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I'm recapping an original Dyna ST70, it has never had any work done previously.
I obtained an exact replacement for the quad filter cap. It is 30/20/20/20uF @ 525VDC.
The original is wired with the 30uF section directly to the 5AR4 rectifier tube, then after the choke is 20uF, which is the point feeding the finals.
My question: would it make sense to use 20uF at the rectifier tube, then 30uF after the choke? I have seen data regarding the maximum amount of capacitance on a 5AR4, but can't put my hands on it now. It seems less capacitance on the rectifier tube might increase reliability.
Any comments? Thx, Al
Follow Ups:
There's some good advice here.
I ended up using the stock value cap. As Paul mentioned, perhaps Dynaco did recognize a need to have 30uF as the first capacitor. And, as others have suggested, that is not too capacitance much for a 5AR4.
One of my concerns was preserving the original Mullard made rectifier. This amp is pristine, all tubes are original. I added diodes in series with the plates as Chip and Eli suggested. I have read other places that this helps prevent arc-overs.
I also added an additional 47uF in parallel with the 20uF for the reservoir after the choke. This gave me 30uF for the first cap, and 67uF for the reservoir, which is close to what Michael suggested.
The next time you get a ST70 in, go "hog wild". :> D There is a good way to convert to SS rectified B+. "Ping" me, if you're interested.
Eli D.
I tried converting my st70 to SS rectifiers and didn't care for it. Really lost a lot of the tube magic. Ended up reversing it back to my good old RCA GZ34
charles
Just as important as getting the caps. right is dealing with the TOTALLY inadequate OEM C354 choke. "Shoehorn" a Hammond 159T in.
Personally, I'd use a 47 μF. 1st filter cap. and install a CL90 inrush current limiter between pin 8 of the rectifier socket and the PSU filter. I'd also install UF4007s in series with the 5AR4 plates, as has been documented repeatedly here on AA. "Fireworks" will not occur, even if a current production Sovtek 5AR4 is used.
Use a substantial amount of capacitance in the reservoir position. Hafler's reasons for the can he used were primarily monetary, not technical.
Eli D.
Just a guess here - it may be that the 30uF is used in front because it has a higher AC current capability. Using the 20uF may save the tube but cause the capacitor to die early. The tube is more easily replaced, and has a less messy failure mode!
Here's another possibility worth remembering - we may know things now that were not known 50 years ago, but it's also possible that the designers of vintage gear knew things that none of us on this forum know. If you can't see why something was done, there was still probably a reason.
...given the cap is in hand, I'd be inclined to put the 20uF at the rect and the 30 after the choke. However, the real world difference is pretty much negligible.
The max cap for a 5AR4 using cap input filter is generally speced at 60uF although there are a ton of factors that may alter this value. Also, contrary to what's stated elsewhere in this thread, the max cap is a limit not only during startup conditions but also during steady state operation unless special precautions are taken to limit peak repetitive current.
"... to limit peak repetitive current"
That's right!
And the more series resistance (DCR of the secondary winding or even added series resistors) the higher the cap value can be but if all one has is the minimum stated 'resistance per plate' then the max input cap is all you can use safely.
One way to solve the problem is to use a critical inductance choke input filter and then there is no current peaking and no minimum resistance per plate requirement.
It's all in the data sheets. One just has to know how to interpret them.
Start up surge is a different issue.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
After seeing a fireworks display on one of my amps putting a 60UF cap on the fier I now never use more than 40UF.
The cap that is a direct replacement for the Dynaco is a bit on the anemic side. I usually add more to the stages(about double) and I try to use film caps when I do.
...but I've seen 5AR4s handle 60uF w/o issue. It all "depends". One example is the Heath AA151. I've had an early Sovtek in mine for ~15yrs of occasional use and no fireworks yet.
The ST70 asks a hell of a lot out of a single 5AR4. Put in 4007s before the rectifier plates.
If that's the case then why didn't Hafler just use a SS rec?
Probably related to the selenium rectifiers of the day not being as robust as the 5AR4. Not the case anymore. 1959 was a long time ago., the 1n4007 came out in 1966.
