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Old 18th July 2010, 10:52 AM   #1
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Default sputtering 5u4 on diy Wheatfield amp

Last weeks I finished my diy headphone amp project based on Pete Millett's HA2 design and the Waarde project.

I only use cans with a high impedance of 250 ohms or more, so I
modified the power supply to L-C-L-C-2xR-C with Hammond 600V-150mA CT & 5U4,
10H/50 ohms choke - 60 uF pio - 2H/50ohms choke - 2x220 uF Electrolytics PhilipsLL nos - 2x 100 ohms - 25 uF pio for each channel.

High voltage is about 220 Vdc, current draw is about 70 to 75 mA for each channel. (10 mA for driver, 60 mAmps for follower)
I build the aplifier with 5687 - 6AS7, this results in a terrific sound with my DT880 and DT931 cans.

During start up of the amp, some sputtering appears on the 5U4 rectifier.
Only a fraction of a second for a RCA 60's nos, with Sovtek or Svetlana 80's model this happens for approx. 2 seconds, it is clearly noticable at the top of the bulb.

I am not sure this is harmfull for the rectifier or the rest of the amp, but in my opinion it does not feel safe to me.
After this "unusual" start-up the amp is operating nice and stable.
No start up thumbs or noise is noticible on the headphone output.

Anyone can advise me to solve this start up issue.
I was thinking of reducing inrush current by placing a series resistor in the CT line, after a defined time (30 seconds?) bypassing this resistor by a timer relay to swich to full high voltage.
(However, inrush current is relatively low with L-C-L-......)

Maybe there are much better solutions, based out of expierience?
Any help is appriciated,
thanks in advance.

Hans.
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Old 18th July 2010, 11:46 AM   #2
chrish is offline chrish  Australia
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You may want to try a CL140 inrush current limiter on the centre tap. It is a resistor that offers some resistance when cold, but that resistance reduces to a very low value when it warms up due to current flow. I have done this on several of my amps. On my latest amp, I have also added some UF4007 in series with the plates of the rectifier tube to help stop any flashover with reverse voltage.
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Old 19th July 2010, 03:40 AM   #3
20to20 is offline 20to20  United States
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Quote:
I only use cans with a high impedance of 250 ohms or more, so I
modified the power supply to L-C-L-C-2xR-C with Hammond 600V-150mA CT & 5U4,
10H/50 ohms choke - 60 uF pio - 2H/50ohms choke - 2x220 uF Electrolytics PhilipsLL nos - 2x 100 ohms - 25 uF pio for each channel.
What was your reason for not using the traditional design with C-L-C off of the rectifier?
40uf - choke - 40uf....? Is that an old stock 5U4?
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Old 19th July 2010, 06:25 PM   #4
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Quote:
Originally Posted by chrish View Post
You may want to try a CL140 inrush current limiter on the centre tap. It is a resistor that offers some resistance when cold, but that resistance reduces to a very low value when it warms up due to current flow. I have done this on several of my amps. On my latest amp, I have also added some UF4007 in series with the plates of the rectifier tube to help stop any flashover with reverse voltage.
Thanks Chrish, great idea.
Let's give it a try.
I measured Rdc of both secundaries 50 ohms, so another 50 ohms cold value for the NTC in Center Tab should work.
First I tried with just a 47 ohms resistor placed in the CT.(unfortunally no NTC available in my junk box...)
Eureka, it works!
No sputtering with RCA's and Svetlana rectifiers, just 1 minor flash during try out of some other Sovtek's and a new ElectroHarmonics tube.

Next time when ordering parts I will buy a couple of 50 ohms NTC's.
For the time being the fixed resitor stays in the CT.
Now I feel more comfortable when using my favorite DT880.
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Old 19th July 2010, 07:22 PM   #5
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Quote:
Originally Posted by 20to20 View Post
What was your reason for not using the traditional design with C-L-C off of the rectifier?
40uf - choke - 40uf....? Is that an old stock 5U4?
Hi 20to20,
The Wheatfield is a great design, perfect and simple!

When stripping a HP CD200 signal generator, I discovered this power tranny.
It was in mint condition, like new out of the box.
I found this tranny deserves a second life.

But with 250 ohms cans like the DT880, all you need is a maximum of 5V/20mA RMS of undistorted signal on the output of your amp.
So, B+ of 300V or higher has quite some overkill.
B+ of about 220Vdc gives already plenty of headroom.


Checking the datasheets,
5687 biased on 100V/10 mA and 6AS7 on 85V/60 mA gives me very nice and lineair operating points in the corresponding graphs.

