Phase Inverter Cathode Bypass

[bobloblaws]

Hello,

I'm looking at a Traynor YGL-2 guitar amp (schematic attached) that has had a few mods added to the circuitry. One of them is to add 4.7uF bypass caps across both cathode resistors on the phase inverter. I understand that in a typical pre-amp stage the cathode bypass cap serves to increase gain and modify the frequency response of the stage. Can someone please explain what is achieved by doing this trick on the PI? Thanks,

- Bob

[Dave H]

I understand that in a typical pre-amp stage the cathode bypass cap serves to increase gain and modify the frequency response of the stage. Can someone please explain what is achieved by doing this trick on the PI?

It's the same I'd guess. V4-A and V4-B look like typical pre-amp stages. The actual splitting (inverting) is done by U4-B.

[Enzo]

Agree^^^

Just because someone printed SPLITTER next to V4 doesn't make it one, the IC is the splitter.

[g1]

For added confusion, the block diagram (pg.4) gets it all wrong too.

[bobloblaws]

It's the same I'd guess. V4-A and V4-B look like typical pre-amp stages. The actual splitting (inverting) is done by U4-B.

Hmmm, that's interesting. So for example in the Fender Twin AB763 schematic (http://www.thetubestore.com/lib/thet...-Schematic.pdf) the PI circuit looks pretty similar. Which component(s) would have the same functionality as U4-B? I'm also curious if I can still use a calculator such as this one https://www.ampbooks.com/mobile/ampl...ode-capacitor/ to figure out the effect it will have on the frequency response. I'm not sure what values to plug in for Rl, Rg and Cg. I'm guessing Rl is 113K and Cg is 22nF. For Rg (grid resistor) I don't know what value to put because the way it looks to me it is looking for the value of the grid resistor on the next pre-amp stage. 10K (R60 and R61)?

[Dave H]

The AB763 PI is totally different. The 12AT7 is configured as a long tail pair. i.e. its cathodes are connected together and there's a large common resistance to ground.

For the calculator you guessed right for RL and Cg but Rg is not R60, R61. They are in series with the EL84 grids (grid stoppers). It's the resistance to ground you want and that's the Master pot (100k)

[Old Tele man]

The two TL072 opamps, U4:A & U4:B (does the inverting), alternatingly drive the two individual voltage-amplifying triode-tubes, V4:A & V4:B, which then each separately drive one pair of the EL84 output tubes.

[Enzo]

The fender circuit and this one only look similar at a distance - they both have two triodes. But the Fender circuit has the two triodes wired together, they share a common cathode circuit. The Traynor triodes are really separate circuits, they share a common ground connection and power supply, that's all.

I tried to explain the operation of the Fender type splitter in another thread. It isn't immeiately obvious to everyone. But I think this Traynor circuit is really simple to understand. U4A feeds the upper triode, but also feeds U4B. U4B inverts the signal to feed it to the lower triode. There it is - phase inversion.

[bobloblaws]

The fender circuit and this one only look similar at a distance - they both have two triodes. But the Fender circuit has the two triodes wired together, they share a common cathode circuit. The Traynor triodes are really separate circuits, they share a common ground connection and power supply, that's all.

I tried to explain the operation of the Fender type splitter in another thread. It isn't immeiately obvious to everyone. But I think this Traynor circuit is really simple to understand. U4A feeds the upper triode, but also feeds U4B. U4B inverts the signal to feed it to the lower triode. There it is - phase inversion.

Got it. Thanks all!