Comments for page: A New Look

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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:

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

Posted by Steve L. January 06, 2016 - 05:22 pm
Dak, I can't find it either. The Distributors link on the IR page doesn't work and I see that they say "Call factory." All that suggests that it might not be a viable part. You know, I didn't mean to butt-in here; I just wanted to correct that error. Sorry.

Posted by dak January 06, 2016 - 04:49 pm
Thank you for the correction. I checked, digikey and mouser for the OM7612ST, but that part is NLA. I also checked google but I did not find anything reasonable. Any suggestions? regards,

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