Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: flora on April 12, 2010, 01:21:25 pm

Title: This isn't correct
Post by: flora on April 12, 2010, 01:21:25 pm

Tried this on my Fender single channel Super Reverb.. sounds terrible. The 50K to ground BEFORE the 220K, as in the Hoffman single channel AB763 , sounds MUCH better.

Title: Re: This isn't correct
Post by: bruno on April 12, 2010, 02:06:14 pm

I don't think that simulates any loading effect, I think that is one big voltage divider... and that's probably why it sounds not so good.

Title: Re: This isn't correct
Post by: sluckey on April 12, 2010, 03:39:27 pm

Quote

Tried this on my Fender single channel Super Reverb.. sounds terrible.

Do you have reverb on your amp? With the extra gain stage? Doug's single channel AB763 does. He chose to build the Vibrato channel w/reverb but no trem. The single 47K resistor is correct in that case.

The drawing you linked is a special case schematic for Dynaman1. He wanted a single channel AB763 that used the NORMAL preamp (not the VIBRATO preamp). To get the loading correct for the missing VIB channel, you have to put a 220K (mixing resistor) and 47K (missing INT pot) in series to ground from the NOR channel 220K mixing resistor. I chose to use a single 270K rather than individual 220K and 47K.

The network in the shaded area is correct for the circuit it's drawn in. It would be incorrect in Doug's single channel AB763 w/ reverb though. It was never meant for the VIBRATO channel.

Title: Re: This isn't correct
Post by: sluckey on April 12, 2010, 03:56:28 pm

Quote from: bruno on April 12, 2010, 02:06:14 pm

I don't think that simulates any loading effect, I think that is one big voltage divider... and that's probably why it sounds not so good.

Sure it simulates the loading of the missing VIBRATO channel. When you're playing through the NORMAL channel with the VIBRATO channel just hanging there, the signal passes thru the 220K mix resistor but it also sees a 220K + 50K shunt to ground (part of a voltage divider) just as shown. Look at the highlighted path on the attached file. I chose to combine the 220K and 50K pot into a single 270K resistor.

Actually the circuit is loaded even more by the plate resistance of the final tube in the VIB channel, but that gets more complicated and 270K is close enough.

Title: Re: This isn't correct
Post by: bruno on April 12, 2010, 04:39:33 pm

Hi thanks sluckey, you're absolutely right!