I'm just gonna go ahead and share my work. My first big mod project. Fender PA 100. Be gentle.

[Whatizitman]

Well…. Making little changes to existing wiring is no longer working. Starting to get over my head. Gonna have to gut the preamp section and start from scratch. Oh well. Always learning something. That’s the upside.

Among other things, keeping track of grounding was getting futile.

 

[Whatizitman]

Starting to take shape, more or less. V2 is rewired as a cathode follower. Working on getting tone stacks set up, and need to figure out how to add the other attenuator resistors. Grounding, too. I adding new 1M pot for preamp volume, and 25k for mids. Lots of good original pots to use, but they’re all 250k. I also have some Marshall friendly cathode resistors and cap to install. But I figured I would keep it as is for now, as they were working ok before I started moving around pots and such. Just not tons of crunch.

Edit: oh yeah. I rewired channel 1 inputs hi and lo per Marshall layout. We’ll see if I did it right. The jacks are j style, btw.

Technically, there is still another gain stage (V5). I wasn’t having any overloading with the channels cascaded before with the TB plate driven. It’ll be interesting to see what happens with the cathode follower and TMB.

Who knows how many cold solder points I have on the old board. Troubleshooting I expect is going to be a nightmare. But this is what I get for being a cheapskate. Gotta learn it eventually, amirite.

 

[Whatizitman]

I have lots of dumb grounding questions I can’t Google my way out of.

1. On the standard Marshall TMB, does middle have to be terminal ground? Or can I use bass pot? See 2 for why.

2. The switching jacks are also isolated types (I believe). Will that be enough to tie in the jack ground to the preamp ground? I’m not using a buss, by the way. But the original bass and treble pots have terminal connections soldered to the backs.

EDIT: No. No it isn’t. Just getting some farting motorboating sound that does change with volume and frequency. But no guitar. Which means no input happening.

 

[Papanate]

I recently came to the conclusion that I'm just way too cheap to buy or build my dream amp (2203/2204). So I'm modding a Fender PA 100. I picked it up a few months ago for $125.

Interesting idea - but why a PA Head?

 

[Whatizitman]

Interesting idea - but why a PA Head?

I wrote my reasons in the main post. If anyone has any insight as to why it’s not a good idea, though, I’m open to hear

 

[Whatizitman]

Works now! Not great. But it is working! Way too much distortion, and still way too hot signal from V1. TMB does work. Will need to review cathode values.

 

[middy]

I recently came to the conclusion that I'm just way too cheap to buy or build my dream amp (2203/2204). So I'm modding a Fender PA 100. I picked it up a few months ago for $125. I had $100 leftover from a recent sale. So, really only $25 out of pocket. Brought it home and it was DOA. No biggie, as I was happy to have something to work and learn on. And just bringing an old amp back to life is rewarding enough in that regard. I'm still very new to tube amp build world. So from the getgo this amp has been a fun adventure.

The first big finds were that the 6L6 bias circuit was open, and at least one of the radioshack 6L6s was toast. No other signs of blown/arced tubes, though. I had a quad of new 6L6s I got with a JCM900 head trade (more on that later), so that came in handy. Replaced rec diode and cap and bias was within reason with the new tubes. But the amp still worked intermittently and blew fuses. After staring and poking at the thing for a few days, I came across a broken chassis ground wire that was very hidden. Bingo. New filter and pre bias caps, and Bob's was my uncle. I love Bob. Really, it seemed like the Radioshack 6L6s were the only thing that were weren't from the factory on the amp. And they were old.

Reverb working ok. Not TR. But ok. With pedals the amp is very workable for guitar. Even better for bass. Even betterer with some external EQ and/or mid-hump pedals. A DOD250? Nice. Rat? Hell yeah. Bridge some channels and it gets a little louder and fuller. It's not a cascade, though. But it's nice.

For those of you unfamiliar with the PA100, it's more or less a standard 100 watt Silverface tube power amp. It has four input channels with Z and tone stacks (~James) suitable for mics. All are shared cathode, each with one triode between two 7025s. Their summed in with a 2nd triode stage. The reverb circuit differs from the AB763 in ways I don't entirely comprehend. Seems like there's an extra triode buffer(?). Anyway, suffice to say that if one were to mod a PA 100, they would find plenty of space (they are huge) and pre-amp tubes to do so. Check out what Fuchs does with them, for example.

