FireFly amp working, but squeals

[ErikMiller]

I finally got my FireFly all put together earlier this morning, and it's sure a nice sounding amp.

I built the version with the cascode boost, and when I turn on the boost and turn up both knobs, it starts to squeal as the knobs approach maximum.

I can get killer usable distorted tones before it starts to oscillate, but if I'm supposed to be able to dime both knobs, I did something wrong.

I remember Paul M. having a similar problem with his FireFly; Paul, if you figured out how to fix it, let me know.

Thanks to Doug H. for the design and assistance, and Ansil for a much-needed part and assistance.

It is said that it can get louder with a 12BH7 in the output spot, and since I have half a dozen 12BH7's, I'm going to try to figure out how to do that. I'll ask over at the amp forum. I imagine it's just a change in one of the resistor values. I specifically chose a filament transformer with enough juice to power a 12BH7 in that position.

I know for sure that you can't just stick a 12BH7 in socket 3 and have it work. Don't ask how I know. :-)

[Joep]

Make sure you check you wiring. Try moving around wires a bit to see it this changes the frequency of the oscillation, or maybe stops it at all.

High gain amps are east victims for oscillation and it can be a pain in the @$$ to get rid of it.....

[Doug H]

Try the "chopstick test". Plug your guitar in, and set it down. Fire up the amp, set the gain up high so it will oscillate, and with one hand behind your back, use a chopstick or some other insulated stick or dowel to move signal wires around. When you wiggle a wire and the oscillation changes in some way, you have found the culprit.

At that point you need to either shorten that wire, move it away from the where the positive feedback is getting coupled into it, shield it, etc. FWIW, on my Firefly, I used shielded wire on all the signal wires going in and out of the cascode and into the first stage of the "normal" channel. Do it like we do for stompboxes- ground the shield on one side only.

Good luck!

Doug

[Joep]

It might be wise to solder the 33k resistors directly to the grids of V1b and V2a. That's an old trick that might help too.

[Doug H]

It might be wise to solder the 33k resistors directly to the grids of V1b and V2a. That's an old trick that might help too.

Yes,  all grid resistors should be tied directly to the grid pins on the tube sockets. I had a problem with oscillation until I did that with the 470K (R9).

Doug

[Paul Marossy]

I'm not the one who had the oscillation problem... I believe it was Alex C.
And, incidentally, I did solder the aforementioned resistors directly to the tube sockets. Maybe that's the why I never had any squealing problems. And I didn't even use any shielded wires anywhere in the amp.

I do get a fair amount of noise when everything is dimed, though. But I wouldn't normally play it that loud anyhow.

[Ansil]

glad to help eric...  i knew that one would work good for ya

[ErikMiller]

Good call, Joep.

The only grid resistor I had not soldered directly to the tube socket was the source of the squeal, revealed by chopstickery.

Now I've discovered that my voltages are too high, but that's another story I'm taking up at AX84 and Firebottle.

What a great project to learn more about tube amps!

[Joep]

GREAT! Enjoy the Firefly!

Are you using a centre tapped powersupply transformer? You can use the trick (with the zener diodes) Doug H uses to lower the voltage a bit.

Joep

[ncc]

...You can use the trick (with the zener diodes) Doug H uses to lower the voltage a bit.

Joep.

This comment got my attention  :?
What is the trick with the zener diodes?

ncc

[Joep]

Sorry, it was a bit unclear what I meant.

The trick is in the Baby Bassman, which you can find here:
http://home.cfl.rr.com/dbhammond/baby_bassman.gif

It's D3-D7, they lower down the B+ voltage.

[ncc]

Cool!
I see what you meant now.
Thanks for sharing this.

ncc

[ErikMiller]

GREAT! Enjoy the Firefly!

Are you using a centre tapped powersupply transformer? You can use the trick (with the zener diodes) Doug H uses to lower the voltage a bit.

Joep

No center tap; rectifier bridge.

I finally used a 1.2K 10W resistor on the B line, which helped, and then a 22V 1W zener between the - side of the bridge and ground.

The grit, crunch, and scream tones are great (very Marshall-y), but I must confess I'm not yet satisfied with the available clean tone.

I'm still trying to figure out what I need to do to get a 12BH7 to work in the V3 position. I guess it's an adjustment of the cathode bias resistor value.

[Doug H]

No center tap; rectifier bridge.
.

My understanding is you can do it with zeners in series on the B+ line too. Ask around ax84, they mentioned it over there.

The grit, crunch, and scream tones are great (very Marshall-y), but I must confess I'm not yet satisfied with the available clean tone.
.

It's not meant for clean. You can dial in the cascode low and use your guitar volume to get "cleanish". The "vintage" channel sounds great for driving with a pedal.

I'm still trying to figure out what I need to do to get a 12BH7 to work in the V3 position. I guess it's an adjustment of the cathode bias resistor value.

I think it's just a drop-in replacement, but you need the filament current budget for the heaters. Ax84 people have used them, so they should know.

Doug

[Lonehdrider]

Ears sprung up! Whats a Baby Bassman?

Regards,

Lone

Ps: I'll check that link from the schematic if I can answer my own question... Thnks..

[Doug H]

Ears sprung up! Whats a Baby Bassman?

Regards,

Lone

Ps: I'll check that link from the schematic if I can answer my own question... Thnks..

It was one of my wacky early Electar 10 mods. I will be moving that to the archive section soon and dumping the sound clip (need the space). I just built something with those electar parts I like much better. Film at eleven...

Doug