Firefly build

[xtian]

I found a problem. Measured resistance between ground and HV wire 1 (where it connects to diode 1) is 141ohms, which I expect because when not hooked to anything, the resistance between the two HV wires is 276ohms.

BUT between ground and HV wide 2 (where it connects to diode 2), the resistance is, um, 2ohms (about the resistance of my test leads). Ouch.

Did I read somewhere it's easy to burn up diodes when soldering them?

[Firestorm]

Sure looks like one of your HV leads is grounded at the terminal strip ...

[M Fowler]

Why do the filter caps ground at the tone board?

[xtian]

Firestorm: Holy patron saints of amp builders, you're right! I was so careful when I was putting the caps into the terminal strip, and then I was blind when I put the diode in. Thanks!

Fowler: The tone board has a ground bus, but it's not grounded TO anything (none of the front panel controls, jacks, switches are grounded to chassis). So I ran that black line from the tone board ground bus to the filter caps star ground point. OK?

[xtian]

It works! But very bad hum. Here are all my voltages:

B+1 = 253 vac
B+2 = 210
B+3 = 200

V1 = 155/89/0.8/89 vdc (pins 1/3/6/8)
V2 = 147/1.1/120/0.8
V3 = 250/8.6/250/8.6

Looks OK?

How do I diagnose that hum?

[M Fowler]

I would ground the caps to its own lug on the chassis away from the volume and tone control.

You made just need to move some wires around with a chop stick to see if it helps. With out a cover on the chassis it could be many things causing the hum, even a tube or tubes.

Mark

[Firestorm]

For sure I'd get the main cap ground away from the preamp cathodes. It's single ended right? You might need to float the AC heater voltage on top of 30-40 volts DC to shut up the heater hum. If the hum is 120Hz, you might add another cap and resistor before the plate supply cap. An extra stage of filtering kills a lot of buzz. Both the cap and the R could be fairly small.

[xtian]

Good suggestions, guys, but I solved the hum by identifying my last remaining bonehead mistake. I forgot to install the two 100ohm resistors from filament heaters to ground. No more hum now!

It works great, with the exception of motorboating and squeal when the Volume is opened all the way. I'll do some research on motorboating, but let me know if you have any ideas.

Thanks, guys! This amp lives! I'll post more photos and audio clips after I receive the wooden base.

[M Fowler]

Did you get rid of the motor boating and squeal?

[xtian]

Did you get rid of the motor boating and squeal?

No. I'm using the 12ax7's from the Packard Bell (they're actually labeled "Packard Bell"). As an experiment, I replaced these with 12ax7's from my Mesa Boogie MkIV. Much worse! Turning the Gain up past 12 o'clock causes a lot of ugly noise when playing (nice and quiet when not playing, though). With both tube sets, there's an element of fizz that stops abruptly when the note drops below a threshold.

I read about this somewhere…cathode lock? It only happens when there's too much signal voltage, and the valves go into an epileptic fit?

[M Fowler]

With or without the boost engaged? Do you have any other 12AU7's?

[xtian]

Here's some audio: Strat > Firefly > 2x12 cab > SM57 > MOTU 828 mk3.

Audio clip opens with two long chords to demonstrate the fizz. Next, a series with increasing gain; at 0:30 it's TOO MUCH gain, and the amp farts. The switch you hear at 0:50 is the Boost switch, and later I turn the Boost gain up. Finally, a little AC/DC at a good working setting.

I should be able to fix these issues, shouldn't I?

MP3 attachment didn't work? Try: http://www.monkeymatic.com/pub/firefly/ ... audio1.mp3

[xtian]

I'm using a $16 JJ 12au7. I don't have any others. I want to order an ECC99 (I put in a switch for the bias). I hear this might be better. Any suggestions?

[M Fowler]

Well I can't get pictures to open up on ampgarage anymore not sure what the problems is but they do not open up to view full picture. So I do not know if there is a wiring errors or just blocking distortion.

[xtian]

Here are photos of the finished wiring:

http://www.monkeymatic.com/pub/firefly/fireflyB1.jpg
http://www.monkeymatic.com/pub/firefly/fireflyB2.jpg
http://www.monkeymatic.com/pub/firefly/fireflyB3.jpg
http://www.monkeymatic.com/pub/firefly/fireflyB4.jpg
http://www.monkeymatic.com/pub/firefly/fireflyB5.jpg