Can cap question; What can it take with it when it goes?

knowledge base for the classic Dynaco ST70

Postby xlr8 » Wed Dec 30, 2009 4:46 pm

Jon,

BTW - sorry to get off topic. Ham radio is fun though.


Not a problem. You tube/radio guys are all the same!!LOL!! Fortunately for me, my Brother-in-law (kj4ama) is the same way. He's actually the real ham guy in the family. I just listen to you guys on an old DX-160 with a half wave, horizontal dipole connected to it. Haven't taken the time to get the license yet. I do play around with an 11 meter set he gave me, from time to time. But....He's the one who loaned me the tube tester.

As an alternative to the can cap, the various cap boards are excellent, these mount under the OPT, and use the same mounting screw hole pattern. I've seen these from Triode (SDS Cap Boards) and Cyrus Chong (cchong on e-bay),


Are you saying the various cap boards that are available are interchangeble with various input boards? This is something I had wondered about, but was almost embarrassed to ask.
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Postby EWBrown » Wed Dec 30, 2009 10:24 pm

If you are referring to "input boards" as being the driver boards, these are separate from the cap boards, and totally different "creatures"...

/ed B

(time for me to go beddy bye, it's sneaking uop on midnight and I'm getting kinda "drifty"...

/ed B
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Postby xlr8 » Thu Dec 31, 2009 5:45 am

If you are referring to "input boards" as being the driver boards, these are separate from the cap boards, and totally different "creatures"...


Actually, I was wondering if it were possible to marry an SDS cap board with another brand of driver board. I mean, I'm sure it's possible, but I'd like to keep things as simple as I can. I spoke to Roy Mottram from tubes for hi fi and he mentioned using a new can cap and adding capacitors under the chassis. This was his reply:

"the only sensible place would be where the two output transformers meet for B+ voltage, you could add 100-200 uf of 450v or 500v there on a terminal strip"

Thoughts?
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Postby mesherm » Thu Dec 31, 2009 6:19 am

If you plan to use Roy's input board then what he suggests is probably the simplest. His board takes care of the last two power supply stages plus the bias circuitry so all you need is the cap before and after the choke.
If you plan to keep the 5AR4 rectifier then the cap value directly after it shouldn't be more than 30 mf. What a lot of tube heads do is to get an 80/40/30/20 multi-section cap that fits in the place for the original.
Wire the 30 mf section directly after the 5AR4 but before the choke. Then on the other side of the choke connect the remaining 80/40/20 sections together giving you 140mf total. This should be plenty and is way way more than it had stock. Roy's board B+ and the OTs would also connect at this point. His VTA board has all the filter caps it needs already onboard.
You can then remove all the bias stuff as the VTA has all that onboard also. The bias supply wire from the PT will be re-routed to the VTA.

You can get one of the multi cap cans at Triode Electronics.
http://triodeelectronics.com/ce80-40-30-20.html
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Postby xlr8 » Thu Dec 31, 2009 7:49 am

If you plan to use Roy's input board then what he suggests is probably the simplest. His board takes care of the last two power supply stages plus the bias circuitry so all you need is the cap before and after the choke.
If you plan to keep the 5AR4 rectifier then the cap value directly after it shouldn't be more than 30 mf. What a lot of tube heads do is to get an 80/40/30/20 multi-section cap that fits in the place for the original.
Wire the 30 mf section directly after the 5AR4 but before the choke. Then on the other side of the choke connect the remaining 80/40/20 sections together giving you 140mf total. This should be plenty and is way way more than it had stock. Roy's board B+ and the OTs would also connect at this point. His VTA board has all the filter caps it needs already onboard.
You can then remove all the bias stuff as the VTA has all that onboard also. The bias supply wire from the PT will be re-routed to the VTA.


OK, this is something else I wondered about. I have the amp in my lap right now, and looking at how the can is wired, It almost seems like I could disconnect the can altogether and wire in a 30uf cap between the rectifier and choke, and then wire in 1 large cap that would cover all three of the other sections. Am I missing something here? I may be totally off base, but it seems that using a 4 section cap was pointless in the first place. One thing that sticks out is the fact that wire # 20 coming off of the stock board is wired to the middle lug of the 20/20/20 section of the can. These same 3 sections apear to be paralleled together via resistors. What am I missing?
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Postby mesherm » Thu Dec 31, 2009 11:01 am

