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Old 2nd June 2010, 06:54 PM   #11
Stixx is offline Stixx  Germany
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You may use tubes you friend gave you then go forward and do something different to compare!
And this goes on and on and never stops... which is what this hobby is about!
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Old 2nd June 2010, 10:15 PM   #12
Bigun is offline Bigun  Canada
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Thanks guys, I will take your advice. Perhaps I should incorporate some additional sockets to allow some 'tube rolling'. I've just come into two 6S19's for example.

I've come to the realization that audio amplifiers is a hobby that doesn't let you go....

Dave - that's a very kind offer, I will certainly plan to include the option in the design.
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Last edited by Bigun; 2nd June 2010 at 10:35 PM.
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Old 2nd June 2010, 10:56 PM   #13
Jeb-D. is offline Jeb-D.  United States
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Sorry to disagree with you guys, but 6as7g can make for a good push pull amp. The one I built measures less than 1% THD at full power using zero negative feedback.

I wouldn't recommend them for single ended use though. They do put out gobs of 2nd harmonic. But that cancels out in push pull.

Last edited by Jeb-D.; 2nd June 2010 at 10:59 PM.
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Old 3rd June 2010, 01:48 AM   #14
Bigun is offline Bigun  Canada
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Default Well maybe you can help me start off in the power supply ?

1) Rectifier:

I'm supposed to be getting a Hammond 270DX which gives me 550Vac C.T. at 104mA. So accounting for the C.T. my 5AR4 rectifier will see 225Vac per plate.

I allow for 60mA per each channel, or 120mA total.

I go and look up the datasheet (see attached) for a full bridge choke loaded supply and it tells me that I'll get maybe 200V for my B+.

Have I got this right ? (it's lower than the 250V I had marked on my schematic)

2) Power rail chokes

Now, it turns out I can't get the 158M supply rail choke but I do have a pair of 1157J chokes. These are limited to 65mA per channel so not a lot of room there. What does this current rating mean ? Is it a thermal limit or a core saturation limit ?

I assume that if it's a thermal limit I'm probably OK because in Class A the load on the power supply is reasonably stable ???

I assume that if it's a saturation limit I need to go get beefier chokes.
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Last edited by Bigun; 3rd June 2010 at 02:08 AM.
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Old 3rd June 2010, 07:01 AM   #15
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Hi Bigun,

you might have a look at "PSU designer II" which is a free download from duncanamps.com, it allows you to simulate all the typical PSU topologies.

Regarding the choke both limits are of course there but while the thermal limit sets in very gradually, saturation occurs much more suddenly. The exact DC current at which a particular choke will saturate is also subject to tolerances so it is hard to tell upfront if it will work in your case. A choke input filter, by the way, is much harder on the choke than a capacitor input filter is. (Greater AC swings)

It would be interesting to simulate the supply in PSU designer and look at the choke current waveform.

Kenneth
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Old 3rd June 2010, 04:42 PM   #16
Bigun is offline Bigun  Canada
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First off, I failed my basic math. I took 550V and divided in 2 to get 225V - doh! I actually have 275Vac per plate at the rectifier. From the published curves, at around 150mA total I'd see 225V d.c. per rail.

I don't fancy using a simulator yet. My SS designs were all heavily simulated in Spice and it's rather a nice opportunity with this simple tube amp to eshew that approach for some good old honest data sheet curves and pen & paper calculations.

So perhaps the psu is settled at this point ?

Well, I have also been given a 5Y4G rectifier tube as an alternative to try. It looks really nice with it's bottle shape.
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Old 3rd June 2010, 05:00 PM   #17
Bigun is offline Bigun  Canada
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Default What about the Input Stage ?

There were some concerns about the gain of the 6SN7 and now with B+ closer to 225V (possibly lower if I use the 5Y4G) we don't want to drop more than 50V from this in setting the plate voltage. With a plate current of at least 6mA this would imply a plate load resistor of 5.6k in series with the 155C (2.75k). This seems awfully low to achieve decent gain and linearity. Something needs to change.

If I remove the plate load resistor (and associated capacitors) and rely on the 155C as a choke load I get better gain, linearity and plenty of plate voltage.

Isn't that looking better ?
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Old 3rd June 2010, 06:09 PM   #18
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5k6 is way too low for the 6sn7. Since you're a little short on voltage, it's definitely worth a shot to go with the choke load. Not a bad idea at all. 60H and 2k7 DCR will get you about 10k at 20Hz which is reasonable.

Don't forget to adjust the cathode resistor too, I think 1k or 1k2 would be about right.

Kenneth
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Old 3rd June 2010, 06:20 PM   #19
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I actually have 275Vac per plate at the rectifier.
275 x 1.4 = 385 V RAW DC less voltage drop across the rectifier, and the Rs & Ls in the series legs of the PSU filter.

dave
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Old 3rd June 2010, 06:20 PM   #20
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Hi Bigun;
since you are fluent with SS, you may go directly to Gyrator load jumping over chokes.
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