Rescue Toaster
100+ Head-Fier
- Joined
- Sep 30, 2005
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Greetings! Here is the design I'm building. The last of the parts should be arriving tomorrow. The design is a parafeed (CCS-loaded) single tube stage with autotransformer output for 300 ohm headphones.
In this circuit there's really only 4 parts that carry AC signal, the tube, the parafeed capacitor, the output transformer, and the cathode resistor. The cathode resistor only carries input signal current though, not output. With good constant current sources above the tubes, those 4 parts *alone* completely determine the performance of the amplifier. There is zero power supply interaction.
The amp:
Schematic
22ma constant current sources load the tubes. The output transformers are any suitable 10K:600 wired as autoformer for 5:1 stepdown, providing a 7500 ohm load to the tube. The bottom of the transformer returns to the cathode instead of ground (neccesitating 4-pin balanced headphones) to eliminate degenerative feedback in the cathode resistor. With the CCS on top, the output current loop is entirely contained and the tube operates with fixed bias. The input volume control can of course be eliminated and replaced with whatever. The two gas tube shunt regulator (with teflon cap bypass) provides further power supply isolation and sets the voltage. The parafeed capacitor has to be tuned to the output transformer chosen.
The power supply:
Schematic
A custom transformer from electraprint w/ electrostatic screen feeds two 6AX4-GTB damper diodes in full wave. They provide a very slow turn-on to ensure the ECC99 is fully heated. The 47 ohm resistors are really just to drop some extra voltage to lower dissipation in the supply CCS. Small film caps can be used because the CCS-fed shunt (the supply CCS & gas reg tubes in the amp) provide excellent isolation. Even so ripple is only 45mA before the regulator. The power transformer's electrostatic screen runs with the heater supply to the ECC99.
I'm using Gary Pimm's cascoded constant current sources, which are probably overkill. You could easily use something like the C4S from bottlehead for each tube, and a simple DN2540 CCS in the power supply.
The basic structure of the amp could be easily setup to run any high-GM triode (ECC99, 5687, probably 6N6P though I haven't done the math, please no 6C45's, too much gain and they won't like the fixed bias). Essentially anything with a plate resistance around 2-2.5K that operates in the 120-180v range and ~20ma could be used. My suspicion is the 5687 is the best choice, but I'm gonna try out the ECC99's first, since I have them.
For transformers I'm starting with some real cheapo Edcor 10K:600's (4:1, wired as autoformer so 5:1). They are nothing special ($12 ea!) but don't really do anything objectionable. The best transformers are probably Lundahl 1674 amorphous cores ($100 ea) wired as 4:1 and then as 5:1 autoformer. If you absolutely must keep your headphones as 3 pin, find an existing ~5:1 transformer, I'd go with the Magnequest B7 15K:500 ($100 ea). Still return the primary to the cathode, but run both secondaries to ground. Another excellent choice is the Magnequest TL-404 which is already an autotransformer, and has many taps for lower impedance phones, but the amp won't have enough power for low impedance, low efficiency phones.
Basically I wanted the ultimate platform for any tube as a single-stage amp. I wanted to be able to use the variety of awesome 10K:600 transformers out there, almost none of which can handle DC current, thus CCS parafeed. It might be beatable with some *extremely* high end SE transformers made to handle the current instead of the CCS's, but you lose a lot of flexibility and they'd be extremely expensive to have them custom made. No-one sells the right transformers out of stock, believe me I've looked.
In this circuit there's really only 4 parts that carry AC signal, the tube, the parafeed capacitor, the output transformer, and the cathode resistor. The cathode resistor only carries input signal current though, not output. With good constant current sources above the tubes, those 4 parts *alone* completely determine the performance of the amplifier. There is zero power supply interaction.
The amp:
Schematic
22ma constant current sources load the tubes. The output transformers are any suitable 10K:600 wired as autoformer for 5:1 stepdown, providing a 7500 ohm load to the tube. The bottom of the transformer returns to the cathode instead of ground (neccesitating 4-pin balanced headphones) to eliminate degenerative feedback in the cathode resistor. With the CCS on top, the output current loop is entirely contained and the tube operates with fixed bias. The input volume control can of course be eliminated and replaced with whatever. The two gas tube shunt regulator (with teflon cap bypass) provides further power supply isolation and sets the voltage. The parafeed capacitor has to be tuned to the output transformer chosen.
The power supply:
Schematic
A custom transformer from electraprint w/ electrostatic screen feeds two 6AX4-GTB damper diodes in full wave. They provide a very slow turn-on to ensure the ECC99 is fully heated. The 47 ohm resistors are really just to drop some extra voltage to lower dissipation in the supply CCS. Small film caps can be used because the CCS-fed shunt (the supply CCS & gas reg tubes in the amp) provide excellent isolation. Even so ripple is only 45mA before the regulator. The power transformer's electrostatic screen runs with the heater supply to the ECC99.
I'm using Gary Pimm's cascoded constant current sources, which are probably overkill. You could easily use something like the C4S from bottlehead for each tube, and a simple DN2540 CCS in the power supply.
The basic structure of the amp could be easily setup to run any high-GM triode (ECC99, 5687, probably 6N6P though I haven't done the math, please no 6C45's, too much gain and they won't like the fixed bias). Essentially anything with a plate resistance around 2-2.5K that operates in the 120-180v range and ~20ma could be used. My suspicion is the 5687 is the best choice, but I'm gonna try out the ECC99's first, since I have them.
For transformers I'm starting with some real cheapo Edcor 10K:600's (4:1, wired as autoformer so 5:1). They are nothing special ($12 ea!) but don't really do anything objectionable. The best transformers are probably Lundahl 1674 amorphous cores ($100 ea) wired as 4:1 and then as 5:1 autoformer. If you absolutely must keep your headphones as 3 pin, find an existing ~5:1 transformer, I'd go with the Magnequest B7 15K:500 ($100 ea). Still return the primary to the cathode, but run both secondaries to ground. Another excellent choice is the Magnequest TL-404 which is already an autotransformer, and has many taps for lower impedance phones, but the amp won't have enough power for low impedance, low efficiency phones.
Basically I wanted the ultimate platform for any tube as a single-stage amp. I wanted to be able to use the variety of awesome 10K:600 transformers out there, almost none of which can handle DC current, thus CCS parafeed. It might be beatable with some *extremely* high end SE transformers made to handle the current instead of the CCS's, but you lose a lot of flexibility and they'd be extremely expensive to have them custom made. No-one sells the right transformers out of stock, believe me I've looked.