B+ operating on 300+Volts gives another 16W of useless HV energy to get rid off. (and it does not improve quality of the output signal).
Even the Van Waarde amp runs on 150V and is working very well!
In my opinion 220Vdc is a very good compromise between signal quality and power consumption.
Another advantage, all components are running on a lower voltage, so this amp will last forever. (which of course will improve relaiability and stability)


Could there be a chance that C-L-C-.... power supply configuration gives a beter sound to my amp?
I did not try it before, so I don't know.

I tried some 5U4's, they are all "bottle" shaped models (like 6AS7G types).
RCA radiotron 60's models, 5u3 (I guess Russian Winged C models)
and a new straight model Sovtek (good!) Electro Harmonics (very noisy!)
I could not notice any difference in sound quality.

For the 5687 I tried some TungSol and RCA nos models, I also tried new 80's Philips and GE. They all sounded great, however the nos are my favorites.
For 6AS7 I tried nos RCA, new 80's models Sovtek and some unbranded (I guess WingedC 6H5(?))models.
I also have some 6080's Sylvania and Philips on stock.
They all sound great, however the 6as7 look more retro!

My current (and favorite) set up is all RCA's or RCA 5U4 & 6AS7 with TS 1965
5687W.
However I finished this amp only a couple of weeks ago, I want to do more tube swapping.
I can recommend this amp to all owners of a high impedance Beyer cans.
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Old 19th July 2010, 08:16 PM   #6
20to20 is offline 20to20  United States
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I am not familiar with the HA2 power supply. Did you replace it with your own design? I think the problem you have is not that your PS needs an inrush limiter. You have other design problems in the PS causing your 5U4 to arc. Your amp has low demands, so an arcing 5u4 seems to suggest a design problem with the input capacitor and choke configuration.
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Old 19th July 2010, 10:40 PM   #7
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Quote:
Originally Posted by 20to20 View Post
I am not familiar with the HA2 power supply. Did you replace it with your own design? I think the problem you have is not that your PS needs an inrush limiter. You have other design problems in the PS causing your 5U4 to arc. Your amp has low demands, so an arcing 5u4 seems to suggest a design problem with the input capacitor and choke configuration.
I fully agree with 20to20 about this: your circuit should not need a NTC or 47 ohms resistor in the transformer C.T return path to prevent arcing in the rectifier tube. Even if this trick works you didn't actually cured the problem and the 5U4 is still probably overloaded during power up. There is obviously a design flaw in your PSU which must be corrected. Maybe your LCLC filter has a critical resonant state which you have partially damped by the added resistor. Undamped resonant condition can produce very high voltages which may exceed the max. P.I.V ratings of your rectifier. As you have modified the original design you should include a drawing (including all parts values and voltages) of your PSU , then someone here can probably debug it for you.
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Old 20th July 2010, 12:44 PM   #8
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Thanks for helpeing me out!

Indeed a design error could cause this issue.
Each component in the power supply I measured seperately before building it into the amp.
To recheck my design I used PSUDII.
You will find a print out attached. the inrush current (IL1) is clearly visible, however it does not exceed the max peak current.
with R 47 ohms included (I simulated this to add 47 ohms as coil resistance to Tr1) the current peak decreased to about 500 mA.

During start up of the amp, I connected my Fluke true RMS multimeter on the PSU.
Switching to "peak hold" measuring for current en voltage does not exceed 700 mA's and 240Vdc.
So I expect, my PSU behaves like the one I simulated.

Did I forgot something?
Any help is apriciated!
Thanks!
Attached Files
File Type: doc PSUDII diy HA2 ps.doc (138.5 KB, 31 views)
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Old 20th July 2010, 01:14 PM   #9
20to20 is offline 20to20  United States
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Quote:
Originally Posted by hansel67 View Post
Thanks for helpeing me out!


During start up of the amp, I connected my Fluke true RMS multimeter on the PSU.
Switching to "peak hold" measuring for current en voltage does not exceed 700 mA's and 240Vdc.

Did I forgot something?
Any help is apriciated!
Thanks!
You are exceeding the 5U4 max. peak current rating.

http://www.r-type.org/pdfs/5u4g-1.pdf

This data sheet specifies max current @ 675ma
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Old 20th July 2010, 02:12 PM   #10
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Originally Posted by 20to20 View Post
You are exceeding the 5U4 max. peak current rating.
http://www.r-type.org/pdfs/5u4g-1.pdf

This data sheet specifies max current @ 675ma
Not necessarily. It depends of which version of the 5U4 you're using:
The peak plate current for a 5U4GA is: 900 mA and for the 5U4GB: 1A
Also keep in mind these are absolute maximum ratings which should never be exceeded.
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