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PostPosted: 26 Oct 2010, 15:34 
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Suncalc wrote:
The PTFWF460 should work with no problems. I'll rerun the power supply output and ripple calculations using this transformer.

Can you confirm for me which Hammond choke you ordered, the 193J or the 193M?

Matt: Fear not.

Here is my order to Hammond:
2 X 1630SEA OPTs
1 X 193M 10H choke
1 X 266K12 filament transformer.

And we are on 50hz. I'll wait for your OK to order the tranni. The company that makes the tranni is only about an hours drive from me.

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PostPosted: 29 Oct 2010, 17:21 
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Matt: In the local power transformer OK. You were going to check that the 50hz operating frequency was OK with your calcs.

I would liek to order the Avat tranni ASAP.

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PostPosted: 29 Oct 2010, 17:59 
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Sorry about that. I was working on some prototyping for the filament supplies and got distracted. :blush:

Main PS: omega-RC = 34.2, <Rs>/R=0.106 We are still on the flat part of the regulatory curve so everything should be OK. Go ahead and order the PTFWF460 (460-0-460)@200mA transformer. This should work great.

I think I'm going to post two PS designs. One for 240V, 50Hz mains and one for 120V, 60Hz mains. The smoothing filters are slightly different.

Sorry again for the delay. :turtle:

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PostPosted: 29 Oct 2010, 20:32 
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Suncalc wrote:
Sorry about that. I was working on some prototyping for the filament supplies and got distracted. :blush:

Main PS: omega-RC = 34.2, <Rs>/R=0.106 We are still on the flat part of the regulatory curve so everything should be OK. Go ahead and order the PTFWF460 (460-0-460)@200mA transformer. This should work great.

I think I'm going to post two PS designs. One for 240V, 50Hz mains and one for 120V, 60Hz mains. The smoothing filters are slightly different.

Sorry again for the delay. :turtle:

All good. I'll order the tranni. The EH Gold 300Bs came today. Look good and I have the Audio Note Silver caps. Still a lot of parts to collect but we are moving forward. The base can't be designed 'till I have all the trannis to see what will fit where.

I'll stick some layouts on the forum when I get to that point.

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PostPosted: 30 Oct 2010, 23:00 
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One quick note about the power supply. Because your mains frequency (50Hz) is about 16% below what I was assuming (60Hz), the damping factors for the filters all come out lower because the primary ripple frequency is 16% lower. The net effect is that we're going to need another 10µf cap in the main power supply. I have both main power supply designs complete (240v, 50Hz mains & 120v, 60Hz mains). I'll post them tomorrow.

I am also prototyping the 300B filament supply to ensure everything works. I'll post that complete design in a few days. Following that post we should have a complete electronic parts list for the amp.

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PostPosted: 30 Oct 2010, 23:32 
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Suncalc wrote:
One quick note about the power supply. Because your mains frequency (50Hz) is about 16% below what I was assuming (60Hz), the damping factors for the filters all come out lower because the primary ripple frequency is 16% lower. The net effect is that we're going to need another 10µf cap in the main power supply. I have both main power supply designs complete (240v, 50Hz mains & 120v, 60Hz mains). I'll post them tomorrow.

I am also prototyping the 300B filament supply to ensure everything works. I'll post that complete design in a few days. Following that post we should have a complete electronic parts list for the amp.

Thanks Matt.

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PostPosted: 31 Oct 2010, 19:53 
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If you’ve been following this thread you seen some of the back and forth concerning the amp design. This is not unusual as design and development always involves trade offs between customer requirements, available components, different design solutions, etc. And, after a couple of embarrassing missteps by yours truly :blush: , we finally have a main power supply design. Actually, I have two. One is for 240v, 50Hz mains and one is for 120v, 60Hz mains. Please allow me to explain.

First, I fell into a common trap when doing the power supply design. Remember way back at the second post in this thread I listed requirements for this amp? Well, I seem to have missed a pretty important one. On that list should have been a requirement for 240v, 50Hz mains. You see Mark, for whom I am designing this amp, lives in South Australia where mains are typically 240v, 50Hz. I, on the other have, live in the Pacific Northwest region of the United States where mains power is 120v, 60Hz. Without the requirement in the list I just assumed that the mains power was just like mine. OOPS! The result of this little error was that I ended up designing the power supply three times! But that’s good for anyone who wants to try building this design because I’m going to post both final PS designs and a builder can simply pick the appropriate design.

My second point is about the order in which things get done. You see, I though I had finished my design to the point of having a set PS topology and told Mark he could go ahead and order the filter capacitors. Subsequently, I discovered the whole mains voltage and frequency issue mentioned above. Having a mains frequency that is 16.7% below the initial design point means that the ripple frequencies are 16.7% lower as well. The consequence is that the filter sections which were acceptable to meet the target ripple voltage at 60Hz, were insufficient to meet the ripple requirement at 50Hz. I had to add an R-C filter section to meet the requirement at 50Hz. And now Mark has to order an additional filter cap.

The lesson here is that all of this could have been avoided if I had thought to ask the question when setting the design requirements. Well this is just another lesson learned. I’ll know better next time (I hope).

