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Parts for a Vacuum Tube amp

Discussion in 'Electronic Basics' started by clRedwolf, Jul 7, 2003.

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  1. clRedwolf

    clRedwolf Guest

    I have a question (don't we all?). I am building (or at least trying to
    get the parts to build) a 'DC Darling' tube amplifier:
    I'm buying the parts I can from Mouser. So far, I have all the stock #s
    written down except the parts I can't find. I need help with these parts.

    First of all, I have only made 3 changes to the schematic, all minor. The
    biggest one was to change it to where the wire between VT2 and VT12 doesn't
    also connect to the left audio transformer (that appeared to be a mistake).
    The others are of no concern (changed Rg1 & 11 to a phenowhatsit (variable
    resister) as a volume knob, and used a DPST power switch.

    Okay, now for the part I need help with. First off, realize this is my
    first time working with AC circuts, but I feel that I can do so safely now
    (if all else fails, I use chopsticks to work with it ;) ). Here's the
    parts that I can't seem to find:

    R2 (1R 5W) - What does the R stand for, as none of the resisters at Mouser
    are rated by that? Or is that a typo for K?

    T1 and T11 - Okay, I know squat about audio transformers. But even then...
    The schematic calls for a 4.8K Primary 15W transformer. I'm going to take
    a wild guess and say that 15W is the for the output, but what the heck is
    the 4.8K primary for? Is that the impedence of the primary? Where would I
    get such a transformer?

    L1-A 5H 75mA Inductor. Mouser doesn't sell them over 2 H it seems. I'll
    look at other places...

    C1a and b - 47 uF 500V capacitors. I just need to go somewhere besides
    Mouser. Know any other *decently priced* electronics vendors? decently
    priced=cheaper than, ugh, Radio Shack.

    Thanks in advance. Oh yeah, and Hi!
  3. clRedwolf

    clRedwolf Guest

    Ahh, thanks! 1 down 3 to go...
  4. Good luck.
  5. google search for "hammond manufacturing" and "antique electronic
    supply" or go here and ask your questions in the antique radio

    also.....this is the best bet for old radio parts.....

    this guy has it all for old radios....I hate giving up this source but
    I am kind person. scroll thru his transformer list.... 30.00 min
    order....he wont answer questions or give must be
    succinct and business like when dealing with him....he is the "soup
    nazi" of old radio parts but he is the best in my opinion.

    ebay carries a lot of tube stuff...just use the search engine or look
    for the tube radio parts section but searching will be your best bet.
  6. crooksie

    crooksie Guest

    why not email bob himself for the info ?
  7. Rich Grise

    Rich Grise Guest

    Actually, I'd say it's rather important - surge protection, yes,
    but even after the heaters are warm, it still participates
    in the voltage divider.

  8. N. Thornton

    N. Thornton Guest

    But there is no need for any such 'surge protection'. Few commercial
    amps use a 1R to reduce the heater surge a bit, as there is no reason
    to. These just aren't particularly well designed amps. They lack a
    fairly essential safety feature, and include assorted sillies.
    I couldnt see any voltage divider on the cct diag which the 1R is a
    part of, nor can I come up with any need for one. Perhaps you could

    To the OP, I would pick the 2nd circuit, labelled 'latest one' rather
    than the dc one. The dc offers no real advantages, and will just
    consume more power and generate more heat. It is also very vulnerable
    to dc on the input, to which it would probably react with a meltdown
    or a fire.

    I built basic valve amps like these some while ago, and was surprised
    at how good the results were, but these days people seem to get weird
    over the designs, and specify 2H chokes when theres no reason for a
    choke at all, and so on.

    I would also use pentodes rather than triodes myself, and I'd use one
    valve per channel rather than 2 or 4. But the design should work OK
    with triodes.

    Regards, NT

    PS always discharge the HT caps before touching anything.
  9. Rich Grise

    Rich Grise Guest

    (N. Thornton) wrote in message
    It could be argued that it protects the cold filaments from a high
    inrush current, which could shorten the tubes' lives. Maybe "inrush"
    would be a better choice of word than "surge." :)
    Happy to! The only 1R I'm aware of was the one in the DC filament
    supply. The voltage divider is the 1R in series with the filament
    array, which is the other resistor of the divider. Tube filaments,
    especially power tubes, consume significant power. About the only
    reason I can think of to do it that way would be either that they're
    able to use a conveniently-available transformer for odd-ball
    filament voltages, or it could be to scale down line variations.
    I didn't delve into it that deep, so really can't give an opinion on
    that item. :)
  10. N. Thornton

    N. Thornton Guest

    It could be argued, yes, but its been found to make no significant
    difference to filament life, thats why it isnt done. If tube life
    could be even a little extended this way we'd have seen heater Rs in
    some high end commercial amps.

    yeah, I dont know the heater V rating on those tubes. Logically I'd
    simply pick the right transformer for the job. I would think the odds
    are the valves'll be 6.3v anyway, but you never know.
    I think it would actually worsen things in this respect. Valve heaters
    are to some extent self compensating, as temp goes up R also goes up.
    The 1R OTOH has no such stabilising effect, and thus reduces the
    amount of stabillisation of heater power.

    Now if the amp were using a series capacitor straight on the mains to
    provide heater power, then yes I'd be all for the series R, and it
    would extend tube life. But the way its set up, I think in truth its

    Also I'd be in favour of a small R in the HT line, so that if the amp
    is wired wrong the R pops instead of the valve heater - but thats
    absent. I think really they're just not very good designs. Should run
    OK tho.

    Regards, NT
  11. clRedwolf

    clRedwolf Guest

    Thanks all! This should really help. In response to using junk parts,
    there's one problem with that...I don't have much in the way of that.
    About 6 months ago I did, but since I never found a use for them...I took
    out some 'stress releif'. I know it was bad...but it was fun!

    Anyway, I should start soon. I just need some money to burn on
    this...which I *should* get later this week (or month, knowing my luck).
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