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ARC SP8 tube reg rebuild
ARC SP8 tube reg rebuild
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Old 22nd September 2015, 01:05 PM   #1
cygnus x1 is offline cygnus x1  United States
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Default ARC SP8 tube reg rebuild

Hi All,
I was considering replacing the 12BH7 pass tube in the regulator. If I do the math the circuit needs 402Vdc at 39.5ma. The 12BH7 has a 228v drop across it, so 9.006W. It's only rated for 3.5W each plate and both plates run in parallel. Most tubes do less than the single plate rating. So it appears that the 12BH7 is not up to the job. Did I miss something? They use a 6550/6H30 in the newer designs. I'm limited by the available heater current and 9 pin socket, so thinking of replacing it with a EL84 in triode? Sound correct? Not sure of the other changes required?
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Old 22nd September 2015, 05:45 PM   #2
cygnus x1 is offline cygnus x1  United States
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Should I post this in the "tubes" section? Someone told me this is a "buffer" style regulator...not sure what that means? I do see the two sections of the 12BH7 are doing different things?
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Old 22nd September 2015, 06:02 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by cygnus x1 View Post
I was considering replacing the 12BH7 pass tube in the regulator.
I wouldn't second guess ARC, they are careful in their equipment design.
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Old 22nd September 2015, 06:34 PM   #4
cygnus x1 is offline cygnus x1  United States
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The reason I ask, even they moved away from this design and are using 6550/6H30 in the newer reference designs.
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Old 22nd September 2015, 06:43 PM   #5
SY is offline SY  United States
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This is an absolutely appropriate place.

Where did you come up with the current draw number?

Audio Research has made some inexplicable design choices over the years, and a lot of their products have poor reliability. There may or may not be a blunder in their circuit.
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Old 22nd September 2015, 06:53 PM   #6
Zen Mod is offline Zen Mod  Serbia
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ARC SP8 tube reg rebuild
poor 12BH7

in fact , poor everything .......

just toss in TL783CKC and be done
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Old 22nd September 2015, 07:58 PM   #7
cygnus x1 is offline cygnus x1  United States
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Voltage drop measured at each plate or cathode resistor. Most of the resistors are 1 watt WW 1% and are still in spec. Current calculated and totaled. There are several opamps and transistors that all draw off of main B+, plus the 12BH7 and 12AX7 reg tubes themselves. Circuit running close to posted voltages on ARC schematic. I don't think 40 ma is out of line.
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Old 22nd September 2015, 08:06 PM   #8
SY is offline SY  United States
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Do they run hot?

A lot of ARC products had failures because of insufficient air flow- I've repaired some of the old tube power amps where the circuit board just eventually started crumbling near the tube sockets. Lots of browning.
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Old 22nd September 2015, 08:56 PM   #9
cygnus x1 is offline cygnus x1  United States
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Yes. I was surprised how hot it was running, felt more like a power amp tube. I can see why people said they were replacing the 12BH7 every year or two. Top and bottom covers are perf metal. I was surprised they used this tube, even the SP10, which is a different design (bunch of 6DJ8s in the phono stage) runs two 6L6 in parallel.
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Old 22nd September 2015, 10:39 PM   #10
SY is offline SY  United States
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OK, so a blunder. It happens. Problem with PCBs is that it's tough to rewire the socket to sub a higher dissipation tube. I'd strongly consider a solid state regulator.
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