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Old 6th June 2012, 03:39 PM   #21
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Quote:
Originally Posted by doozerdave View Post
The circuit looks okay to me, but a note about the "bootstrapping" on the cathode follower. I tried this in my amp and found it to be excessive and unnecessary. It added a fuzzy/scratchiness that I didn't care for. Obviously I'd encourage experimentation for your own tastes. For reference, my pre-amp is essentially a cross between a Bogner Shiva and a Soldano SLO. Gain ranging from blues/classic rock to thrash metal.
Could you be more specfic here? Are you talking about C9, R25, and D1 in my schematic?

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I've got "Chinese", Svetlana and E-H tubes in my high gain pre-amp and they're nice and quiet. They're cheap from thetubestore.com. I've got the E-H in the DC coupled cathode follower position and it sounds great. I tried 6 different types in that position (in all positions actually, but focused on that one) and found it to be the richest, most widely usable at different gains. FWIW, I found the Sovtek sounded like complete crap compared to Svetlana and E-H. Cold, no character, and too compressed.
Noted... I'll have to buy a few different kinds once it's up and running.
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Old 6th June 2012, 03:57 PM   #22
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Quote:
Originally Posted by Stormrider View Post
Could you be more specfic here? Are you talking about C9, R25, and D1 in my schematic?
C9 is the bootstrapping cap. I also tried the R25/D1 thing to protect the cathode follower, but ended up removing it also. It's said that it has no effect on tone but I though I could hear/feel a difference. I have my heaters elevated to 70Vdc and I'm not terribly concerned about damage at power-up since my B+ is only 280Vdc at the DCCCF. Not sure if it was already covered, but are you planning to elevate your heaters? If your transformer has a centre-tap on your heater winding then it's super simple to do.
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Old 6th June 2012, 04:49 PM   #23
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The Antek transformers have two separate 6V windings.

In my last amp I created a virtual CT with a couple 100R resistors and elevated them that way. For this amp I was thinking about going with regulated DC heaters for the preamp section on one winding, and the power tubes on the second winding running on the straight AC. This will be another thing I won't worry about until after I have it up and running. I don't want to add complications for no reason. If the amp doesn't hum with the elevated AC heaters and a good layout, I see no reason to go any further.
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Old 6th June 2012, 05:28 PM   #24
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I agree, keep it simple. My main point wasn't so much about hum, but about heater to cathode maximum voltage on the cathode follower. I'm sure you're aware that the max is +/-200Vdc+pk.

Good luck with this project. It sounds very similar to what I just built (high gain pre-amp with quad-6v6 output).
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Old 6th June 2012, 06:35 PM   #25
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Do you have a schematic for your amp? How about construction pics?
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Old 8th June 2012, 03:33 PM   #26
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Here is the power supply schematic:

Click the image to open in full size.

Pretty straight forward; the "HV" and "LV" transformers are actually going to be a single Antek AS-2T300, which should give me a B+1 of about 410V - 420V with tubes. The caps should probably be 500V, which means I'll probably use 47uF instead of the 100uF shown for each of the main filter caps. Between the choke and the cap multiplier I think it will be plenty stiff/quiet enough.
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Old 9th June 2012, 01:46 AM   #27
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Again, that depends on the sound you want. Many tube guitar amps purposely allow the power supply to sag, which adds another source of compression with another more controllable time before distortion sets in and time before the gain recovers and a different kind of "old-amp" distortion as the bias dips and the tubes get outside their linear range. It's not my "cup of tea" but you might consider a really hefty switch to throw another bank of caps in parallel, so you can have it both ways. You can put a large-value resistor across the switch which won't affect the power supply sag when you want a smaller capacitance, but the 'extra' caps will charge slowly (like a few seconds) so the switch contacts don't weld together when you decide you want the extra dynamics instead of compression.
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Old 9th June 2012, 03:00 AM   #28
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Do you have a schematic for your amp? How about construction pics?
Sure I do. Here is a google docs link, hopefully it works.
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Old 9th June 2012, 03:09 AM   #29
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Here's the power amp/power supply too.
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Old 12th June 2012, 08:51 PM   #30
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Quote:
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Again, that depends on the sound you want. Many tube guitar amps purposely allow the power supply to sag, which adds another source of compression with another more controllable time before distortion sets in and time before the gain recovers and a different kind of "old-amp" distortion as the bias dips and the tubes get outside their linear range. It's not my "cup of tea" but you might consider a really hefty switch to throw another bank of caps in parallel, so you can have it both ways. You can put a large-value resistor across the switch which won't affect the power supply sag when you want a smaller capacitance, but the 'extra' caps will charge slowly (like a few seconds) so the switch contacts don't weld together when you decide you want the extra dynamics instead of compression.
The sort of sound I'm going requires very little power supply "sag", and a very clean B+. Hence the over-sized toroidal transformer, large caps, choke, and cap multiplier. When it comes to extremely high gain amps for modern metal, I think you almost need to think in terms of hifi for the power supply. If I was designing an amp for classic rock/blues I would be thinking in much different terms.
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