Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: tubedood on June 27, 2018, 09:39:27 pm

Title: .047 cathode bypass cap for V1a in a champ type circuit
Post by: tubedood on June 27, 2018, 09:39:27 pm

Hi again!
I am looking thru some ideas on my little 5F2-A build and I came across an interesting "voicing" switch I guess we can call it on the 1st stage of the 12axy. It has 3 choices, one is just the 1.5K resistor, and the other two are either a 10UF or a .047UF either with the resistor. Tonally what can I expect from the .047? Wil it be a nice chimey treble sound? I don't think Ive seen a bypass of that value before...

Dale

Title: Re: .047 cathode bypass cap for V1a in a champ type circuit
Post by: PRR on June 27, 2018, 10:24:30 pm

A rise above 4KHz. Try it. You may not hear it on guitar.

Title: Re: .047 cathode bypass cap for V1a in a champ type circuit
Post by: 66Strat on June 28, 2018, 11:02:22 am

Hiwatt used .047 uF bypass capacitors. The early Hiwatts used them to bypass a 2K2 cathode resistor. Later circuits used them to bypass 1K and 1K5 cathode resisters.

Title: Re: .047 cathode bypass cap for V1a in a champ type circuit
Post by: tubedood on June 28, 2018, 12:49:27 pm

Thanks for the replies! It seems a little small but I saw it in a build years ago it was a project Princeton non reverb called the Prince o Wails. It was switchable and added a great treble boost to the sound, but it also had an inductor as a mid cut along side of it. I hope this sounds similar I will try it and see!

Dale

Title: Re: .047 cathode bypass cap for V1a in a champ type circuit
Post by: dude on June 28, 2018, 01:53:19 pm

Might sound pretty bright with little bass response, if so try a 1K5 parallel with a 4.7uf cap. Marshall uses a .68 with a 2K7. With a 1K5 you can try anything from 1uf to 10uf, I think the stock was 25uf, too bassy for me. I don't think you'll hear any different unless you can go at least 5uf difference each change.


al

Title: Re: .047 cathode bypass cap for V1a in a champ type circuit
Post by: tubedood on June 28, 2018, 05:30:38 pm

Thanks for the replies and suggestions. I appreciate it!

On the same note has anyone got an idea to allow me to choose the 3 options of this "voicing" mod without drilling the chassis or using a hole for a purpose not intended? I have seen the push/pull switch potentiometers but they always seem to have 2 positions... I think I need the on-off-on switch in a potentiometer if they make it. The circuit is first half 12axy with a 1.5K resistor on the cathode always to ground, and a switchable 10uf or the other value we are discussing to bypass. So we get 3 choices, the resistor alone or each added cap. I would like to not drill the new chassis I hope to get this week!


Dale

Title: Re: .047 cathode bypass cap for V1a in a champ type circuit
Post by: sluckey on June 28, 2018, 06:38:03 pm

Gator clip test leads. If you gonna do this stuff get a dozen.