Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: archaos on May 11, 2010, 05:44:21 am

Title: Hoffman AC30 board ; question about a bypass cap value
Post by: archaos on May 11, 2010, 05:44:21 am

My question is about the bypass cap value of the Brilliant channel of the 1st preamp tube. Why does a .1 µF is used instead of a 22/25 µF one, like with the Normal channel ? What's the consequence ?

Thanx a lot.

Title: Re: Hoffman AC30 board ; question about a bypass cap value
Post by: FYL on May 11, 2010, 07:29:17 am

With 100 nF mid/treble is boosted from around 750 Hz on, the higher frequencies at + 6dB or so.
 
Some versions used quite small bypass caps, down to 1 µF, AFAICT none went down to 100 nF (but I'm not a Vox specialist). Maybe it's a typo on Doug's schematics.

Title: Re: Hoffman AC30 board ; question about a bypass cap value
Post by: archaos on May 11, 2010, 07:59:54 am

Thanks a lot !  :thumbsup: Very helpful.

Title: Re: Hoffman AC30 board ; question about a bypass cap value
Post by: archaos on May 11, 2010, 03:22:48 pm

Have finally got the explanation concerning Doug's schematicsin Stephen Grosvenor's AC30 guide :

"...the AC30/6 Treble model has a different arrangement for the cathode biasing on V, where it has separate cathode components for each triode sction (rather than the shared cathode components in the Normal & Bass versions).
The cathode resistor for the triode of the brilliant channel is 1.5 k (as in the 2nd section), but the cathode capacitor differs in value & is .1µF (as oposed to 25 µF). This to provide the extended high frequency response of the brilliant channel."
 :glasses10: