Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Voxbox on October 27, 2010, 04:16:40 pm

Title: Firefly with dc on input
Post by: Voxbox on October 27, 2010, 04:16:40 pm

Hi All

I've just built my first Firefly, Rev.3 and noticed that I get dc on the input when the boost circuit is switched in. It's ok when in clean mode.

The dc is -0.4V which causes crackling when I turn the guitar's volume pot. :sad:

The pre-amp voltages when in boost mode are here -

voxboxffly.htm
If the link doesnt work, here are the voltages anyway

V1a
a 204V
g 26V (and decreases the longer the probe is left on)
k 32V

V1b
a 32V
g -373mV
k 298mV

Is this a "function" of the cascode boost pre-amp, or is there something else going on?
Help!

Title: Re: Firefly with dc on input
Post by: Tiny_Daddy on October 27, 2010, 07:14:33 pm

Have you tried a different preamp tube? Or swap the tubes.

Title: Re: Firefly with dc on input
Post by: Voxbox on October 28, 2010, 05:55:10 am

Thanks Tiny Daddy

Tried an 12AX7 and ECC83 from different manufacturers - Sovtek and JJ.

Not sure how this part of the circuit works actually so I dont  know what to expect.

Any ideas?

Title: Re: Firefly with dc on input
Post by: OldHouseScott on October 28, 2010, 12:34:07 pm

Put a .1uF cap between the input jack and R1 and it won't matter if you have a bit of DC on the grid of V1.

Title: Re: Firefly with dc on input
Post by: HotBluePlates on October 28, 2010, 01:01:17 pm

Did R1 go on the correct side of the Boost switch?

As drawn, it should be on the input jack side. If it is on the V2a side, the grid reference may be tentative (it should be present through the guitar volume control).

Title: Re: Firefly with dc on input
Post by: Voxbox on October 28, 2010, 04:55:42 pm

Thanks guys,

Sure the blocking cap is an option, but I dont think the dc should be there at all....

HBP,
R1 is across the jack, so no chance of being on the wrong side of the switch.

I'm wondering if I've got a duff valve base? I had to take it apart as I used far too hot an iron and got solder tracking all the way up some of the pins so far that the I couldnt insert the valve. I cant see any tracking, but you never know - think I'll change the base anyway.