Combed through the Electra-Print schematics and this caught my attention. I've been looking at it off and on for a few years and might move forward. I have a load of 6BL7 tubes so I don't care that they're obsolete. I have just as many 6BX7's that I could swap out. Power supply will need to be beefy. Other than that, it looks straight forward.
I love the simplicity and how hard it's being driven. Anyone ever heard of this being built?
I love the simplicity and how hard it's being driven. Anyone ever heard of this being built?
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Knew the power supply was the biggest challenge. Don't know of anyone making a power transformer with a 100V bias tap. Never considered making my own. Looks like it's just the right resistor values to end up in the ~ -100V range based on this example. 4.2A of DC heater current for driver circuit. Think I'm looking at monoblocks or separate power supplies for an insanely heavy stereo build.
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"Don't know of anyone making a power transformer with a 100V bias tap."
You don't need 100V AC for -50V DC.
See my #30 post here.
Hammond 300 series transformer has 50V AC bias tap.
Hammond 300 series
The 373BX:
You don't need 100V AC for -50V DC.
See my #30 post here.
Hammond 300 series transformer has 50V AC bias tap.
Hammond 300 series
The 373BX:
| Primary VAC | Universal - 100, 110, 120, 200, 220,230, 240 VAC 50/60 Hz |
|---|---|
| A.C. High Voltage Secondary RMS | 700V C.T. @ 201ma. |
| Bias Tap on High Voltage Secondary w/ C.T. Grounded (VAC) | 50 |
| Filament #1 (VAC) | 5V C.T. @ 3A |
| Filament #2 (VAC) | 6.3V C.T. @ 5A |
This looks like a fine circuit, designed by Jack Elliano and gives you a nice breadboard for a monoblock 300B design. Unfortunately, I am not familiar with LTspice (as of yet) but the famed @jcalvarez is and he may be willing to model it, giving you voltages to look for as well as overall gain. At that point, working backwards and designing up the supply will be a fun exercise.
The 6BL7 and 6BX7 are excellent driver tubes in this application. If you have enough source voltage (i.e. common modern D/A converters), the 6BX7 may be as much to your liking…but I do like them both.
Best,
Anand
The 6BL7 and 6BX7 are excellent driver tubes in this application. If you have enough source voltage (i.e. common modern D/A converters), the 6BX7 may be as much to your liking…but I do like them both.
Best,
Anand
Quick question/clarification.
I see that the overall gain is 6.836 which I am going to round up to 7 for simplicity sake. A gain of 7 is 20 log (7) = ~17 dB. Is that gain including the step down imposed by the output transformer? And what turns ratio did you use? Hmmm....the Hammond 1630SE has a 3.5K:8 ohms impedance ratio, which is a turns ratio of 20:1. Just figuring out the AC signal voltages in the circuit which is also a fun exercise.
Let me know if I am wrong about this.
Best,
Anand.
I see that the overall gain is 6.836 which I am going to round up to 7 for simplicity sake. A gain of 7 is 20 log (7) = ~17 dB. Is that gain including the step down imposed by the output transformer? And what turns ratio did you use? Hmmm....the Hammond 1630SE has a 3.5K:8 ohms impedance ratio, which is a turns ratio of 20:1. Just figuring out the AC signal voltages in the circuit which is also a fun exercise.
Let me know if I am wrong about this.
Best,
Anand.
Yup, it does. It is calculated as rms(Vout)/rms(Vin)
~21 turn ratio for the 8 ohm tap, yes.
Appreciate the simulation and other feedback. Seems to me that Electra-Print was in the unique position of having no limitations around transformers, since they could just design what they needed. That 450V and -100V bias are too much to overcome with stock solutions. Jcalvarez, any way you can run that again with 400V and -50V bias? I'm into stock transformer territory with those numbers. Much appreciated!!
Why not just use a small toroid trans for the neg. bias supply?
jeff
jeff
I'm looking at some sort of auxillary trans (or two) no matter what. I'd love to pull this off as a stereo SET. A monoblock with a single 300B sitting there seems kinda silly. Multiple power problems to overcome. 450V, -100V bias, and 4.2A of DC just for driver tubes in stereo.
No need for simulation. You will fry the 300B and probably the OPT too at ~400V and -50V Bias 🙂
About bias voltage, I have used a small EI 6VA transformer, VTX-120-4206-420, 20V+20V secondary, with a doubler. You get more than 100V, and plenty of current for bias.
