Carlsbro CS60 PA/Reverb, modify for guitar?

[Wigankev]

Hi,
I have a Carlsbro CS60 PA/Reverb.
Has anyone modified 1 of these before for use with Guitar and Bass?

I was thinking maybe gutting it and starting scratch with a layout on here. But can't seem to find anything?
Does sound good as it is. Just I'd like a bit more grit to the sound. It's just very clean with absolutely no overdrive.
So would actually like get some gain out of this thing.

I've also posted the schematic for the amp.

Regards
Kevin

[shooter]

Does sound good as it is.

then mod don't gut
if it was mine I'd gut V1 (1st pic)
and make it look like (2nd pic)
then I'd start tweaking

the 1M to 47k to ground can just go to ground, the 100k plate I forgot to include - you only need 1

[tubenit]

You have a really nice amp to work with there.  IF it were mine,  I'd consider something like this. 

Parallel V1 & change V2 to more of a Marshall inspired topology …………….. then change the cap going into the LTPI to a .02.

I'd try that leaving the rest of the amp the way it is and see how it sounds?   Does the reverb work on it?

With respect, Tubenit

[Wigankev]

You have a really nice amp to work with there.  IF it were mine,  I'd consider something like this. 

Parallel V1 & change V2 to more of a Marshall inspired topology …………….. then change the cap going into the LTPI to a .02.

I'd try that leaving the rest of the amp the way it is and see how it sounds?   Does the reverb work on it?

With respect, Tubenit

Yeah the reverb works great. I've just had it serviced as the negative bias cap exploded! I bought it from eBay, apparently working but requiring service.

All seems to be working now. Some one seemed to have added a wrong value resistor plus added a pot to it. That was probably the reason it exploded.

I did change out most of the filter caps before service, replaced all the Erie 68uf to 100uf at 450v, also replaced the 47uf negative bias capacitor that exploded, along with a 220k resistor it took out in the explosion.

[Wigankev]

Also, 1 downside is, I can't read schematics. So this is going to make modding this a little difficult. Plus, my CS60 PA/R model is PCB based.

[tubenit]

Since you posted the schematic to the amp, I presumed you knew how to read schematics?  It's going to be difficult to explain how to modify the amp IF you are unable to read schematics. I am truly not being critical.  I'm simply stating a respectful comment on the challenge in helping you.

Any chance you have a layout for the amp?  OR ………. can post a good resolution photo of the inside of the chassis overall and a 2nd good photo of V1 and V2 tubes.

With respect, Tubenit

[Tony Bones]

Also, 1 downside is, I can't read schematics.

Some call it a downside, others call it an opportunity to learn. 

[Wigankev]

Also, 1 downside is, I can't read schematics.

Some call it a downside, others call it an opportunity to learn. 

I've tried to learn the basics of a schematic, just can't get the hang of it. Just trying to get recommendations on here what people may have done to theirs. Any information I get here I'll send over to the guy who serviced my amp.

[shooter]

Some one seemed to have added a wrong value resistor plus added a pot to it

That was probably done to easily adjust the bias Voltage for the 34's.
when you were tinkerin, did you check/verify the -34vdc at pin 5 of the 34's?

[shooter]

here's a 2 min search link

https://www.wikihow.com/Read-Schematics

from there;

https://frank.pocnet.net/

grab the datasheets for ALL the tubes in your amp (I like RCA or GE 1st choice )

then go get really confused, til it starts to make sense 

[Wigankev]

Some one seemed to have added a wrong value resistor plus added a pot to it

That was probably done to easily adjust the bias Voltage for the 34's.
when you were tinkerin, did you check/verify the -34vdc at pin 5 of the 34's?

I only started tinkerin with the amp once the cap exploded. Never did any tests prior to receiving it. As listing said working order but required service.
I just replaced the resistor the capacitor took and the cap in question which exploded. While I was in there I changed out most of the high voltage filter caps. But when I powered up again just got hum, with nothing on input, also EL34's was red plating. So shut off immediately. Then took to amp tech.

So now working order and would like to get some gain out of it.

[shooter]

Until you understand why those conditions existed, and what the tech did to resolve them, your probably better off working with that tech, otherwise things can go sideways fast.

[Wigankev]

Until you understand why those conditions existed, and what the tech did to resolve them, your probably better off working with that tech, otherwise things can go sideways fast.

Was repaired by SAI Amplification. He's back building amplifiers again. I'll send the modified schematic which Tubenit did to him. See what he can do.

[shooter]

In the mean time, if this is something that gets you all itchy thinkin about fixin n tuning and building, take the time to read up.  Find some stray family that has a meter and knows why 

read through a few of the threads here where heading is something like "Just finished my build HELP"

[Wigankev]

In the mean time, if this is something that gets you all itchy thinkin about fixin n tuning and building, take the time to read up.  Find some stray family that has a meter and knows why 

read through a few of the threads here where heading is something like "Just finished my build HELP"

I already own a multimeter. As I repair TV's and old valve record players.

[Wigankev]

Here is a picture of the innards of the amp.

[shooter]

Nice guts

If you wanna do this you ARE at the right place, here's my suggestion;
take your schematic to your favorite office supply, have them print it out ~~ 11 X 14
keeping the aspect ratio

read up on schematic reading, at the same time pencil up your new schematic.
 1st is to Identify the power supply section (PS)

next find the input section (typically called out V1 on schematic) This is typically the "1st gain stage"

ask questions you can't find answers to, grab the tube datasheet
tuning is a better/worse endeavor, but knowing where "original" is is important, so you'll be taking lots of notes, or you'll nail it 1st time 

make sure your meter is "calibrated", measure enough DC things to be happy, AC things, If you got a good one Freq comes in handy.
 

