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Carlsbro Amps

12/12/1997 3:36 AM
Preben Hansen
Carlsbro Amps
Some time ago, i think it was in the start of october this year, a guy from Finland, Jukka, asked some questions about a Carlsbro CS 100 TC.  
I have a CS 60 TC head myself. 2x EL34 and 3x ECC83, configured in the same way that Jukka describes. I bought it as new, and have owned it for about 25 years or so. It served me well in the first years, a really nice sounding amp. For the last 15 years or so it has not been used, but now I have started using it again, because I have started playing guitar again.  
I have chaged some of the cabs, due to bad condition. Checked resistors and so on.  
I have discovered the same problems as Jukka describes, plates sligthly read, alot background hiss. The phase splitter valve, ECC83, is set up in a normal Fender/Marshall configuration, but a pot on the front, labeled LIMITER, is interacting with the 1 meg resistors in the splitter, and can adjust the output and "softclipping range", no diodes involved.. The setting of this pot seems to change the phase-splitting. Limiter set to max, causes the plates going read and volume down.  
This sounds really bad, sometimes the output is totally gone, and then comes slowly back.  
The amp has 2 clean channels each with high/low sensivity inputs. The signal chain is the following:  
input1 --- ½ECC83 -- Tone network1 - Volume1 --  
input2 --- ½ECC83 -- Tone network2 - Volume2 --  
Tone network1 has Bass, Treble and Responce,  
responce acts as some sort of Middle control.  
Tone network2 has Bass and Treble.  
Both volume controls are mixed at the input of the second ECC83. The first half is a normal gain stage, the second half is a cathode follower. The  
cathode resistor here is a 47k, connected to ground, in series with a 100k trim-pot. The wiper of this trimpot, (output of the preamp) goes to the phase splitter. (I think this trimpot is a hidden master-volume). The Presence control goes from input to output of this second valve, (i.e from grid of first triode to cathode of second triode).  
I am going to mod this amp in the following way:  
1. Separate cathode resistor/cap on each input stage. Perhaps a switch to change cab values.  
2. Move the Precence control to the feed-back loop.  
3. Disconnect the Limitor function.  
4. Install Master volume pot on front, at the place where the Limiter was.  
5. Experiment with input grid resistor on EL34, it is now 10k.  
6. Experiment with Triode/penthode switch.  
7. Experiment with fixed/cathode bias. Bias voltage comes from separate windings at the power transformer.  
8. There is a spare hole in the chassis for a valve socket. I am thinking of installing a gain stage, ECC83 with relay switching, to make some sort of "Brown Sound", and an effect-loop for my Dynacord S65 Tape Echo Machine.  
Any further suggestions, exept for reverb and tremolo ?  
And now the god news: Carlsbro has a web-side, under construction. I e-mailed the sales department, and told them my problems. They asked for my home address, and a few days later a nice copy of the schematic arrived to me, at my address in Denmark.  
Carlsbro is still im bussiness, making amps. They are known here in scandinavia of making good amps.  
I hope this help's Jukka, and anyone else that ownes a Carlsbro Amplifier.  
12/15/1997 9:06 AM

I'm glad ,that someone noticed my message :)  
My CS 100 TC did have an open grid resistor,  
on the one EL34 and by then the tube was shorted by an overload,  
causin burned screen res and OT winding also (last one I did managed to fix,luckily).  
Mine has no "Limiter" pot ,but I think you've  
got somekind of a bias shifting problem with the power tubes,if the plates are going red ,when the pot is turned to max.  
You did checked those coupling caps after phase splitter,right ?  
Mine lacks the trim pot (the one you described  
as a "hidden master-volume"),no presence control,  
just a "Top Cut" pot (with a hi cuttin' cap to the ground ),no "response" pot at all just a bass and a treble in a tone network of each channel (I think ,this model is designed for a bass)  
This one seemed to use some more gain (added 12ax7 ) at the preamp ,I did it,sounds great ,(cranked marshally like somehow),maybe some kind of a diode clipping circuit is going to be in a question ? I added a NFB loop on/off switch too,and the amp sounds different and of course ,louder ,not necessarily better tonally when the NFB is switched out  
Let us know ,about your experimentations. (exspecially positions 6 and 7)  
12/16/1997 1:36 AM
Preben Hansen

