"Vintage" Hoffman Plexi-100 board - mod to 1987/2204?

[PharmRock]

Recently on Reverb I came across a listing for a never-used Hoffman Plexi-100 kit.  In my GAS-induced state I made an offer and am now the proud owner.  When opening up the instructions, a Hoffman business card falls out, and its from way back when he was in Florida....so I am guessing this thing is a good 20 years old or so.  It looks like a freakin' work of art.  It also came with the control pots with a pre-wired ground buss.

Anyways, I'm thinking that a 50-watter is more in line with what I would like to build, and am contemplating either a 1987, a Plexi-50 with hot-switch, or a 2204.  I've looked over the Plexi-100 and Plexi-50 pages on the site multiple times, and I know the power supply of the Plexi-100 is rather unique, but the on-board components of the plexi-100 and the plexi-50 boards look pretty much the same, except the 100 watt board has the 2 extra screen resistors and 1-ohm bias resistors for the 2 extra EL34s.  Also, the board I have has 2 diodes for rectification, whereas I've seen the plexi-50 layouts with either 2 diode rectification or a bridge rectifier.  Is this difference explained by whether the PT has a center tap and/or the secondary AC voltage of the PT?

Thanks and I appreciate the feedback.  This will be a good project, and I'll try to post progress on it in the forum. 

[EL34]

Good find!

Pre 2000 board for sure

On the library page somewhere I have info on Marshall power supplies
The power transformer dictates how the board is built

Go here and find that info
https://el34world.com/schematics.htm

[PharmRock]

Hi Doug,
Yeah I am pretty happy to be the new owner of the board.  At one point I had one of your smoked-plexiglass 50 watt amps that I bought off craigslist here in Birmingham.  Unfortunately I sold that about 10 years ago.

I scoured over the Layout and Schematic for the Plexi-50.  I notice that the layout for the "standard" plexi-50 has a bridge rectifier, and the schematic shows a center-tapped PT.  The power supply layout specifies the MPT50 power transformer.  However, the layout for the Plexi-50 with hot switch gain mod shows a dual-diode rectifier.  I guess that's what is throwing me off. 
I've attached screenshots of the PDFs below.

[sluckey]

I notice that the layout for the "standard" plexi-50 has a bridge rectifier

That's not a bridge. It's simply two diodes in series to replace your single diode. This is to increase the reverse voltage rating (PIV) of the single diode. You really need to replace the single diodes on your antique board with series diodes. A single diode may be very close to having the PIV rating exceeded. You will still need the PT with a center tap.

[PRR]

> I had one of your smoked-plexiglass 50 watt amps ....  Unfortunately I sold that

He still has a chassis. Look in store catalog Yardsale.

[PharmRock]

Awesome...that makes sense...thanks!   Obviously I need to brush up on the correct terminology... I was thinking "4 diodes = bridge", without looking more closely at the arrangement.

I'll install a couple of extra turrets and get the other two diodes mounted in series...along with getting the remainder of the parts ordered.

[PharmRock]

> He still has a chassis. Look in store catalog Yardsale.

That's pretty tempting.  I've got an empty sourmash head cab and blank chassis I was going to use for this build.  May have to rethink that.

[EL34]

> I had one of your smoked-plexiglass 50 watt amps ....  Unfortunately I sold that
He still has a chassis. Look in store catalog Yardsale.

Yeah, just one of the original chassis left
The boards for those chassis were way different than what Pharmrock has
They had all the tubes and parts on one board
The filter caps were under the board.

This section of the library has some pics of my old stuff
https://el34world.com/schematics.htm#Hoffman%20Amplifiers%20stuff

This page shows the original boards for that chassis
https://el34world.com/Hoffman/Original50.htm


They looked like this

[EL34]

I notice that the layout for the "standard" plexi-50 has a bridge rectifier

That's not a bridge. It's simply two diodes in series to replace your single diode. This is to increase the reverse voltage rating (PIV) of the single diode. You really need to replace the single diodes on your antique board with series diodes. A single diode may be very close to having the PIV rating exceeded. You will still need the PT with a center tap.

Sluckey is correct
That is why I went to double diodes much later
I never heard of the original single diodes failing, but just to be safe.

Edit, The feedback resistor from the speaker jacks will be a different value from the 50 watt
Or it just went onto a different speaker tap?
I can't remember
other than that, the 100 watt board just had two extra screen grid resistors and two more one ohm resistors

[EL34]

That board was made way back when I used a mask and a drilling template to drill all the holes on a drill press
The mask went on top of the template and showed me where I had to drill holes in the board material

The mask was held in place on the template by a couple steel pins

Here's the plexi 100 mask and aluminum drilling template


I also had to rip 3 foot x 4 foot sheets of G10 on a table saw
I am glad those days are long gone

[PharmRock]

I notice that the layout for the "standard" plexi-50 has a bridge rectifier

That's not a bridge. It's simply two diodes in series to replace your single diode. This is to increase the reverse voltage rating (PIV) of the single diode. You really need to replace the single diodes on your antique board with series diodes. A single diode may be very close to having the PIV rating exceeded. You will still need the PT with a center tap.