[bmccowan]

Wow - what a nice chassis to work with! I too had a hard time learning to read schematics. I could only learn by doing, and correlating a schematic to the layout (either/or on paper or in the chassis.) People learn differently, but you will find your way and all of a sudden it'll start to make sense. For me, it was dividing the circuits into elements: the plate and cathode connections in the preamp; the biasing of the power section, etc. As Shooter said, you are in the right place - very helpful folks here.

[Wigankev]

Really considering gutting this amp and starting fresh. May change this to a JTM 45, but instead of KT66, I'll be using my EL34's. Should just be a case of getting the correct bias to run these EL34's. I'm not too bothered about the reverb. So I'll just take the reverb tank out.

[JayCobie]

That thing looks like Carlsbro's late-60's-early-70's amps. It's not going to take much to modify that thing for more overdrive, and personally I think it'd be a shame to start from scratch on that amp. But then again, it's not my amp.

What Tubenit and Shooter suggested are on point.

My suggestions for mods:

• Gut and rebuild V1A and V1B, just make V1 like any marshall, fender or Carlsbro TC60/TC100 from the same era.
  
• If not rebuild V1A/B then at the very least take away the 2x 220k resistors that are providing local negative feedback. They're causing that stage to be more clean and have less gain
  
• Try reducing cathode resistor on v2a and v2b from 3,3k to maybe 1,5-2k, thus increasing their bias and increasing gain
  
• There is some local negative feedback going on around V2A (2x1M resistors and a 50pF cap) that can probably be safely removed since they're doing the opposite of what you want: cleaning up the signal and reducing gain.
  

If I'm wrong about those NFB stages then someone please correct me, I'm still learning :D

[Wigankev]

That thing looks like Carlsbro's late-60's-early-70's amps. It's not going to take much to modify that thing for more overdrive, and personally I think it'd be a shame to start from scratch on that amp. But then again, it's not my amp.

What Tubenit and Shooter suggested are on point.

My suggestions for mods:

• Gut and rebuild V1A and V1B, just make V1 like any marshall, fender or Carlsbro TC60/TC100 from the same era.
  
• If not rebuild V1A/B then at the very least take away the 2x 220k resistors that are providing local negative feedback. They're causing that stage to be more clean and have less gain
  
• Try reducing cathode resistor on v2a and v2b from 3,3k to maybe 1,5-2k, thus increasing their bias and increasing gain
  
• There is some local negative feedback going on around V2A (2x1M resistors and a 50pF cap) that can probably be safely removed since they're doing the opposite of what you want: cleaning up the signal and reducing gain.
  

If I'm wrong about those NFB stages then someone please correct me, I'm still learning :D

It's just the fact it's PCB board based. Very hard to work on. Lift traces due to heat etc. Seen this era Carlsbro with eyelet boards. Wish this 1 was!!! It would make modding this 100% easier.

[shooter]

with eyelet boards

Too much heat will crap out pads n runs, I've had luck using hi-temp epoxy to "glue" pins onto the board then just use wire n leads to re-connect.  I have a set of itty-bitty drill bits to "ream-out" the old hole.
Patience is key, gremlins feed on frustration 

[jjasilli]

Looks like somebody already rebuilt it.  Those Ecaps are not original.  My guess is that it was rebuilt to spec; but this should be verified first (except for a gut job).


Also, if you have any need for a PA amp, voice or acoustic guitar amplification, the amp might be useful as-is.

[Wigankev]

Looks like somebody already rebuilt it.  Those Ecaps are not original.  My guess is that it was rebuilt to spec; but this should be verified first (except for a gut job).


Also, if you have any need for a PA amp, voice or acoustic guitar amplification, the amp might be useful as-is.

I changed out all the main high voltage filter caps. Originals was 68uf. Upped to 100uf for better filtering.

[shooter]

I like a 25W weller with a 1/8" chisel tip.  I slather on some flux, lay a rosin soaked wick, heat, it melt point I feed in the wick rolling it around the pad.  before I lift I free up my left hand, snag the component n pull pull the heat.  aim for ~~ 3sec start to finish. move on, come back an clean, move on, come back solder new thing

[JayCobie]

It's just the fact it's PCB board based. Very hard to work on. Lift traces due to heat etc. Seen this era Carlsbro with eyelet boards. Wish this 1 was!!! It would make modding this 100% easier.

I just recapped all the electrolytics in my Carlsbro TC100 from the same era (1968 to 1972ish) about 2 months ago with no problems. PCB and construction looks much the same as in mine, just different circuit.
My board did have a single trace that had come loose from a previous amp tech that had changed out a couple of components, but it had been stabilized, so I get that the board can possibly do that. It is very much possible to avoid that though.

You may need to check your soldering iron heat or work on your desoldering technique. See shooter's comment for one method.
I had good luck keeping my 40w iron on the board for about 2 seconds at a time, using a spring loaded desoldering pump to remove the solder. Usually I need a couple of tries per solder joint, but I let the contact point cool down in between, so as not to overheat the area.

[jjasilli]

Another alternative is to work on top of the board, point-to-point. Solder the lead of a new component directly to the lead of an old component; or rune a wire lead from component to component on top of (or under) the board.