5881 cathode resistor in a DIY Magnatone 260 amp

[dinkotom]

Hi,
I've just completed building this amp
http://www.schematicheaven.com/barga...atone_260a.pdf
and I don't have a 250 Ohm cathode resistor, but I can put a pair (paralleled) of 470 or 560 Ohm. With a 470 Ohm pair I'd get approx. 21W dissipation per 5881 (Tung-sol reissue) - is that OK, or should I try a 560 Ohm pair?

[RWood]

Using the two 470Rs in parallel looks okay on paper, as that's 91% of max and with a cathode-biased power amp you can go up to 100%, or 23 watts for this tube.

BUT

why not try both and listen to the results? You can measure actual current draw in the circuit, too. Using the 560Rs in parallel would lower the current somewhat but will increase the plate voltage, so I suspect the differences between the two will be minimal.

Where did you find varistors for your build?

RWood

[loudthud]

At 21W you are very close to the 5881 rating. Traditionally, the 5881 was a very rugged tube that could withstand abuse. How good the reissue can withstand abuse is unknown. You might try the 560 ohm for a slight reduction in dissipation but in the long run it probably won't make any difference.

If you can, please post pictures of your amp. It's always interesting to see builds from outside the USA.

How does the vibrato work? What did you use for the varistors?

[tubeswell]

(edit - found it another way) What type of "5881" is it BTW?

[dinkotom]

thanks for replies!
I tried 3 values of resistors, and you are correct, it sounds the same to me...
I used Zenith 63-5314 varistors (from Talonix.com, no longer available), the vibrato works fine. I'll also try a quartet of original Maggie varistors I bought at ebay from Warbler Muse (Juke amps owner) to see if there's any difference to vibrato.
but in the meantime, it seems I have a textbook case of parasitic oscillation - the sound cuts out when I hit the E note on the third string, so I have to bring the amp to a friend with oscilloscope to fix this.
if you look at the wiring, no wonder it oscillates, (though it doesn't look worse than in a real Magnatone, I must have done something wrong):

[Diablo]

I used Zenith 63-5314 varistors (from Talonix.com, no longer available), the vibrato works fine. I'll also try a quartet of original Maggie varistors I bought at ebay from Warbler Muse (Juke amps owner) to see if there's any difference to vibrato.

Do you mind telling us how much you paid for the varistors from Warbler Muse? I heard they are fantastically expensive.

[dinkotom]

$32, but the last time I saw them listed, they were $50 if I remember correctly