Bassman Plexi-ish micro build

Len058

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I'm building Rob's EF80 LPT Bassman micro with a few tweaks. I want a switchable cathode on the V1b, so I can switch to a more Marshall style. I also like the 4 input principle. With an AB-switch pedal I can go between treble and normal, or I can use a jumper between the channels, like my heroes. The switch will decouple the shared 820ohm and 250uf to V1b and hook a 2,2K and a .68uF to the V1b cathode. That should get it more into JTM superlead territory.

I've already made the turretboard and placed almost all components on it. But recently I've been messing with my Champ micro and found that I botched the layout and other things. There was a strange oscillation because the wiring is sloppy and the wires have to be too long. I fixed some of the issues but it's not the way I'd like.

Moving on I decided to really pay attention to detail with this build. I already made a kit amp, changed the EL84's to EF80, and that went fine. So I know I can do it.

I'm using a JTM chassis and before I drill the holes I want to be sure that there are no obvious mistakes in the layout that can cause extra hum, noise or oscillation.


The first Idea: The OT next to the choke, close to the filter caps and choke so there's a short run of wires between the B+ node and the tubes.

IMG_20220505_222031_7.jpg



Option 2: The OT is in between the output tubes and the pre-amp tubes. The OT is further away from the choke and the PT. Maybe it's easier to manage the transformer spaghetti and there will be less interference of the AC stuff.

IMG_20220505_222142_2.jpg


Idea's comments? Any help would be appreciated!

Lennert
 

Tom Kamphuys

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You could use the headphone trick to find out whether the OT picks up hum from the power transformer. When I did that with my guitar amp (jcm800), I found it needed some space (or a weird orientation). My hifi amp uses a toroid and in that case there was no hum.
 

Len058

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You could use the headphone trick to find out whether the OT picks up hum from the power transformer. When I did that with my guitar amp (jcm800), I found it needed some space (or a weird orientation). My hifi amp uses a toroid and in that case there was no hum.
How does that trick work? Just curious because the Champ and this amp have a toroid PT. The choke is not though.
The problem with the champ is in the wiring, not the transformers.
 

dan40

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As Tom mentioned, space is the key to prevent transformer coupling. I have found that once I get at least 4" between the PT and OT, the level of noise is greatly reduced. Being that your PT is a toroid, you may not have much of an issue. I have never had much problem with the OT being within an 1" or 2" of the choke though.

For the headphone trick, you need to wire a pair of headphones to the secondary wires of the OT. You can cut the end off of the headphone cable and connect the wires to the OT wires or, you could wire the correct jack to the OT wires and plug your headphones in to it.

You then need to wire up the primary side of your PT so that you can energize it with 120v. Your secondary wires can be insulated with heat shrink to keep them safe. Once the PT primary is wired and you have 120vac going through it, you will be able to hear "hum" through the headphones as you move the OT around on the chassis. The closer to the PT you get, the more the hum increases. You may also find that rotating the OT can cause the hum to change. In one build it was necessary to mount the OT at a 45 degree angle to lessen the hum. Being that your PT is a toroid, the hum may not be that noticeable through the headphones.
 

Paul-T

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The headphone trick does work really well. I simply used two crocodile clips on the OT secondaries (they have little screws on the top to which you attach the wires) and clipped them onto the earbud headphones jack. In practical terms you normally only have a couple of possible orientations and headphones show up the difference really easily and quickly.

Looking forward to seeing this progress as I'm planning to convert my EF80 Deluxe to a JTM.
 

Len058

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Got the headphone trick figured out. As I said, I'm more concerned about the wiring. I've read about someone who had interference between the OT and the phase inverter components. So I need to consider that.

In my champ I have trouble with the preamp cathode wire running to close to the wire between the plate of the output tube and the OT. If I move the OT wiring, it gets too close to the heater wires.

I'll do the headphone test, shooting for option one. The larger distance between the pre-amp section and the OT feels safer than putting the OT next to the phase inverter. The placement of the EF80's will depend on the outcome of that test.
 

jtcnj

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you could also move the 2 pre-amp sockets farther left away from the power PI / power section.
 

Len058

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you could also move the 2 pre-amp sockets farther left away from the power PI / power section.
Yes, but that would mean longer wires from the board to the tubes or if I move the board the wires to the pots will have to be longer.
Maybe longer wires to the pots would be better than a short distance between the power section and the pre. I have a feeling it would be better to also have the phase inverter further from the OT and powertubes.
 

Nicko_Lps

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Video in post 19
Im not sure if this is the best way..
I remember a few months ago seeing a youtube repairsman that makes amplifiers as well considering this setup wrong.

He even made a video showing less noise when bobbin positions something that looks a bit like this:
chassis-layout.jpg

But please, dont ask thy Name... Its been long that i deleted youtube, all i recall is that he was focusing on Vox and plexi style amps
 

Len058

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But please, dont ask thy Name... Its been long that i deleted youtube, all i recall is that he was focusing on Vox and plexi style amps
This guy? He's got another video, same build but a bit more about the design of the rest of the amp.
 

