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Old 17th April 2003, 12:26 AM   #31
PassFan is offline PassFan  United States
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I am glad you like the transformers from handwound. I have a set that I'm going to use on a 6L6 pp project. They lost my order and it took them 6 months to get them to me. It looks like they might be worth it. I told you I was patient.
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Old 17th April 2003, 12:47 AM   #32
mig-ru is offline mig-ru  Russian Federation
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hehe, yeah i had to wait two months for mine, versus the advertised four weeks; it was worth the wait though, very high quality stuff. then one of the companies i ordered the other parts from had four pin sockets on back order so that added another week, so i guess im somewhat patient. when i work i try and pust my best effort into it right away, and i get really stressed out if it doesn't pay off that's why i built it so quick, and as you can see it works!
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Old 17th April 2003, 03:24 AM   #33
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Patience??? That's not what I remember on ICQ!

Tim
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Old 17th April 2003, 02:46 PM   #34
Gunders is offline Gunders  Norway
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Default I'm a bit confused

Well, I must say that the result looks very good, but I just have a question about your design.

It's the biasing for the 2A3, I see in the schematic that you have used an unbypassed cathode resistor.
That means that you have a series current feedback in your output stage. Maybe you hoped to improve your design a little bit by using feedback.

The problem is that series current feedback is not desirable in choke or transformer loaded amplifiers, it will actually worsen the the design.

The feedback will decrease the damping factor of your output stage.

I can also refer to page 329 in the Radiotron Designers Handbook which says:

"Current feedback is undesirable in transformer loaded amplifiers for it tend to stabilize the output transformer's magnetizing current(i.e. make it sinusoidal) and thus produce a distorted output voltage"

So maybe you should consider to bypass the cathode resistor?
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Old 17th April 2003, 03:22 PM   #35
Gunders is offline Gunders  Norway
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Eeehh.. I meant Radio Designer's Handbook
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Old 17th April 2003, 04:20 PM   #36
Morse is offline Morse  United States
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Hi Dmitry;

>>>...added the 100 ohm resistors to the 6SL7 filament connections, barely any hum now!!! now i can enjoy it 100%....<<<

Great! Glad to see it's working out for you! The 'hum monster' can be a tough one to kill....

All the best,
Morse
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Old 17th April 2003, 05:46 PM   #37
mig-ru is offline mig-ru  Russian Federation
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Gunderz, wouldn't adding a capacitor also add distortion, or is that only for a resistive load? It sounds quite well, so I don't see a need for a capacitor. One of the main reasons for leaving it unbypassed (if there is such a word) is that I didn't have the proper voltage rated capacitors on hand. In the future I plan to add a bigger choke to the PS stage and DC the filaments, might just also bypass the cathode resistors. Thanks for the input.
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Old 17th April 2003, 07:41 PM   #38
Gunders is offline Gunders  Norway
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Well.. normally cathodebiased output stages does have higher distortion compared to similar output stages with fixed bias.
In your case you are only using a resistor for biasing, so I don't know if the distortion will be greater or less than "normal" cathode biasing.
But since you have feedback it will probably give you less distortion.

One thing that is obvious is that the capacitor will cause more stress to the tube during start up. Just after start-up the cap will work as a short, but it will soon begin to charge up to the proper voltage and biasing.

If you would consider to bypass the resistor I will recommend you to use a polypropylene capacitor since they tends to have a better high frequency response.

Maybe you also could change the caps in your PS to some polypropylene types, giving the amp more transparency.
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Old 19th September 2011, 11:16 AM   #39
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It has been a long time since this thread was done. I am now getting ready to build an amp that will use 6A3s. I am wondering if there is any difference sonically in using a 2A3 vs a 6A3. 6A3s and the octal version 6B4G can still be bought for reasonable prices for NOS US made triodes and there are a lot of the Svetlana early 70s version selling in pairs, quads or any number you want matched up for around $20 to $25 each.
Getting new in the box US 2A3s can run the price of the amp up quickly and tube rolling is a thing needed to tune the system to the room. I can buy 6A3s but not old 2A3s. Is there a difference in sound beyond the brand? More hum? Any negative of the 6.3V vs the 2.5V ?
I bought a pair of the James J123HS so that I have multiple primaries and can use anything that needs 2.5K, 3.5K or 5K and still have 4,8 and 16 ohm secondaries. Seem like the SE DIY OPTs from Heaven ready to give me some musical Nirvana.
One thing I read early on was about NFB and choke loaded PSs not being a good thing. Tube amps are very close to being oscillators and I was thinking of using a 10K stepped attenuator and running the NFB like it is done in PP amps which I think is all the way back to the grid of the amplifier triode and not to the power tube. I would like some feedback on that, pos or neg.

Thatch
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Old 19th September 2011, 08:21 PM   #40
mogliaa is offline mogliaa  United Kingdom
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Hi there,
Great work, well done!
What is the operating point of the 6LSL7 driver? Not sure that with 300V of HT supply you can achieve the output swing required by the output valves without lowering the bias point, increasing distortion and potentially running positive grid current... See my posts in 'back at the 45 SET' as I built a similar SET with 45s BT And got an optimal point for the 6SL7, but then changed it to 6J5 to lower the gain of the amp.....
Cheers,
Ale
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