Heya, newbie in need of help and ideas here! 🙂
I have a Carlsbro 50 TOP coming in, and someone has done some pretty sketchy mods to the old chap. I haven't got the amp yet, but I've been trying to figure out what I'm going to do with it based on a gut shot sent by the seller. However, I'm having some difficulties understanding what has been done to it. Here are some images:
Gut shot:
https://dl.dropboxusercontent.com/u/1809705/carlsbro/sisukset.jpg
The original schematic:
https://dl.dropboxusercontent.com/u/1809705/carlsbro/kytkis.jpg
Some of the mods I *think* have been done:
https://dl.dropboxusercontent.com/u/1809705/carlsbro/modit.jpg
And a gut shot of an untouched one for reference:
http://uraltone.com/info/wp-content/uploads/2012/04/IMG_4694.jpg
It's pretty obvious that I have to at least remove the shoddy parts and protect and attach them properly. It seems there's a pre phase inverter master volume and some tone stack modding done (I couldn't see enough from the photo to figure out just what it is), and some other changes I have no idea about. This is what's got me a bit baffled:
1) What's up with the added parallel resistors and capacitors?
2) Why has the original 68K resistor been replaced with what seems like a 10K and a 22K in series?
3) Should I keep the mods by tidying them up or just try and get it back to original state?
I'd be really, really grateful is someone could shed any light on this one! Thanks! 🙂
I have a Carlsbro 50 TOP coming in, and someone has done some pretty sketchy mods to the old chap. I haven't got the amp yet, but I've been trying to figure out what I'm going to do with it based on a gut shot sent by the seller. However, I'm having some difficulties understanding what has been done to it. Here are some images:
Gut shot:
https://dl.dropboxusercontent.com/u/1809705/carlsbro/sisukset.jpg
The original schematic:
https://dl.dropboxusercontent.com/u/1809705/carlsbro/kytkis.jpg
Some of the mods I *think* have been done:
https://dl.dropboxusercontent.com/u/1809705/carlsbro/modit.jpg
And a gut shot of an untouched one for reference:
http://uraltone.com/info/wp-content/uploads/2012/04/IMG_4694.jpg
It's pretty obvious that I have to at least remove the shoddy parts and protect and attach them properly. It seems there's a pre phase inverter master volume and some tone stack modding done (I couldn't see enough from the photo to figure out just what it is), and some other changes I have no idea about. This is what's got me a bit baffled:
1) What's up with the added parallel resistors and capacitors?
2) Why has the original 68K resistor been replaced with what seems like a 10K and a 22K in series?
3) Should I keep the mods by tidying them up or just try and get it back to original state?
I'd be really, really grateful is someone could shed any light on this one! Thanks! 🙂
Hi BaronVonKale,
Welcome!
Personally, I would restore it back to original plus rebuild to remove age related problems.
If you make circuit changes, make notes and corrected schematic drawings. In a few years you might need this info to troubleshoot a new problem or a mod failure.
In a nutshell, remove the changed wiring and components. Return the amp to factory state. After you have used the amp in stock trim you can decide on changes.
-Chris
Welcome!
That depends on what you want. Do you want the make and model of amplifier you bought, or is it so bad that you got it with the intent to strip it and make something else?
Personally, I would restore it back to original plus rebuild to remove age related problems.
Unless you can talk to the person who did that change, you will never know. My guess is that they didn't have the value needed so made one up sorta close. The replacement is only 32K, less than 1/2 of the original value.
Who knows but the designer of the "mods"? But then, you have no idea whether you would like the changes, so return the amp to its normal factory state. This way you are starting from a known point in the event you wish to try changing some things.
If you make circuit changes, make notes and corrected schematic drawings. In a few years you might need this info to troubleshoot a new problem or a mod failure.
In a nutshell, remove the changed wiring and components. Return the amp to factory state. After you have used the amp in stock trim you can decide on changes.
-Chris
Thank you very much, Chris, I think you're absolutely right. Actually, I was more worried about the safety of the mods in case I end up liking how it sounds as is. Like you said, only the modder would know what he was going for, but all the suspicious joints and series/parallel substitutes had me wonder if the modder actually had an idea himself.
