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Rectifier with slow start up?

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#2
Damper diodes give some of the slowest warm up times, and are among the lowest resistance vacuum diodes. Warm up one the order of 15 seconds. Filament supply for them may be an issue.

To emulate copper cap youd probably get close with a pair of diodes, a thermistor and a resistor.
 
#3
Dampers vary. I like the 6D22S from Svetlana, which has about 25 secs warm-up time. It's no problem feeding its 6.3v heater, which is very well insulated from the cathode, hence the slow warm-up. It can tolerate 600v between heater and cathode, which means it can share the heater circuit of the other tubes.
 
#4
Surprised that no one has mentioned the venerable 5AR4/GZ34, currently available from at least 3 sources this tube has relatively long warm up times of up to 30 seconds or so.. Vf is typically 25V or less compared to some of the better dampers at 15V - 20V.
 
#6
burnedfingers said:


The 5AR4/GZ34 is what I am trying to replace.😉
Click to expand...

Hi burnedfingers,
Any particular reason why you want to move away from the 5AR4? Might give us some sense of what you want to accomplish by so doing, and hence we could fine tune our recommendations, or provide possible reasons for reconsidering the decision..

😀
 
#7
Sure, it probably would help if I did give my reason for wanting something else. I just finished putting in my 3rd 5AR4 in my modified Dynaco stereo 70 amplifier. It toasted 2 in less than 6 months.

The power supply board is a SDS and has a first capacitor value of 40mfd. Well within the limit for this rectifier.

THe heater draw is 2.4A per side as I am running 6BG6GA's for output tubes. I am running 1) 6SJ7 and 1) 6SL7 per channel for the front end.

The 6GA6's are biased at 50mA per tube.

I shouldn't be going thru rectifier tubes this fast. Thus the reason I would like to come up with something that will give me a slow start up for the B+ and a life longer than what I have experienced so far.

You would think this combination would be easier on things since the current draw for a channel (stock) is 3.45A.
 
#10
I haven't tried an inrush limiter. I didn't think I needed one because of the fact the tube has a slow warm up. At this point I am ready to try about anything except for a high dollar Mullard.

I have had both the Sovtek and the JJ in the amp. The last tube was a JJ.

I am thinking about putting diodes before the 5AR4 in hopes of making it last longer.

I'm up for about anything:cannotbe:
 
#12
Hi burnedfingers,
I have found adding a single UF4007 or equivalent in series with each plate lead does wonders for tube life with current 5AR4 since doing this I have not lost a single tube. You still get tube like forward conduction behavior, delayed warm up and a lot better behavior in terms of piv rating and cold start arcing.
 
#13
audiowize said:
There are little timer kits floating around that might help. They can switch the CT on the power transformer and give you 15 seconds or so of warm up time before closing the CT to ground.
-Paul
Click to expand...

Bias tap is on this winding so no stable bias while contacts are open, plus the whole winding is now floating on the bias rectifier and input filter, some odd things may happen under some conditions. (I think there could be a potential for exploded bias components depending on other current paths during warm up. Spelling it out, but I could be wrong as this isn't something I have tried or would try with a bias tap in use on the same winding.)
 
#14
kevinkr said:


Bias tap is on this winding so no stable bias while contacts are open, plus the whole winding is now floating on the bias rectifier and input filter, some odd things may happen under some conditions. (I think there could be a potential for exploded bias components depending on other current paths during warm up. Spelling it out, but I could be wrong as this isn't something I have tried or would try with a bias tap in use on the same winding.)
Click to expand...


This is a good point, you might then consider using two units, with one switching on the center tap at 20 seconds, and the other switching on the bias supply at 22 seconds....

Just a thought,
-Paul
 
#15
audiowize said:



This is a good point, you might then consider using two units, with one switching on the center tap at 20 seconds, and the other switching on the bias supply at 22 seconds....

Just a thought,
-Paul
Click to expand...

Good idea except that the bias must always be on first, and there is no need to delay it, particularly important because if the bias relay fails there will be B+ with no bias, and unlimited plate current will flow at relay closure.
 
#16
a few years ago I ran a '70 with the SDS board and it worked just fine with a Sovtek rectifier - but at the time I was running a 110V/10A AC regulator on all my equipment. You may want to remove the 40uF cap (well the SDS has two 80uf in series if I remember correctly) and try something lower like 20uF. Squeeze in a 630V Solen or something like that. It should help.

You can also try another brand of 5AR4 - I've found the 70s Matsushita and Hitachi ones to be very good and still sanely priced (for now).

With all these new power tube 're-issues', you would think someone could make a good / long lasting 5AR4. Its 'just a diode' for goodness sake.
 
#17
kstagger said:
a few years ago I ran a '70 with the SDS board and it worked just fine with a Sovtek rectifier - but at the time I was running a 110V/10A AC regulator on all my equipment. You may want to remove the 40uF cap (well the SDS has two 80uf in series if I remember correctly) and try something lower like 20uF. Squeeze in a 630V Solen or something like that. It should help.

You can also try another brand of 5AR4 - I've found the 70s Matsushita and Hitachi ones to be very good and still sanely priced (for now).

With all these new power tube 're-issues', you would think someone could make a good / long lasting 5AR4. Its 'just a diode' for goodness sake.
Click to expand...


Yeah, it seems like the current production from several vendors is quite suspect. I'm still using mullards or chinese made types and have not had any recent problems.
 
#18
I could see the first cap as the problem if the 70 were set up like the 8417 mono block Quicksilver amp. The Quicksilver used a 330 mfd cap as the first cap. The 70 with the SDS board uses two 80's in series and I would certainly think the 40 mfd cap is well within the capability of the 5AR4's rating. I believe the 5AR4 is spec'd for a maximum of 100 mfd as a first cap. I can't see the need for a high dollar Mullard GZ34 tube to replace the Sovtek, JJ, or run of the mill average low dollar tube. I will solder in a couple of 1N4007 diodes per side and let them take the start up surge and see how long the rectifier tube survives.
 
#20
I really don't want to cobble things up with another board. I put a couple of diodes in the circuit before the 5AR4 and everything is fine now. The diodes do the rectifying and the tube smooths the ripple and nasties caused by the diodes. Gives me the best of both worlds with the diodes taking the brunt of the load and the tubes slow warm up passing the DC slowly to the tubes. Best of all its .03 per diode as opposed to a higher price for a board. I really do not believe in cathode stripping as long as signal is NOT being fed to the tubes before full warm up. Have played with diode rectification in other amplifiers for thousands of hours and NEVER experienced a case of cathode stripping.
 
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