FUCHSAUDIO wrote:While I'm sure bucking is a possible solution, the easiest way (IMHO) is as shown by the attached diagram.
I dunno. I suppose it depends on ones definition of the word "easy".
Those zeners are going to dump some serious heat and need sinking at the very least. Bucking requires only tying a pair of wires together.
Never the less. An elegant solution Andy.
Dan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Firstly, the zeners do not get hot at all. They are about the size of a 5408 diode (not a stud mount diode at all). They do get warm during amp warm-up (about 20 seconds before the preamp tubes start drawing current), but during normal operation, they are cold, since the actual preamp power voltage is less than the total of the string. The preamp feed is about 400 volts after warm up, so the zeners are basically fully out of circuit when the amp is running.
Remember, if you use series filter capacitors in the preamp (like 47 @ 350V x 2) you do not need the zener string. If you use 450 or 500 V caps, the zeners are a wise idea to extend capacitor and preamp tube life. I always wondered how 12AX7's liked 700 volts on their plates then the amps were cold !
This big resistor was used in the EL-34 high voltage Bruno D-clones and in real EL-34 Dumbles with Music Man iron.
As far as plate voltage: People forget the EL-34 is rated as a 1000 volt tube in most tube manuals. 700 volts (especially with 350 volts on the screen), is not tough on it, assuming it's a good quality tube like a JJ and bias conservative.
The music man amps with transistor drivers did indeed use positive grid bias, but the cathodes (going through the transistor) were modulated by the audio, and the total current was low (about 20-ma I think I remember). It sounds worse than it was in actual practice...lol.
Ok, after a little more time spent looking instead of talking, I see how the zener string works. They aren't really conducting much current once the tube starts conducting. Because of the 25K resistor.
But I'm just not used to lookng at this sort of supply. The screen supply is a very simple half wave recitfier. I'm having trouble visualizing current flow in the main power supply. The bottom transformer tap seems out of place. Really, it all looks like a half wave rectifier. How does the bottom filter cap work?
No arguement going here Andy, I'm just too stupid to see how this works right off. I'll noodle for awhile.
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
skyboltone wrote:
But I'm just not used to lookng at this sort of supply. The screen supply is a very simple half wave recitfier. I'm having trouble visualizing current flow in the main power supply. The bottom transformer tap seems out of place. Really, it all looks like a half wave rectifier. How does the bottom filter cap work?
Maybe the MMan drawing will help:
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While they have added the capacitor "sharing resistors" it's basically a simple voltage doubler. What's cute is the screens are fed from the mid point (1/2 output voltage). It's a pretty clever way to do things imho.
BTW: Some music man's have an additional filter cap and resistor or choke feeding a separate cap on the screen supply. It depends whether the amp uses a tube phase inverter or the solid state driver circuit.
What if you have the pt that has the hi low power switch on the primary side? Can you just use the low setting and leave the high one unconnected? I have a hd130 that is doing nothing and has the switch on the primary side and would like to convert it over.
What if you have the pt that has the hi low power switch on the primary side? Can you just use the low setting and leave the high one unconnected
Nah!!! go with the full 700...More dynamic/ touch sensitive.. bigger sounding and a midrange tone you can stick your head in.. These are my favorite D-style amps..
If you don't want to use the 34's you can also use Tung Sol winged C's and the Solvtek 5881's..My favorite with the MM's
Thanks that's kind of what I was thinking. I've built a lot of amps and I just can't get away from the el34 sound. I got this hd130 with a jbl 15 and its just calling for a conversion.
I imagine it would sound good with both.. Another cool sounding tube for MM style are the JJ KT 77's...More of a rounder top end vers the 34's better crunch tube..
go with the full 700...More dynamic/ touch sensitive.. bigger sounding and a midrange tone you can stick your head in.. These are my favorite D-style amps..