From what I gather, these amps are quite frequently modded with a master volume and a different tone stack, so I thought maybe someone would spot something familiar in there 🙂
From what I gather, these amps are quite frequently modded with a master volume and a different tone stack, so I thought maybe someone would spot something familiar in there 🙂
Hi,
It's brave to assess things from pictures.
Did you only look at what appear to be the modded parts? It's best to trace the schematic as it is on a blank piece of paper. It's more work but you avoid assuming things that are different after all.
About the things you spotted:
- changes in bias resistors could have been done to accommodate new output valves that needed a different bias setting. There are no pots to adjust the bias. Another guess is that perhaps one of the mods is another type of outputs? Is it still running EL34s? 6L6 or KT88 would require more negative.
- the change from 68k to 32k increases the negative feedback around the power section. Resulting in cleaner sound. This also could have involved different output valves.
A mod (as in moderator 😉 ) probably will move this thread to the Musical Instruments forum where there are more people with experience with this particular amp.
It's brave to assess things from pictures.
Did you only look at what appear to be the modded parts? It's best to trace the schematic as it is on a blank piece of paper. It's more work but you avoid assuming things that are different after all.
About the things you spotted:
- changes in bias resistors could have been done to accommodate new output valves that needed a different bias setting. There are no pots to adjust the bias. Another guess is that perhaps one of the mods is another type of outputs? Is it still running EL34s? 6L6 or KT88 would require more negative.
- the change from 68k to 32k increases the negative feedback around the power section. Resulting in cleaner sound. This also could have involved different output valves.
A mod (as in moderator 😉 ) probably will move this thread to the Musical Instruments forum where there are more people with experience with this particular amp.
I compared the gut shot to the original schematic and photos of untouched units. Yeah, it's brave (and borderline stupid), but I'm just trying to get a general grasp of what's up with this one before I take it to the bench 😛
Might have to do with the bias! However, the bias resistors themselves seem untouched. The outputs should be stock, and the amp still rocks EL34's!
Ah, of course! Now that I had another look, I'm not actually worried about that part. It's of course a bit shoddy to replace a single resistor with such an arrangement, but I don't think there's anything really wrong with halving the negative feedback resistance.
Another thing I noted, the added resistor and cap in parallel with the 1K resistor on the board is just halving the cathode resistor. Not sure about the cap though, shouldn't that be unnecessary since the other half of V1 already has a DC coupled cathode follower, or am I way, way off here?
The parallel resistors in V1's grid should make the preamp stage a bit cleaner as well, right?
Ah, thanks for pointing out, stupid rookie mistake by yours truly!
The resistor in parallel to the 4k7 in the negative power supply makes the bias more negative, hence less current through the EL34s.
A matter of taste I guess. Personally I prefer less NFB than more.
Smaller cathode resistor = more current. Too much for an ECC83 I'd say. Because it reduces the dc difference between grid and cathode it can accept less ac before it distorts.
The added bypass cap increases the gain.
Both increase distortion/overdrive.
Those could be to suppress high freq oscillations.
Welcome on board 🙂
Thanks, that was extremely helpful! From what I've heard, this amp is notorious for high freq oscillation problems, so the added grid resistors would make sense.
I got the amp now, and here a couple more notes:
- The work done is actually a lot cleaner and sturdier than it appeared on the photo. Still needs some work to give me peace of mind, though.
- The 0.1uF cap parallel to the 4k7 resistor has apparently been replaced and then cut altogether:
- The 330 ohm resistor in the power supply looks burnt:
- The seller included a couple of spare fuses, although there was no talk of it burning them 😱
I got the amp now, and here a couple more notes:
- The work done is actually a lot cleaner and sturdier than it appeared on the photo. Still needs some work to give me peace of mind, though.
- The 0.1uF cap parallel to the 4k7 resistor has apparently been replaced and then cut altogether:
An externally hosted image should be here but it was not working when we last tested it.
- The 330 ohm resistor in the power supply looks burnt:
An externally hosted image should be here but it was not working when we last tested it.
- The seller included a couple of spare fuses, although there was no talk of it burning them 😱
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