Now on the the actual design. I have set the requirement for ripple at 0.003% or ~ -90dBv. This is a level suggested by Herbert Reich in the book “Theory and Application of Electron Tubes” for microphone circuits. Normally one might think this excessive for a simple audio amplifier however, as this is a high fidelity amplifier, I want to err on the side of caution. In addition, the step down for the 6SN7 drive will include and additional stage of filtering to ensure that the noise in the gain stage is as low as possible.

The one thing to note is that these designs assume some things about the characteristics of the power transformers themselves and the output transformers. On the design sheets you’ll see where I have estimated the Rs parameter based on some assumed primary and secondary resistances and the windings ratios. Also, in setting the requirement for 454v B+ I have assumed a voltage drop in the output transformer of 19v based on an effective resistance of 200Ω (364v+71v+19v=454v). I have asked Mark to measure the transformer DC resistances after he receives them so that I can makes sure everything is ok. If not, it will just mean that the dropping resistors in the power supplies may need to be altered some what. This is not really a big deal.

Here is the design sheet for the original 120v mains supply.
Attachment:
Final 120v PS Design.jpg

Here is the design sheet for the 240v, 50Hz mains supply.
Attachment:
Final 240v PS Design.jpg


A couple of quick points. First, these designs are based on the 5U4-GB rectifier tube. Contrary to what some people believe, the 5U4-GB and the old 5U4 are not the same tube. The 5U4-GB has much lower internal voltage drop then the old 5U4. If one were to use a 5U4 in this design, it would probably still work, but the B+ voltages would be as much as 30V lower. Second, the output voltages and ripples are not the same between the two designs and are not exactly what was specified. This is ok. The differences are minor and once built you’ll be very hard pressed to tell the difference even with two otherwise identical amps side by side.

And now for what everyone has been waiting. The final main PS schematics.
Attachment:
PS Schematics.png


Now we are almost done with the design. The only thing missing is the heater design for the two 300B filamentary cathodes. I have a preliminary design that I have posed a little earlier in this thread. However, I am prototyping it right now to make sure that the circuit responds correctly at startup. I should be ably to post the final schematics for this last bit in a few days. At that time I’ll also post a complete schematic of the design including all components.

Comments?


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PostPosted: 01 Nov 2010, 13:20 
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Suncalc wrote:
A comment about the filter caps. I understand the desire to use motor run caps for filtering. The low cost and ready availability make them seem an attractive option. However in my experience these motor caps tend to be noisy in audio circuits. This is definitely a component construction issue with the capacitor design being optimized for other parameters. I'll post my power supply design in a little while. But I'll tell you up front the capacitors called for are 40µf, 100µf, and 47µf all at 500v minimum.
Hope this helps.

I've used Ronken and GE motor runs and did not experience any noise issues. I've also seen them used in Phono preamps with no problems. Which brands did you have the noise problems with?


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PostPosted: 01 Nov 2010, 19:09 
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Quote:
Quote:
Suncalc wrote:
A comment about the filter caps. I understand the desire to use motor run caps for filtering. The low cost and ready availability make them seem an attractive option. However in my experience these motor caps tend to be noisy in audio circuits. This is definitely a component construction issue with the capacitor design being optimized for other parameters. I'll post my power supply design in a little while. But I'll tell you up front the capacitors called for are 40µf, 100µf, and 47µf all at 500v minimum.
Hope this helps.

I've used Ronken and GE motor runs and did not experience any noise issues. I've also seen them used in Phono preamps with no problems. Which brands did you have the noise problems with?

I wish I could remember which brand they were but this was maybe 25 years ago. We were retrofitting a PA system in a church and were experiencing intermittent wide band noise interspersed with what sounded like shot noise. It took a while but we finally tracked it down to the power supply and the motor run caps we were using in the supply filter (because of their low price). We replaced the caps with some made for audio amplifier power supplies and all the noise disappeared. When we contacted the manufacturer a member of their design staff they told me that this was not unexpected given how we were using the caps. In short, this is the explanation he gave (as best I can remember it).

Motor run caps are designed with only three requirements in mind. First they must impart the appropriate phase shift to auxiliary motor windings dependent on the motor speed requirements. Second they must support high current draw without damage. And third, they must be robust enough to stand large voltage and current spikes (as can occur with step torque loads on the motor) without failure. There are no formal requirements for leakage, capacitance drift, or uniformity throughout the capacitor. The net effect is that many of these caps, while meeting the requirements for their intended use, fail in regard to what we would consider audio performance. This may be due to non-uniform plate material and dielectric properties, inexpensive bonding techniques within the caps, or even non-uniformity in physical construction.

I am not saying that some motor run capacitors can't be used in an audio amp with acceptable results. All that I'm saying is that these caps are not designed for this type of use and you have no way of knowing what tradeoffs the manufacturer used to meet his cost and yield targets. I had a bad experience and after a discussion with the manufacturer decided that it is simply not worth the risk to save a few dollars on an amp. I've used JJ, F&T, Jensen, Sprague/Vishay, and IC all with excellent results at a reasonable cost. So as a matter of personal preference I simply stay away from motor capacitors in anything other then a motor application.

Just my opinion. :soapbox:

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PostPosted: 04 Nov 2010, 15:42 
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Matt. Don't worry about the extra filter cap or two, I ordered a couple extra which all arrived yesterday. The only one I don't have is the 10uf. But I'll find a work around or trouble a few tube friends for one.

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