Green Beret. The amp sounds absolutely amazing! I've been playing it every day since I built it and I can't get enough of it. It sounds just like it's bigger brothers.

Thanks again to everyone on this forum who's helped me out when I run into problems. And now for the pics....


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Paul-T

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That looks amazing. Well done

Can someone school me on Marshall master volumes? On the JCM800, pre amp is initial gain, and the master is pre phase inverter, correct?

Whereas on the Bassman/JTM, you get gain (initial gain) pre amp (pre phase inverter) and PI volume (post phase inverter).

If you want only 2 volumes, is the JCM800 arrangement the best one?
 

Len058

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That looks amazing. Well done

Can someone school me on Marshall master volumes? On the JCM800, pre amp is initial gain, and the master is pre phase inverter, correct?

Whereas on the Bassman/JTM, you get gain (initial gain) pre amp (pre phase inverter) and PI volume (post phase inverter).

If you want only 2 volumes, is the JCM800 arrangement the best one?
The Bassman/JTM had no master volume. Rob's design added some master volume options. You can easily change from pre to post PI master volume.

The big difference between the JCM and the bassman/jtm is cold clipping in the pre-amp. So if you had to choose, I wouldn't be bothered by the knobs, but what kind of overdrive you like. Modern high gain> JCM. Classic rock, blues 60's 70's >JTM/Bassman. There's the Marshall Superlead inbetween and with a little modding, you can make the Bassman circuit into that.

 

owlexifry

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Did you do a voltage chart? My voltage was always low, so considering going back in to fix things up (mine was a bassman, but voltages should be about the same). I ask because there are very few EF80 builds with voltage charts included.
agree.
hardly anyone ever seems to list/discuss the voltages people are achieving with EF80 power amp builds.

perhaps for the blackface / tweed builds it's less of an issue, where B+ for preamp stages doesn't need to exceed 300V to match 'original specs' (i think?..)

but for the bassman, JCM800 or SLO micro EF80, I'm still yet to figure out how anyone is going to achieve 'original spec' plate/B+ voltages when EF80s are limited to 290-300V max.

eg.
jcm800/bassman
V3 phase inverter B+ supply, per original is ~380-390V.
1653958305612.png
5F6A schematic
2204/JCM800 schematic

And then for the JCM800, the entire preamp B+ supply is supposed to be 350-330V.
1653958606481.png

I understand most people probably don't give a rats if it's off spec, but it'd be great if the issue could at least be acknowledged and discussed.

With what limited knowledge I have, I've previously attempted to suggest solutions to get around this and somehow achieve >300V B+ supply to the preamp stages whilst supplying <300V B+ to EF80 power amp, but have so far failed to gather any interest.
 

pelayostyle

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Took some measurements this morning and this is what I found....

Filaments 6.5V
B+ 272.5V
B+2 270V
B+3 262.5V
B+4 250.8V

V1 Pin1 176.2V Pin6 213.8V
V2 Pin1 134.4V Pin6 250V
V3 Pin1 168.3V Pin6 158.3V
V4 Pin7 268.8V
V5 Pin7 268.2V

@owlexifry I can be your guinea pig and try out your suggestions to get the voltages up on V1 - V3. I could also record some audio before and after to see if it makes a significant difference.
 

2L man

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A higher voltage output power transformer is needed to increase pre amp voltage! When power amp use very low current its voltage is very easy to made adjustable lower using MosFet circuit. This make output power adjustable as well. When Fet does not heat much it can be screwed to the chassis without cooling fin and instalation take very little room. One MosFet should be max $5 but for low power amp perhaps only $1.

Another way to increase pre amp voltage is to install additional small transformer, bridge-rectify and filter its output and connect it series "on top of" original power supply to feed pre amp higher voltage.

