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Direct coupled screen drive amp... http://www.dcaudiodiy.com/phpBB3/viewtopic.php?f=4&t=1825 |
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Author: | ratbagp [ February 25th, 2021, 5:01 pm ] |
Post subject: | Re: Direct coupled screen drive amp... |
Thanks for the update. I am progressing on the build. I have more or less completed the power supply and am about to start wiring the circuit. Late February is the time of year here where the heat (90+) and humidity make conditions tough for enjoyable soldering so I can only work in the early mornings. Still, I progress slowly and once March arrives, things should be much more pleasant. ray |
Author: | Roscoe Primrose [ March 2nd, 2021, 2:41 pm ] |
Post subject: | Re: Direct coupled screen drive amp... |
Some random thoughts after 5 days with these amps.... Most of you probably want to know how they sound.... Kinda hard to describe that, but let me tell you this: I plan on spending quite a bit of time going forward optimizing this design. This design, as it sits now, is merely a proof of concept. No effort has been put into choosing the best tubes for the job (yet), so it's quite possible that there are better choices. The EL84 for a driver comes to mind, and there's a whole host of sweep tubes that might be better choices for output tubes. I haven't tried it yet, but the math suggests an EL34 will bias up correctly as an output tube if the cathode bias resistor is changed to 250ohms. Time will tell. Being direct coupled, the amp can be twitchy to get the values right since any change to the driver stage affects the output stage. Those of you who primarily build existing designs, rather than doing your own design work, would probably be better off building known working variants of this design rather than wandering off on your own. Roscoe |
Author: | SoundMods [ March 2nd, 2021, 4:22 pm ] |
Post subject: | Re: Direct coupled screen drive amp... |
Roscoe Primrose wrote: Some random thoughts after 5 days with these amps.... Most of you probably want to know how they sound.... Kinda hard to describe that, but let me tell you this: I plan on spending quite a bit of time going forward optimizing this design. This design, as it sits now, is merely a proof of concept. No effort has been put into choosing the best tubes for the job (yet), so it's quite possible that there are better choices. The EL84 for a driver comes to mind, and there's a whole host of sweep tubes that might be better choices for output tubes. I haven't tried it yet, but the math suggests an EL34 will bias up correctly as an output tube if the cathode bias resistor is changed to 250ohms. Time will tell. Being direct coupled, the amp can be twitchy to get the values right since any change to the driver stage affects the output stage. Those of you who primarily build existing designs, rather than doing your own design work, would probably be better off building known working variants of this design rather than wandering off on your own. Roscoe I found that when I was re-building a Dynaco ST-70 many moons ago and later when I had a Williamson-design stereo amp. that could take EL-34s, if I chose to do so, the EL-34s had a sweet spot at 38-ma cathode current. Any less and the amp would get kind of "dry" and edgy -- any more and the sound would get "dark" and smothered (relatively speaking - no pun intended). At 38-ma the mid-range got "creamy open" with great sound-stage presence. |
Author: | Roscoe Primrose [ April 14th, 2021, 10:24 am ] |
Post subject: | Re: Direct coupled screen drive amp... |
Made some minor updates to the amps yesterday. Not really changes looking for improvement, but changes to make them serve better as development platforms. Replaced the 220ohm cathode bias resistor for the output tube with a three resistor network consisting of three 345ohm resistors and a switch. Now, I can change the cathode bias resistor from 230ohms to 345ohms at the flick of a switch (two 345ohm resistors in series which can be optionally switched in parallel with the third 345ohm resistor). The 230ohm value is good for 6550s at near max dissipation (should also be good for KT88s, KT90s etc), the 345ohm value is good for 6550s at reduced current, or EL34s, 6l6s, 5881s etc at near max dissipation (voltage is a little higher than the 400v spec for 5881s, make your own decision...). Ray, you making any progress? Roscoe |
Author: | ratbagp [ April 14th, 2021, 4:45 pm ] |
Post subject: | Re: Direct coupled screen drive amp... |
We went away for a trip and will get back to it shortly. Thanks for the info. ray |
Author: | Roscoe Primrose [ July 31st, 2021, 1:28 pm ] |
Post subject: | Re: Direct coupled screen drive amp... |
Which tubes are you using for outputs? I've tried 6550s, two pairs of 5881s, and a pair of EL34s, the 6550s were definitely best in my system.... Roscoe |
Author: | ratbagp [ July 31st, 2021, 4:36 pm ] |
Post subject: | Re: Direct coupled screen drive amp... |
6550. I don't have 5881 or EL34 tubes. It's been an interesting process building in a different country. One of the best things is that I can still order from Mouser and Digikey. Provided the total cost of the order is above $50, shipping is free. Delivery takes about 7 days and is often faster than delivery from Sydney or Melbourne. I have no idea why. So thanks for the schematic. Now there are two of the amps on either side of the world. ray |
Author: | Roscoe Primrose [ July 31st, 2021, 5:19 pm ] |
Post subject: | Re: Direct coupled screen drive amp... |
ratbagp wrote: So thanks for the schematic. Now there are two of the amps on either side of the world. ray At least the world is balanced now Roscoe |
Author: | tomp [ July 31st, 2021, 9:16 pm ] |
Post subject: | Re: Direct coupled screen drive amp... |
Roscoe Primrose wrote: ratbagp wrote: So thanks for the schematic. Now there are two of the amps on either side of the world. ray At least the world is balanced now Roscoe Even if us folks are not. |
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