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Old 12th April 2011, 10:35 PM   #1
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Default Those using a Hammond 374-BX...

...on their SSE, I need your inputs!

I am baffled that the filament voltage on my trafo is 6.7 VAC instead of 6.3VAC as specified. Source is 120Vac(wall). Any of you have a record somewhere when you were initially testing your amp of the filament voltage you got (across T1 GRN Connector if you prefer)?

Thanks in advance!


Abe
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Old 12th April 2011, 10:51 PM   #2
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is this with a load or without?
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Old 12th April 2011, 11:02 PM   #3
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I have that transformer but haven't yet wired it into an amp yet, nearly there though!

When you say "initial testing" of the amp it sounds like you don't have the valves installed and therefore there's no load on the (nominal) 6.3V taps. Once the valves are installed and the filaments glowing that voltage will drop under the load. I don't think you have anything to worry about with those figures.

Does it make music?
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Old 12th April 2011, 11:21 PM   #4
tomchr is offline tomchr  United States
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Have you confirmed with an AC voltmeter that the voltage at the wall outlet is indeed 120 VAC? The nominal range for the AC voltage in the US is 120 V +/-5 % so anything from 114 V to 126 V is valid...

If you measure 6.7 V without load, expect the voltage to drop to within the 6.0~6.6 V range specified by most tube manufacturers under load.

6.7 V is only slightly more than 6 % high. That's not bad, actually. Even if the filament winding measured 6.30000000 V with 120 V AC in, the output voltage could vary from 5.98 V to 6.62 V depending on time of day, etc. if the AC in was varying within the 114~126 VAC range.

Don't worry about it. If you're picky, I suggest using regulated DC for your filaments. When you do the math, you'll find that you need a 9~12 V transformer to keep the regulator happy under all input voltage and load conditions. You'll also find that unless you go with a switching regulator, filament regulators quickly become an exercise in thermal management.

~Tom
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Old 13th April 2011, 12:30 AM   #5
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I have a Hammond 374BX. My line voltage generally tends to run anywhere from 120VAC up to 126VAC. I do not recall the voltage of the heaters under full load. With no load on the PT, they were around 6.5~6.6 on this particular day:

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Checkout :: volt01.jpg picture by Ty_Bower - Photobucket
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Old 13th April 2011, 12:35 AM   #6
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Hello guys!

Thanks for the quick responses!

Yes, that figure is under load, using a variac set and measured at 120.1 Vac! I noticed it last night, even when music was playing(Yes! It makes music!).

Even tried it with no HV, so no rectifier (that's 5V anyway), but with the input/output tubes. I can see that the filament supply on this circuit is lifted ~26 Vdc off GND(my B+ is 410 with /GZ34-5AR4).

Ahh, I should just ignore it and play music then!

tomchr, nic6paul,Horsebox, and Ty thanks again guys!


Abe

Last edited by amandarae; 13th April 2011 at 12:51 AM.
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Old 13th April 2011, 02:13 AM   #7
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I've found that while all of my multimeters are remarkably close in their measurements of DC, they tend to have some error in their low voltage AC readings. Do you have a different meter you could try?
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Old 13th April 2011, 06:26 AM   #8
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Quote:
Originally Posted by Ty_Bower View Post
I've found that while all of my multimeters are remarkably close in their measurements of DC, they tend to have some error in their low voltage AC readings. Do you have a different meter you could try?
Hello Ty,

Thanks for the heads up!

Yes, I do have two meters and the difference was +/- 0.08 Vac.

Abe
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Old 13th April 2011, 01:26 PM   #9
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Quote:
I am baffled that the filament voltage on my trafo is 6.7 VAC instead of 6.3VAC as specified.
Unfortunately this seems to be the case with almost all Hammond transformers. The HV winding will be above spec too. The voltage is usually within the +/- 10% spec most tube manufacturers quote.
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