Fender Super 60 to Hoffman Plexi 50 Conversion

[edthill]

Hey all,

Just scored a Fender Super 60 for cheap, fully functioning too. It sounds like doo doo, so I did what any amp crazed voltage monkey would do and tore everything out minus the PT and the OT.
I'm happy to keep the 6L6's as a part of the circuit as I'm not entirely sure the PT could support EL34's also curious where I should steal my bias from? will the brown taps on the PT give me what I need? or should I tap the Red wires feeding the B+ circuit and adjust from there.
Also, if I keep the 6L6's I would use 470 Ohm instead of 1K5watt for the screens?
Attached is a schematic of this monstrosity.
Also open to suggestions and mods or other circuits. The B+ off the FWR is about 480V. Going to make for a loud amp. OT has 2/4/8 Ohm taps too. Nice beefy looking Iron. Curious to hear how she sounds with a nice handwired circuit board in her with top shelf components.
Thanks for looking,

Ed

[sluckey]

Use the brown wires for the bias.

[SolidStateCircuitReject]

yes, use 480 instead of 1k on screens. Though 1k will work fine as well.

I just went through a similar thing with bias tap vs secondary HV, stick with the bias tap, it's probably similar to what I just did where the brown wires will yield around 100 VAC. If that is the case, you should have no problem running a plexi style bias circuit, at least I think... I havent seen bias taps on the power tx until the JCM800 when it historically comes to marshall oem tx'ers... but no doubt Sluckey will know a hell of a lot more than I - he just helped me sort a similar issue yesterday.

[DummyLoad]

yes, use 480 instead of 1k on screens. Though 1k will work fine as well.

not familiar with that value - perhaps you mean 470?

--pete

[SolidStateCircuitReject]

yes, use 480 instead of 1k on screens. Though 1k will work fine as well.

not familiar with that value - perhaps you mean 470?

--pete

Indeed I meant 470... it was 5:30 am when I wrote that  ;-)

[edthill]

Thanks for the input, All.
I will see what brown can do for me.
Ordering parts today.
Will hopefully start the build this weekend.
I'd love to a 480Ohm resistor. I don't know about you guys but I'm a sucker for blondes and even numbers.

[Tony Bones]

Grab any NOS 470 CC resistor and measure its actual value. I'll bet lunch money it'll be well over 480 ohms. That doesn't stop me from using 'em. 

[edthill]

Just got my Hoffman Plexi 50 board and other parts today.
I have a few questions for the Guru's out there.
I'm not used to building an amp with these weird fender switches, nor am I used to building an amp with a standby switch, as weird as that sounds. All of my latest builds have had no standby.
I'm also trying to figure out the wiring of my lamp and fuses.

In the picture attached, Both hot and neutral wires go to the top of the switch. The white/neutral wire at the bottom of the switch goes back to the power transformer. Coming off of the transformer I have a black wire free to go somewhere I also have a brown wire at the bottom of the power switch in need of a home. Which of these wires goes to the lamp, both I'm assuming? Which goes to the fuse and in which order. Lamp then fuse?

Then my next question would be, I have one of red high voltage wire's coming from the PT going to the standby switch, there is also a red wire coming off of the top of the switch, do these both go to their respective diodes? Also the PT for the Super 60 is Full-wave bridge, correct?
I'm just a tad confused.
Thanks in advance for any and all help.

[edthill]

Grab any NOS 470 CC resistor and measure its actual value. I'll bet lunch money it'll be well over 480 ohms. That doesn't stop me from using 'em. 

I wouldn't take that bet. Those old CC resistors drift like a rear-wheel drive car on ice. I pulled a 470 Ohm resistor out of ab AB165 Bassman that had gone up in Value to 610 Ohms. The do sound good though.

[sluckey]

You really need to study the Super 60 schematic. It will show you exactly how each PT wire is used. Then study the Hoffman Plexi schematic. This is not gonna be plug 'n play. You will have to make some adjustments, especially with the rectifier circuit.

[Tony Bones]

Have you seen the schematic? Ugh!



My interpretation is that you are correct about the black and white wires from the power cord going to the top terminals on the switch. The terminal below the black wire should go to interconnect CP101 that is connected to a fuse (XF1).

On the schematic, it looks like the terminal below the white wire gets grounded. That's nuts! (Maybe my interpretation is wrong?) I would connect the terminal below the white wire directly to the white wire on the power transformer, either by crimping a connector on the white wire (if there isn't one already) or by splicing into a wire that has a connector. The black wire from the transformer should plug into a junction of some sort (CP102) that has that also has a fan connected to CP104. I don't know what any of this actually looks like. You'll just need to poke around and figure it out.

IF you have an international transformer then you're on your own.

http://schems.com/bmampscom/fender/super_60_rack.pdf

[edthill]

Have you seen the schematic? Ugh!



My interpretation is that you are correct about the black and white wires from the power cord going to the top terminals on the switch. The terminal below the black wire should go to interconnect CP101 that is connected to a fuse (XF1).

