Bassman Plexi-ish micro build

dan40

Friend of Leo's
Joined
Aug 19, 2015
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Richmond Va
I will dive into that. I'm using smaller transformers so I don't have to make the compromises Fender and Marshall made. Although they made great amps, it doesn't mean they're perfect. Leo often cut costs, where he could, and Marshall also did some weird stuff.

When you watch amp-tech's and builders on youtube, you learn that the existing designs can be improved. But if you improve them too much, you'll lose the original quirks that makes the tone.


Yes...all of this is true. I also enjoy taking these old circuits and throwing in some modern twist to come up with something unique. It's important to remember though that the original designers often went through multiple prototypes until they came up with a proven layout that was quiet and dependable in every chassis that they wired. It's perfectly acceptable to throw in some modern changes but just try to stick with some of the tried and true rules of lead dress and you will end up with a great finished product.
 

RicciAntoni_07

TDPRI Member
Joined
Apr 3, 2022
Posts
24
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39
Location
france
So I did the headphone test. Made a video for people who are curious about the hum of a toroid PT. I thought I could do a quick video but you can never do that. It always takes hours to make even the simplest video.


Cool! I'm going to have to go back over this thread! Nice job!
 

Bitsleftover

Tele-Holic
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Oct 17, 2016
Posts
681
Location
The North of England
I'm planning to convert my EF80 5E3 micro into a JTM...
Sorry Paul, I missed this. I did start a build thread, but as I ran into problems, both my build, and it’s thread turned very messy!
Once the problem was fixed I tidied the build up but the thread was abandoned. In promised to post some video clips but never got round to it.
I might get the hangar to myself this weekend so if I do I’ll try and record something.
 

Len058

Tele-Meister
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Jan 31, 2021
Posts
373
Location
Netherlands
So here it is. It's working but needs a cabinet and I need to check the bias stuff. I measured the plate voltages and all seems to be fine.
Bassman-flits.jpg


I [email protected]#5 up the chassis so I had to improvise a face plate. There's a reactive attenuator underneath it.

I'll make a layout of what it was supposed to look like but doesn't because I didn't do enough homework before I made the turretboard.

It has the four inputs, the JTM45 extra bright cap on a switch, the superlead bypass cap on a switch and the Superlead split cathode on a switch. So other than the 12AY vs AX, I can switch between a Bassman, JTM45 and a Superlead. But isn't a 12AY7 just a 12ax7 with the gain turned down?

First impressions: The Superlead is very bright, too bright for playing a Strat at home. If you only noodle at home the Superlead sounds like razorblades. It would be great in a mix though. It doesn't fart out at high gain, but it lacks low end and warmth. So if you only want a nice amp to play at home, without other instruments or backing tracks, the superlead stuf is not necessary.
Jumpered channels are the way to go with a Strat, even for cleanish tones. You can add some brightness without hurting your ears.
I need to learn how to use this amp and I think I need a Les Paul or SG....

So here is a clip. I didn't put in too much effort getting the mic right. No delay, or other effects.


To be continued.....
 

Len058

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Jan 31, 2021
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373
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Netherlands
That looks great. But this is shock brothers… ”Gutshots or it didn’t happen!” :)
You're right. Warning, it ain't pretty. Oh yeah, I also added an 10K extra nfb resistor on a switch. The Superlead has les NFB than the bassman. The JTM has more than the Fender but I don't think I'll like that very much.

IMG_20220522_233655_2.jpg
IMG_20220525_155050_8.jpg
IMG_20220525_155022_9.jpg
 
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Len058

Tele-Meister
Joined
Jan 31, 2021
Posts
373
Location
Netherlands
So the power transformer is a bit weak for this build. I used the TubeTown tordoidal 30VA because it's the closest to the Antek Rob recommends, I can get in Europe. The AnTek isn't for sale over here.
The voltages drop too much. I had 90V on V1. I must say that's with the 100K plate load, per half. Rob uses 47K for the both of them. I removed the dropping resistor and changed the PT to the 230 taps. Now the EF80's are about 295V and V1a with 56K plate load is somewhere around 120V.
I'm chasing my tail a bit so I'll put the OT back at 240V after I checked all voltages in the current state and write them down. I want to know how much effect it has at the V1. I would feel more comfortable with the EF80's at 285 ish, as it was with the PT at the 240 taps.

I couldn't get a clean tone. I suspected the 1K screen resistor, but also thought the low voltage at V1 could be the problem. I finally bypassed the 1K's and got nice clean tones. Yes! Result! So I'll put those on a switch and maybe lower their value.

I don't know what to do about the plate load resistors. I have ordered some 68K and 47K resistors, and I've got some 82K's. At the moment I'm leaning towards 82K's. A bit higher plate voltage and a bit more signal leak. Lowering the values probably gives more headroom but also less output. But that's how I understood the function of the plate resistors.

I promise a proper demonstration, when I'm done.
 




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