Modding/Converting 100w Carlsbro amp

[JayCobie]

Ahoy! First time poster here.

I've got an old Carlsbro TC-100, 2 channel 100w amp head from circa 1969-1972 (I think) that I don't find useful but I can't allow myself to sell off.
It has a pair of original Partridge transformers that can take 110-230v input, as well as outputting in 4, 8 or 16 ohm so I feel obliged to somehow make those work well in an amp that I want to own and use.

Here is the scematic: http://www.chambonino.com/carlsbro/carl100tc.html
I've heard that it's close to being an old JTM Plexi but it has some small differences, I believe.
That schematic also has a Gain pot before the slave output/phase inverter that I don't have on my amp but the rest is the same.

The amp works fine as it is but it's 100w and it has no master volume. Good lord is this thing loud.
It has a "Limiter" pot that I've never seen on any other tube amp (pre-phase inverter volume?) that does a little bit of damping but it's not a volume pot and it doesn't damp enough.

Either I need to add a master volume or I need some sort of low power/voltage scaling/bias changing to tune it down to manageable (i.e. Useful) volume levels.
What would you guys recommend? I would love to be able to somehow have power tube distortion but I don't want to pull tubes if it means possible damage to transformers.
Is post-phase inverter master volume the way to go? I've never heard how voltage scaling affects a tube amp's volume, would that be the way to go?

[PRR]

Welcome.

> I can't allow myself to sell off.
> Good lord is this thing loud.

Hasty lookover-- sell it to someone who plays stadiums without PA. It's a beast. You can't turn an elephant into a lap-pet.

The "limiter" strangles the driver stage, an attempt to limit the power output. Such things can work, but proportions are critical. Fiddling this could be deafening.

> a Gain pot before the slave output/phase splitter that I don't have

Easy to hack that in; however the stage driving it has gobs of headroom so it may not reduce the maximum.

[JayCobie]

Thanks for the reply!

I live in Iceland and I'd get pennies for this amp. Already tried selling it for a reasonable price (400 bucks) and there were no takers. The only way I'd sell it is if I'd stoop to a 200 buck price range. I'd rather just use the amp as an experiment.
I atleast want to see how a master volume makes this thing behave. If it doesn't even make it into a semi-manageable high-headroom pedal platform then I'll change it back and sell it.

Anyone able to tell me what values I'd use for a post-phase inverter master volume?
I'm assuming I'd use 2 ganged pots for a volume control and I've heard of using caps to mitigate high end loss. I'm just lost when it comes to calculating values. Anyone able to help?

[TIMBO]

Hey mate, This might give you some ideas.
http://el34world.com/Forum/index.php?topic=20932.0

[tubenit]

If it were mine & I liked the tone overall but wanted less volume,   I'd go cathode biased and remove two of the EL34 tubes and install a PPIMV.   I think those would be very easy mods to make.

You could also install a pre LTPI master volume.

OR you could gut it and build it into whatever you wanted.  You could do a Plexi 50w with an active FX loop or one tube reverb. 

The topology isn't much like a Marshall at all with a James tone stack after V1  vs.  typical TBM Marshall tone stack off a cathode follower.

With respect, Tubenit

[tubenit]

He is an idea that would work with the number of tubes you have.   Paralleled V1 for 30% more gain and no increase in floor noise.

An active FX loop that also acts somewhat as a pre-LTPI volume control.   A PPIMV.   Going cathode biased for less volume with only two EL34's.

Not pushing an agenda, just offering a consideration. 

With respect, Tubenit

[tubenit]

Again, making use of all the preamp tubes and keeping PPIMV and cathode biased.

This would use a pseudo Dumblish tone stack which has a mid-boost and a preamp boost that could use a foot switch to active relays.  I have midboost and PAB on both of my amps and like these features a lot. They add a considerable amount of fatness with some overdrive even on the clean channel (before the OD channel is kicked in).

with respect, Tubenit

[PRR]

> I live in Iceland

OH!!

I thought I'd have a hard time finding it a new home here in coastal Maine, where venues are small and everybody is poor. Yes, even tougher where you are. Have to wonder how it even came to Iceland. Maybe one party THAT big per year?

That OT impedance, it isn't going to cathode-bias happy unless you waste-off a lot of B+, like down to 300V-330V. Whether VVR or big resistor, a lot of heat. (But this is less a problem for you than in say south California.) It would run the 30W-40W output range (for 130+W of heat!). Is 30W that really much less than "good lord loud!"?

[tubenit]

The Matchless Chieftain uses EL34's that are cathode biased at around 455v, I think?

The Matchless Clubman uses EL34's that are cathode biased at around 430v (if I am reading the schematic correctly)

http://el34world.com/charts/Schematics/files/matchless/matchless_chieftan.pdf

http://el34world.com/charts/Schematics/files/matchless/matchless_clubman.pdf

350v X 1.3 GZ34 (5AR4) rectifier = 455v.

With respect, Tubenit

[DummyLoad]

and a 6.6K ra-a OT with 450V B+ and go to go. live on the edge, use a 5K ra-a.   


--pete

[DummyLoad]

jay, that is a collectible amp if left unmolested. you should just consider a power soak and leave it as is, or sell it to an ensku or trade it for something more to your liking. just my worthless 2 cents worth.   


respectfully,


--pete

[MartinB]

I had a four channel Carlsbro PA from the same era with a similar circuit, minus the presence and limiter controls.  It made a good clean bass amp; good volume for 100 watts and it didn't break up easily.  I think maybe that's their niche, and I regret selling mine.