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Old 3rd April 2004, 08:45 PM   #11
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Default transformer

Hi,

I was talking about the power transformer, not the output transformer.

You may not have a problem with your build, after looking at the original schematic a bit more, I see a few things that could cause some voltage differences. For one, there will be increased drop across the 5U4 if the output current is doubled. Add to that the fact that the original transformer is spec'd at 300-0-300 and run at half rated current, which could result in the real AC output voltage being considerably higher than the spec'd value. Those two taken together could probably add to ~60V of the difference seen between your readings and the original's. Add a higher DCR choke and you might have most of the rest.

A different point, the 5U4 is running much closer to it's current limit now and the 100uF input cap could be too large to be conducive to a happy rectifier. The spec sheet shows a suggested value of 40uF for the 5U4GB.

A third point, changing the feedback hookup may have gotten rid of the oscillation, or it may have just moved the frequency up to something you can no longer hear. You should check with a scope if possible. Especially if your tubes are running hotter than you think they should, or your DC current draw is higher than the idle operating point would suggest it should be.

Needless to say, your tubes are operating in a significantly different area of their curves than the orginals were. This could result in significant increase in distortion, as well as a drop in output power.

Not trying to be negative on your build here, just trying to suggest some of the potential problems I see with present situation.


Michael
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Old 3rd April 2004, 11:00 PM   #12
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Quote:
My transformer is not the suggested one. The original is a Tango U-808, but I believe the Hammond 1627SE have similar electrical characteristics; they both have a 2,5K primary impedance.
That the transformers have same primary impedance is not enough to make them interchangeable, leakage inductance and interwinding capacitance probably differs a lot between these 2 transformers and is the reason why you have oscillations in the amp, (The Tango transformer probably have much better high frequency response and therefore lower leakage inductance)

That the oscillation is dependant on the setting of the volume pot suggest that the frequency of the input stage pole at the grid is critical, try by adding a 10kohm resistor between the pot and the first grid, that will lower the pole and hopefully make the amp stable.

If you have a oscilloscope you can then check the square wave response at 10 kHz and look for ringing, if there is ringing you can try adding a small cap ~100pF maybe in parallell with the 1.5kohm feedback resistor.

I really advice you to check the square wave response as the amplifier is clearly very close to oscillation and is bound to have very small phase margin which will effect the sound in a very negative way.

Regards Hans
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Old 4th April 2004, 04:43 PM   #13
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Wow! My first amp is definitely not in a bed of roses
Quote:
I was talking about the power transformer, not the output transformer.
Yep, sorry Michael, I was sleepwalking at that time of the night.

I believe you are right, there maybe an ultrasonic oscillation, since the tubes are getting quite hot, but I don't have a oscilloscope to check the wave response. I will try to find a second hand one.

If I connect only one channel, the current is 73 mA for left channel and 72,2 mA for the right channel. When I connect both channels together, the total current drops to 123 mA.

Quote:
The spec sheet shows a suggested value of 40uF for the 5U4GB
So Hiraga is running the 5U4G out of the specs . I will try with 50 uF, since I am using 50+50 caps in parallel. Do you know where can I find the spec sheet for this rectifier?

Thank you very much for all your helpful comments I will be back with the results when I have the oscilloscpope and a tone generator.
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Old 4th April 2004, 04:55 PM   #14
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Err... Do you think that this KT88 amp maybe more suitable for the Hammond 1627SE? I've got the Rainer zur Linde's book "Audio Valve Amplifier", but the only thing it mentions about the OT is that it's 2,3kOhm.

Cheers,

Manuel
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Old 4th April 2004, 07:01 PM   #15
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Hi,

Quote:
So Hiraga is running the 5U4G out of the specs
No, not really.
Current draw is a constant 100mA at 380VDC.
This well within ratings for a 5U4G.

Cheers,
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Old 4th April 2004, 08:19 PM   #16
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Default psu

Hi Manuel,

Do you have the program Power Supply Designer? It can be downloaded for free from http://www.duncanamps.com/ It is very easy to use and has been quite accurate for me in the past.

You can use this to model your power supply, also you should find a link to the datasheet for the 5U4 datasheet there.

I ran a quick model of your supply and your output seems a little low, but not too much. The program does give a warning that you are exceeding the forward current maximum on startup. Most likely due to the large first cap. What is the DCR and the current rating for your choke? Are you sure your power transformer is rated for 300mADC output? The per channel current drop when you connect both channels seems to indicate a further drop in output voltage with larger current demand. Some addt'l drop is to be expected through the rectifier but yours may be a bit higher than the expected. What I would do is make a list of voltage test points for the amp, such as plate, screen, cathode of KT88, plate and cathode of preamp, before choke, after choke, power transformer secondary AC. Then record the values with one channel, both channels, zero signal, full output. For me these measurements, and the current calculations you can do with them, does a lot to tell me if the circuit is operating as expected and where voltage drops are taking place, just a general feel for the amp, if you know what I mean.

I think the power transformer in the original schematic must have actually been 325-0-325. You could go to a solid state full wave rectifier if you want to raise your voltage closer to what is shown on the schematic.

Might as well get this design working well before changing horses to another design. At least then you can give a fair comparison to the two and decide which you prefer.

Michael
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Old 4th April 2004, 09:38 PM   #17
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Default Oscilation pot position dependent..

Hi Manuel!!

Put a 1k Ohm resistor ,in series between the grid of the input tube and the center pin of the RCA input socket.
This resistor must be located near the tube pin.

Jorge
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Old 5th April 2004, 05:33 AM   #18
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Hi Dr. Frank, Michael, Jorge, Thank you very much for your replies. I will do all this homework next weekend. It's really getting very interesting. I've never liked "easy listening" music so, why to have an easy made valve amplifier?

Quote:
Might as well get this design working well before changing horses to another design
Yes, you are right. Anyway, it will be very easy for me to fast change to another design since I am using a "rack-mount" assembly with different modules for the amp and the PS. But I will first fight with this one until everything is ok.

When I ordered the transformer I did specify 300 mA. But because it is a noisy transformer (though I did ask for it, at the factory they told me it was not necessary to encapsulate it with epoxy) I will complain and order a new transformer, maybe 325-0-325.
Is there some safe way to calculate the maximum current output of a transformer?

Thank you,

Manuel
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Old 5th April 2004, 04:18 PM   #19
Gerry is offline Gerry  Germany
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Default Cap too big

Yes, you seem to hurt the rectifier tube with the inrush current. The capacitor should be max 50uF

see here:
http://www.triodeel.com/5u4_p1.gif

Sincerely,

Gerry
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Old 13th April 2004, 08:17 PM   #20
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Default ¿Did I order the wrong transformer?

Hello,

I suspect that the guilty of the voltage drop is the supply transformer. This is the first time I use PSU designer (thanks for the link, Michael) and if I'm not wrong I have the following parameters:

Primary Supply Voltage: 220V
Primary resistance: 5 Ohm

Secondary off-load voltage: 312V
Secondary resistance: 143 Ohm.

With these values, PSUD gives me about the same result as the actual values that I can measure. (325V at C1, 322V at C2)

So, ¿how can I tell the manufacturer of the transformer what do I really need? I see that it's not enough to specify 2x300V/300 mA

Here is the schematic with my latest modifications and measures.
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