Thank you very much!
This sounds exactly like what I was looking for, a simple parts swap that could easily be reversed if it doesn't work out as well as I'd like.
Would you be able to provide the resistor values, and their positions in the circuit based on the ID codes on the schematic I'd initially provided?
While I haven't heard the Saga+ I've read comments from owners who say the buffer mode, which includes the tube, doesn't really sound any different than the passive mode. And, if you read the description on their website, they make no claims that the tube alters the sound at all. They only say that it's handy if you need to drive long cables.
So if you're looking for something that offers "tube sound", the Saga+ doesn't seem to and $400 seems steep to me. But then I'm cheap.
If you don't care about "tube sound" and just want the convenience features I'd suggest this little non-tube Chinese preamp. I bought one a few years ago simply for the convenience features. At the time it was about $60 shipped but, of course, prices have gone up. Looks like ~$100 now.
I mainly wanted something with remote control volume and mute but it also has 3 analog inputs and bluetooth. It does have gain but the volume adjusts in 0.5db steps and you have to turn it up a lot before the gain kicks in, so there's plenty of adjustment range. The volume control is "digital", not a motorized pot, but I have no idea if it uses relays or what. It's quiet and has been totally reliable. And it's 1/4 the price of the Saga+.
https://www.ebay.com/itm/293406152320?hash=item44505e9680:g:s6kAAOSwJBdeDwxv&amdata=enc:AQAHAAAAwKiNdbM968BRqGoaqpUk6RkoF7QjaEnKF/rTsBgYpQjwxSJWQMcjN6rajZ9um39y+f/DeLi0qO/I9cVYOAoZmfwn46VNmqnlCTqg+JCA1YqeXnATEChIRP+XjvxQxcPPIAqWlq01YmtqsK9oG51Mx6B5u0kFhdyjkU9S5NxDILGFYxC23KRm2ivz1MYlwBuWeKL4p6GiSr4eG1LclRoO0h+wGH7n9RHI5j1mnNIx8129i7PDuASMY7COBHW0K2cejA==|tkp:Bk9SR8q5kKvqYQ
This is what I did when I built a preamp and wanted some tube character. The only tubes I considered had a mu (amplification factor) under 10. You'll probably have to build one yourself, though. Most commercial designs use tubes that are currently being manufactured. If you build one yourself you have many more choices, which allows you to start with the right tool for the job.
Barry NJ,
1. First, carefully do a few measurements, before modifying:
(Note: carefully means safely, and also means not letting the meter probes slip and short something)
DC voltage at the + of 3C2 (that also connects to 3R5 and 3R11).
DC voltage at 3R2 and 3R3.
DC voltage at 3R4 and 3R10.
2. Now, replace the 12AX7 with a 12AU7.
Again, carefully do a few measurements:
DC voltage at the + of 3C2 (that also connects to 3R5 and 3R11).
DC voltage at 3R2 and 3R3.
DC voltage at 3R4 and 3R10.
3. Listen to the preamp, and see if there is any improvement (less buzzing and spitting noises).
Is it good enough now?
Report the voltages for 1. and 2., and the listening test 3. above.
Based on those measurements, and the listening test, we will see if any resistors should be changed.
1. First, carefully do a few measurements, before modifying:
(Note: carefully means safely, and also means not letting the meter probes slip and short something)
DC voltage at the + of 3C2 (that also connects to 3R5 and 3R11).
DC voltage at 3R2 and 3R3.
DC voltage at 3R4 and 3R10.
2. Now, replace the 12AX7 with a 12AU7.
Again, carefully do a few measurements:
DC voltage at the + of 3C2 (that also connects to 3R5 and 3R11).
DC voltage at 3R2 and 3R3.
DC voltage at 3R4 and 3R10.
3. Listen to the preamp, and see if there is any improvement (less buzzing and spitting noises).
Is it good enough now?
Report the voltages for 1. and 2., and the listening test 3. above.
Based on those measurements, and the listening test, we will see if any resistors should be changed.
So currently...
3C2 = 283v DC
3R2 = 1.17v DC
3R3 = 137v DC
3R4 = 144v DC
3R10 = 147v DC
Note, I'll need to acquire the parts before I can swap them in, don't have those type of things handy here.
I'll order from Mouser or Digi-Key to hopefully ensure I get quality parts.
So, do magic pixies come into the system after a few days and just fix problems?
No changes made, and the noise is pretty much gone(?) I can hear a little something with my ear against the speaker grill, but there are no longer any nasty noises heard at the listening position...
I did order some NOS RCA 12au7 tubes that should be here in a couple of days, and I'll see how they effect the gain and overall sound of things.
12au7_tested by Barry, on Flickr
No changes made, and the noise is pretty much gone(?) I can hear a little something with my ear against the speaker grill, but there are no longer any nasty noises heard at the listening position...
I did order some NOS RCA 12au7 tubes that should be here in a couple of days, and I'll see how they effect the gain and overall sound of things.
12au7_tested by Barry, on Flickr
Not sure where from, my system is in the basement here, all below grade, and I've made no changes to electronics in the house(?) Also, it's still quiet...