It was actually cheaper and more space efficient at the time for Dynaco to use the 4 section cap can.
The way the stock power supply is wired is rectifier tube/30 mf/choke/20mf/6.8k resistor/20mf/10K resistor/20mf.
Thats a pretty standard way to supply different voltages to different sections of the amp. The 6.8k resistor drops the voltage down to feed the phase inverter (triode section of 7199) and then the 10K drops the voltage even more for the voltage amp section (pentode section of 7199).
The reason there are filter caps on each stage is to de-couple those sections from the previous sections. Without them the supply voltage would fluctuate with the signal causing feedback.
The values of resistors are chosen based on the current draw of each section of the 7199.
The power to drive the EL34s comes from the 20mf section immediatly after the choke.
You can do all this without a can cap but don't tie anything to a bad one. I was assuming you were talking about the unit with the burned out cap.
If you are installing Roy's VTA board in the good one then you won't need the last two 20mf sections and you can probably replace the resistors with solid wire (or just jumper them) to parallel the last two 20mf sections with the 20mf section after the choke. That would give you 60mf of filtering to the power tubes which is 3 times the stock value. Run the amp and add more caps only if you think it needs it.
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Postby xlr8 » Thu Dec 31, 2009 11:47 am

You can do all this without a can cap but don't tie anything to a bad one. I was assuming you were talking about the unit with the burned out cap.
If you are installing Roy's VTA board in the good one then you won't need the last two 20mf sections and you can probably replace the resistors with solid wire (or just jumper them) to parallel the last two 20mf sections with the 20mf section after the choke. That would give you 60mf of filtering to the power tubes which is 3 times the stock value. Run the amp and add more caps only if you think it needs it.


I certainly did not mean to cause any confusion. The amp with the bad cap is the one I am looking at. I'm just trying to be sure of what I need before I go out and buy parts. I'd like to do it all as a package deal (keeping it simple) and as of right now, the only can caps Roy has are stock values. If I'm understanding you correctly, even if I were to buy a stock value can, I'll still end up with a few more mf's using Roy's board? That should be fine.

Let me ask this question: How much of a sonic difference does all of the extra PS capacitance afford in reality? I mean, at what point do the laws of diminishing returns kick in? I do not subscribe to the theory that if a little makes a big difference, then a lot should be sonic nirvana. I don't want to spend money I'll never hear.
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Postby mesherm » Thu Dec 31, 2009 12:23 pm

Well all of Dynaco's published performance specs were with the stock 4 section capacitor that had only 20mf in the power tube supply.
Push pull amps are inherently immune to B+ ripple since it is cancelled out in the output trannies. The low frequencies are helped out the most by the extra capacitance. IMHO something like 80 to 100mf in the second section would be more than sufficent. Some people will also parallel that B+ cap with a 2 to 5mf foil cap on the theory that it will discharge quicker than the electrolytics and help the high frequencies. I've never seen any tests that confirm this but I do it sometimes just in case...LOL
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Postby xlr8 » Thu Dec 31, 2009 1:44 pm

The low frequencies are helped out the most by the extra capacitance. IMHO something like 80 to 100mf in the second section would be more than sufficent.


Perfect. These amps will ultimately be used to vertically bi-amp the mid and tweeter sections of a set of speakers we built last year. I'm currently running the midrange with the working amp. I can't wait to see what the tweeters sound like when I'm done with this.

Thank you guys so much for helping me make my decisions on what to buy. I'll be back when I get all of the parts together. My business has been very sporadic lateley, so I don't know when I'll actually be diving into these amps, but at least I think I have the parts list figured out!!
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Postby EWBrown » Thu Dec 31, 2009 2:25 pm

If you're not using these to drive the woofers, then the extra PSU capacitance won't buy you anything, the can cap will be more than sufficient.

My own "big" setup, I'd probably use an ST35 for the mids/tweets and MK3s or Ikes for the woofs. But then I've got some seriously efficient speakers, so even 10WPC generates "threashol of pain" SPLs :-) And the darn things can handle 400Watts if necessary. (Home made Karlsons with very good 12 inch "pro" drivers, and Altec 800 Hz horns and drivers, weith an SPL of at least 98dB w /m.

My usual "daily driver"is 2A3 oe 300B monoblocks and BK-16 FR speakers and they ar eplenty loud enough, though a tad bass-shy, but I can "cheat" and use two powered subs if I really want to "feel" the lower octave of the bass.

/ed B
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Postby xlr8 » Sat Jan 02, 2010 3:32 pm

Well, we replaced an old set of Maggie MG3A's with a set of home made line arrays. It started as an experiment. The first go-round was 12 vifa logic 5.5" drivers and one of the maggie tweeters hung on a piece of cardboard. I got all those drivers for a song and had to do something with them. This showed enough potential early on for me to try with real wood and a crossover and the results of that have spurred me on further. The array and tweeter panels are now made of 2 pieces of 3/4" MDF glued together with plenty of liquid nails to form a 1-1/2" thick baffle. The baffles (open/dipole) sit atop 2 cubes which house 4 Vifa Logic 7" drivers in a compound/push pull configuration. Still a work in progress. [/img]
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