Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ACHIEVEIT on December 11, 2012, 06:59:52 am

Title: AB763 Deluxe Reverb Plate Voltage OK??
Post by: ACHIEVEIT on December 11, 2012, 06:59:52 am

Hi guys,

i totally rebuilt a DRRI using a hoffman ab763 board from theairtightgarage along with a 5x22uf filter cap board. I used the stock power transformer. I measure plate voltage around 404 volts with bias set to 23 ma. I use jj 6v6s power tubes and a jj gz34 rectifier tube. The tubes are good - i ve tested with others as well. Is that a little too low for a gz34 equipped ab763 or is this normal??

The reason i am asking is because although the cleans are gorgeous, the overdrive i get when i push either channel above 7 is somewhat zingy, farty and has a metallic edge which is hard to express with words. It is not smooth at all and it is somewhat fuzzy and ear-piercing. I don't have another dr to stack it against but it is not smooth even with the bright switch disengaged (i have added a push-pull bright cap on the second channel). I usually set the bass/treble at 5 to have a somewhat neutral eq. What do you think? Is it the nature of the amp? Could it also be that the preamp tubes need replacing (they are the fender branded ones and they have seen some action)?

Thanks! John

Title: Re: AB763 Deluxe Reverb Plate Voltage OK??
Post by: tubenit on December 11, 2012, 07:16:21 am

What speaker are you using?  This can be a huge factor in tone.

Have you tried a 5751 in V1 position?  And/or a 12AT7 in the LTPI position?

IF you type in "enchance cap" in the search function, you may find several ideas for smoothing the high end hash tone.  I typically will put a enhance cap across the LTPI entrance plate resistor.

You can also try lowering the coupling caps between the LTPI and power tubes to .047 or even .033.

You can also try lowering the tone stack bass cap from .1 to .047.

With respect, Tubenit

Title: Re: AB763 Deluxe Reverb Plate Voltage OK??
Post by: ACHIEVEIT on December 11, 2012, 07:22:05 am

Sorry i forgot about the speaker. It is the stock speaker - it is a Jensen C12K. I have tried a Cannabis Rex in this amp as well but eventually sold it as it didn't sound as 'big' as the Jensen. I have tried a cab of mine loaded with Scumback m75/h75 speakers and while the overdrive was definitely better than the stock jensen, it still had a few of the qualities mentioned. But it was definitely better although it had lost some of its sparkle.

Also, i haven't tried any other preamp tubes. I ll search for the enhance cap, thanks for that!

Title: Re: AB763 Deluxe Reverb Plate Voltage OK??
Post by: phsyconoodler on December 11, 2012, 01:58:58 pm

There are a lot of things that could make the amp sound wonky.The original amps uses 16uf filter caps,that's one thing I'd change.
 What coupling caps did you use and what dropping string on the filter caps?
What are the voltages on the preamp and phase inverter tubes?Also a wrong value in the negative feedback area can do that too.
 Lots of things.