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| Direct coupled screen drive amp... http://dcaudiodiy.com/phpBB3/viewtopic.php?f=4&t=1825 |
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| Author: | Roscoe Primrose [ December 9th, 2020, 3:15 pm ] |
| Post subject: | Direct coupled screen drive amp... |
So, while the rest of you were arguing about guns & who has the best geriatric friendly exercise program, I built an amp. Yes, AN amp, can't find the parts to build a matching channel using the same parts... Mouser here I come   Dead quiet, and gain is pretty low, but definitely worth building a second one, will give another update when I get the other one built... Roscoe |
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| Author: | ratbagp [ December 9th, 2020, 3:32 pm ] |
| Post subject: | Re: Direct coupled screnn drive amp... |
Interesting. What made you go for the 6V6 input? ray |
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| Author: | SoundMods [ December 9th, 2020, 3:44 pm ] |
| Post subject: | Re: Direct coupled screnn drive amp... |
Roscoe Primrose wrote: So, while the rest of you were arguing about guns & who has the best geriatric friendly exercise program, I built an amp. Yes, AN amp, can't find the parts to build a matching channel using the same parts... Mouser here I come Dead quiet, and gain is pretty low, but definitely worth building a second one, will give another update when I get the other one built... Roscoe Hmmmm. Do I see some David Berning thinking in this piece? Just a note -- if you have room for 100-ufd film caps (50-wvdc would do fine) you can use them for the cathode bypasses and they will take you down to about 6-Hz. and provide speed where those giant 3,000-ufd caps will slow it down. Here is an interesting example: https://www.ebay.com/itm/CAP-FILM-100UF ... SwB6RevXiu |
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| Author: | Roscoe Primrose [ December 9th, 2020, 3:51 pm ] |
| Post subject: | Re: Direct coupled screnn drive amp... |
ratbagp wrote: Interesting. What made you go for the 6V6 input? ray The screen grid current on the 6550 is about 5mA, and varies with drive voltage. I wanted something with enough standing plate current that the variations in 6550 screen grid current wouldn't create too much of a problem. I'm actually using 6005s right now, which are industrial 6AQ5s. 6V6s will drop right in, but of course you'll need a different socket. Roscoe |
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| Author: | Roscoe Primrose [ December 9th, 2020, 3:53 pm ] |
| Post subject: | Re: Direct coupled screnn drive amp... |
SoundMods wrote: Hmmmm. Do I see some David Berning thinking in this piece? Just a note -- if you have room for 100-ufd film caps (50-wvdc would do fine) you can use them for the cathode bypasses and they will take you down to about 6-Hz. and provide speed where those giant 3,000-ufd caps will slow it down. Here is an interesting example: https://www.ebay.com/itm/CAP-FILM-100UF ... SwB6RevXiu I don't generally worry about parts quality until I've gone through the parts value shakeout... Roscoe |
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| Author: | SoundMods [ December 9th, 2020, 3:58 pm ] |
| Post subject: | Re: Direct coupled screnn drive amp... |
Roscoe Primrose wrote: SoundMods wrote: Hmmmm. Do I see some David Berning thinking in this piece? Just a note -- if you have room for 100-ufd film caps (50-wvdc would do fine) you can use them for the cathode bypasses and they will take you down to about 6-Hz. and provide speed where those giant 3,000-ufd caps will slow it down. Here is an interesting example: https://www.ebay.com/itm/CAP-FILM-100UF ... SwB6RevXiu I don't generally worry about parts quality until I've gone through the parts value shakeout... Roscoe Got it. I have done the same thing -- first make it work -- then make sound good.  |
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| Author: | ratbagp [ December 9th, 2020, 4:41 pm ] |
| Post subject: | Re: Direct coupled screnn drive amp... |
Is there any sonic or power advantage to screen drive or is it just a case of doing something different for the fun of it? ray |
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| Author: | Pelliott321 [ December 9th, 2020, 5:00 pm ] |
| Post subject: | Re: Direct coupled screnn drive amp... |
why can you get away with no coupling cap |
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| Author: | Roscoe Primrose [ December 9th, 2020, 6:35 pm ] |
| Post subject: | Re: Direct coupled screnn drive amp... |
ratbagp wrote: Is there any sonic or power advantage to screen drive or is it just a case of doing something different for the fun of it? ray Paul got close to it. If you direct couple to the control grid, then if you want the output tube to operate at the same conditions, you have to either: 1: use separate supplies for the driver and output tube and stack the power supplies. or 2: Raise the B+ by 250V, raise the cathode by a corresponding 250V by increasing the cathode bias resistor, which will now be dissipating 22W; Increase the value of the driver plate load and screen grid resistors to get the voltages back where they were, or figure out another way to get the driver voltages where they need to be. Roscoe |
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| Author: | Roscoe Primrose [ December 9th, 2020, 6:40 pm ] |
| Post subject: | Re: Direct coupled screnn drive amp... |
SoundMods wrote: Just a note -- if you have room for 100-ufd film caps (50-wvdc would do fine) you can use them for the cathode bypasses and they will take you down to about 6-Hz. and provide speed where those giant 3,000-ufd caps will slow it down. I went back and did the math. To get a 5Hz F3 for the driver, the cathode bypass would need to be 220uf, for the 6550, it'd need to be 360uf... I used the first thing I found that I was sure was big enough Roscoe |
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