• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

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.
 
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:

[IMGDEAD]http://gboers.xs4all.nl/daisy/home/332/version/default/part/ImageData/data/CRCfilterDifHEX.png[/IMGDEAD]

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.
 
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
 
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....:D
 

Geek

Member
2004-09-08 7:17 am
Hi Erik,

ErikdeBest said:
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 :)

Cheers!
 
Hi Gregg

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

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
 
ErikdeBest said:
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.
 
according to MJ, the Vrrm should be 2*square2 ~ 2.8x the Vrms: 2.8 x 240 > 600. So 600V is underrated.


i believe this should be 2.8 x 480 x 1.1 = 1.478 volts, this is the voltage a reversed biased diode will see in absolute terms....this is why a FWCT configuration is not so good in terms of required diode ratings...this is the reason one diode burned out....

1.1 is form factor of a sine wave, 2.8 is the peak to peak value....
 
Tony said:



i believe this should be 2.8 x 480 x 1.1 = 1.478 volts, this is the voltage a reversed biased diode will see in absolute terms....this is why a FWCT configuration is not so good in terms of required diode ratings...this is the reason one diode burned out....

1.1 is form factor of a sine wave, 2.8 is the peak to peak value....

I don't know where the 1.1 factor comes from (unless it's 10% transformer regulation) - and the peak voltage is 2.8 * 240 for each diode... still greater than the 600V rating, even ignoring possible surges and inductive spikes.

Design practice where I work is to use only avalanche rated parts for line voltage rectification. For small supplies (<500W) we use IXYS DSA-2 series (1600V) or MUR4100E (1000V).

When an input choke is used, there will be a snubber cap or an MOV to limit the turn-off spikes. Because there ARE diode failures without them.
 
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.

I few users have experienced blown IXYS DSEI12-12 FRED diodes in SimpleSE amps when the standby switch is flipped. One user had a 5AR4 fatally arc over. I suspected back EMF and set some scope probes up to catch the spike. I set the scope up to trigger on the CT and looked at each HV lead with the two traces. Sometimes there was no spike, usually there is a small spike (300 to 500 volts), but occasionally there is a spike of 2500 volts or more. The 1200 volt FRED diodes don't stand a chance. The spike seems to be related to the individual power transformer used. Some surplus transformers that I have been using (from HP equipment) rarely spike, and the spike is under 500 volts, others (Hammonds) seem to deal out lightning bolts with regularity. I caught a 4000 volt spike on my scope.

Older slow diodes seem more able to handle the spike and seem to absorb it, the spike voltage is lower with the diodes in the circuit. I will try the IXYS DSA-2 series.
 
Fred's are by far the quietest of all the solid-state diodes I've tried. Make sure your PIV rating isn't close to being exceeded. To help suppress turn off transients, you can try adding .01uF cap (103) across the switch and from each B+ winding to the center tap. You can also try a .001uF cap (102) across each diode. Also, as mentioned by Mr. Bavis, MOV's can be helpful if you find one with the appropriate parameters.
 
Tony said:


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....:D

Tony, I have another power tranny with 0-230 and have used the motorola diodes as bridge, I still fry them that way.

Am using a 1.2kV bridge from Semikron now, no problem.

Thanks.