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Old 22nd March 2012, 03:12 AM   #11
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Quote:
Originally Posted by Eli Duttman View Post
No way 300 mA. of voltage doubled B+ from an 850 mA. RMS rectifier winding. Approx. 225 mA. is the limit. Will 225 mA. of B+ be sufficient to feed 4X 6B4s and the small signal circuitry?
Eli: Is this a rule-of-thumb? Voltage doubler yields 25% of initial current capability? I haven't had to voltage-double anything yet, just curious......


225ma for a stereo MM would be tough. Would be plenty for a monoblock.
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Old 22nd March 2012, 12:31 PM   #12
mfaughn is offline mfaughn  United States
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Quote:
Originally Posted by Eli Duttman View Post
AFAIK, Poinz has never done a DHT MM.
I was referring to the breadboard that nerdorama mentioned above. Wondering if he had sketched out his changes or if they were just in his head.


Quote:
Originally Posted by Eli Duttman View Post
No way 300 mA. of voltage doubled B+ from an 850 mA. RMS rectifier winding. Approx. 225 mA. is the limit. Will 225 mA. of B+ be sufficient to feed 4X 6B4s and the small signal circuitry?
Well, the iron came from an HK TA260, a receiver feeding a quad of 7355 tubes idling at 60ma each (unless I don't understand datasheets at all yet...which is entirely possible) as well as the usual pile of small signal tubes that would be found in a receiver. Could be the Photofact isn't quite right?

This is the main reason I started this post in the first place...to help me figure out what I could and couldn't do with this iron and very hopefully to help me to come closer to figuring it out on my own the next time I have a similar question

And also...Thanks Eli. You are always answering my neophyte questions. You are one of the folks that make this forum (and by extension the web) a great thing.
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Old 23rd March 2012, 12:48 AM   #13
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Quote:
Eli: Is this a rule-of-thumb? Voltage doubler yields 25% of initial current capability? I haven't had to voltage-double anything yet, just curious......
Yes, it's a good, conservative, rule of thumb. Remember, TANSTAAFL always applies. When a full wave rectifier feeds a cap. I/P filter, approx. 1/2 the AC RMS capability of the rectifier winding is available as DC, as a consequence of I2R heating considerations. All voltage multipliers are cap. I/P filters and the energy stored has to come from somewhere. I divide the 50% cap. I/P filter basis by the number of multiplication stages. For a doubler, we end up with 25% of the AC RMS current available as DC.

Quote:
Well, the iron came from an HK TA260, a receiver feeding a quad of 7355 tubes idling at 60ma each (unless I don't understand datasheets at all yet...which is entirely possible) as well as the usual pile of small signal tubes that would be found in a receiver. Could be the Photofact isn't quite right?

This is the main reason I started this post in the first place...to help me figure out what I could and couldn't do with this iron and very hopefully to help me to come closer to figuring it out on my own the next time I have a similar question
I think you misinterpret the 7355 data sheet. Notice that the Class "AB1" anode currents shown are for [b]2/b] tubes. You can bet H/K went for the power, by using a near Class "B" operating point that resembles the right hand column for "AB1". A 225 mA. B+ rail would have been adequate to power both the 7355s and the small signal circuitry.

With its near Class "A" O/P tube operating point, a MM based on EL34s or 6B4s is pushing your luck, in combination with the power trafo you have. Don't give up the ship! The "Musical Machine" and "El Cheapo" are kissing cousins. Both are a differential splitter/driver and PP "finals". I think we can execute an "El Cheapo Grande", with your power "iron", although 225 mA. is cutting things a bit fine. EC's 12AT7 based splitter/driver works with "12" W. tubes and 7591s. Now, you know where the "Grande" comes from. Let's look at the 7591 data sheet to find a reasonable set of operating conditions with approx. 360 V. on the plate. My initial guess is that 40 mA. of cathode current per tube, in either triode or UL mode, is reasonable. PURE fixed bias will be essential, as zero B+ Volts can be spared. 7591 grid to ground resistance limits will force the buffering of the LTP anodes. Otherwise, gain will be insufficient. DC coupled source followers made from little ZVN0545A MOSFETs will do that job very well.
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Old 24th March 2012, 12:39 AM   #14
mfaughn is offline mfaughn  United States
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Well, that was totally dumb on my part with the 7355 sheet. Pretty obviously says that values are for two tubes. I think I was blinded by wishful thinking.

I am really more interested in using the OPT iron than the power iron. I think the best thing is to just pair it with a new power transformer that is *well* suited for whatever I decide to build. Likewise I'll someday use that power transformer with something that will be happy with what it can provide.

