Hammond 290-series Guitar amp PTs

[prosounddaryl]

Hi Daryl

The mod to the feedback loop of the PA is as simple as I've described. Can you read a schematic? This is different than wiring diagrams for PA systems, or for wiring guitar pickups, as those are closer to layout drawings - pickup wiring are in fact just layouts.

In most Fender amps, the feedback resistors are 820R in series from the speaker output working against 100R in shunt to ground at the bottom of the splitter. These values give a gain of 9.2 - colloquially referred to as" a gain of 10". Were the amplifier inverting the gain would be 820/100=8.2, but then the input impedance facing the preamp would be way too low. Most PAs are non-inverting, and gain is 1+(820/100)=9.2. With tubes, things are never quite as it should be, so even with this strangle hold, and an open-loop gain of just 100, the real gain will be a little bit off.

Increasing the values by 10x says use 8k2 and 1k instead. This does nothing to the gain as the ratio is still the same. It does nothing to the bias of the splitter. The effect is to loosen up the sound and make the amplifier even more stable than it already is. Note that this works for tube circuits and not for solid-state, for reasons too involved to get into here.

---

Regarding the public address systems, the bulk of installations are small-scale and intended for low-key SPLs. The 70V North American standard and 100V Euro standard is in fact intended to allow the use of small wire over long distances, the same as using higher voltages for electric railway power distribution allows thinner catenary, or simple mains power distribution using extremely high-voltage for long runs.. Lots of power can be distributed this way These PAs come in all sizes, and you have been lucky to work on some impressive ones .

It is simple Ohm's Law. 100W at 100V requires 1A and thus can be transmitted via a thin wire. 100W at 10V requires 10A and thus a thicker wire is needed. Obviously, the length of wire between the source and load introduces its own resistance and a power loss to the load. In your big system, you may need very thick wire close to the source, but outlying branches can be handled with smaller gauge wire. The compromise of loss versus cost of the copper  comes into play here. Whether the "allowable" thinness is taken advantage of or is seen as a detriment in reality, is up to whoever oversees the installation.

This a factor in wiring guitar amps, particularly for heaters, where high currents are needed. The PT fly-wires must handle the total current of the load, and these leads are often quite thick. The wiring can be fanned out from one point, allowing thin wire to each socket, or if the sockets are daisy-chained, graduated gauges of wire can be used as you move away from the PT to the end of the chain. TUT3 shows both methods.

Fidelity is a moving target with public address systems, and this includes stage PAs.The required fidelity also varies with each installation. A train station only needs intelligible voice. A hockey rink in the 1950s might have only needed the same, where one wired today wants more of a hifi sound.

Hey Keven;

"Can you read a schematic?" No, not very well. By staring at it long enough I can likely identify basic signal flow, or recognize resister values when I see them, but no, I would be at a grade 1 level in this.

"This is different than wiring diagrams for PA systems, or for wiring guitar pickups," Absolutely correct. 

Having agreed to all that, would I recognize "Increasing the values by 10x says use 8k2 and 1k instead" when I see them? If I understand this correctly, I would simply replace a few of the resistors near the amplifier output transformer. I would recognize these resistors by the color of the bands around the resistors?

The documents below indicate they are from a Super Champ X2 service manual. Is there anyway you can draw a big circle around the parts I need to swap, and, in a perfect world, sell me the parts and bill me for your help? Not a novice to soldering, but I just don't work on componenet level stuff.

Thanks for your help in all this.