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Thread: 5C3 harp amp question

  1. #1
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    5C3 harp amp question

    hi,

    i am considering a 5C3 Weber kit for a harp amp. would like to get the most headroom i can. would a 5u4 and 5881 tubes work? what speaker would be your choice in this amp? any suggestions greatly appreciated.


    thank you,
    maxamp1951

  2. #2
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    To be 100% honest, I don't know that many serious harp players the really like the tweed Deluxe for harp.

    The 5C3 is a bit different of course.... I don't know if the current run of Weber transformers for the 6V6 amp can hold up to the 5881s and a 5U4... I would think so but ???
    I'd ask them first.
    You must know they do sell a 5C3 P 6L6 amp kit that you could convert to harp playing.
    For harp playing, I would bias the power tubes colder then the 250-270 ohm resistor normally found would allow.
    Speaker for harp... it depends on your playing level, technique and the mics you will try to use, but you can't go too wrong with a 12A100-O or 12A125-O from Weber... other speakers from other vendors are just as good but sound different. Do another search on Google.
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

  3. #3
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    What Bruce says...

    To realise the additional potential power of the 5881s you need to significantly raise the B+ voltage, so a 5C3P style set up would be the better option. As you say that you want "most headroom as I can" I would be tempted to aim for a PT with a B+ secondary of around 375-0-375VAC (for a plate to ground voltage of ~500-510vdc, or maybe a shade more (speak to the PT manufacturer, you might even be able to bump up the B+ by using the next lowest wall AC primary for your region - go by heater voltages for the limiting factor, aim for <6.9VAC, assuming that you will not have a fixed bias option & therefore no negative voltage supply).

    Start with a ~680ohm cathode resistor (10W minimum), try it bypassed & unbypassed with an electrolytic cap (100uf/100v). I would also try a switch that shorts out the grid of the "out of phase power tube" (next to the rectifier), DO NOT TRY THIS IF YOU HAVE A FIXED BIAS OPTION. With an unbypassed cathode resistor the amp will now run in self-split, it will have a more transparent & touch sensitive tone. With a bypassed cathode resistor, shorting the "OOP" tube grid will give you a single-ended (champ-like) option for more grind at lower volume. Between the "cathode resistor bypass switch" & a "OOP tube grid short switch", you get 3 modes of operation.

    I'd also set the minimum cathode resistor value using a solid state rectifier, aim for a workable compromise with the 5U4 fitted. Perhaps >15W dissipation with the 5U4 and <22W with the SS rectifier? The softer sound of cathode bias usually does not necessitate a tube rectifier in a harp amp.

    Rewire the volume pots as regular voltage dividers, signal from the 1st stage in on the RH pot tab, signal out from the pots out on the middle tabs via 270K mixer resistors (look at the 1st stages of a tweed bassman), tone pot goes at the junction of the 270Ks with just a hi cut cap to ground from LH tab, no 500pf bright cap.

    I'd maybe also consider a 12" to 10" adaptor plate for the baffle board maybe try some higher headroom 10" speakers, like the Fane AX10 ceramic/Medusa 75? If the cab is a little oversized (5E4-A sized), you might fit 2x10"?

    Unless you solely use crystal & ceramic mics I would convert the 1st stage to cathode bias rather than grid leak, as CM/CR & dynamic mics aren't so compatible. Use a 1K common cathode resistor (again see the tweed bassman, or the 5D3 Deluxe for an example, keep the 5Meg grid resistor values, you can also keep the 250K plate resistors.

    Personally, I'd aim to use 9 pin preamp sockets as you will have much greater flexibilty of potential tube choices compared to octal twin triodes.

  4. #4
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    thank you for the information! i am a novice amp builder. i'm not a harp player but did a build for someone. it was basically a tweed deluxe circuit with grid leak octal front end, 5d3 phase inverter with the 56k resistor through a 100k pot as per gerald weber, cathode biased 5881's , 5u4g, in a pro sized cab and original jensen c15n. it sounded amazingly good and he didn't have the feedback issue he had with other amps. yes, every player has their own style, and this amp suited him.
    i think i understand most of the technical info you shared. wow, very cool options on the power tubes and preamp tubes. rewiring the volume pots as voltage dividers i don't quite get, but i wiil look at at the tweed bassman layout and maybe it'll make more sense. lots of new thinks to try! thank you again!

    maxamp1951

  5. #5
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    oh yeah, i used ted weber wo25130 power, and w404248 trannies. i think i got 425 volts on the plates, 340-340 .

    maxamp 1951

  6. #6
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    ok, another question. in eliminating the 500pf tone cap , could you also use a 250k tone pot instead of the 1meg? how would that affect things?

    maxamp1951

  7. #7
    Old Timer
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    I'd want more than 425vdc (plate to ground) on the 5881 plates if it's a stage amp, unless the band is very restrained. 425v will be OK for a recording/mid powered amp.

    Yes, a 250K tone pot would be a good idea, will give a more useful sweep on the tone pot than 1Meg.

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