This is the closest schem to mine. Some of the tone stack values were... different. Everything seemed original, though.

https://schematicheaven.net/fenderamps/pa100.pdf

I could have left it at that and been happy enough for my daily noodling. I then turned my attention to 2204 build ideas, and put my also fairly recently acquired JCM 900 4500 up for sale/trade, in hopes of procuring some dough toward that 2204 build. No bites. There are DRs polluting Facebook Marketplace everywhere, just sitting. There's a 50watt combo near me for $550 that's been sitting for a month now. Nothing 900 is moving. I took it off market and started getting together plans for modding the 900 to an 800. Short story is that it's just not worth it, as I'm sure y'all familiar with the dual reverbs can attest to. It is a good amp for a working pro. Just not good for my tinker and noodle avocation.

I needed to work on something. So back to the PA100. I figured I could cut my teeth on some simple things first, and see how it goes. The first mod was to change the grid leaks and stoppers for standard values. Not my best lead dress work. But it did work. Better response for sure. Changed the bias resistor and cap to a more typical fender value (split four ways). That helped, too. By this point, including the amp, I'd probably spent around $150-160ish. Not bad for a working vintage 100watt Fender beast.

After some digging, I concluded the tone stacks were James style, and set about to figure if I could mess with the mids.

And it's all gone downhill from there... I need to stop writing here, but will post some pics and 2203 molestation status later today for anyone along with me for this ride. I took a break from TDPRI posting for a while. Happy to be with y'all again!

Wow.. I guess nobody realizes how great the JCM900 clean channel is! I might have to pick one up cheap.

 

[Whatizitman]

Wow.. I guess nobody realizes how great the JCM900 clean channel is! I might have to pick one up cheap.

They’re not as cheap as they were a few years ago. But they’re clearly not selling at the prices folks are listing them for, either. Tons on my FB marketplace. And it seems like more everyday. All just sitting. “Cash only! No trades!”

 

[Whatizitman]

Gonna take a day or so to process. My next plan of attack will be to fix all the preamp cathode values to proper. I had initially used what I had on hand. I also think I will add an attenuator resistor from the treble pot output. The PA 100 circuit has 120k mixer resistors on each channel going to the 2nd pre stage. Well, I have a lot more stages now before that, and no 120k out of the channel bus. So the signal will need to cooled down a bit, in several places.

Right now, for a example, I have 1.8k/10uf on V1A, 15k/1uf on V1B. I added the cap on the “cold clipper” before I moved the TMB, so it was helping with some gain. I have way too much now. So when I put in the 10k I will leave it unbypassed. I have 680uf on V2A, which is the first half of the cathode follower. Marshall specs 820uf. I have a proper .68uf for V1A to install. I will double the resistor.

So what did I learn this last go around? How these crazy 4 tab switching jacks work, for one thing. Learned some about preamp grounding. Only enough to make it functional, though. It’s very noisy and oscillates now. Hopefully bringing down the signal and clipping a bit will help. But given how many stages it has, it might just be a high gain amp in the end. I’m a classic crunch kinda guy. Not a metal head. So we’ll see how satisfied I am with it once some bugs are worked out.

 

[chezdeluxe]

I know nothing about the technicalities of amps.

I do own a PA 100 where a previous owner had modified the first channel to mimic a Twin Reverb. ( What that involved I don’t know)

It works as a head over two PA cabs I have loaded with JBL K120s and tweeters. It is loud and clean and I can’t imagine ever using it again but that is a reflection of the venues I now play occasionally.

 

[Whatizitman]

I know nothing about the technicalities of amps.

I do own a PA 100 where a previous owner had modified the first channel to mimic a Twin Reverb. ( What that involved I don’t know)

It works as a head over two PA cabs I have loaded with JBL K120s and tweeters. It is loud and clean and I can’t imagine ever using it again but that is a reflection of the venues I now play occasionally.

I think you can get pretty close to a Twin Reverb with some mods. But the reverb section is a little different, and definitely doesn't sound the same.

These are so big, heavy, loud, and clean that as a PA they are really only good for leaving in place in a practice room or whatever. No XLR inputs. As a guitar or bass amp they are great with some pedals with no or minor changes to the grid stoppers and grid leaks. But there are plenty of better or more practical options for that nowadays. For a home mannish cave noodler like me they are great. I see a couple locally that are priced at $500-700 with or without the original speaker columns (also useless). Not sure what people think they are worth. But it ain't that.

