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Old 22nd February 2008, 06:20 AM   #1
alexg is offline alexg  Philippines
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Default FREDs on PSU

I was experimenting with fast recovery diodes on my 6080 parafeed's PSU (240-0-240 secondary).

I am using a Motorola diode with 600V Vrrm and current capacity of 1A. Everything works find, it seems to sound better than a standard diode. But there are times when I switch off my standby (it disconnects the center tap of the HT from ground) while the main switch is on, one of the Motorola fast recovery diodes goes pop!

I was talking to a friend who is a switching mode power supply expert and he told me that the frying of my diodes is due to back EMF of the transformer.

He also told me not to use fast recovery diodes because they are noisy. How true is this?

Thanks.
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Old 22nd February 2008, 07:18 AM   #2
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I used HEXFREDs for both the high voltage and heater supplies in my 300B amp. It's running for a year now and there's no problem at all. For the HV supply I used IR HFA06PB120 (1200V 6A) with 22nF snubber caps. I did not measure the difference the HEXFREDs made for the HV supply but for the heater supply this was the result:

Click the image to open in full size.

What you see is the spectrum of a CRC filtered heater supply. The red line is for a simple 25A bridge rectifier, the blue line is for the HEXFREDs. In this case the HEXFREDs have the least harmonics.
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Old 22nd February 2008, 08:02 AM   #3
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Hi Alex

Maybe it is unrelated, but I will cite it anyway.

From your post I understand that you are using full wave rectification, that is, two diodes in series with each 240VAC tap, CT grounded!? If so, according to MJ, the Vrrm should be 2*square2 ~ 2.8x the Vrms: 2.8 x 240 > 600. So 600V is underrated.

And now another question, out of curiosity. Is this the power supply of the amp you recently posted in the photo gallery? Are you using a capacitor or choke input? A capacitor input would yield about the maximum plate voltage for the 6080, that is fine, but some people here prefer this valves run 'low and hot': lower plate voltages (~100VDC) and higher currents (~100mA). Have you tried that to? (I suspect it would be hard, as the chokes may not be up to the task of passing 100mA without saturating). I am not recommending anything as I have not built a 6080 amp myself, just curious as to your experience with this valve.

Hope this helps, Erik
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Old 22nd February 2008, 08:08 AM   #4
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Hi,

I've tried a lot of SS rectifiers and found that avalanche rectifiers like the 1N5062 are just the prescription for tube amps.



Cheers!
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Old 22nd February 2008, 08:36 AM   #5
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Hi Gregg

Thanks for the input. Any additional consideration for best performance with the use of the 1N5062? I mean, snubber capacitors, resistors in series with the diode, whatever?

Many thanks, Erik
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Old 22nd February 2008, 09:08 AM   #6
AJT is offline AJT  Philippines
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Quote:
was experimenting with fast recovery diodes on my 6080 parafeed's PSU (240-0-240 secondary).
your diode should be at least 1.2kv piv, series connect those diodes for safety....use an equalising resistor of 1meg across diodes....

or you can just use one half of the transformer and use a full wave bridge rectifier using 4 of those 600piv diodes....

with today's setting and availability of rectifier devices, i see no reason to use the FWCT circuit unless you have the transformers on hand....

your choice....
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Old 22nd February 2008, 09:43 AM   #7
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for new designs, i believe a FWB or a voltage doubler, will be better than a FWCT psu......

transformer utilization is a lot better....
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Old 22nd February 2008, 10:27 AM   #8
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Hi Erik,

Quote:
Originally posted by ErikdeBest
Hi Gregg

Thanks for the input. Any additional consideration for best performance with the use of the 1N5062? I mean, snubber capacitors, resistors in series with the diode, whatever?

Many thanks, Erik
The advantage of an avalanche rectifier is the snubber is built-in. It is also very effective. I measured ~25dB less noise than a 1N4007 and 10dB less than a UF4005 in a preamp circuit for example.

No need for resistors either. I had an amp that was blowing fuses (fast blow) and rectifiers (1N4007, 1N5408) on power up with everything but a tube rectifier and .... a 1N5062

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Old 22nd February 2008, 10:43 AM   #9
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Hi Gregg

Thanks for the reply! Achieving 25dB noise reduction is surely worth the extra dollar spent in the amp

From my online electronic shop I can source the BYT52 (Fast Avalanche Sinterglass Diode with soft recovery characteristics) and the BY228 (Standard Avalanche Sinterglass Diode). But e-bay got the 1N5062, for an even better price. I will pick some of them.

Many thanks, Erik
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Old 22nd February 2008, 02:20 PM   #10
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Quote:
Originally posted by ErikdeBest
Hi Alex

Maybe it is unrelated, but I will cite it anyway.

From your post I understand that you are using full wave rectification, that is, two diodes in series with each 240VAC tap, CT grounded!? If so, according to MJ, the Vrrm should be 2*square2 ~ 2.8x the Vrms: 2.8 x 240 > 600. So 600V is underrated.

And now another question, out of curiosity. Is this the power supply of the amp you recently posted in the photo gallery? Are you using a capacitor or choke input? A capacitor input would yield about the maximum plate voltage for the 6080, that is fine, but some people here prefer this valves run 'low and hot': lower plate voltages (~100VDC) and higher currents (~100mA). Have you tried that to? (I suspect it would be hard, as the chokes may not be up to the task of passing 100mA without saturating). I am not recommending anything as I have not built a 6080 amp myself, just curious as to your experience with this valve.

Hope this helps, Erik

Ok thanks for the computation, no wonder I was popping one diode after another.

I tried what some others have observed on the 6080, low plate voltages and higher currents, from 150V to 280v and I liked what I get at around 240V of plate voltage on my parafeed circuit.

Thanks.
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