Paul-T

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With the switchable cathode... do you mean a lead-channel style mod?

'The switch will decouple the shared 820ohm and 250uf to V1b and hook a 2,2K and a .68uF to the V1b cathode."

I guess you mean the original 25uF , which you're switching for a Marshall-style .68uF, a bit like on Rob's 5E3 lead channel mod?
 

Len058

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With the switchable cathode... do you mean a lead-channel style mod?

'The switch will decouple the shared 820ohm and 250uf to V1b and hook a 2,2K and a .68uF to the V1b cathode."

I guess you mean the original 25uF , which you're switching for a Marshall-style .68uF, a bit like on Rob's 5E3 lead channel mod?
Yes, only the bassman micro has 250uf instead of 25 in the deluxe. Thanks I'll start reading about the mod at Rob's site. I know what I want and added the components to the board but haven't desided how to switch between the stock and mod.

I based the 4 way input on the deluxe micro and the Marshall mods on a video by Kley de Jong. I might add another cap further down the circuit. And maybe switchable a bright cap somewhere. I reserved space for the switches in the front panel.


Rob increases the V1a cathoder resister but the marshall 1987 still uses the 820 for V1a
Marshall_1987_jmp_lead_50watt_el34.gif
 
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Nicko_Lps

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This guy? He's got another video, same build but a bit more about the design of the rest of the amp.
YEP! I recall his voice. This was indeed the video, yet in your case it does not matter because you have a toroidal PT.
My modded amp as a toroidal PT that i ordered from the UK, its placed on the other side of where the OT stands and the choke is oriented wrong. yet in all of its 50 watts maxed can barely listen to it hissing without a guitar plugged on the amp on clean channel.
 

Bitsleftover

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I recently built a Plexi Micro. I had a switch to split the cathodes like you are planning. It works well.
Another thing to think about would be another switch to change between the JTM45 tonestack and the later JMP tonestack values. That gives it a considerable kick in the pants and compliments the split cathode switch perfectly.
It really highlights the Marshall tone evolution.
I added the V2a .68 cathode bypass cap and I experimented with a .022 and .0022 coupling cap on the bright channel.
.0022 with the .68 was a bit too much treble To me. I must check next time I pull my chassis. But I think I settled on .022 and .68
I also added a tube/diode rectifier switch….Waste of time!
I love mine. It’s really versatile. Being able to jumper inputs made the 4 hole 2 channel approach really worth the extra effort.
Good luck with Your build.
 

dan40

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Yes, but that would mean longer wires from the board to the tubes or if I move the board the wires to the pots will have to be longer.
Maybe longer wires to the pots would be better than a short distance between the power section and the pre.

Aiming to keep wires short is a worthy endeavor but keep in mind that many of the larger amps are built in a very large chassis with much longer wire runs than yours will be. The important thing is to keep the layout and wiring very close to the original amp's design. When you start moving components around and changing the wiring layout from the factory version, you run the risk of added noise. Sticking with the original layout will give you a much better chance of building a quiet, great sounding amp. Find as many pics as you can online of original Bassman amps and make a note of how Fender ran each wire to the individual sockets and potentiometers.
 

Paul-T

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I recently built a Plexi Micro. I had a switch to split the cathodes like you are planning. It works well.
Another thing to think about would be another switch to change between the JTM45 tonestack and the later JMP tonestack values. That gives it a considerable kick in the pants and compliments the split cathode switch perfectly.
It really highlights the Marshall tone evolution.
I added the V2a .68 cathode bypass cap and I experimented with a .022 and .0022 coupling cap on the bright channel.
.0022 with the .68 was a bit too much treble To me. I must check next time I pull my chassis. But I think I settled on .022 and .68
I also added a tube/diode rectifier switch….Waste of time!
I love mine. It’s really versatile. Being able to jumper inputs made the 4 hole 2 channel approach really worth the extra effort.
Good luck with Your build.
Is there a build thread, @Bitsleftover? I remember your PT thread but don't recall seeing the results!

I'm planning to convert my EF80 5E3 micro into a JTM...
 
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Len058

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Find as many pics as you can online of original Bassman amps and make a note of how Fender ran each wire to the individual sockets and potentiometers.

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.

I'm using a JTM chassis. It will be a Marshall style head, with the inputs and controls oriented like a fender. So everything is slightly different. Original Bassmans are combo's with the chassis against the back of the cabinet. Later Fender amps moved the chassis to the top, which forced it upside-down, compared to a Marshall head. Those Fender amps have the filter caps in a dog house.
Marshall just flipped the chassis, so the inputs are on the right. Mine will be on the left. They used big canned filter caps, mine are on the board.
Both the Bassman and the JTM/JMP had big transformers, mine are tiny compared to those. I have more room to play with so I can probably improve the layout. The fact I use a Toroid also makes a big difference.

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.
 

Len058

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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.

 




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