 

[2L man]

No help for amp weight and size but you could make it bit quieter making other power tube pair switchable and then two remaining can be driver harder. Only about 3db effect but it is still 3db and when you drive same volume it means more tube distortion.

Using half the power tubes count will increase voltages but you can install "sag" resistors which increase distortion and also make sound more interesting "sag'n bloom". This drop volume only perhaps only 1db but there come more distirtion. Also filter capacitor must be changed switchable to lower capacitance to strengthen the sag-effect.

Often when "half the power tubes" are used the OT impedance is is kept the same for remaining tubes connecting speaker to next lower output jack. But if this is not done the loadline turn steeper up and make tubes work harder. Together with sag-circuit this produce interesting exhausted "sag'n bloom" sounds. This steeper loadline will drop outout power a lot when tubes do not operate at their optimum load and perhaps another 3db comes down and as again much distortion when volume is brought back. Naturally this is just one more option and normal OT load can also be used when cleaner sound is needed.

One easy method to improve sound (increasing distortion) is to install ajustment potentiometer (or switchable resistor) to Phase Inverter which make imbalance to power tube drives. Now when OT size is double for half the tubes it won't saturate. This unbalance produce mostly warm 2nd harmonics. This is about the same when guitarists have said that amp did sound best just before it broke and it was when other power tube had come much weaker and volume knob had to open more. Possibly just this did happen to me when I was a teen.

 

[Whatizitman]

Thanks @2L man . All good things for me to think about down the road. To me the benefit of running on two power tubes would make it possible for use with less speakers and/or less efficiencies. I currently have it in a 2x12 with a V30 and VOTW speakers, both ~60watt and pretty efficient. I would love to use some greenbacks. As it is it may already be a bit mismatched at 8ohms (2x 16ohm parallel). The OT has two 8ohm taps for a 4ohm load. I don't think they're built to use only one tap. I'm not playing at Lemmy levels, so I'm not too concerned.

At the moment I don't need anymore distortion. I need far less LOL. A thought I had today was that I likely have too much plate voltage on the V2B, which is the cathode follower. All I did was swap the load resistor to the cathode. Not sure how much that will affect things. When I fix all the cathode values I'll check all the pre plate voltages. They were all good stock. But it ain't stock no more. So something else later to tweak if need me.

Never finished, amirite.

 

[Whatizitman]

I changed the cathode values and caps to Marshall specs, more or less. I also added 470k from the treble pot to the next stage. It’s a little better, but still way too much distortion. Basically no clean setting whatsoever. With the preamp pot set barely off of zero, it’s a nice stock 2203 crunch. Anything higher is super high ultra mega gain, like a DSL red channel and beyond. Not my thing. TMB section will need some good tone caps at some point. But it’s ok for now.

The cold clipper cascade is definitely doing its job, though. It’s a nice Marshally asymmetric crunch.

The biggest issue is a major oscillation that occurs after a moment of a note or chord sustained. The amp is noisy anyway, so you can hear it start to oscillate when idle. It’s also frequency dependent. The more bass roll off the less prominent and slower to start oscillating. With the bass down all the way there is very little oscillation. But you gotta keep playing to mask what you can hear LOL.

I pulled tubes starting with V1, and it’s isolated to V2, which is the cathode follower. The voltages are fairly low compared to Marshall specs. I thought they would be high, actually. Would lower voltages on the CF plate and cathode make that much of a difference?

 

[2L man]

You immediately found another very valid benefit which I missed for lower amp power

Amp seem to use chassis for many stages return currents. That is definite no no for me because it open possibilities for hum, osillation and transmitting noise to mains.

It should be quite easy to install one return current wire for all pre amp and PI stages electrolyt negatives. First to B01 and then to PT. If B01 electrolyt electrode is too small to solder all current wires just install something isolated close to it what you can use to collect all return current wires. Do resistance measures when you cut current chassis connections to verify there is no leak path left and few times when you install new wires. Then last step connect input "ground" to chassis on jack which is farthest from power supply and there is no possibility to leak current to chassis because it does not have another return path. Also less noise is transmit to guitar (where some inevitably would come back to amp input) when amp input does not leak operative current to chassis

Your amp has lots of steel input jacks and I think you can still keep them connect to chassis because there does not flow operative current when a wire is used for that.

