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I was looking for some 6x4 rectifier for my eico st84 preamp which i just finished rebuilding. I found that i have quite a few 12x4 but no 6x4. the eico uses 6.3vac heater voltage. So, i wondered if i did a voltage doubler circuit with the 6.3 and appropriate rectifiers and caps if the resultant DC voltage would be adequate for the 5 12ax7 tubes and a 12x4 rectifier. I would have to rewire the sockets for 12 volts.
I know a lot of you are thinking 6x4 tubes are pretty cheap why go through all the trouble? As a matter of fact i am going to get some but still i wonder for argument's sake if this is a valid thing to do? In case i have a situation where i needed the lower noise of DC heating of the preamp tubes. Cheers, Dak
You can't use a voltage doubler, with that filament winding. When a winding is rectified and cap. I/P filtered, only approx. 1/2 of the AC RMS current is available as DC. A doubler is even worse. Only approx. 1/4 of the AC RMS current is available as doubled DC. TANSTAFFL! Your current needs are halved by switching to "12" V., but the necessary current is not present.
As a temporary expedient, until a known good 6X4 is acquired, use 2X UF4007s in series with dropping resistors.
Although it is just for the signal tubes.
The power trafo's "12" V. winding feeds the 12X4 rectifier and a doubler. The doubler feeds series wired pairs of signal tubes.
Series, parallel arrangement. I was just checking that, and on each circuit board, the 2 tube's heaters are in series, and the other circuit board is paralleled with the tube heater again in series. I have not noticed this type of an implementation in other preamps or amps. Is this kinda clever or mostly and economy move or both? I really did not notice this before and the PAS was the first tube preamp i owned over 30 years ago, which i was given by my father in law. And this explains why i always had trouble rebuilding that circuit, because i failed to notice that it was a voltage doubler and when rebuilding, its design always threw me off.
Ok, so, if the winding could supply 1 amp of 6.3vac , using a voltage doubler on it would only supply .25A?
I am already using a SSR replacement for testing but i don't like using it for real listening in my home where the extra voltage could jeopardize some of my lower voltage signal caps. I will just wait a bit until my tubes come in.
Put resistors in series with the SS diodes to make them resemble the 6X4. That will allow for safe listening, until the new tubes arrive.
"Ok, so, if the winding could supply 1 amp of 6.3vac , using a voltage doubler on it would only supply .25A?"
You could get a bit more, but 25% is the safe "guesstimate". Also, keep in mind that, less losses, doubling 6.3 VAC yields 17.8 VDC. The losses include the forward drop of both diodes. Schottkys exhibit an approx. 0.5 V. forward drop and are switching noise free. IMO, that's 2 good reasons to favor them. My guess for doubled 6.3 VAC, via Schottky's, is 16.4 VDC.
FWIW, I like doubling a 6.3 VAC winding of adequate capability with 2X Schottky diodes and regulating with a 7812 IC, for 12AX7 phono preamps. A high performance, modest budget, PSU for RCA setups, tweaked or original, can be built around Allied's 6K1VF power trafo. Tie the rectifier winding's CT off and bridge rectify the end to end voltage. Voltage double and regulate the filament winding. While the 1 A. filament winding is a bit undersized for this job, the heat generated (a critical factor) is compensated for by the lightly loaded rectifier winding. Contact "inmate" Jeff Yourison for more details.
It's surprising to me that the Eico ST84 powers the 6X4 with the same transformer winding as used for several preamp tubes. There have been many reported failures of the 6X4 when used with its filament grounded. Apparently, the heater/cathode insulation isn't what it should be. This has been known for a long time - all the old Akai reel decks powered the 6X4 from a separate floating winding. I don't know if the 12X4 has the same issue, but it's worth considering. Eico might have used a particular brand or style of 6X4 that was more resistant to this failure mode.
Buy Chinese. Bury freedom.
Actually, the rectifier tube gets its own filament supply. So, my whole premise is not very feasible.
The schematic I posted is the old Akai. The Eico schematic I found online shows the rectifier winding powering several tubes in parallel, and the winding is balanced to ground.
Buy Chinese. Bury freedom.
I was just in there yesterday, but maybe i did not remember the filament circuit going from the rectifier to other tubes. Anyway it was mostly a mental exercise as it turns out. I have pretty much nixed that idea.
The schematic does have errors in it and frequently EICO would make running changes to the circuit without updating the schematic. For example, on my copy of the owner manual the filament circuit is biased with positive dcv from the last tap of the B+ supply. the 2 resistors used are 100k and 27k But it appears that the schematic has the 27k as the dropping resistor and 100k to ground. This, is opposite to what it should be, but if you check the schematic one would think that is how the resistors are labelled. thanks for the support, Dak
So, maybe the Eico schematic I saw was somewhat preliminary, and they changed it later. About your original question, I don't think it's worth the trouble VS buying the 6X4. If you decide to pursue it, some of the 12V switchers on eBay are very small and inexpensive. The output can be left floating (ungrounded), so this might be a much less painful way to heat the 12X4.
Buy Chinese. Bury freedom.
Yeah, i was just brainstorming some ideas to get around the bump in the road. And if it was feasible to do the voltage doubler circuit, which had the potential to yield additional benefits, besides allowing the use of my 12x4 tubes, i would have pursued that. But as you and others pointed out the EICO tranny is probably not capable of doing such a conversion.
You mentioned about using low voltage SS power supply for powering the filaments and i am intrigued about that. I need to rebuild anohter vintage preamp that i have postponed for over 10 years and i think using something like that would be a neat way to make the rebuild more in line with current trends. cheers.
Don't these work by taking AC input and doubling the DC output? You need 12.6 VAC for a 12X4 rectifier.
Good point, hence my question was not only regarding the current loss but also what would be the DCV from running the 6.3vac into a doubler circuit.
You don't need a rectifier. At least, not for the heaters. As long as the source has enough current to handle the heaters of all tubes.
To get 400+ VDC. You start with 6.3 VAC for heater/anodes of the rectifier and at least 400 PIV AC on primary line. Then, rectify it to 400+ VDC. Right?
Hi FL, i was wondering about the feasibility of using a voltage doubler on the 6.3v filament line and getting 12.6 to run 12x4 rectifier. But it was pointed out that i would lose 1/4 of the current in the circuit.
I'm not familiar with how to double 6.3VAC to 12.6VAC. Not without putting another transformer inline.
I don't know if the voltage doubler circuit will exactly double the 6.3vac into 12.6 VDC. Eli, says it won't be that much, but i guess i should breadboard it and see what i get. I have all the parts and i will put it together and let you know. regards.
Like any other fully indirectly heated type, DC on the filament will energize a 12X4's heater, without incident. Of course, AC is fine and DC is "overkill".
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