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Old 3rd July 2012, 05:51 PM   #1
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Default Heater regulator for guitar amp?

I have been thinking of pulling a pair of power tubes to reduce the power from a Peavey Valve King guitar amp, but the heaters are series connected. Could someone lead me to information on the technical difficulties of just throwing in a regulator to keep the voltage at 6.3v? The heater supply is feeding 2 heaters per side, and I figure I would rather not just throw in resistors like the valveking wiki mod page suggests. I understand the high voltages in this amp do not bleed off on their own and plan to take precautions, but see no immediate problem with a regulator addition to the low voltage before the heaters. Any help would be appreciated.

Josh
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Old 3rd July 2012, 06:39 PM   #2
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The schematic for a 100W 212 combo found here: Peavey valveking 100 head blowing the fuse for the 14V circuit shows all four 6L6s in series and three parallel connected 12AX7s in series with that. Total current 0.9A, total voltage 31.5V. What exactly do you want to do? Regulate the 31.5V down to 18.9V?
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Old 3rd July 2012, 06:55 PM   #3
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You can do a simple way by cutting active elements of the tube, and maintain them in his sockets.
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Old 3rd July 2012, 11:10 PM   #4
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No need to pull 2 tubes - just switch the voltage to the screens of the 2 tubes you don't want to work to 0V, that will effectively turn them off.
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Old 3rd July 2012, 11:45 PM   #5
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Quote:
Originally Posted by Osvaldo de Banfield View Post
You can do a simple way by cutting active elements of the tube, and maintain them in his sockets.
Did this for my niece's husband's Classic 30. He replaced his tubes and I used two of the old ones. You also have to plug into the external speaker jack with an open male plug. It switches the windings and gives the correct impedance to the tubes. I am guessing the two amps are similar.
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Old 3rd July 2012, 11:54 PM   #6
Simon B is offline Simon B  United Kingdom
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Or perhaps a current source? Which would give a rather less brutal startup for the filaments.
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Old 4th July 2012, 06:46 AM   #7
Enzo is offline Enzo  United States
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I really can't say the tube heaters are under any stress in these amps. Tubes fail in all tube amps, it isn't rare. But what is rare is a failed heater. They happen, but are way down the list of failures.

This amp is a series string of 0.9A.

The Classic 30 trick was to take old EL84 tubes and cut off all the active pins except the heaters. Then those completed the series string. You could do something similar with the 6L6s, but what a pain in the butt it would be.

Gingertube's Idea is simple, each tube has its own screen resistor in this model (not all do), so removing it is just that simple, pull the resistor.
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Old 4th July 2012, 03:45 PM   #8
Simon B is offline Simon B  United Kingdom
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I don't know of any valves designed for series heater wiring currently in production, but when they were, they not only had specified heater current, (though various voltages) but controlled warm-up time too.

This was viewed as worthwhile, because otherwise a faster heating filament will end up with a higher than rated voltage across it, increasing the thermal shock at switch on. Additionally, series connected heater equipment often included an iron-hydrogen resistor, usually referred to as a "barretter", to regulate current flow.

I doubt that 6L6 and 12AX7 will have similar warm-up times, series wiring will exacerbate that and cause additional switch-on stress.

If a regulator is to be used, I suggest it be wired for current, not voltage
, regulation. Apart from a somewhat higher voltage across the regulator until all the filaments are hot, which might mean a slightly beefier regulator being required to cope with the temporarily increased dissipation, it doesn't add complexity over choosing to regulate voltage, but does do something that Peavey should have done in the first place.


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Old 5th July 2012, 11:19 AM   #9
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So if I am understanding this, the easy way would be to use 2 of the 4 tubes as a "heater dummy" either by cutting pins off an old set or removing just the 330ohm screen resistors (R5 and R6 on the schematic) to half the output tube circuits. Leaving the 1.5Kohm resistors from V3 to the output tubes wont hurt anything?

The other way would be to wire in a current regulator for the heaters that puts out .9A, allowing me to add or remove the tubes as output requirements change. If I went that regulator route, it would need to dissipate a good deal of power when first powered up, and as the heaters warmed, settle down, right? I wonder if one could be built with an LM350 circuit.
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Old 5th July 2012, 01:41 PM   #10
Enzo is offline Enzo  United States
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For the old series heater tubes for radio and TV, that controlled heater warm up characteristic thing mattered, especially with a many-tube string and with disparate heater voltages. And I am sure in this instance, they donl;t all heat up in identical fashion, but whatever they do, it is over in seconds, and doesn't hurt the tubes.

Yes, the suggestions are either make heater dummy tubes, or disconnect the screen resistors for two of them. I suppose a resistor could be calculated for each missing tube instead of actually making a dumkmy, but haven;t thought about it.

If you open the screen circuit of the 6L6, it will stop conducting. No, nothing will be harmed by leaving the control grid connected via the 1.5k stopper. IN fact if you look at the schematic, you will see that that is how Peavey implements a standby, by removing screen voltage. The power tube plates remain live. Won't hurt it to have onloy two of the tubes thusly turned off.
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