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.
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.
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.
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.
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.
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!
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!
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.
Well, everything looks fine and with a choke input filter and 100 µF first cap you should not exceed the maximum ratings of a 5U5G by far. Problem is that in true life the circuit doesn't behave as expected. I would not rely 100% on the PSUD simulation (some parameters may be ignored and/or tube modelization may be crude) , also I'm not sure the capture speed of your FLUKE DMM in peak hold mode is fast enough for short transients. (a digital storage scope would be much better for this purpose).
If I were you, the first (easy) think I would do before going any further is:
Discard ALL previously used 5U4's and try again with a fresh N.O.S US made 5U4GB. Don't use any Chinese or Russian tubes in your circuit,they're know for being unreliable and prone to arcing. Beware of rebranded tubes. There are good chances your circuit will work very well now. If the problem still persists you may try to reduce the value of the 1st filter cap to 50 µF to see what happens...
Keep us posted about your results.
Hi Hansel,
I have had some sputtering on a rectifier too, though it is a 5R4WA the issue might be similar.
When I start up the amplifier (separate mono power amps) there is a brief sputtering in one block. I added resistance as the specs require (in your case, the 5U4 requires a total resistance of 75 ohms and a max capacitor of 32 muF), but it still happened a bit.
Then I dropped in a mint tube (Chatham) and [presto, no problem.
Inspecting the tube I saw that there was white dust on the inside of the glass. This meant that the coating of the cathode had been blasted off once. Then there are hotspots. Suggest too to discard such a faulty unit.
albert
I have had some sputtering on a rectifier too, though it is a 5R4WA the issue might be similar.
When I start up the amplifier (separate mono power amps) there is a brief sputtering in one block. I added resistance as the specs require (in your case, the 5U4 requires a total resistance of 75 ohms and a max capacitor of 32 muF), but it still happened a bit.
Then I dropped in a mint tube (Chatham) and [presto, no problem.
Inspecting the tube I saw that there was white dust on the inside of the glass. This meant that the coating of the cathode had been blasted off once. Then there are hotspots. Suggest too to discard such a faulty unit.
albert
Thanks for your advise Albert,
My next plan is to rebuild the PS on a wooden board, just for testing.
It is more easy to swap components and doing measurements.
I also want to try another choke.
The one I used is a 10H/200mA model, Rdc = 50 ohms, I want to try a more heavy model, bigger size, 10H/500 mA, Rdc= 40 ohms.
I will keep you posted.
My next plan is to rebuild the PS on a wooden board, just for testing.
It is more easy to swap components and doing measurements.
I also want to try another choke.
The one I used is a 10H/200mA model, Rdc = 50 ohms, I want to try a more heavy model, bigger size, 10H/500 mA, Rdc= 40 ohms.
I will keep you posted.
I just downloaded and tried PSUDII with your circuit and values. The peak charging current computed by default (ID1=IL1= 746 mA) is for what is called "Hot switching" condition and doesn't take into account the warmup time of the rectifier, this would happen only if you put a (stand-by) switch between the rectifier cathodes and input to filter and suddenly close this switch when the rectifier is already fully warmed up and all capacitors discharged.(worst case situation) In real life when you switch the mains power on from cold, the slow warm up of the (tube) rectifier will limit the peak charging current to a much lower value (this is not valid with s.s rectifiers). Fortunately, the program can simulate this but you must select (check) soft start in the menu options and re-run the simulation. The (fixed) 0.5s ramp-up time is a rough approximation (tube rectifiers usually need much more time to fully warmup) but will give a more realistic value for your circuit: 452 mA peak,which is well within the maximum ratings of any 5U4. Also, you must be very careful when using a constant current load in your simulation which can play havoc with the program and lead to false or impossible results. Better to use a resistive load corresponding to the current drawn by your circuit at the nominal working voltage.
Before using any circuit simulation software you must be aware of all his limitations and fully understand the meaning of all the parameters involved (and sometimes ignored): if the models are lousy the results will be... lousy.
PSUD II is usable for simple power supply circuits simulations but far from perfect: this is clearly stated in their program limitations you should read.
Nevertheless, I had some fun (and frustration too) with this little freeware.
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