Comments for page: A New Look

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Posted by Bill September 30, 2016 - 03:22 pm
How far can one deviate from the 100R value for the screen stability resistors in this circuit. I have read where it "might" be better to go to a higher value, like 220R or so.

Posted by Dave July 12, 2016 - 07:54 pm
Hi Allan -- We need a lot more information to diagnose your problem, that would include:

1. Is the motorboating in both channels?

2. Is it present on all inputs?

3. Is it present at all volume control settings?

4. Do the tone controls change the nature of the motorboating?

5. Have you always used electrostatic speakers -- or did you just recently change to this type of speaker? If so, did the motorboating start when using electrostatic speakers?

6. Is your amplifier an SCA-35 or ST-35?

7. Are your electrostatic speakers (likely) internally transformer coupled with their own HV power supply?

The answers to these questions can shed significant light as to just what -- or what not -- the problem might be. However, if the output tubes are biasing correctly, this would indicate that the EFB(tm) portion of the amplifier is operating correctly. Any more detailed suggestions at this point however would represent no better than stabbing at the wind. With more information, more precise suggestions can be made.

Dave

Posted by allan rosenthal July 12, 2016 - 06:21 pm
hi- i bought your boards some time ago. the amp was performing well until recently. there is now an intermittent motorboating. i am using a matched quad of el84's and they bias correctly. my speakers are electrostatic. any ideas for fixing this?

Posted by dak January 08, 2016 - 12:03 am
Hi Steve, on my amp there are several extra 6.3v filament taps, and if I need to have a bias supply I can reverse install a filament transformer to get 120 volts out. I guess I will break down and go with fixed bias for the outputs and try the EFB for the screens..Thank you for all your efforts. take care and have a prosperous new year

Posted by Steve L. January 07, 2016 - 06:52 pm
Hi Dak, Yes, all is well here. Having too much fun for a grown-up :) Thanks for asking. I see, about the amp. By the way, I had a thought about how you could develop a negative bias supply with the existing transformer. Since it doesn't require much current, you could capacitively couple from one of the HV windings to a rectifier, as shown in the sketch here: http://www.tronola.com/misc/Negative_bias_supply_concept.GIF.

Sorry that I can't sign up to work out the details but it should be amenable to experimenting. With the desired load on -Vb, C1 can be varied to adjust the DC voltage. Regulation might be a good idea to help get rid of hum and to keep the negative supply constant. C1 should be nonpolar. Just a thought. Best wishes, Steve

Posted by dak January 07, 2016 - 04:35 pm
hi Steve, I hope all is well with you and yours in this new year. I meant to thank you for the suggestion in the first place.
About my amp, I believe the Dynaco A420 output trannys were designed for cathode bias operation. According to the specs, the primary impedance is 6600 ohms into 8 and 16 ohm secondaries. It is the main reason I wanted to keep its operating conditions close to cathode bias and hoping to improve that with EFB. take care, Dak


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