firefly cap q

[jimmy]

hello

i was wondering about this 0.02u/1KV cap in the firefly schem. does this HAVE to be 1KV??? i cant get them anywhere close by. would 630V be alright? and also with the power supply caps (22uF) can that be like 160uF because ive got a few of those lying around.

cheers
Jim

[Peter Snowberg]

630V will be fine for the .022uF. For the PS, there isn't any reason you couldn't use the larger caps, but you'll end up with more bass response and a MUCH darker sounding amp. It may be too much. It may also be just right.

Just make sure they're all high enough voltage and that the resistors and diodes in the power supply are oversized. When you switch on from a cold start the caps will all look like dead shorts until they take a charge. The surge this places on the diodes and supply resistors can be enough to explode them.

You can always use two caps in series, with a 470K 1/2 watt bleeder resistor across each one. That will give you 80uF units. Are you using photoflash caps? If so, I would wire three in series (each with the bleeder) just to get a solid high voltage rating and less capacitance. Those things aren't made with the same care as "regular" caps. (I like running at Fender Super voltages)

You'll have to let your ears too you how well this substitution works. It will work better in some PS stages than others, but it will work.

-Peter

[jimmy]

yup im using photoflash caps. so i wire 3 in series for each of the 22uF caps in the schem? and the bleeder resistor is just a resistor that shorts the legs of the caps together isnt it?

oversized diodes you say... is there a difference between a 1N4004 and a 1N4007?

cheers
Jim

[Peter Snowberg]

If you use 3 in series, the value you'll end up with will be roughly 1/3 of the uF value so you'll see a hair over 50uF here which is great for an amp with a full sound. If you have enough caps, you could use 4 in series and get down to roughly 40uF which for my ears is just about perfect in most cases. The tolerance on thise things is LOOSE! check them with a meter if you can. You can always bring your caps to RadioShack and test out their nice meters too.

Correct on the bleeders. They're just resistors (~470K 1W ideally) resistors wired across each cap. The value is big enough to not affect operation, but small enough to bleed the charge out when it's off.

For HV diodes, I like the 1N5407 which is rated for 3A at 800V. They're cheap in quantity too. You can also use two 1N5404s in series (3A @ 400V), but each one should have a 220K 1/2W resistor and .01uF HV cap across it. 1N5407s are super strong and robust in tube amps (and laser power supplies). I don't like 1N400x diodes in HV circuits with big caps. The voltage isn't the main issue; it's the surge current. I've poped too many 400x parts.

Good luck on your build!

-Peter

[jimmy]

excellent!! and another quick q, the pots im intending to use are these alpha ones that are rated at 1/2 a watt MAX.  all good?

cheers
Jim

P.S. Peter, you'll get a thankyou in the operating manual  

[Peter Snowberg]

Those should work just fine. As long as you don't try to use them for adjusting biases or high voltages (I think they're only rated for 350V MAX), you sholdn't have any problems at all.

Thanks for the kind words Jim, but if anybody should get a thank you, I would nominate Aron.

-Peter

[Doug H]

If you're building the original FireFly, note that the 3 22u's feeding the output stage are in parallel. That amounts to about 66u of capacitance. The only reason I used the 3 caps in parallel is because that's what I had lying around. If you're building the FireFly Boost version, I use 47uf's for all the filter caps. The boost circuit required more filtering to tame the hum and noise.

As Peter mentioned, you really can't go too wrong by using larger value filter capacitors, but you might notice a tonal difference. In my experience I have found that the preamp filter caps are more sensitive in this way than the filter caps which are feeding the output stage. I have found I can adjust bass response pretty effectively by just tweaking the size of the preamp filter caps.

Doug

[jimmy]

YAY!!! today i drilled the chassis and mounted all the jacks, pots, switches, lights and transformers, except the output transformer because thats still in the mail :x

its starting to look like a tube amp, and im getting way pumped up for the finished project. 200 bucks well spent.

cheers
Jim

[Ansil]

doug when can we get a layout for the firefly....  is it possible, or am i just being greedy..

[Doug H]

doug when can we get a layout for the firefly....  is it possible, or am i just being greedy..

I never did a layout because I wired it point-to-point. I had a layout when I started then I realized I wired all the tube sockets upside down and bassackwards. So it ended up being "plan as you go" on terminal strips. That's how I did the booster mod too.

Doug

[Ansil]

i know i remember asking you, but i was wondering if there was one.  someone else had done.

[jimmy]

ive got a pretty solid layout on tagboard. if youd like to have a look let me know.

cheers
Jim