Fender PA100

[jake19840]

I got a great deal on a PA100 for modifying and it seems to be all original and untouched. I am looking for ideas about modding this amp, and I also would like to know if you guys have seen this type of bias circuit before. I checked the transformer codes and this amp is a 1975 model. It uses the same trannys as the twin reverb and so on according to the numbers on the trannys. Any suggestions or ideas would be great. I tried to upload the amp schematic but it is to large. Here is a link. http://www.prowessamplifiers.com/schematics/fender/pa100.html
Thanks
Tony

[HotBluePlates]

The bias circuit is simple, if you carefully following the diagram's lines. Things are positioned in a way that you'll not be used to seeing in older schematics. Basically, the bias supply is a bias tap -> rectifier -> cap to ground -> series resistor -> cap to ground -> mid tap on a bias pot. The bias pot itself is a bias balance or hum balance type arrangement.

The amp itself is 4 channels, with each channel having a dedicated input stage and tone circuit. From there, all channels are mixed to the input of the 2nd stage, reverb input is split off and sent to the reverb circuit prior to the 3rd stage, where the dry and reverb signals are mixed and amplified. Then a standard phase inverter and output section. Overall, this thing is a Twin Reverb with only the reverb channel and no tremolo, and with a tone circuit that's not typical for guitar.

Start out by plugging in and seeing if you like the action of the tone circuit. If not, pull out the tone circuit and rebuild with normal Fender tonestack values and arrangement, including replacing some pots with the correct value. You'll need a fixed mid resistor instead of a mid pot, unless you replace a pot with a suitable one and accept that the faceplate marking doesn't match the function.

It's probably not a bad clean amp as-is.

[jjasilli]

great platform!  search the forum.  somebody i think maybe plexi50 did a great set of mods.

[PRR]

The 47K at the input jacks: make one 470K or 1Meg and see if it sounds different. (Will depend on the guitar: some won't care, some will.)

Mostly: this is a geetar amp with more jacks and knobs, not a PA amp. And it has been used as a guitar amp. I would not do any major change.

[bigdaddy]

I can see some basic stuff to the power supply and bias circuit and a lot for the preamps. But basically it's a Twin reverb. Before I would mod anything though I would make sure everything is in working order. Remember that it will be a very loud amp so take that into consideration and maybe you want to add a master volume.

Since there are 4 channels maybe knocking it down to three and making 3 different sounding channels would be a great idea. For instance one being a Plex/Marshall type, the other a simple BF Fender and the other another flavor. If you're skillful enough add channel switching and a footswitch. Even a switch to knock some power down and go cathode bias. This will make the amp very useful.

If your not into the more modern high gain amps and channel switching just make each channel another basic Fender flavor such as Tweed, BF and then Brown/white era. The tone stacks do not look guitar friendly to me. You can remove one channel and add tremolo. Just follow other Fender schematics. One thing is to remove one input jack and add a mid control if you change the tone stacks.

That can be a fun amp to have with the ability to have completely different sounding channels.

[jjasilli]

Or lot's of cascading preamp gain stages, like a Mesa Boogie, Soldano or later Marshals.  E.g., 4 stages > tonestack > tone recovery > PI Could be hi gain if you're into Metal, or creamy like Allman Bro's @ the Fillmore.

[plexi50]

This is going back 4 years now i believe but it is a great platform

The hardest part for me was getting the (2) EQ's layed out as all the holes are already there to have two seprate tonestacks

Looking back it seems like that would have been the easy part but there are a whole lot of pots and the faceplate can confuse you constantly looking at it

I will see if i can find what i did to mine on disc. Everything else i have is on a 80 Gig backup disk except the PA-100 and a few others

One half of the preamp board was JTM45 and the other High gain 2204 all on the dsame board / It ios a long board with just enough room to do this

Belive it or not that 100 watt OT did breakup earlier than i expected it to and it turned out to be one of the best toneful amps i have ever built

I will go to photobucket. The build process should all be there

 

[P Batty]

That's a nice amp as-is for an acoustic with a pick-up.  All I'd do is use the ground switch as a three-position NFB selector (2kΩ-820Ω-6kΩ) and use a pedal for dirt. I knew a couple of guys who used them for a simple multi-keyboard set-up with 15" JBL.

[jake19840]

I really appreciate all the interest in this post. Please keep the ideas coming. I would really like to make this amp a Black Face twin reverb on one channel and something high gain on the second channel with reverb for both channels. I plan on having a faceplate made by BitsandPieces and having only one input per channel. This will make more room to add pots into one of the two existing inputs by drilling it out a little larger. I will be using Corel Draw to do design the faceplate. I also would like to see more of what Plexi50 done with the JTM 45 and the 2204. My plans are not set in stone. I will also post all mods and pics for every one to see as the project progresses.
Thanks,
Tony

[topbrent]

I use a PA-100 to run my Hohner D6 Clavinet, 73 Rhodes, and Wurlitzer 200A.

It is absolutely ideal in this setting. 
Each keyboard has its own input/tone controls. 
One input left over to run a guitar through if the need arises.

Got the idea from seeing Robert Walters live.

[plexi50]

Tony i cant find my Fender PA 100 pics or info. If i do find them i will post them for you. You may want to consider leaving the amp as it is with minor changes. Myself i have gotten into some crazy experiments over the years and this amp just happened to give me a whole lot of those crazy options / ideas.