Sluckey is correct
That is why I went to double diodes much later
I never heard of the original single diodes failing, but just to be safe.

Edit, The feedback resistor from the speaker jacks will be a different value from the 50 watt
Or it just went onto a different speaker tap?
I can't remember
other than that, the 100 watt board just had two extra screen grid resistors and two more one ohm resistors

Thanks for the info!  The diodes on there now are huge 1N5408's.  The schematic on Mojotone's website for the 1987 kit just shows just 2 1N5408's for rectification, whereas other schematics show 2 x 2 1N4007's, yet others show 2 x 2 1N5408's.  I'm taking the advice of you guys and will insert 2 more diodes in series to be on the safe side.

Regarding the feedback resistor...I've checked over a few schematics, and even hunted down the PDF build manuals from around 2007-2009 from Metro Amp for the 100 watt and 50 watt kits they used to sell.  Both of the boards (100 watt and 50 watt) have a 47K feedback resistor, although the 50 watt schematic has 100k in parentheses next to the 47K, making me think it is an option.  Both manuals discuss options for hooking up the NFB to either the 4, 8 or 16 ohm taps, or just making the connection at the jack so that the feedback changes depending on the impedance of the speaker cab.
Edit...looks like the 2204 has 100k for the feedback resistor.

[PharmRock]

I'm scrounging together a parts list to complete this build.  I've got a blank 17x8x2.5" chassis that I will attempt to punch out with my newly-acquired Greenlee chassis punch set.  I think I've settled on going with the 2204 circuit, and using the Mojotone faceplates.
The cap cans and small parts hardware is on the way (thanks Doug).  I've already got some tube sockets, and I will be ordering some "bolt on" turrets to make it easier on myself when I have to install extra ones seeing that the board is already populated.

So the big high dollar items I need to settle on are the transformers.  One thing for certain is that I will be using a stand-up PT since it will be nearly impossible for me to cleanly cut out the chassis for a laydown version.  It looks like Doug doesn't carry these anymore and the 50 watt Mojotone transformers are on back order/out of stock. 
Being that the Marshall plexi-type circuit is completely new to me, I've checked all the forums for suggestions on quality iron.  In all my other builds, it always seemed to me that the OT made the biggest difference, so if I need to take out a loan to get one, that's where I'd rather spend the money.  Mercury seems ridiculously expensive to me, but Metro, Marstran and Merren have transformers that have a really good reputation and are in the same general pricepoint.  ClassicTone is lower priced but I haven't been overly impressed with their 18 watt OTs when compared to Heyboers. My mind can be changed though.
 This may be opening up the ultimate subjective/biased can of worms, but any recommendations for transformers for this build? 

[DummyLoad]

but, but... the board says 100W plexi. anything less would be false advertising!    build the 100W and pull 2 tubes and adjust load accordingly if you want the 50W model.   

--pete

[PharmRock]

but, but... the board says 100W plexi. anything less would be false advertising!    build the 100W and pull 2 tubes and adjust load accordingly if you want the 50W model.   

--pete

I did give that some thought initially, but that puts me into finding a chassis that will accommodate the extra tubes and cap cans. And I'm not sure the 100 watt chassis would fit inside the head cab I am repurposing for this build.
From what I can tell, the Plexi 100 "on board" power rail components is the same as the 50 watter.  So I can save on parts costs and bypass sourcing a chassis going the 100% 1987 or 2204 route. 
Maybe I can ship the board back to Doug and ask him to write in "/2" under the "Plexi 100". 

[PharmRock]

So I've been doing a little internet detective work and happened upon sluckey's awesome website and found this:
http://sluckeyamps.com/misc/Marshall_Dual_50.pdf
This may be exactly what I am looking for, as I can't quite decide between 1987 vs 2204....why not have both? 
One thing that really caught my attention is that instead of making the 1987/2204 switchable is that a 470K resistor could be used in the 2204 signal path after V2B and have 2 input jacks..one for 2204, one for 1987, and as the schematic says, use an ABY box for channel switching.  Seems like this would be like having a lead "boost" channel for soloing and/or more aggressive tone.

This might be straying a little too much from my pre-populated plexi-100 board, but looks like a pretty cool design.  Curious if anyone has built this "dual 50" amp and if you could offer your opinion on how accurately it gets both the 1987 and 2204 sounds.