Hi Jukka.  
Thank's for yuor responce, and hints.  
I will check the caps between phase splitter and output tubes once again.  
By the way, have you mailed Carlsbro sales department ??  
A friend of mine, bass player in my band 25 years ago, had a CS100 TC. A 100w head. It was a 60TC with 2 more EL34 added in the output section. It did'nt have the limiter pot, but instead there was a SLAVE output Jack on the front.  
It sounded VERY good with a FENDER JazzBass, and 2 homemade speaker cabinets, each with 4 10" SEAS Speakers. I dont remember any other differences from the 60TC.  
Fore about ½ year ago, I discovered a CS 200TC in a little musicshop. It had the same "front" as the above mentioned CS 100TC, but cabinet a bit larger. I was determined to buy it, it could have been the perfect base for mods, but the day after it was gone; price 2500DKr.  
If you want to add a diode clipping circuit, look for JCM900 preamp on this site. Or you could go for Peavey 5150 preamp alike design.  
About point 6 and 7 in my previous mail, i have some ideas:  
6. I will experiment with the configuration shown in Marshall JCM900 50W power stage, triode - penthode switch, (half power switch). I have to make some calculations conserning the 10k control grid resistor, and the screen grid resistor, to keep the output tubes operating within specs.  
It is the original MULLARD tubes, so I want them to live as long as possible. (I have 2 spare NOS Phillips, sounds fine too).  
7. Kevin O'Conner outlines some soloutions conserning this topic, as can be seen at the London Power on this net. There are 3 soloutions, primarily for old Fender amps, but none of them tell's anything about the bias comming from a separate winding on the power transformer. Here I also have to calculate a little bit to ensure the correct bias value for both triode and penthode mode.  
I will let you know the results of my experiments, some time next year.  
12/17/1997 3:02 AM

Hi again ,Preben  
>By the way, have you mailed Carlsbro sales >department ??  
No I haven't ,yet ,thanks for the info  
The "SLAVE output" jack as you mentioned isn't  
available on my machine .There must be some amount  
of a sub revisions under the model descriptor CS100TC or it's just related to the make era ,BTW I haven't been able to dated my amp  
exactly , make from the someday of the high heels 70´s ,as you said you bought yours 25 years ago  
Maybe those guys in Carlsbro can help me out .  
CS 200TC ? what a monster that could have been :)  
I already checked those JCM schem's ,thanks anyway  
I've got 4 Philips NOS tubes too , but haven't used those in my experimentations ,nor repair testing either ,I'm using cheapos (Chinese National's) for that ,actually chinese ones are not too bad at all  
Other things what we could do ,is adding a fuse and a "Stand By" switch for the B+ ,  
good place for a switch is in the indicator bulb  
place, (at least on CS100TC)  
Happy Xmas and New Year  
12/17/1997 3:59 AM
Preben Hansen

Hi again Jukka  
I will try to find out when I actually bought the  
60TC. Maybe I could find som old notes in my archive.By the way, then I will look inside the amp for any hints concerning "date of birth".  
My 60TC HAS a Stand By switch, and there are a fuse for the B+.  
I will explain it, the following way:  
Power transformer: single vinding, no center tab.  
Bridge rectifier: 4x BY238 diodes  
goes to H.T.Fuse, 1Amp anti-sourge - signal gnd.  
2 single 50uf in series,(25uF total) to signal gnd.  
Positive: to stand-by switch  
Paralel to the 2 terminals of the stand-by switch are mounted 10 Kpf capasitor, (spark killer)  
The other terminal of the STB switch goes to the the filter-choke. Parallel to the choke are mounted a 1K 2w resistor, ( to kill reverse current when switch are operated)..  
The other terminal of the choke goes B+ connection of output transformer and to 2 single 50uF in series to gnd, ( 25uF total).  
You can try to make this arrangement in to your amp over the chistmas, I am going on a small vacation during Xmas and new year, so happy Xmas and new year to yuo too.  

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