What are you thinking of doing?



 

[jake19840]

 I would really like to make this amp a Black Face twin reverb on one channel and something high gain on the second channel with reverb for both channels. I plan on having a faceplate made by BitsandPieces and having only one input per channel. This will make more room to add pots into one of the two existing inputs by drilling it out a little larger. I will be using Corel Draw to do design the faceplate.

[zapped]

FWIW, I got got a PA100 reverb free and made a huge PITA project by using all the components for a Blackface TR clone. 2 years and $300. later i'm still working and learning on it. I wish i had PRR's advise before i started. Good luck with your project..

[RicharD]

>I would really like to make this amp a Black Face twin reverb

That is doable but with quite a bit of work.  At a glance, the output stage is pretty much the same.  The LTPI is different, but not all that different.  The big differences are the summing stages and the reverb driver.  The summing stage is quite different because it has to recover more loss due to 4 inputs.  The reverb driver does not use the classic 12AT7 in parallel.  I'm guessing it's less in your face of a reverb.  mmmmm... there's no vibrato circuit.  The tone stacks look more Princetonish which personally I like better, but that's neither here nor there.  Look closely at how they're shorting unused input jacks.  They're actually shorting unconnected 1st stage outputs.  I think those type of input jacks are getting scarce.  So to get a Twin out of the deal, you're gonna hafta completely rewire V1 thru V6, you have no tubes left for anything else, & a whole lotta holes left in the face plate.

Now you really need to consider the collector's value of this head.  I looked on fleaBay and there are none.  Maybe nobody wants them.  The closest thang is a PA135.  I think it might be fairly valuable, especially if it's not cosmetically beaten and in mostly original condition.  You say you got it for cheap... you might be able to flip it for a good profit and buy a real Twin.  Find one with good speakers and black face that.  I'm a big fan of restoration over modification.  I'd be interested to know if it has any historical significance.  It is CBS era only (I think).

If you're hell bent on modding this amp, I have to concur on K.I.S.S. so you can put it back.  The 1M grid leak is a good idea.  Disconnecting 1 or more of the preamps should get you some more gain due to less summing load.  You could use one of the extra stages for an over-drive gain stage.  Note that all the input stages share a common cathode resistor and bypass cap.  You'll wanna break that up into individuals before getting too sexy with the preamps.  If I was gonna mod this amp (which I probably wouldn't), I'd change both of the grid leaks of V1a&b to 1M.  I'd give each 1/2 their own 1k5 cathode resistor.  I'd probably give 1 a 15uF bypass cap and the other something bright and crunchy like a .68uF.  With V2 I'd either do a 2 stage over-drive circuit (ala Stout) or go out there and do Geezers EF86 front end.

[zapped]

If anyone has plans on building a huge amp head i have the cab and chassis for a PA100 and the speaker collumns i'd like to transfer ownership. PM me   thanks  Ed

[plexi50]

I wish you had all the transformers on that PA-100 Ed /

[zapped]

Sorry Dave i used all the good stuff for my clone. Still interested?

[plexi50]

Im thinkin on it Ed. If someone else is ready to jump though let it go. What kind of parts or tubes are you loking for?

[jake19840]

Great Ideas, Please keep them coming so the thread will continue....

[plexi50]

Ed do you still have the eyelet fishboard and bottom from the PA-100?

[zapped]

Yes Dave, i still have the circuit boards. I have removed the caps and resisters..

[jake19840]

Thanks Guys.....

[djcguitar@yahoo.com]

Looks like plenty of power.. A Passive FX LOOP and use an amp simulator box/pedal for the pre-amp..Plug guitar directly into the PEDAL BOX and output of the PEDAL BOX can go into the power amp in on the FX LOOP , and you bypass the pba preamp altogether..Plus If you put an FX LOOP on your Princeton or JTM45 clone build with a vibrolux xfrmr set with a multiohm output "T" running an 8 ohm  single 10" , you can use the power amp of the PA 100 for a slave amp.. So you will have a mini 35 watt combo around 30 lbs..and a decent 12" 8 ohm for the pa 100. As I see it is 8 ohms also..  there are plenty of options you can do..but you will definitely be heard...BUT the PA 100 CAN MAKE A KILLER SLAVE AMP ! Without the weight of Twin..A  twin head power amp for $400? I want one of these...Twin is a top dawg !

[djcguitar@yahoo.com]

Oh just wanted to add...HANDWIRED . Amp IS NEARLY ETERNAL...Would have to fish it out of the river years after a flood before it would ever see a LANDFILL.. So the head might weigh 3O lbs.. it's still portable. A 1-12 cabinet should not be over 25 lbs..I have  a JTM45 CLONE with Vibrolux 35 watt xfrmr set & 6L6's , with a 75 watt RAMROD spkr and it weighs 25 lbs -13.9 oz .Passive FX LOOP is only two jacks & two main wires and a jumper between the jacks .And you can certainly figure out how to implement the reverb into the game..PA 100 is big boy power and easy to transport.. Extremely versatile all the way around..VERY MULTI PURPOSE FOR CONCERT WORK !

[joesatch]

i have been eyeing these. I would make it a 4 channel head with clean, voxy, marshall, and slo each channel with it's own EQ, gain and level. If you look at the front panel of the amp it's almost asking for that