Barry NJ,
When you get your 12AU7 tubes, just plug them in to the 12AX7 socket.
Then listen.
Gain OK, Sound OK, hum OK, noise OK?
Problem solved?
Done.
Or if not, then . . .
Is it more distorted, versus with the original 12AX7 tubes?
You might change 3R5 from 5k to 2k.
After that,
You also might see what happens if you change 3R4 and 3R10 from 100k to 47k.
Remember, these things may change the sound, more to what you like, or less than what you like.
When you get your 12AU7 tubes, just plug them in to the 12AX7 socket.
Then listen.
Gain OK, Sound OK, hum OK, noise OK?
Problem solved?
Done.
Or if not, then . . .
Is it more distorted, versus with the original 12AX7 tubes?
You might change 3R5 from 5k to 2k.
After that,
You also might see what happens if you change 3R4 and 3R10 from 100k to 47k.
Remember, these things may change the sound, more to what you like, or less than what you like.
So, 12au7 tubes installed into 12ax7 sockets...
3C2 = 272.7v DC
3R2 = 4.85v DC
3R3 = 123.1v DC
3R4 = 134v DC
3R10 = 136v DC
Listening was horrible!
The tubes seem to be so microphonic to the point I was getting feed-back.
Lightly taping the preamp caused a clearly audible ring from the speakers.
Could it be that the tubes are bad, or is it a reaction of the circuit to the different tube type?
P.S. I'll add that the gain was now much more reasonable, with normal listening levels reached with the volume knob at about 11 o'clock. But the microphonic sensitivity was ridiculous. I didn't really get to evaluate any of the other audible characteristics for fear of entering a bad feedback loop and damaging something.
Okay, so some GE5814 Triple Mica tubes installed and I think all is as it should be. The 5814 tubes seem to be running a lot hotter than the 12ax7 tubes did, just by the evidence of their glow if nothing else. The tube on the right has less silver coating on the top, hence its more obvious glow. But the 12ax7 tubes had no discernable glow when they were in there, and the rear 12au7 doesn't glow either...
2023-04-16_09-01-06 by Barry, on Flickr
The 12AU7 and 5814 will draw a lot more plate current, versus the 12AX7 plate current.
That is the main addition to the heat you noticed.
Example; 3R2 1.5k Ohms, and the voltage across it is:
With a 12AX76 1.17V; that is 0.78mA
With a 12AU7 4.85V; that is 3.2mA (and with the 5814, 3R2 is probably about 4.85V too)
4.1 times more current, 4.1 times more heat.
The 12AX7 and 12AU7 filaments wired for 6.3V draw 300mA each tube.
The 5814 filaments wired for 6.3V draw 350mA each tube (not a lot more current).
That is the main addition to the heat you noticed.
Example; 3R2 1.5k Ohms, and the voltage across it is:
With a 12AX76 1.17V; that is 0.78mA
With a 12AU7 4.85V; that is 3.2mA (and with the 5814, 3R2 is probably about 4.85V too)
4.1 times more current, 4.1 times more heat.
The 12AX7 and 12AU7 filaments wired for 6.3V draw 300mA each tube.
The 5814 filaments wired for 6.3V draw 350mA each tube (not a lot more current).
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Are you still looking to reduce the gain in this amplifier?
It is hard to change the schematic since it is all mounted on a circuit board. Otherwise I might propose to change it from SRPP to MU-Follower.
Swapping out the input valve/tube from 12ax7 to 12au7 might still work, so stick with 6A3sUMMER's help here.
Another option would be to add some global negative feedback to the circuit. I never did it for SRPP but it is an option, and it does work nicely. I would take the feedback loop from after the cathode follower's output cap through to the input (if it can be done).
Morgan Jones has a nice write up on this I think.
Ian
It is hard to change the schematic since it is all mounted on a circuit board. Otherwise I might propose to change it from SRPP to MU-Follower.
Swapping out the input valve/tube from 12ax7 to 12au7 might still work, so stick with 6A3sUMMER's help here.
Another option would be to add some global negative feedback to the circuit. I never did it for SRPP but it is an option, and it does work nicely. I would take the feedback loop from after the cathode follower's output cap through to the input (if it can be done).
Morgan Jones has a nice write up on this I think.
Ian
Thanks Ian, I appreciate your input, but at this point, I'm happy with the 5814 tubes in the circuit where the 12ax7 tubes were.
Another possible issue I didn't see discussed in this thread is over driving the preamp. I built a 12AX7 based preamp feeding a 12AU7 crossover chain and with standard 2v input levels the 12AX7 input stage was producing distortion in the 4% range. I had a very lossy bandaxell bass/treble control circuit and needed the gain. Not one of my better projects, both sonics and measurement wise
Thanks, I tried reducing the input to the preamp by using the variable output from my CD player, but that actually made things worse.
You had a very lossy tone control circuit. The Baxandall circuit isn't lossy: it has a gain of 1, which can be increased. You didn't have a Baxandall circuit.