I'm still liking the idea of a PP 6B4G. That pile of EL34's might just stay in the drawer for a long time.
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Old 24th March 2012, 01:31 AM   #15
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The O/P "iron" from the H/K receiver that used 7355s should be OK with PP 6B4Gs. Current production Sovtek 6B4Gs can take a higher plate voltage and a higher plate dissipation, compared to NOS. They go for about $80 a tube. I wish you luck in finding matched pairs of NOS, with all 4 tubes from the same manufacturer and the same internal construction. The really inexpensive way to get close to 6B4Gs is by triode wiring 6AV5Gs. Sylvania actually did that strapping internally to satisfy a U.S. gov't order. No, the triode strapped pentodes will not be as nice as true 6B4Gs, but you can debug your setup without the worry over destroying COSTLY DHTs.

BTW, "Uncle" Ned Carlson has a way to setup an amp, along Dyna ST70 lines, that works with both Sovtek 6B4Gs (no NOS) and EL34s.
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Old 24th March 2012, 02:04 AM   #16
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Hmmm, $320 for a quad of Sovteks or play dice with pricey old tubes...vs. the nice quad of =C= EL34's. Perhaps that won't mean so much after I've dug back out of the hole that going back to school put me in. I have some other horizontal deflection tubes in the drawer -- compactrons. I suppose the possibilities are quite large. Really, I want to start understanding how to build a single stage and how to get different stages to play nice with each other. I know there is a ton of literature and such out there. It is just a matter of picking it up and reading it.
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Old 24th March 2012, 02:28 AM   #17
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I don't know that Poinz ever actually built it, but here's a link that shows a 6B4 Musical Machine schematic with an interstage transformer coupling.

EL-34 PP schematic - mqracing - MagneQuest/Peerless Forum

and a response from Poinz to the above post:
Ver-ry Kewl, Mikey. - Poindexter - MagneQuest/Peerless Forum

All the best,
John
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Old 24th March 2012, 04:09 AM   #18
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Quote:
Originally Posted by mfaughn View Post
Hmmm, $320 for a quad of Sovteks or play dice with pricey old tubes...vs. the nice quad of =C= EL34's. Perhaps that won't mean so much after I've dug back out of the hole that going back to school put me in. I have some other horizontal deflection tubes in the drawer -- compactrons. I suppose the possibilities are quite large. Really, I want to start understanding how to build a single stage and how to get different stages to play nice with each other. I know there is a ton of literature and such out there. It is just a matter of picking it up and reading it.
The O/P "iron" you salvaged from the H/K receiver will be fine in combination with triode wired EL34s. I would, ala "El Cheapo", use a small amount of NFB around the trafos to linearize them and improve damping factor. The 12AT7 based splitter driver does not have sufficient gain to drive both EL34s and the NFB loop. You get around that by using 1 section of a 6SN7 or close relative wired common cathode and don't bypass the cathode bias resistor. Such a setup is a linear stand alone (not inside the NFB loop) voltage amplifier feeding yet another "El Cheapo" variant.

BTW, what sort of preamp do you have or plan on building? The "El Cheapo" variants I've been discussing are "integrated" amps that need only a CDP or phono preamp/TT/cart. as a "2" VRMS signal source.
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Old 25th March 2012, 01:18 PM   #19
mfaughn is offline mfaughn  United States
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Yea, I had figured out that pentode wired EL34's were not in the cards for these OPTs.

Well, it seems that I consistently find myself really wanting to better understand the whats / whys / and hows of amplifiers. I think I'm once again changing my mind about building 'as drawn' from a tried and true schematic. Having this ostensibly decent but somewhat unusual iron is a driving factor. I could just get some 7355 tubes and build the HK A500 power amp section but probably won't. I'm going to slow down and learn more about how these things work so I can actually know why I would choose a tube for a particular section and whatnot. I've got a DCPP and an RH84 going at the moment anyway
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Old 25th March 2012, 05:38 PM   #20
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I'm getting around $140 from the Siberian Shop on ebay for a quad of 6B4G types. I'm using PP2a3s right now with a differential pair of 4P1L. This is a very detailed sound - your total Russky quality! Got those from the Siberian Shop too. They do need a bit of current, though. Reckon on 15mA each. Mu is around 11, so it needs a preamp. I use a 26 preamp, as described in the "26 pre amp" thread here.

andy

Last edited by andyjevans; 25th March 2012 at 06:05 PM.
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