First you need to choose the OPT.
The OPT recommended primary current is definite, this is an important parameter.
If you have it the load of 300B (3k...5k OPT, for example max. 80mA primary current), must to draw the loadline.
As you can see in this theoretical simulation, the 300B distortion tolerable over 3k load.
300B tube, fixed bias 416V, 80mA, at 81Vp grid swing (A1)
Loadline drawing is possible on hardcopy curve sets (see #12 post) or with appropriate program.
"Triode loadline" is very good aid: triode loadline.
Choose output tube, choose load (for example 3k3), set the loudspeaker impedance to 8R, find the operating point (for example Ia 70-80mA and resumed Ua anode voltage - OPT primary DCR*Ia lower than B++ -).
The operating point must be around the middle of the available curves.
If it's between 0V and minus 140V, the supposed "middle" at about -70V curve.
The suggested -sample- operating point is 357.7V, 80.34mA, -70.92V.
At 3k3:8 OPT load the tube gives 6.28W (with 4.63% THD) up to A1 limit (up 0V grid swing).
If the Ua is 357.7V, assuming the OPT's primary DCR is 110R, with 80mA static current the required B+ is:
Ua+ (Ia*DCR) i.e. 366.5V.
These calculations can be done with different parameters (larger B++, another anode current, another operating point etc.).
The OPT recommended primary current is definite, this is an important parameter.
If you have it the load of 300B (3k...5k OPT, for example max. 80mA primary current), must to draw the loadline.
As you can see in this theoretical simulation, the 300B distortion tolerable over 3k load.
300B tube, fixed bias 416V, 80mA, at 81Vp grid swing (A1)
Loadline drawing is possible on hardcopy curve sets (see #12 post) or with appropriate program.
"Triode loadline" is very good aid: triode loadline.
Choose output tube, choose load (for example 3k3), set the loudspeaker impedance to 8R, find the operating point (for example Ia 70-80mA and resumed Ua anode voltage - OPT primary DCR*Ia lower than B++ -).
The operating point must be around the middle of the available curves.
If it's between 0V and minus 140V, the supposed "middle" at about -70V curve.
The suggested -sample- operating point is 357.7V, 80.34mA, -70.92V.
At 3k3:8 OPT load the tube gives 6.28W (with 4.63% THD) up to A1 limit (up 0V grid swing).
If the Ua is 357.7V, assuming the OPT's primary DCR is 110R, with 80mA static current the required B+ is:
Ua+ (Ia*DCR) i.e. 366.5V.
These calculations can be done with different parameters (larger B++, another anode current, another operating point etc.).
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If you choose -for example- Hammond 373BX power transformer (2*350V, 50V for negative bias), the estimated HV output about 385V at 174mA load.
The 300B tube's supposed anode current is 80mA, 6SL7 eats 1mA, the 6BL7 about 6mA, so one channel current requirement about 87mA.
A possible solution for PSU:
The 50V AC too low for -70...-80V bias, so must to use simple voltage doubler.
With estimated 3k3:8 OPT (DCR 110R, 80mA), this is the loadline:
As can you see, this operating point requires about -76V bias, which the PSU can produce.
The 300B tube's supposed anode current is 80mA, 6SL7 eats 1mA, the 6BL7 about 6mA, so one channel current requirement about 87mA.
A possible solution for PSU:
The 50V AC too low for -70...-80V bias, so must to use simple voltage doubler.
With estimated 3k3:8 OPT (DCR 110R, 80mA), this is the loadline:
As can you see, this operating point requires about -76V bias, which the PSU can produce.
The 300A (electrically the same as the 300B) datasheet has a very useful table (paghe 4) that includes voltages, biases and 2nd/3rd harmonic levels.
https://frank.pocnet.net/sheets/136/3/300A.pdf
Think I have a plan to run this in stereo. Going with Hammond iron. A 300BX for the B+ and 5V. That tube puts out 508V after a GZ34 rectifier tube (Have one in a KT88 setup and just measured). A 193M choke for the first CLC and low value resistor for the next CRC should get me to 450VDC. A second small transformer can feed the 6.3VDC and the 300B filaments. That will be 3 separate low voltage power supplies. Thoughts?