On the schematic, it looks like the terminal below the white wire gets grounded. That's nuts! (Maybe my interpretation is wrong?) I would connect the terminal below the white wire directly to the white wire on the power transformer, either by crimping a connector on the white wire (if there isn't one already) or by splicing into a wire that has a connector. The black wire from the transformer should plug into a junction of some sort (CP102) that has that also has a fan connected to CP104. I don't know what any of this actually looks like. You'll just need to poke around and figure it out.

IF you have an international transformer then you're on your own.

http://schems.com/bmampscom/fender/super_60_rack.pdf

Tony,

Thank you for the reply.

This thing is domestic.

It has what I see as a typical Power transformer set up. two red high voltage taps. A black 120V tap. A White lead(neutral perhaps?), which goes to the power switch. Two Green filament taps. and two brown leads for Bias. Although I'm not sure why it has two brown wires for bias, and do I need to use both?

So I ripped every thing out of it minus the transformers.
I'm just trying to use the existing PT, if I can, to save some money, and also these ugly 80's Fender switches.
The schematic is a nightmare. I think I pretty much have it figured out. Although, I don't know why there are two fuses coming off the power transformer and the switch. Why would Fender do that? It's on the board so I don't need to use it.
I'm just trying to get a proper sequence for switch, standby, mains fuse, HT fuse, and the lamp which is a marshall style that doesn't run off the heaters. The rectifier appears to be full wave bridge so I know I'm going to have to ground one end of the HT side.
The heaters don't have a center tap so I will have to create an artificial one with the 100 Ohm resistors to ground.
Anyway, thoughts, or any input is very welcome.
Thank you,

Ed

[PRR]

> Maybe my interpretation is wrong?

Line cord. Green to chassis ASAP. Double-Pole power switch for added safety. Fuse(s). PT primary.

> why there are two fuses

There is a fuse for the User to blow and wrap in tin-foil. Experience says there should be a back-up fuse inside, not easily tinfoiled to a 99-Amp rating. This has become common on factory designs. You are free to use one fuse, just don't tinfoil it.

[sluckey]

Here's how you need to wire your PT to the Hoffman board. The pic also shows the simple mods you must do.

[edthill]

> Maybe my interpretation is wrong?

Line cord. Green to chassis ASAP. Double-Pole power switch for added safety. Fuse(s). PT primary.

> why there are two fuses

There is a fuse for the User to blow and wrap in tin-foil. Experience says there should be a back-up fuse inside, not easily tinfoiled to a 99-Amp rating. This has become common on factory designs. You are free to use one fuse, just don't tinfoil it.

Thanks PRR,

You're funny. Yeah I was really scratching my head because it was on the circuit board but not soldered to where the connections should be.
Don't worry, there will be no tin foil in its new incarnation. I'm putting nice things inside this time.

Am I soldering both brown wires to the bias range resistor? or just one? I am throwing the hoffman Plexi 50 board in this beast. I am assuming I can change the 220K/3watt bias range resistor to something a lot smaller in value? 470 Ohm perhaps?

Thank you for the tips,

Green wire of power cord is attached to the chassis,

Thanks again,

Ed

[edthill]

Here's how you need to wire your PT to the Hoffman board. The pic also shows the simple mods you must do.

Thank you Sluckey,

Very Much appreciated,

Ed

[super&plexi]

Hey greetings...thats the amp (mine is combo) that i built my TBM in. As soon as its plugged in to wall the switch lights. On or off its always on. Hopefully your results will be better.

[sluckey]

Hey greetings...thats the amp (mine is combo) that i built my TBM in. As soon as its plugged in to wall the switch lights. On or off its always on. Hopefully your results will be better.

You have the switch wired backwards. A lighted switch has a line side and a load (switched) side. The light is internally connected to the load side. Just reverse the line and load connections.

[edthill]

Hey greetings...thats the amp (mine is combo) that i built my TBM in. As soon as its plugged in to wall the switch lights. On or off its always on. Hopefully your results will be better.

Thanks,

I'm definitely not used to these style switches, but we are in good hand on this forum.

How does the amp sound?

Ed

[edthill]

Finally completed the assembly portion of this build/conversion. Thanks Sluckey and other for your help. Not getting much sound though. My Voltages are strange however. My B+ rail is High from the A point all the way to the E point on the power supply.  It goes from 472V at point A, right off the standby and only down to 438V on the first tube, V1. I have been probing the amp looking for strangeness. I wish I had a schematic with voltages. I noticed there are no voltages on the coupling caps that go to the pots?
Is this normal? should I be seeing something at least on one end of the cap?

What should I be seeing on the pins of my preamp tubes. I'm only getting voltage at the plates and on the heaters. The heater voltages seem fine, Measuring 2.9V AC. Plate voltage on the preamp tubes seems awful high though.

Where should I be looking?
Thanks,

Ed

[sluckey]

Click the link at the bottom of this post.

[edthill]

thanks, it's the logical next step.

[super&plexi]

I think it's still up on YouTube...maybe TBM with 335 style, or under AmpCordAxe... My nom DE Tube

[super&plexi]

And as always... Thx Sluckey...