Skunkie and Forum,
I just built the EL34/12AX7 amp and it sounds really good. Thanks for everything you do to help us with this Hobby Skunkie!!! I am interested in trying to learn more about how to tune it for my preferences. Although with all the work you did it may be the best version as it sits.
I have it set up to switch between two different Schade resistor values and I was wondering if I can disconnect the Schade feedback completely while the amp is running or if I need to power down first. In reviewing the circuit I can't think of a reason I would need to power down unless it would cause a current/voltage spike. I'd like to do this to understand better how the amp sounds under different configurations. It is hard to compare if you have to remember the sound between subsequent power cycles. Does anyone have experience with a mod like this?
I am also planning to add the ability to change out the driver tube for 6-volt tubes like the 6AQ8 and 6DJ8. I plan to add relays and have the configuration controlled by a microprocessor. It would switch the heater wiring and required loads in/out.
any input would be appreciated!
Here is a picture of the build. The hole in front will be filled with an LCD display.
well, I put a relay in and could switch between different values I started with 510K and 340K. 510k was dull with the need of more sparkle on top. 340k felt just a touch bright. So I switched it and tried 360k and 480K both of these seemed fine with only a small difference. with the current speakers and tubes, I do think I like 360k best.. However, I am putting a microcontroller in to reconfigure the preamp tube, I'm planning to leave both values in. I will allow them to be switched in a configuration mode where the tubes aren't powered along with some preamp settings like 12/6 volt heaters. I didn't try the amp without feedback but I'm planning to try it..
I also plan to drop the value of Rd when I put KT88s in, which will slightly increase the voltage. I used an Edcor power supply with a little more voltage than the Hammond so I had to drop it a little more. I found the Edcor doesn't buzz audibly if you mount them on the rubber pads they sell and pay close attention to the resistance on the secondary. I had to add a 75-ohm resistor on each output of the rectifier to ensure the load was correct for a GZ34 rectifier with a 22uF load.
I am using good tubes (I have several sets) and capacitors. I like the Clarity caps better than Mundorf or Orange Drops, but both sound good (it's all a matter of opinion). I am surprised at how good orange drops sound for a few bucks.
The amp sounds really good. Thanks for everything you have taught me Skunkie! I would have never built a tube amp if it wasn't for you and a couple of other people you referenced on your channel (BlueGlow and Cascade Tubes) and I have an Electrical engineering degree but they don't teach this in school.
When I first started playing with this on a KT88 amp, I used a center off DPDT switch and would "hot switch" two values of feedback + zero feedback to listen to the changes A/B. That amp also had a triode/UL switch. I learned I don't really care for the sound of a triode trapped pentode in a SE amp. Yeah you will hear a "pop" but didn't seem to hurt anything. For this amp, try a 5751 tube in place of the 12AX7, the gold pin JJ sounds really nice! I tried between 300K and 470K and ended up with 360K as the sweet spot for me.
Ok, I am going to offer my opinion (remembering opinions are like backsides....... everybody has one).
Typically a circuit design is a middle of the road kind of thing, there are vast quantities of tubes you can plug in, and they all sound a bit different, so you design to give an acceptable sound for the maximum amount of choices. That is where the value of 820k comes from for the Schade resistor.
That said, the consensus in these parts is that a value of 620k does outstanding things there.
I have an amp here, with a rough sounding JJ tube in it (or a pair of tubes) that 550k seems to work well, but upon advice of the guy who sells me tubes, I switched the JJ's out for the recommended tubes, leaving me on the edge of a muddy sounding mess.
So, my advice to you is, pick a value, solder it in, and be done with it.
You can tune the sound with different output tube choices. EL34, 6CA7, KT77
There are benefits to be had by changing the coupling capacitors. Don't go all crazy here, just change out that Chinese stuff for "known good" stuff. Solen, Sprague Orange Drop, and Mojotone Dijon are all affordable and good ($4-$6 each).
Your 12AX7 is a high gain tube, giving you enough gain to drive the output tubes without a separate pre-amp. My tube guy sells 42 different types. Why fix something that is simple and not broken? (I use JJ E83CC tubes)
Class A tube sound is all about speed, short simple audio paths through your amplifier, as untouched as possible.