So called "circuit board transformers" come at least 2 x 24V and should cost max $10. 48VAC turns to about 67VDC, possibly higher when output current is low. If its power rating is for example 2VA its max current for 67VDC is 30mA. I have used one as a Bias supply and recall its output was 75VDC but bias current is much lower than pre amp current.
 

owlexifry

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A higher voltage output power transformer is needed to increase pre amp voltage! When power amp use very low current its voltage is very easy to made adjustable lower using MosFet circuit. This make output power adjustable as well. When Fet does not heat much it can be screwed to the chassis without cooling fin and instalation take very little room. One MosFet should be max $5 but for low power amp perhaps only $1.

Another way to increase pre amp voltage is to install additional small transformer, bridge-rectify and filter its output and connect it series "on top of" original power supply to feed pre amp higher voltage.

So called "circuit board transformers" come at least 2 x 24V and should cost max $10. 48VAC turns to about 67VDC, possibly higher when output current is low. If its power rating is for example 2VA its max current for 67VDC is 30mA. I have used one as a Bias supply and recall its output was 75VDC but bias current is much lower than pre amp current.
many thanks for your input!

so we've got:
1) mosfet circuit to adjust / regulate voltage supply to power section?

2) separate PT - but im not sure i understand this concept you've described, "connect in series "on top of" original power supply"

3) circuit board PT?

im thinking 2x PTs might be the simplest way to go.
 

owlexifry

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Took some measurements this morning and this is what I found....

Filaments 6.5V
B+ 272.5V
B+2 270V
B+3 262.5V
B+4 250.8V

V1 Pin1 176.2V Pin6 213.8V
V2 Pin1 134.4V Pin6 250V
V3 Pin1 168.3V Pin6 158.3V
V4 Pin7 268.8V
V5 Pin7 268.2V

@owlexifry I can be your guinea pig and try out your suggestions to get the voltages up on V1 - V3. I could also record some audio before and after to see if it makes a significant difference.
great idea! if you can, try and keep the variables all the same with the "before and after" (recording device (mic / phone) position, controls etc.)
 

andrewRneumann

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this was another idea i had.... yet to receive feedback/comment:

split / parallel B+ - would this work?
View attachment 989293

This *can* work if the resistor feeding “B+2a” is small. But since you are dropping more than 80V, it will probably be a couple kilo-ohms. This makes it suitable for Class A but probably way too saggy for Class AB.
 

owlexifry

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This *can* work if the resistor feeding “B+2a” is small. But since you are dropping more than 80V, it will probably be a couple kilo-ohms. This makes it suitable for Class A but probably way too saggy for Class AB.
many thanks for your review!

what if the filter cap for B+2a was massive? like 100 or 220uF?
would this address the sag in any way?
 

andrewRneumann

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many thanks for your review!

what if the filter cap for B+2a was massive? like 100 or 220uF?
would this address the sag in any way?

The capacitance controls the rate of sag and the rate of sag recovery, but not the depth of sag. A larger capacitance will sag at a slower rate and take longer to recover from sag.
 

owlexifry

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The AS-05T240 has a 240V secondary with a 220V tap. Maybe that coupled with two rectifier bridges?
not a bad suggestion, could use the 220VAC winding for ~300V B+ and the 240VAC winding for ~340V B+

ive just been looking at this one:

-190V winding; ~270V B+ -> a little bit low, but could work for the EF80 pair
-275V winding; ~385V B+ -> pretty much spot on for the preamp

geez, this PT might just be the one!

The capacitance controls the rate of sag and the rate of sag recovery, but not the depth of sag. A larger capacitance will sag at a slower rate and take longer to recover from sag.
ah yep. that makes sense. thanks again for sharing your knowledge :)
 

owlexifry

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I would worry under load that the PT cannot put out that much for both secondaries. I could be wrong though.
apparently it can?
in the description for this product, it's claiming that these two windings have a full 160ma each.
1654053044573.png

and even if it's actually just 160ma over both the windings, surely 160ma is more than adequate?
 
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