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Old 22nd December 2011, 07:48 PM   #1
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Default ST70 Inrush Issue

Hi,
I am new to tube audio and re-built a dynaco st70 with a triode electronics capacitor and driver board. I am using 5Ar4 rectification. When I turn the amp on, the 12au7 (hammond) filament lights up brighlty and then dims and I hear an inrush sound on the transformer (brief hum) at the same time (<1second). Then the other filaments come up fine. Transformers are all the old clothe lead style. No sparking or tracking is heard/seen. If I bring it up on a poor mans variac and then shunt the variac (go to full power), the affect is minimized greatly.

Amp sounds fantastic. Virtually no hum at zero input. The bias is holding nicely at 1V. There is a ground loop if I use the 3 prong plug I installed and using with pre-amp. No hum with just ipod and 3 prong. No hum if I let the amp float on it's own ground with pre-amp.

The hum I am not concerned about as I think it is a simple ground loop. The in rush stress to the transformer and 12au7 over time, I am worried about.

Since I am new to tubes, I wondered if this is just normal. All I can figure is that the power transformer is saturating briefly to charge the cap board and then there is somehow some bleed over to the filament taps. 12au7 having the smallest filaments light up faster. As the saturation recovers the filament voltage stabilizes.

What do you think?
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Old 22nd December 2011, 08:29 PM   #2
SY is offline SY  United States
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That sounds perfectly normal. Relax and enjoy some tunes.
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Old 22nd December 2011, 08:40 PM   #3
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The valve heater is virtually a short circuit at power up as it is cold.
This causes a rush of current to start with, but as the heater warms up it becomes more resistive and the current settles down to a normal value.
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Old 22nd December 2011, 08:59 PM   #4
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Default Thanks!

Thanks, I am enjoying very much on digital and vinyl. It sounds fantastic compared to my Adcom GFA555 and chipamps.
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Old 23rd December 2011, 12:31 AM   #5
20to20 is offline 20to20  United States
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Quote:
All I can figure is that the power transformer is saturating briefly to charge the cap board and then there is somehow some bleed over to the filament taps. 12au7 having the smallest filaments light up faster. As the saturation recovers the filament voltage stabilizes.
The tube filament "flare" is probably a small manufacturing anomaly with the filament centertap not being perfectly in the center. If one side is really 5V and the other is 7v then you'll have a small overcurrent on the small side until the heater temp evens out.

I think your biggest concern should be for the 5ar4 if your input capacitance is much higher than the stock 30uf of the ST-70. If you search the boards you'll find plenty of complaints about arcing 5ar4 in new kit and modded amps with high cap boards.
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Old 23rd December 2011, 12:40 AM   #6
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Quote:
Originally Posted by heiligenwho View Post
When I turn the amp on, the 12au7 (hammond) filament lights up brighlty and then dims
Sounds like a Mullard, or made with their tooling. Kinda cool how they flash like that, perfectly normal. Enjoy your amp.

jeff
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Old 23rd December 2011, 08:24 AM   #7
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3 prong is not really designed for that amp/transformer combo. disconnect green wire( ground prong) if you want to be fancy, install a ground lift switch.

Last edited by DavesNotHere; 23rd December 2011 at 08:47 AM.
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Old 23rd December 2011, 09:56 AM   #8
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Quote:
Originally Posted by heiligenwho View Post
Hi,
I am new to tube audio and re-built a dynaco st70 with a triode electronics capacitor and driver board. I am using 5Ar4 rectification. When I turn the amp on, the 12au7 (hammond) filament lights up brighlty and then dims and I hear an inrush sound on the transformer (brief hum) at the same time (<1second).
What do you think?
Can be a common problem with transformers when the vacuum impregnation has gotton old and brittle and didn't quite get to the core lamination internals or ..can be a symptom of overheating.
A near loose lamination activated by the start up load, especially a GZ or U series rect when the warm up has a touch of assymetrical half wave loading that can aggravate a springy lamination in the core centre... Nothing to worry about except if it gets seriously bad, a replacement made tranny is the only solution.
I've got loads of good running but noisy transformers, some I've tried dismantling and then given up trying to get the laminations out to put more varnish in, so they get used for door stops. Some are so noisy with buzz that is still annoyingly disconcernable listening at the 5W amp level... Yup that bad.The cloth bound toroid is an impossible task.

The other question is how hot does your tranny get after a couple of hours running ?.....an older transformer conservatively wound should be just warm to the hand, however more recent ones with class E winding materials can accept a stewing 60 deg C temp rise without problems.

richy
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Old 23rd December 2011, 11:06 AM   #9
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quot #7


3 prong is not really designed for that amp/transformer combo. disconnect green wire( ground prong) if you want to be fancy, install a ground lift switch.

Not the smartest thing to say.... The ground needs to be connected for safety sake. Have been shaken in the past by the DIY Audio police for my comments on removing or not having the safety ground attached. I have 3 wire setups on ALL my equipment. Have put the signal ground at a different potential and the noise is gone.

Install 2 1n4007 diodes at the rectifier socket and these will help take the brunt of the load off the 5Ar4 rectifier.
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Old 24th December 2011, 02:42 AM   #10
roline is offline roline  United States
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I used a CL80 as an inrush limiter on all of my amp designs.
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