08-23-2009, 12:08 PM
I've done a few of these myself, and when I see some ringing on the scope, (don't trust my ears any more.) it can come from a number of sources.
1. Did you use grid stopper resistors in the output section - Say, 1K 1 watt soldered really close to the first grid of the 6L6 ahead of the coupling capacitors. And don't make the coupling capacitors too large, .047 ususally is enough, you're asking for trouble if you use above .1.
2. Lead dress is always an issue. Nobody likes to cut the leads short on an output transformer, I know that, but ... Try a .002 cap (1KVor better!) plate to plate on the output tubes.
3. Play around with the negative feedback resistor value. Too much feedback is almost as bad as hooking the PP primary backwards, that 50-50 chance we all know about the first time you fire the thing up. Seems like I have a coin with two heads, (or two tails, don't know which,) but it never fails...
4. If you are using an ultralinear output transformer and you get the screen windings switched, you either get fireworks in the output tubes, or miserable sound. A lot of people also use stopper resistors on the screen grids as well.
5. Star grounding really rocks. EVERY GROUND TIE should return to that point where the the first electrolytic is soldered to the chassis, and the center tap of the HV winding is. At first this sounds silly, but prove it to yourself.
6. I really love the sound of 6SJ7 wired as a triode. Learned that trick from the Shulmerich Carrilon (Church Bells) amps I acquired. Now that's some really Old School Brick S**t house design. These things sat in dank basements or in belfreys and played on for decades after decades. There is a "Red tube" equivalent that is available from time to time on Ebay at reasonable cost, (5693 if memory serves.) They are a treat, if they are not microphonic, and most of 'em are not.)
7. You won't be able to tell what stage the distortion is coming from by ear. It can be anywhere from the input jack to the speaker terminal. What I like to do is hook up some non inductive resistors to the output transformers, put in a sine signal, get it to clip, and then look at each stage to find where it went south. It is not always in the final stage. Hint: The phase inverter is especially suspect.
8. If you're wondering, I have a little time to participate on the forum, but not enough time lately to get down to my own workbench where there is no clock, boss, GF, or family. Either way, please let me know if this helps at all.
1. Did you use grid stopper resistors in the output section - Say, 1K 1 watt soldered really close to the first grid of the 6L6 ahead of the coupling capacitors. And don't make the coupling capacitors too large, .047 ususally is enough, you're asking for trouble if you use above .1.
2. Lead dress is always an issue. Nobody likes to cut the leads short on an output transformer, I know that, but ... Try a .002 cap (1KVor better!) plate to plate on the output tubes.
3. Play around with the negative feedback resistor value. Too much feedback is almost as bad as hooking the PP primary backwards, that 50-50 chance we all know about the first time you fire the thing up. Seems like I have a coin with two heads, (or two tails, don't know which,) but it never fails...
4. If you are using an ultralinear output transformer and you get the screen windings switched, you either get fireworks in the output tubes, or miserable sound. A lot of people also use stopper resistors on the screen grids as well.
5. Star grounding really rocks. EVERY GROUND TIE should return to that point where the the first electrolytic is soldered to the chassis, and the center tap of the HV winding is. At first this sounds silly, but prove it to yourself.
6. I really love the sound of 6SJ7 wired as a triode. Learned that trick from the Shulmerich Carrilon (Church Bells) amps I acquired. Now that's some really Old School Brick S**t house design. These things sat in dank basements or in belfreys and played on for decades after decades. There is a "Red tube" equivalent that is available from time to time on Ebay at reasonable cost, (5693 if memory serves.) They are a treat, if they are not microphonic, and most of 'em are not.)
7. You won't be able to tell what stage the distortion is coming from by ear. It can be anywhere from the input jack to the speaker terminal. What I like to do is hook up some non inductive resistors to the output transformers, put in a sine signal, get it to clip, and then look at each stage to find where it went south. It is not always in the final stage. Hint: The phase inverter is especially suspect.
8. If you're wondering, I have a little time to participate on the forum, but not enough time lately to get down to my own workbench where there is no clock, boss, GF, or family. Either way, please let me know if this helps at all.
