Fender 1970s PA 100

[bobtait]

I just got a 75 PA 100 I am going to play with and maybe mod it some. I noticed that a lot of people are experimenting with these amps. And most of them are looking for the odd tone knobs. I got mine and started looking and playing it some to see what needed to be fixed and I came to conclusion that I didn't even like those tone knobs. I prefer simple 1-10. Even if they are technically cut and boost circuits I don't know if I will leave them like that. So, if anyone else in here is working on or thinking of working on an amp like that, I would love to hear what you guys are thinking and doing and I want to tell you that if you are, and you need and want some of those tone knobs, I have 7 and I am not trying to make any money. I would like to offer them up to people who might really desire them in this group. Maybe we can get some talk back and forth of what cool things we could do with the 4 channels. So get in touch if you have been searching for those knobs and can not find them.

[Bieworm]

Do you have the schematic? It’s easier to determine what is possible.

[bobtait]

I do have the schematic and there is lots of stuff online documenting things you can do. I just haven't decided what to possibly try first. It is like a blank canvas. I am going to get matched tubes in it and bias it and try to get any hums and such fixed before I dive into the channels. There is a video on you tube of a guy going through the schematic and talking about stuff he would change in the channels to make them better for guitar. The amp has lots of possibilities.

[JMPGuitars]

Bieworm was asking for you to post the schematic so he could share his opinion.

[bobtait]

bassman_100_schem.pdf

I am trying to add the schematic

[bobtait]

I think I got the schematic in. What I am wondering is what makes sense to improve the channels for guitar. They actually dont seem to sound bad as they are. They are a little dry and generic sounding. Pretty close to just plugging a guitar into any dedicated sound system. I have played bass many times just through a direct box into a sound system but guitars are different a bit. Any input is appreciated.

[Bieworm]

I’m confused. You ask questions about the fender PA amp and you post a schematic of a bassman bass amp. I don’t think they’re the same amp. A bassman sounds terrific with a guitar plugged in. What is the condition of the components and tubes? Do you have a photo of the amp that’s discussed here?
If it is in fact a PA amp, well that’s designed to be very clean and flat sounding. Add the 100 watts power amp to the coctail and you will inevitably have a generic flat sounding amp. To turn this into a guitar amp you might look into rebuilding (part of) the preamp to an AB763 circuit. Then you will have a Twin amp.

[bobtait]

I am so sorry, I posted the wrong schematic. Here is the PA 100 schematic. I downloaded other amps to look at their channel wiring.

[bobtait]

Here is a photo of the amp I am restoring. And you were absolutely right about the PA nature of the amp. Kind of clean and no real edge. But I was surprised that it did not actually sound bad, but I am sure it can be better. It kind of reminded me of when the band Mountain got an endorsement from Sunn on their first tour. And Sunn sent them a bunch of Sunn Coloseum PA heads. But, the band discovered that those amps had a master volume and they could crank them. So that i what Leslie West played through on that tour.

[Bieworm]

Ok that looks more like it. I would start with separating the kathodes of each 1st preamp stage. And then rebuild one of the preamps to AB763 specs. Leave the power amp alone for now

[bobtait]

OK, now I am real new at all this so and had to look that up so: "put individual cathode resistors and bypass capacitors on each channel"? Would I use the same values on those parts as what they are sharing at the moment?

[bobtait]

Also the amp has what looks like brand new MOD filter caps. I haven't haven't removed the chassis to inspect the other components yet and see what else is new. The amp when you first turn it on sounds like some other caps are struggling. Way too much hum. But after it warms up completely it settles down alot but it still is too noisy. I have not set the bias yet or the hum balance or put new matched output tubes in. So some of that might go away. Just a report of the current state of the amp.

[Bieworm]

OK, now I am real new at all this so and had to look that up so: "put individual cathode resistors and bypass capacitors on each channel"? Would I use the same values on those parts as what they are sharing at the moment?

No, the resistors should be between 820 ohm and 1.5k. The caps are usually 25uf. Check the AB763 schematic online for a better idea of what I mean

[bobtait]

Thank you, I can see that amp being useful with maybe 2 guitar channels and 2 that would work for vocals or keyboard, maybe steel guitar, things that sound better in flat PA channels. I have done a lot of audio mixing, ran a sound company for a while done a pretty big handful of major artists off and on and I know what a good PA can do with certain instruments. But they have never been a great place to plug in a Les Paul or Strat LOL. At least not directly.

[bobtait]

Update on this 50 year old amp. I put a brand new quad of JJ5881 power tubes in left the bias a couple of points hot til I can get in there. Replaced 12AX7 that was making all the noise. And man, this amp is dead quiet. With a channel on 10 and the master on 10 it is dead quiet. It is still as expected Clean, Clean, Clean and not edgy at all but the tone controls work really well. I mean they really change the bass and treble, so that is useful. I am thinking about Black Facing part of it. I have had some good suggestions already in here. First though, I am going to put Compressor and an OCD pedal in front of the channel just like we used to on twin reverb and let the OCD set the breakup according to the guitar volume and let the compressor do what it does and see what I can get out of it along with the master volume. It is a heavy beast for sure but I found a 2-12 cabinet that look identical to a Bassman 2-12 except it is closed back and only 16 inches tall and 30 inches wide. The whole rig will look like a Bassman piggyback on steroids LOL. I put handles on the side of the head for ease of lugging around. I will keep this page posted. The cabinets in the photo go with several of my 18watt Marshall and Tweed Deluxe heads.

[bobtait]

So I am looking at the schematic to this PA 100 and I can see the Kathodes of each pair of output tubes share a common path to ground but there is not any kathode resistors or bypass caps. It is a straight path to ground. So, if I am going to separate the kathodes am I needing to add resistors and caps to match whatever bias point the amp will be set at? I checked the plate voltage 448V and the math for 6L6 at 70% is -46mV. The bias in the amp was set at -45mV. which is right at 70%. Right now I have 5881 tubes in it and their math at 70% is -36mV. Max plate dissipation would be -51mV. So -45mV is a little warmer with 5881 than it would be with 6L6. This is all from the published specs. Once I go inside I will measure from pin 8 and figure out that math. So if I don't have a kathode resistor to compare to, how do I figure what value resistor and cap based on the current plate voltage? Also if I do this will I have an amp that is a combination of fixed and katode bias? Or does the bypass cap eliminate that?

[Bieworm]

It’s a fixed bias amp. So the power tubes have their cathodes connected to ground.
I meant the cathodes of the 1st preamp stage. The 4 triodes of the 2 first tubes all share 1 cathode resistor and 1 cathode bypass cap.

[bobtait]

Thank you, I was researching separating the Kathodes and it said separating at the power tubes, then I read somewhere else to do it at the preamp tubes so you are right and whatever I was reading was dead wrong. But I was trying read the schematic and thus it promped the question about there are no Kathode resistors there. So, back to the schematic.

[bobtait]

I have not removed the chassis yet to examine the amp. I know it has new filter caps and 3 prong ac cable so if done right the suicide cap is removed. It may or may not already have other mods to it. It sounds pretty darn good right now. But, gonna go slow and assemble all the info on this amp correctly. And thanks for all the responses.

[bobtait]

looks like all the channels share a single 270 ohm kathode resistor but each channel has a 220pf bypass cap. So, I will have to figure what values when I split them apart since there are 4 channels on the kathode resistor. Please correct me if I am wrong per the schematic. I am just learning how to read this stuff.