The Amp Garage

Yes, you can tie the MOSFET to the chassis electrically if you put the whole thing in the ground side and tie the negative return of the rectifiers to it. That's how the drawing at geofex actually shows it. I drew that up nearly 15 years ago, and I've had some time to think since then. There are som...

Maybe I can help. This idea is often thrown out as an inexpensive way to reduce B+ voltage, but I can't find any practical, proven examples. So I hope to detail my build here for the community. Good! I've done it, but sometimes I forget to write down exactly what I did, so a third party build is a g...

Forgot to mention: For signal tracing, you often need a signal. :D Here's one: ... uh, apparently that file is gone from my page. I'll go see if I can find it. It's a one-transistor oscillator that makes a test tone suitable for running into an amp input. And here's a fancier one that makes individu...

See:
http://www.geofex.com/FX_images/audioprb.gif

You might also like:
http://www.geofex.com/FX_images/xform_test.gif

The only good ways to tell are (1) study the schematic and evaluate what the circuit does to make a good guess and (2) try it and see if it works. Older pots were often +/-20% on the end to end resistance anyway. It depends on what the circuit does as to whether there's a problem. If it's volume-con...

At the place I used to work we referred to this situation as a self-eating watermelon.

Try 60650, IIRC. The requirements are real, even if I didn't come up with the exact standard number. I dug out my guidebook. It's 60950 or 60065. The guidebook references 60950, but I think 60065 is similar. Class I equipment requires a protective earthing conductor ( EN60950 reference 1.2.4.1). EN6...

Frequency response questions aside, vacuum triodes input-clip more softly when driven from a low impedance source. This is because when the grid goes positive, the grid goes from near-infinite impedance to a few K quickly. A low impedance source can drive this change in impedance with less distortio...

Bipolar caps are literally back-to-back (well, OK, + to + or - to - ) electrolytics inside. If you use them with only one polarity of voltage across them, one of the oxide layers stays unused, and may deteriorate faster - but that's OK, as it's never used. The biggest issue is that a bipolar or non-...

If you wanted to be Really Sneaky, you could use a 1200V MOSFET to actually switch the high voltage, and an LED-to-photovoltage module to turn it on an off. This also has the advantage that you can incorporate a few parts with the MOSFET to make the MOSFET clamp the current through it to being more ...

I get a lot of criticism about it from Facebook tube hobbyist types - same as you would if heaven forbid you ground under a transformer lug (which I do not do, but its perfectly fine if you do it because there are no rules in this game) Well, in this case there are rules. If you mean attaching AC s...

There is a long dissertation on the mojo of carbon comp resistors at http://www.geofex.com . Carbon comp *was* cheaper to make than other forms of resistors back in the 40s, 50s, and 60s. Improved manufacturing made carbon film cheaper and tighter tolerance after about the 1980s. Metal film is becom...

You're right pdf64 - there are cases where a short on a heater cannot cause enough primary current to blow the mains fuse. Even more importantly, the purpose of the AC mains fuse is to stop electrocution and fires, so it's sized differently than it would be for protection of the transformer. The AC ...

The "silver foil" caps are aluminum foil with polystyrene insulation. Good caps, low leakage, low memory, low dielectric absorption. You can still get polystyrene caps, but they're expensive. Most PET/Mylar will be as good in audio circuits. Back then, they couldn't make the polystyrene thin enough ...

I have often thought that modern manufacturing techniques could make dynamite tubes, and probably cheaply in modest quantity. The things that were heartbreakingly difficult back in the 50s and 60s are done effortlessly by programmable and automated devices these days. Even things that require many r...