Selmer Zodiac Twin 30 Power Amp Restoration odd ball...

[harryandaguitar]

Hey guys!

I'm very new to changing things on amps and want to learn as much as possible, so sorry in advance if any stupid questions here...

I recently get an old Selmer Zodiac Twin 30, the schematic says there's a 0.0022uf Cap in parallel with a resistor before going to the output, after looking at some gutshots of various units, it sometimes appears to be there, and other times not, can you explain to me the affect this cap would have, both tonally and circuit wise?

On top of that, I'm getting continuity to ground on both output terminals on the output plug before it goes to the wires that connect to the speakers, is this normal?

Schematic and photos of internal below, the extension outputs on the amp have been disconnected at some point before I got the amp, hence the resistor soldered directly to the output...

https://schematicsonline.com/wp-content ... win30_.pdf

[xtian]

The 3K3||2n2 from output secondary to PI is the global negative feedback loop. The resistor sets the amount of NFB and the cap helps shape the frequencies fed back to the PI.

Yes, because one end of the output secondary is grounded, the other end of the winding will appear to be grounded also, because the winding measures just a few ohms.

[harryandaguitar]

The 3K3||2n2 from output secondary to PI is the global negative feedback loop. The resistor sets the amount of NFB and the cap helps shape the frequencies fed back to the PI.

Yes, because one end of the output secondary is grounded, the other end of the winding will appear to be grounded also, because the winding measures just a few ohms.

Amazing thank you, is the capacitor necessary? I see so many amps without it but have one I can put in if needs be.

Any Tonal affect I would hear whilst playing if it’s not there versus is there?

[trobbins]

Yes tonally the amp may oscillate wildly and grossly depending on the speaker and leads used - its called unstable operation.

[harryandaguitar]

Yes tonally the amp may oscillate wildly and grossly depending on the speaker and leads used - its called unstable operation.

I guess I should install the cap then! I wonder why so many of these amps are out there without the cap though…

[martin manning]

Easy enough to tack a cap in and try it out.

[harryandaguitar]

Easy enough to tack a cap in and try it out.

Done exactly that, the amp seems to be a bit darker/less gain with the capacitor in the circuit, almost like a plexi presence control has been turned down slightly…

[nworbetan]

Amazing thank you, is the capacitor necessary? I see so many amps without it but have one I can put in if needs be.

Any Tonal affect I would hear whilst playing if it’s not there versus is there?

The cap provides frequency compensation, which like you've already noticed isn't used or talked about much in the guitar amp world. It's way more commonly used in op-amp circuits, but it works and gets used in tube amp circuits if/when high frequency (mis)behavior is a concern.

https://en.wikipedia.org/wiki/Frequency_compensation

[martin manning]

Easy enough to tack a cap in and try it out.

Done exactly that, the amp seems to be a bit darker/less gain with the capacitor in the circuit, almost like a plexi presence control has been turned down slightly…

As expected. You have increased feedback of higher frequencies, basically the opposite of turning a Presence control up.

[trobbins]

The feedback stage has a 470pF bypass on its 220k anode load, which would start rolling off gain above say 1kHz.

The 2.2nF introduces a corner frequency at 22kHz, which is quite low. That seems to be too low to use to improve stability margin for one common compensation scheme where the network changes the phase well below that corner frequency in order to improve margin at an instability resonance (ie. in the region of 5-10kHz).

Without knowing the response of the output transformer, or the total output stage, I'm not too sure of the intent.