When the ST70 was designed and introduced, 1959 or thereabouts, selenium rectifiers with the nec current and voltage ratings were large and had high voltage drop. Silicon rectifiers new and really expensive and usually required 2 in series to get the nec voltage rating. So, considering the budget nature of the amp, the 5AR4 was a logical choice even if run pretty much at absolute max rating. Too bad Hafler didn't opt for dual 5AR4s and more filtering beef like some other stereo amps of the time did. That would have been a giant killer.
So you are saying that two parallel 5AR4's would do wonders to that amp? I have pulled out the choke on many occasions and it woke the amp up much more than I thought.
Think solid state in an ST70
"Wonders" might be a little bit of hyperbole but dual rectifiers and beefed up PS would be a big improvement. Or, if you're not committed to 100% tubes, go silicon for the rectifier with due precautions for higher B+. Dave Gillespie covered the SS route in some detail @ Audio Karma a while back. Worth checking out!P.S. Don't mean to imply a stock ST70 can handle 2x5AR4...PT isn't up to addl filament load. Just that ideally, dual rectifiers would have been the way to go at the time. SS can be done with the stock PT.
Edits: 07/11/14
Hi,
Dave Gillespie is a very frequent poster on AudioKarma. Can
you please provide a link to the specific SS discussion you
mention????
Thanks and regards to all,
John
...a long thread but worth a read. Interesting conclusions about the stock circuit of the amp.
Edits: 07/12/14
Thanks, Steve. I appreciate you.
I thought the maximum input capacitance for a 5AR4 is 50uF. Now I can't seem to find that datum easily. I can tell you we rebuilt an ST-70 and used a 50 uF input cap and the amp is a daily player for our customer with no problems after several months.
The size of the capacitance in the power supply is only a start-up issue; after the first few seconds the value harldy matters to the rectifier.
"The size of the capacitance in the power supply is only a start-up issue; after the first few seconds the value harldy matters to the rectifier."That's not true.
With a capacitor input filter every half cycle one diode "turns on" for a brief time.
The peak current is high, much higher than with a choke input filter, and that peak current (that happens twice each cycle) is what can not be exceeded.
BTW, for capacitor input filters the data sheets state the minimum DC resistance per plate. If you have more than the minimum DC resistance per plate (either as drop resistors or as DCR in the HT winding of the power transformer) you can use a larger value cap and still not exceed the max peak current limit.
As an aside, you will notice (on the data sheets) that there is no minimum DCR per plate requirement when a critical inductance choke input filter is used. That's because the current flows for the entire half cycle for each diode so there is not large peak, in fact the current through the power transformer secondary and the diodes is just a little over the load current.
Start-up is a different issue.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 07/10/14 07/10/14 07/10/14
it's been while - C1 gets slammed(charged) 120 times per second (mains Freq. x2)
The size of the cap in uf determines the amount of current drawn from the secondary, required to charge C1. Which in turn, (ohms law) the primary sees that current as well.
This is why increasing the already huge C1 bank on SS amps can cause the fuse to blow.
I gave the simplified answer. As the capacitance goes up, the conduction angle goes down, and peak current goes up.
Rectifiers can take significant repetitive short-term overloads due to the electron cloud surrounding the cathode, so in most cases there should be no problem. If it does cause a rectifier to have a short life, an input choke can be inserted (low resistance, moderate inductance, enough to increase conduction angle).
Al
Putting in the exact that it originally had is insane.Those small original values are simply not enough to run that amp in this day and age when you have listening material in the 90db and 100db range...If you are wise,I would get an 80/40/30/20 and put the 40uf off the GZ34 and the 80uf on the other side of choke and the 30 and 20 for the other sections.If you bought the cap you have now,you can probably send it back and pay the difference and get the 80/40/30/20.
Honest amplification is better than excessive 2nd order distortion anytime.
Mikey,
That is exactly what I have done to an ST-70 unless the rec is SS. Then I would use about 100UF on the input and go from there, probably take out the choke and use a resistor of a proper value and power handling. I feel the choke suffocates the amp to a certain degree.
Exactly Paul..A poly there is best if you can fit it in.
Honest amplification is better than excessive 2nd order distortion anytime.
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