Balanced vs Unbalanced?

OK Herman, yeah maybe a little, or maybe more than that :) But really, its pretty easy. If you drive the cathode of the input tube rather than the grid you have what is called a grounded-grid circuit. You need a coupling cap to do it. Turns out that the value of that cap is the same if that cap were to be used as a cathode bypass cap, in the case of normally driving the grid.

So, why not drive both the grid and the cathode in opposition? In practice, this works quite easily. You install an XLR on the amp, and attach the cathode bypass cap (if there is one, if not you have to supply one) to pin 3 of the XLR rather than ground. Pin 1 of the XLR goes to ground, and pin 2 goes to the RCA input. Now if you drive the grid only, single-ended, you will want to put a jumper between pin 1 and 3 if the cap was a bypass cap. If not, you don't need the jumper.

The only trick is that the grounded-grid input is pretty low impedance. You might want to have an auxiliary resistor that plugs into the RCA input that sets the normal input (pin 2) to the same value. This is not a problem if your preamp supports the balanced line standard.

I have this hookup running right now on a set of DIY amps I built, which are a set of push-pull amps using 45s for output tubes (class A), and a single-ended driver circuit similar to a Dyna ST-70, only all-triode. I didn't bother with the extra resistor, as this input behaves a lot like a differential input. I guarantee you that if you tie the two inputs together and try to inject a signal, you will find there is a substantial CMRR.

Atmasphere; one question. are you saying, in an xlr set up, that the interconnectors, as long as they are of reasonable quality, really do not matter? I have found this to be true. you seem to know a lot about this. I would value your opinion and any explination. not to technical please. thanks

Koegz, in a nutshell, that is the whole point of balanced line operation. The system was created to eliminate interconnect cable problems.

There is a standard that has to be conformed to in order for this to be true. If the conformance is not there, you will still hear differences in the cable.

The primary hanging point with most high end gear is the 600 ohm termination standard. What this means is that the equipment should be able to drive a load of 600 ohms (which might be at the input of the amplifier) without difficulty (for example loss of bass). For tubes this usually means an output transformer, or very large coupling caps, 100uf or more. Most high end manufacturers don't want to put in such a large capacitor at the output of the preamp, as it will color the sound and for the same reason aren't usually too keen about an output transformer either. That's why conformance to the standard is rare.

Ralph, I get what you are saying, and I may try it sometime, but I don't use a bypass cap. I have the filament current of my input tube going through the cathode resistor to establish bias. That R is only a few ohms so too low to do what you say.

Thanks for the idea though.