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After a long period of continuous usage, one of the 270K screen dropping resistors in my prototype Fangblenny failed. The one in the other channel was clearly on its way out. These resistors only dissipate about 156mW (calculated), but some half-watt resistors apparently can't take it. The lack...

I have a strong preference for triode operation of the finals because I've seen major improvements in OPT performance relative to the other configurations. I tend to add powerdrive to the grids as well, which overcomes the power output disparity relative to pentodes.

My original intention was to use a conventional power supply circuit for the low-current -96V rail. I never did build that amp design into a chassis, so I don't have a final power supply circuit design to show. The breadboard was always operated from benchtop PSUs.

-6dB is the same as divide-by-two. A simple L-network made with two 10K resistors would work well in most cases, unless the downstream load impedance is under 50K.

It looks like a good general-purpose introductory model to me. Make sure the traces are bright and focus well. Try to get the original probes if possible.

The reason to use mixing resistors is to isolate upstream outputs from one another. If you don't do this, then the outputs can overload one another and cause distortion whenever there's a difference between left and right channel signal content. Mixing resistor values should be at least equal to...

AUX inputs are equivalent to tuner inputs in every way.

Smart power strips can sense load current on one outlet and automatically apply power to other outlets. And they're cheap.

Peter's right, especially considering that you could sell the MK-II OPTs to cover a good portion of the MK-III OPT cost.

There's nothing on the primary side of the power transformer except a fuse and line cord in this model. Have you checked DC resistance between primary and chassis? It should be infinite on a standard ohmmeter.

That's correct, but you end up listening to the sum of the outputs of these two filters, which will be down 3dB at crossover frequency (ignoring phase). Harrison applies a correction for this error. You don't have to understand all of his math --- just use the simple formulas given in the figures.

I believe that circuit yields about 3dB suckout at crossover frequency. If you're cascading two RC sections for each filter, you want each one to be down 1.5dB at crossover frequency, not 3dB. See the Harrison article.

When you're driving the grid positive, into the grid current region.

Tube cathode followers can be used very effectively in active filter circuits of the type with unity-gain op-amp buffers, i.e. unity-gain Sallen-Key type. You could easily de-stabilize the amplifier if you start fiddling with its feedback loop or internal gain parameters. Don't do this unless...

It's important to understand the distinction between bridging and paralleling. Bridging requires one channel to be driven out of phase with the other, which calls for an external phase inverter circuit with most tube amps, and the load is connected across (bridging) the output terminals labeled...

I've thought for a long time that the C8 would make a great homebrew project, especially for playing very old records that might not be RIAA-equalized. The main thing it lacks is a high-frequency noise filter, but that's not a big issue if you're sticking to vinyl that's in good condition.

This schematic at 4tubes.com shows something similar to what you're looking for. It uses a line-level L/C crossover with inductor values not given, but your crossover frequency is probably different anyway. This doc gives design info for sharp R/C crossovers.

Yes, at the full clockwise setting. Is there some reason you need greater input impedance at the power amp? Most preamps can easily drive 100K, and it's a good value to keep tube input capacitance from causing excessive high-frequency loss.

10uF between the preamp output and power amp input sounds about right. To be more certain, we would need to know the power amp's input impedance.

Basic volume control hookup is discussed here: http://www.angelfire.com/electronic/funwithtubes/Amp-Volume.html I would suggest 100K as a good compromise value for this application.

Not terribly common, but it does happen. The noise might go away after a while, or it might not. There's not much you can do other than replace the tube, which isn't likely required for electrical reasons.

Computer tubes have special cathodes that resist failure caused by long periods of operation without current flowing through the tube. As in a flip-flop circuit that idles with one side in cutoff. They're often processed for extra reliability as well.

That's not a problem as long as the base remains firmly attached to the glass envelope.

Welcome to AK. It appears that you have the winding axis of the OPT closest to the power transformer aligned with that transformer's winding axis, which could result in hum pickup in one channel. The spacing might be sufficient to prevent that, so perhaps no cure is necessary.

If cleaning the FM local oscillator tuning gang doesn't fix it, then it's likely that one of the components in the LO has changed its value vs temperature characteristic. This is a devilishly difficult problem for an audio repair technician, because conventional analog LO circuits are made...