Dealing with High B+

[Leo_Gnardo]

Are you saying that this Zener Method will change, clip, distort the Sine Wave as it comes from the wall source.? That is to say...will this add "noise" to the wall source.? It seemed like such an easy solution. Thank You

Yes, as Steve Connor and others have noted the zener will take a nice square "bite" out of the AC waveform, and that will likely show up in the audio, and maybe send noise into other things on the AC line as well.

Distorted AC power has been an issue with a couple items I've been asked to look at recently. A gentleman of "retirement age" who has taken up electronics as a hobby brought me 3 amps he modified - a Dynaco Stereo 70 and two Dynaco MkIII - he installed new drive boards but noticed the power transformers making a "grinding" noise when he fired up the amps to test. He has a 'scope so I asked him to put the probe on the AC line. He reported that it was anything but a sine wave. Very distorted. So it wasn't the mods. His power is dirty as could be, and bad enough to make the windings in his power transformers rattle audibly. So he will have to get a serious filter or even an AC power "regenerator."

The NTC "inrush limiter" is intended (on amps that have one) to soften the startup voltage & help preserve filament life. After warmup the NTC reduces the line voltage that the amp sees by about 1 volt. That Peavey amp I mentioned (sorry can't remember the model) that had 8 or so of these definitely was trying to make a major reduction in line voltage. The NTC's were all on a separate mini PC board @ the size of a playing card. And got so hot the solder was melting and the NTC's were falling off the board. What a farce. You're better off with a bucking transformer or autoformer.

[Ted]

You're better off with a bucking transformer or autoformer.

There shouldn't really be any need for an isolating transformer here should there?
Autotransformers are significantly smaller (like half the size) than an isolating transformer for the same VA rating.

[Leo_Gnardo]

There shouldn't really be any need for an isolating transformer here should there?
Autotransformers are significantly smaller (like half the size) than an isolating transformer for the same VA rating.

Correct, and the way chinrest & others have created a bucking unit essentially are autoformers also. In jolly old London you should be able to find a selection of 240/120V autoformers at several locations up on Edgeware Road a couple blocks north of the Marylebone station. In the good old days there were a couple of excellent surplus shops there but I think they're long gone by now. The modern electronics shops there will have them. Probably can be found in other parts of town too but Edgeware had a concentration of electronics shops last I looked. And treat yourself to an excellent fish & chips at the Sea Shell takeaway just 2 blocks or so north of the station and a block east of Edgeware. (Sitting down there will cost a fortune but the takeaway is much more affordable and has the best finch & chimps I ever found in London.)

If you can find an autoformer with multiple taps you can fine-tune the voltages inside your amp for near-perfect operation.

[chinrest]

Introducing the new
Buck Henry

[olddawg]

Why not just put a small, thick O-ring under each screw and suspend the sockets like a Reverb tank? You could even use just a small rectangle of metal stock on each side with 2 holes drilled in them and use the original mounting holes. It would look clean if mounted from the bottom of the chassis. Sounds like a lot less work if you have a drill press and a hacksaw.

[Justin Thomas]

Wrong thread?

Justin