AB763 layouts are... different.

[MrCoolGuy]

AB763 Deluxe Reverb layouts and schematics show different values for several components. I've figured out a few of those, but I have a question about one.

Should V5B have 56K or 100K cathode resistor?

Why the change?

 

[Lowerleftcoast]

I would *assume* the change was made to reduce the cathode voltage on V5. The cathode cap was rated for 25V. That particular cathode can go higher than 25V. Rather than change the cap they changed the resistor?

I suggest a higher voltage rating for that cap. OMMV.

 

[Ten Over]

AB763 Deluxe Reverb layouts and schematics show different values for several components. I've figured out a few of those, but I have a question about one.

Should V5B have 56K or 100K cathode resistor?

Why the change?

The grid on V5B is connected at different points with the AA763 and AB763 versions. The AA763 has the V5B grid connected to the third RC combo of V5A and the AB763 has the V5B connected to the second RC combo. The signal is larger at the second RC combo as compared to the signal at the third RC combo. In both cases, V5B is biased so cold that only the peaks of the positive side of the grid signal get amplified which creates a series of pulses at the plate. V5B in the AB763 version is biased colder with the 100k cathode resistor than with the 56k cathode resistor in the AA763 version, but the incoming signal is also larger in the AB763 version.

Whether V5B should have a 56k cathode resistor or a 100k cathode resistor depends on what the V5B grid is connected to.

 

[Lowerleftcoast]

The grid on V5B is connected at different points with the AA763 and AB763 versions.

Thanks Ten Over. That helps to clear things up.
Your post made me seek out the Deluxe Reverb AA763 schematic. I've seen the different layouts before but I don't think I have seen a schematic with the 56k resistor (AA763). For me it is much easier to see the circuit in schematic form.

 

[MrCoolGuy]

The grid on V5B is connected at different points with the AA763 and AB763 versions. The AA763 has the V5B grid connected to the third RC combo of V5A and the AB763 has the V5B connected to the second RC combo. The signal is larger at the second RC combo as compared to the signal at the third RC combo. In both cases, V5B is biased so cold that only the peaks of the positive side of the grid signal get amplified which creates a series of pulses at the plate. V5B in the AB763 version is biased colder with the 100k cathode resistor than with the 56k cathode resistor in the AA763 version, but the incoming signal is also larger in the AB763 version.

Whether V5B should have a 56k cathode resistor or a 100k cathode resistor depends on what the V5B grid is connected to.

Yes, but... unless it's a mistake when they switched from AA763 to AB763...
SOME AB763 show 56K, some AB763 show 100K... worth the same V5B grid connections.

 

[Ten Over]

Yes, but... unless it's a mistake when they switched from AA763 to AB763...
SOME AB763 show 56K, some AB763 show 100K... worth the same V5B grid connections.

If you think that's bad, you should try building a house from a set of plans that a registered architect drew.

 

[MrCoolGuy]

If you think that's bad, you should try building a house from a set of plans that a registered architect drew.

Yeah, I hear ya. I'm a structural steel inspector. The engineers drawings always look fine... until you get out in real life situations.