Twin Reverb AB763 Dropping Resistor

[Amp Kat]

Anyone use a different value dropping resistor for the D node on these amps ? I have two in the shop and one has a 27k instead of the 4.7k and it sounds pretty good and seems to put the preamp voltages a little closer to schematic values. Pretty cool having two identical amps next to each other doing comparison test. Another difference was one has 100k plate resistors for the splitter instead of 100k/82k. Not sure how much effect it has but not hearing much. Any other mods you guys may know about these ?

[MWJB]

Are you sure one isn't an AA763? These had 100K at both PI plates & a 27K tail rather than 22K (according to the schem anyway).

"Not sure how much effect it has but not hearing much." Answered your own Q there :-). I know of a respected tech who holds the same contention.

You could probably drop preamp voltages (V1/V2) to the 220-230v range and the amp would still sound characteristically BF, just a little warmer.

[Amp Kat]

Are you sure one isn't an AA763? These had 100K at both PI plates & a 27K tail rather than 22K (according to the schem anyway).


"Not sure how much effect it has but not hearing much." Answered your own Q there :-). I know of a respected tech who holds the same contention.

You could probably drop preamp voltages (V1/V2) to the 220-230v range and the amp would still sound characteristically BF, just a little warmer.

I couldn't find a Twin AA763 schemo but even the blackface amps listed in the Field guide were AB763's and I was under the impression that the AB meant Silver Face and the AA was Blackface but these are both identical silver faces however I have seen some blackface chassis in that transision period that have blacface cicuits in them and I'm thinking someone may have blackfaced it because there is signs of some work in it. Both power transformers are identical with the same numbers and choke and OT too. The preamp voltages sit at about 210
on V1 and V2 and seem to work quite well there. The 27k I was referring to is in the power supply in between node C & D which is listed as a 4.7k but has a 27k in one.

Ok I did just find the Blackface schemo AA763 and it shows 4700 ohms for node D too and not alot different from AB763.

[MWJB]

"I was under the impression that the AB meant Silver Face and the AA was Blackface but these are both identical silver faces" Hi Amp Kat, both AA763 & AB763 are blackface amps, the only difference between the two is the PI plate & tail resistors mentioned.

[Amp Kat]

Thanks Mark and I did get that amp squared away and it sounds exceptional. The problem was a previous tech had subbed an .0002 cap for the 10pf paralleled with the 3.3 M and twisted the wires on both ends adding some inductance along with it. I put the 10pf back and it cleared up nicely. I've worked on lots of Silver faces in the last month that have had some strange circuits and different components than the schematic.

[Bruce / Mission Amps]

Thanks Mark and I did get that amp squared away and it sounds exceptional. The problem was a previous tech had subbed an .0002 cap for the 10pf paralleled with the 3.3 M and twisted the wires on both ends adding some inductance along with it. I put the 10pf back and it cleared up nicely. I've worked on lots of Silver faces in the last month that have had some strange circuits and different components than the schematic.

200pF... wow that would be like very little dry signal mixing attenuation from 300-600Hz on up. What did that sound like?

Years ago I would find amps here in Denver modded by locals with all kinds of crazy RC combos in that mixing stage and huge +.022uF PI input caps replacing those 500pF to 1000pF caps... all via nut news letters from D Torres and G Weber.

[Rick Erickson]

and twisted the wires on both ends adding some inductance along with it.

Twisting leads together isn't likely to add any appreciable inductance. I can't imagine this even approaching the audible range. It does look sloppy however and shoul be avoided whenever possible.

RE

[Amp Kat]

200pF... wow that would be like very little dry signal mixing attenuation from 300-600Hz on up. What did that sound like?

Years ago I would find amps here in Denver modded by locals with all kinds of crazy RC combos in that mixing stage and huge +.022uF PI input caps replacing those 500pF to 1000pF caps... all via nut news letters from D Torres and G Weber.

Bruce, the main problem was it was doing the ocean sound like you get when theres to much gain from say, too large of a cathode or plate resistor but also the intensity pot when cranked caused a high hz pitch squeal that was motor boating and this was with the volume barely cranked and of course any treble added made it worse. Drove me crazy until I found it.

[Amp Kat]

and twisted the wires on both ends adding some inductance along with it.

Twisting leads together isn't likely to add any appreciable inductance. I can't imagine this even approaching the audible range. It does look sloppy however and shoul be avoided whenever possible.

RE

I was thinking like a coil of wire Rick and which is what it is basically. So how much do you think the inductance would be ? Maybe a.001uh or so ?

[stokes]

Thanks Mark and I did get that amp squared away and it sounds exceptional. The problem was a previous tech had subbed an .0002 cap for the 10pf paralleled with the 3.3 M and twisted the wires on both ends adding some inductance along with it. I put the 10pf back and it cleared up nicely. I've worked on lots of Silver faces in the last month that have had some strange circuits and different components than the schematic.

Many of the early Silver Faces were Blackface circuits in the Silverface package,as well as varying degrees of both circuits as they transitioned from Black to Silver.Its common to see a lot of discrepencies from the schems during that period.