All other jacks should be isolated so there comes a need for small investment but then they are new and should provide many years of throuble free use.

 

[Jowes_84]

There's another TDPRI thread going on, right now, and one of the posts is by someone using a PA135 as a bass amp.

Hi
That might be me.
My PA 135 has two channels modified, one with a TB stack and one with the inputs modified (470k to grounds instead of 47k stock)
I also added a bright switch in the TB channel and a Rivera-Style lead mode using the reverb tube, but that is as just fun. I don’t use that.
I use it every week at rehearsal and when gigging. Works great for bass.
If I was using it for guitar I would first reduce the negative feedback (a lot), second, you could Marshallise one of the four channels using either a tube from another unused channel, or an IRF820 or similar MOSFET cathode/source Follower and bypass the reverb circuit directly to the phase inverter. That would put you in Super Lead territory. Or you could cascade one channel into another. Many options.
I discarded all this as I love this thing for bass AND it is as a very gracious gift from @jhundt

 

[Whatizitman]

You immediately found another very valid benefit which I missed for lower amp power

Amp seem to use chassis for many stages return currents. That is definite no no for me because it open possibilities for hum, osillation and transmitting noise to mains.

It should be quite easy to install one return current wire for all pre amp and PI stages electrolyt negatives. First to B01 and then to PT. If B01 electrolyt electrode is too small to solder all current wires just install something isolated close to it what you can use to collect all return current wires. Do resistance measures when you cut current chassis connections to verify there is no leak path left and few times when you install new wires. Then last step connect input "ground" to chassis on jack which is farthest from power supply and there is no possibility to leak current to chassis because it does not have another return path. Also less noise is transmit to guitar (where some inevitably would come back to amp input) when amp input does not leak operative current to chassis

Your amp has lots of steel input jacks and I think you can still keep them connect to chassis because there does not flow operative current when a wire is used for that.

All other jacks should be isolated so there comes a need for small investment but then they are new and should provide many years of throuble free use.

I've verified the input jacks ground to chassis. But I'm still not getting the hi and lo wired right. I'm really confused, as they have four tabs. The way I have it now the hi is the only jack that gets guitar input. Plugging into lo is just noise. No guitar signal. Since V1A (first stage in my layout) plate connects to the lo input jack per marshall spec, could there be grid leaking or something else going on? Clearly too much signal and/or clipping is getting to V1B.

As to the oscillation, I'm suspecting my lead dress first and foremost. In efforts just to get the mod working in a general sense, I really took no thought of placement and length of wiring. The oscillation is very audible and sounds kinda like a dysfunctional vibrato/tremolo LOL. My next phase will be lots of reading and questions until I come up with better overall layout. I am proud of myself for getting this far! But I want an amp I will want to play regularly on.

All my preamp plate voltages are a bit low. Not significantly for V1A and B. But still lower than marshall spec. I kept the stock 100k plate loads. Maybe I need to change them? I don't want more gain. Just less clipping.

 

[jhundt]

My PA 135 has two channels modified, one with a TB stack and one with the inputs modified (470k to grounds instead of 47k stock)
I also added a bright switch in the TB channel and a Rivera-Style lead mode using the reverb tube,

ha ha! I also have an old Rivera design Princeton Reverb II, and an early 80's Priceton Reverb with that reverb tube switchable as a distortion/boost. Horrible! I gutted that circuit on both amps. I'm glad to hear that you are enjoying the Fender!

 

[Whatizitman]

Wait. My V1 voltages are high compared to @robrob annotated 800 preamp schem. But they're low compared to mojotone's 2204 schem. Hmmm.

Here are mine. Kinda smack dab in the middle of the two schems. Does that mean anything?
V1A plate 253v
V1B plate (cold clipper) 291v

EDIT: Nevermind. robrob's marshall load line graphic uses 280v at plate for the cold clip. I'm close enough I think.

 

[Lowerleftcoast]

1. On the standard Marshall TMB, does middle have to be terminal ground? Or can I use bass pot?

This depends on your ground scheme. If all of the other grounds are willy nilly attached to the chassis, this *middle* can be done so as well. Back of pot is usually connected to the chassis when the pot is screwed to the chassis.
If you have a preamp ground scheme, it should be attached with these grounds.

Have you changed the old filter caps? They could have something to do with motor boating.