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Old 9th July 2012, 10:08 AM   #1
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Default Heater voltage drops?

Hi all,
Is it normal that the measured heater voltage sags considerably when the tube is in place vs. not connected?

I'm servicing a Microphone preamp that uses three 12AX7 tubes at certain points. I'm trying to check heater voltages, here's what happens:
-The DC heater supply consists of a full wave diode rectifier, a filter cap (4700uF) and two 1kOhm resistors that crate a center point to GND
- the heaters of the 3 tubes are wired in parallel. AFAIK the heater center tap is not connected.
-When I measure PSU output with NO TUBE connected, the voltages are +6,8 and -6,8 VDC
-When I measure with all three tubes connected, the voltages drop to +4,6 and -4,6 VDC.
The service manual states it should be +-6,3 V (+-1)

What could be wrong, could it be the transformer not giving enough current? or is it normal?

Thanks for any insight!!
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Old 9th July 2012, 10:18 AM   #2
DF96 is offline DF96  England
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If it is a cheap preamp then the transformer is not up to the job. Perhaps the 4700uF cap could be bigger too, as there could be about 1V of ripple which might do more harm than 6.3V smooth AC.

If it is a decent preamp than it is possible that the reduced heater voltage gives less shot noise because the cathode wil be cooler. A balance may have been struck, between reducing the temperature-related noise and losing the space charge smoothing effect.

My guess is that it is a modern cheap preamp, using valves as a fashion statement? I say this because back in the days of valves nobody used DC heaters; instead they just knew how to wire AC heaters correctly and had valves with good heater-cathode insulation.
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Old 9th July 2012, 10:35 AM   #3
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Thanks DF for your reply
It's a TL Audio 5051 mk1 channel strip. Not the cheapest but neither a top preamp. Hybrid solidstate and tubes (but marketed as FAT TOOBE SOUND of course ) , which are run at 150V plate voltage.
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Old 9th July 2012, 10:58 AM   #4
DF96 is offline DF96  England
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The maker's blurb does not mention under-running the heaters. Strange that they stabilise the HT but not the heaters. The result could be that sound depends on local mains voltage. There does not appear to be any user adjustment for mains voltage; the manual just says 220-240V. 10% variation on an under-run heater will affect valve characteristics. This is an FX box, not a preamp. The manual specifically mentions turning up the gain to get "valve sound".
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Old 9th July 2012, 12:50 PM   #5
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Hi,
Thanks again for the insight! For now I will go on with the service procedure, I hope nothing's seriously wrong.
Cheers!
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Old 9th July 2012, 03:58 PM   #6
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Check that cap... could be open or high ESR. I'd expect more like 8-9V open circuit...

If you replace it, be sure the cap is is rated for at LEAST 500 mA ripple current.
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