What to build with Fender PA 100? ADVICE NEEDED

[Davidg]

I recently acquired a silverface fender pa 100 -1oowatt head with four inputs. It is in good cond and fully functional other than needed new caps.I originally got it to cascade the channels after reading an article about converting these to guitar use (I think its on the geofex page) but I also have lately considered building a trainwreck clone out of it. The conversion article suggested cascading preamp tubes with a gain/volume control at each stage which seems like would give u more control over your overall tone but I am not sure how this would sound in real life.Any advice/experience would be greatly apprecciated (esp. you Tubenit as it would be your Wreck I would be building).Also if I wanted could a wreck clone sound decent with 6L6s or would I have to go with EL34s

[Dave]

It uses the exact same transformers as a 100 watt (silver face) twin reverb. Does that help?

Dave

[jjasilli]

Not the power amp.  The real question is what to do with the preamp -- several different ones; or lots of cascading gain stages.  I think plexi50 did a creative build with this amp.

[Davidg]

Im sorry if it wasnt too clear but yes I am curious as what to do with the pre-amp stages.I already have a Traynor YGL that i am converting to a twin(not much of a conversion really).To me the Fender PA head is just begging to be made into a screamer- but what type-SLO,PLEXI,WRECK,ODS, or something original. Thanks for the replies and please keep them coming! I saw an earlier post about the build Plexi50 did with this head but unfortunately he did not have records or remember exactly what he did.

[plexi50]

Yeah i remember and loved how great this amp sounded when i was finished with it. It was one freakozoid build. But not really. You can only achieve so much using a single OT. And theres absolutley nothing wrong with a single OT. Thats the norm. I used a 50 watt schumacher OT and a 100 watt schumacher OT in that build. I called it Frankenstein. It was a franky freaky great sounding amp. I have to dig into a hardrive in the closet to post all that info. In short i built a JTM45 circuit and a 2204 circuit. The cool thing about the PA 100 is if you layout and reposition the tonestack pots it will look normal and you have independant EQ's for both channels. It was something new to try. I think schumacher transformers are extra great. Insane but i saw a 50 watt schumacher sell for $450.00 last month or so on dinalingbay. I think i have something to post now. Looking. Man this was a long time ago /

[tubenit]

(esp. you Tubenit as it would be your Wreck I would be building

What Wreck are you referring to?  I've never built a Wreck.

With respect, Tubenit

[quayhog]

Note that V1 and V2 all share the same cathode resistor.  seperate out each cathode with its own resistor/cap and voice each channel to suit taste.  As built the tone controls are very neutral, play around with component values.

recap and get the power supply stable and mod the bias circuit for adjustment and not balance.   If I remember correctly I left the balance but added a second pot to adjust supplied bias voltage.   

I eventually just used it for bass and never used any of the other channels.  I won't advise you on what to build.  I never used any of the other channels after I seperated the Kathodes.   It was always a work in progress.

Its been close to twenty-five years since I played around with one of these.  I lost it to my son's band and am not sure where it is anymore. 

There were two different models of PA-100.  One had the pull boost on the volume pots.  make sure you have the correct schematic.  All PA-100 amps had reverb.

[Davidg]

(esp. you Tubenit as it would be your Wreck I would be building

What Wreck are you referring to?  I've never built a Wreck.

With respect, Tubenit

           I apologize Tubenit, apparently I confused you with someone else, I saw a thread giving credit to a trainwreck layout to someone and I thought it was you.I dont think I will go that route anyway as I think I would rather create something at least somewhat original.Thank You so much Plexi50 for posting that schem and I will apologize in  advance for stealing some of your ideas! I am sure I will use some of it maybe alot of it I have it narrowed down to this or cascading the stages with a ind. gain control for each stage-this is what I would really like to do but I worry about stability/and voicing with this many stages as I have never worked on anything like this. Thanks to Quahog for your input as well. Can anyone tell me the biggest pitfalls with cascading stages? would the individual gain controls take care of these? 

[quayhog]

Biggest pitfall of cascading gain,   Motorboating. 

If it were me I was going through this drill again.  Keep in mind my goals are tempered by age (+60).  I now like low power and clean.  Its real hard to build clean amps.   You can dirty them up with a pedal.

I'd voice one channel for low gain,  build it around a 12DW7 using the low gain (12AU7 equivalent) side in V1a. ref Vox AC50.   To do an AC50 you'll need to sacrifice a second channel and use its socket for cathode follower and tone stack.   Great classic amp.   

Use the left over Hi gain side (12AX7) equivalent add it to the next left over channel and build something crazy.  The last remaining channel build a tweed circuit.

My experience (long time ago), was there was no practical way to use all four channels.  Not enough tube sockets to get inventive.

[Davidg]

Thanks again quahog! Thats a good idea,vera versatile. I am really obsessed with the idea of cascading stages with individual gain for each stage. It seems to me that would be the best way to control the response and tone of the amp. By using different cathode bypass values and a few other things it seems that there would be a tone for any occasion in there. I don't want tons of gain but to get a slo or boogie sound if I want to that day as my preferences are constantly changing day to day. Do u think the ind. gain controls would help prevent motor boating or, at least be able to control it,I think it would because u could control gain at each stage, what do u think?

[quayhog]

Before you rip and tear.  use patch cords to combine extra channels gain.   Less risk and it would give you an idea of how the amp may sound cascaded.   The power section is very clean.  If it were me I'd use that to your advantage.  build two (possibly three) completely different pre sections feeding the power amp. 

If you find you need extra sockets, try running two 6550's and free up two octal sockets.  There would be very little difference in overall volume.    I love octal preamps.

I used mine mostly for bass with two single 15" cabinets loaded with D-140 JBL speakers.  I had grand visions about the amps possibilites too.  Ended up spliting out the Kathodes with individual Cap/resistor and modded the tone stack on one channel.  With the exception of bias adjust mods I left the other channels as they were.  It was very hard to get it to break up which was allright by me.   My guitarist at the the time liked the other channels as they were with his 335.   Sounded like crap (to my ears) with single coils.

[Davidg]

Before you rip and tear.  use patch cords to combine extra channels gain.

Do you mean just jumping the channels like on a Plexi?-I  just assumed that that wasnt possible with this amp. Or running jumpers inside the chassis? This is a great idea and as u said would give me an idea of what could be done.As u may have noticed I am definitely not a guru and am still very much a learner so octal pre-amp tubes may be a little outside my comfort zone but I wouldnt mind cutting this sucker down to 50wts.Did I mention that along with this mint head it is also loaded with all RCA Blackplate tubes pre and power, the power tubes are the big bottle 6L6GC.Thank you again quayhog and pleaseclarify the patchcord idea if u dont mind.Anyone else sees this feel free to chime in!

[quayhog]

plug into channel 1 input #1, run a patch cord from input#2 to channel #2 input #1.  Then repeat down the channel, change to suit. 

I never tried it on this amp.   It worked like a charm on our old bassmans.  Not sure if you'ld have signal phase issues.  You may need to work these out internally.

As far as octal preamp tubes.  6SN7 make great PI tubes, it's a robust equivalent to a 12AU7.  The 6SL7 is much like a lower gain 12AX7.  They are great to work with.  Start with surrounding component values for 12AX7/5751.  The 6SN7 while lower in gain than a 12AT7 PI, the component values are ballpark correct in the typical Fender amp.

If you only use two 6L6 you'ld want an 8 ohm speker load instead of the 4 ohm a twin requires.  The transformers are the same as the twin reverb.   

Its a beastly heavy amp.

Jonathan