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Thread: The carlsbro 100 pa/r amplifier

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    Red face The carlsbro 100 pa/r amplifier

    Hi

    This is my first post.. I'm new to the land of valves really. Ive got a Calrsbro 100 watt Valve PA Head which i use for bass in a 60s/70s band in UK.. Its got a set of quad JJ's EL34s. Is there any tweek i can do to make it better for the bass. I aint touched the biasing ..It seems to be over driven, possibly without being that loud. As i only play with drums and guitar...I use Fender P Bass with Bareknuckle pickups and the amp goes though a warwick 2 x10" Looking on line today saw videos about changing caps/ resistors in tone stages. any help with biasing too would help... I only have a Multi meter.. I dont have an Silly scope or signal gen's
    can anyone help ?

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    Old Timer olddawg's Avatar
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    Assuming that this PA amp is working correctly, (Easy enough, run a mic into an input and see if it is clean and loud) I would just get one of those $25 Art tube preamps and run it into a line level in jack or main amp in. The Art preamp even has a voicing switch with a bass selection. This way you don't need to modify anything or waste any time at all.

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    Hi.. Many thanks the amp is working fine. I will look into that Art tube John

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    Noodle of Reality Steve Conner's Avatar
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    Back in those days, PA amps, bass amps and guitar amps were all kind of similar. If the power tubes haven't been replaced in many years, they may be worn out and a new set could make it louder and cleaner. Failing that add more/bigger speakers.

    The schematic:
    Carlsbro 100 PA/R Amp Schematic

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    Hi many thanks i dont want more speakers as i live in 2 nd floor flat.. I have replaced valves new set of JJ's.. will have a go at Biasing i will changed the caps anyway i think.. thanks for the schematic .. i already downloaded it from champ

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    Here's a link for the Art preamp. They make several variations. I have one that has a selector switch for a bunch of different voicings including bass for the same price:

    http://www.musiciansfriend.com/pro-a...dio-mic-preamp

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    Bent Member Chuck H's Avatar
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    I don't think a preamp that's more refined is a bad idea at all. But I don't see how it's going to make the amp louder. The OP should add a 15" speaker to the pair of 10"s he's using now. No rules about when he needs to use all the speakers. That is, only use the big guns at practice and gigs.

    On the note about adding speakers. It's possible that the load your 2x10 cab presents is less than ideal for the amp. A good impedance match is important for max power. After that I'll +1 on needing good tubes. A proper bias and good filter caps are also paramount to achieving max power.

    There are some minor changes that might be made too. But it won't amount to a hill of beans compared to what's already been covered. Different film cap, types or values and different resistor types or values absolutely will NOT make the amp louder.

    If the amp is tube rectified AND the other components in the amp are up the it you may be able to change to a diode rectifier. This could allow more filtering (better for bass) and an increase in voltage (tighter bass, more power).

    On the note about NOT wanting to add more speakers... If the amp gets louder because of more speakers OR because of circuit changes, what's the difference??? The amp will still need to be turned down more to practice at home, right?

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