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Step 24Update, V0.3
Sept, 2008
I continue to refine the amp. It's been several months since the initial build, and it's still going strong. Any concerns I had-- for instance the power tube cathode bias being too "hot"-- are gone... Here are the next round of changes (see the new schematic):
(***there's another change I won't be adding to the schematic yet...we'll see if I like it first..***)
1) Added a screen-grid stopper resistor to the ultra-linear tap of the output transformer. It's there to prevent too much current from frying the screen.
It's added only in series with the tap, not with the main screen supply. The main supply already has the current dropping resistors in the power supply.
If anything, it's addition has brightened and added gain to that option of the screen supply (there are two options available.)
2) Removed the extra cathode bypass cap on the second power tube. It just didn't make a large difference.
It's still not a bad idea--only have a double pole switch add / remove capacitance from both 6DG6GT's at the same time. That would make a noticeable difference. Maybe start with two 10uF, and switch in another pair of 30uF caps....
3) Added an external speaker jack. I've been experimenting with two cabs.
-- a 2X12 cab with two old CTS organ speakers (with the "whizzer cones.") It's an old bass cab.
-- a 4x12 cab with two Eminence speakers (and the other two holes empty.)
It's pretty freaking amazing how loud this amp is with a 2x12 cab. I literally can't hear myself shouting...
Of the two cabs, the Eminence speakers have a harder, more aggressive sound. Which you would expect. What I didn't expect is how much I like the old CTS speakers--especially when the amp isn't cranked. It's hard to describe--but a very full, vintage sound that loves single-coil pickups....
I continue to refine the amp. It's been several months since the initial build, and it's still going strong. Any concerns I had-- for instance the power tube cathode bias being too "hot"-- are gone... Here are the next round of changes (see the new schematic):
(***there's another change I won't be adding to the schematic yet...we'll see if I like it first..***)
1) Added a screen-grid stopper resistor to the ultra-linear tap of the output transformer. It's there to prevent too much current from frying the screen.
It's added only in series with the tap, not with the main screen supply. The main supply already has the current dropping resistors in the power supply.
If anything, it's addition has brightened and added gain to that option of the screen supply (there are two options available.)
2) Removed the extra cathode bypass cap on the second power tube. It just didn't make a large difference.
It's still not a bad idea--only have a double pole switch add / remove capacitance from both 6DG6GT's at the same time. That would make a noticeable difference. Maybe start with two 10uF, and switch in another pair of 30uF caps....
3) Added an external speaker jack. I've been experimenting with two cabs.
-- a 2X12 cab with two old CTS organ speakers (with the "whizzer cones.") It's an old bass cab.
-- a 4x12 cab with two Eminence speakers (and the other two holes empty.)
It's pretty freaking amazing how loud this amp is with a 2x12 cab. I literally can't hear myself shouting...
Of the two cabs, the Eminence speakers have a harder, more aggressive sound. Which you would expect. What I didn't expect is how much I like the old CTS speakers--especially when the amp isn't cranked. It's hard to describe--but a very full, vintage sound that loves single-coil pickups....
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Congratulation for your proyect!!, i like me a lot. Before start I need you that I don´t speak well English I speak Spanish, so, perhaps you find a lot of mistakes in the comment :). I write for two comments, the first, I want share a program with you, its name is ‘TDSL Personal Edition 1.0.1 Duncan Amplification’, it´s a database of vacumm valves. look at!
The second comment, about technical questions , I´m beginner in electronic topics. So I don´t understand which is the capacitance unit (µF,mF,nF,pF,etc.) of red circles, neither resistance unit (kΩ,MΩ,etc) in blue line and finality about audio transformer, I sought its datasheet (http://www.edcorusa.com/products/522-gxse15-4-1_7k.aspx) and I don´t understand which is the correct wiring. So please help me with my questions. Thanks for your time (I image the time that you spent to understand me):)
My bad...the coupling caps are within the typical ranges for tube amps; I just assumed most people would savvy. That's sloppy on my part. So the capacitors circled in red are "uF" (microFarads).
The resistors in the blue boxes have no suffix--that's the actual value. So 180 = 180 ohms, and 150 = 150 ohms. The wattage is noted.
Looking at the Edcor site, I see the link I provided no longer works. Also, the transformers themselves have changed--no solder tabs, and now have end-bells. They are also a little more expensive. Here's a current link for the equivalent transformer.
http://edcorusa.com/products/522-gxse15-4-1_7k.aspx
The 10 watt version should also work fine (and is cheaper).
http://edcorusa.com/products/500-gxse10-4-1_7k.aspx
The hookups should be fairly obvious from the diagrams on each page... If not, let me know, and I'll see if I can help.
Note: I didn't include exact links before, simply because not everyone wants a 4 ohm output impedance for their amplifier (like if you have an 8 ohm cabinet)...
Interesting idea, but I believe the suppressor grid is internal connected to the cathode in the 6DG6GT (like a 6L6, etc.)
Incidentally, I tried playing around with a "triode mode" for this tube, but couldn't get it to work--when you lift the plate voltage above 110V, the screen voltage (g2) needs to be kept well below plate voltage, which is unusual for pentodes... The datasheets show this--and sure enough, I needed to adjust the screen supply to something like 120V max.
But the ultra-linear tap on the transformer works nice as an alternative...