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Old 15th September 2014, 04:33 PM   #1
cmerdan is offline cmerdan  United States
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Default Source for least expensive tubes.

Where can I find a 12SL7 tube for my Heathkit A7 amp?
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Old 15th September 2014, 05:17 PM   #2
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$4 here:

Electron Tube List range 12A - 12Z5 @ ESRCVacuumTubes.com
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Old 15th September 2014, 05:27 PM   #3
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cmerdam - where are you living ?
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Old 15th September 2014, 05:44 PM   #4
tomchr is offline tomchr  United States
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Vacuumtubes.net is another good source. There's always that auction site...
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Old 16th September 2014, 01:14 AM   #5
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Cheapest solution: get a NOS russian 6N8 tube, and put two 10 ohm 2W resistors in series with it's heater. =)
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Old 16th September 2014, 02:36 AM   #6
RTF671 is offline RTF671  United States
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If you were brave you could rectify it for just that tube and use a zener to drop the voltage across the heater. You'd get the benefit of a quieter tube too.

Just a random passing thought! ;D
Otherwise the resistor will drop your voltage for the heater just as well.
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Old 16th September 2014, 04:02 AM   #7
kevinkr is offline kevinkr  United States
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I'm fairly familiar with the A-7, and running a 6SL7 with a dropping resistor given the increased filament current is probably not a good idea - the stock power transformer is hanging on by a thread.. The 12SL7 is very cheap here, it has not attained collector status because of the filament voltage. The OP appears to be located in the U.S.
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Old 16th September 2014, 04:37 PM   #8
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Old 16th September 2014, 05:46 PM   #9
GoatGuy is offline GoatGuy  United States
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You know, the "cheap solution" for using 6 volt tubes as substitutes for 12 volt tubes is to do 2 things:

Full-wave bridge rectify the 12.6 VAC winding (this drops the 1.414 12.6 = 17.8 peak voltage by 20.7 = -1.4 volts. 16.4 volts - if you use a 2,200 F 25 V electrolytic to smooth things out. Now, string downwind of that 5 or 6 of the standard 3 amp 100 PIV silicon diodes in series. That'll drop the DC to 12 volts, for all the 12-volt tube filaments. Now another string of 8 diodes, to get the 6 volt needed for the 6-volt substitution tubes.

Sounds like a lot of diodes? Nah... they're less than 3 cents each. Further, having a bunch of 'em in series takes care of the power-dissipation problem. Each is capable of about 2 watts (0.75 volts times 3 amps = 2.25 watts). So, having 8 in series is like having a 18 watt resistor up there.

Sometimes the "old, simple solutions" are warranted. Oh, sure, you could put in a fixed-value regulator, and a heat sink, and ... and ... and ... but why? Its just a heater circuit, dude!

GoatGuy

PS: if you go this route, say, "on the test-bench" ... don't forget to LOAD the 6-volt output with a nice resistor between 5 and 25 ohms. It is only under load that the diodes have a voltage-drop of nearly-constant 0.75 volts, nominal. If you leave it open, the voltage will drift up to near-12 volts! A life of dealing with quixotic semiconductors uncovered this truth some 45 years ago, to me.

LOL
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Last edited by GoatGuy; 16th September 2014 at 05:49 PM. Reason: caveats, quixotic semiconductors, loads, etc.
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