I discovered a half-dozen new VT-33 pentodes while digging through yet another carton of my uncatalogued tubes. The datasheet looks interesting – a directly-heated pentode with a very small heater current, and a gain of about 70. Max plate voltage is only 180VDC, so the tube should operate nicely at 150 volts or less. The triode-strapped curves look very nice.
This got me thinking about a simple, low-gain headphone amp. My CD player puts out a maximum of 2.2 volts, with only a 300-ohm output impedance. My headphones are 250-ohm Beyerdynamics, and should need only 6 or 8 volts to drive them to uncomfortable levels.
A simple rectified and well-filtered isolation transformer could provide the 150-volt B+, and DHT heater problems could be avoided by using a Gates Cyclon 2V, 5Ah battery, with a charging circuit that switched on when the main power was switched off.
Edcor makes some very inexpensive line matching transformers (20-20K for less than $13) if one was needed for the input. Since the output power would be in the range of 200mW, an OPT would be inexpensive as well.
I'm wondering if its worth crunching some numbers to use this tube in a headphone amp. Possibilities would be single-tube triode-mode, single pentode, pentode-wired tube driving triode-wired tube, and others.
Does anyone have any experience using old DH pentodes in low-power amps? I'd like to pursue this, but not if I'm going to run into problems with microphonics or other noise, or some other pitfall I'm not experienced / knowledgeable enough to recognize.
Thanks in advance for your (no pun intended) feedback...
This got me thinking about a simple, low-gain headphone amp. My CD player puts out a maximum of 2.2 volts, with only a 300-ohm output impedance. My headphones are 250-ohm Beyerdynamics, and should need only 6 or 8 volts to drive them to uncomfortable levels.
A simple rectified and well-filtered isolation transformer could provide the 150-volt B+, and DHT heater problems could be avoided by using a Gates Cyclon 2V, 5Ah battery, with a charging circuit that switched on when the main power was switched off.
Edcor makes some very inexpensive line matching transformers (20-20K for less than $13) if one was needed for the input. Since the output power would be in the range of 200mW, an OPT would be inexpensive as well.
I'm wondering if its worth crunching some numbers to use this tube in a headphone amp. Possibilities would be single-tube triode-mode, single pentode, pentode-wired tube driving triode-wired tube, and others.
Does anyone have any experience using old DH pentodes in low-power amps? I'd like to pursue this, but not if I'm going to run into problems with microphonics or other noise, or some other pitfall I'm not experienced / knowledgeable enough to recognize.
Thanks in advance for your (no pun intended) feedback...
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I take it that the solid lines on the graph are the Triode mode curves, which look OK. At first glance looks like a gain of about 4.25 and an Rp of around 2800 Ohms?
So a 1:2 step-up transformer at the input and a 4:1 step down on the output will net you about 30 mW into 250 Ohms for a 2.2V input. Whether 30 mW is enough depends upon the sensitivity of the 'phones and how loud you like it. On most new high-sensitivity 'phones 30 mW is too loud, but on your 'phones I have no idea.
The loading on the tube would be 4800 Ohms via the transformer, which might be a bit heavy as it is only 1.7 times the Rp of the tube.
So a 1:2 step-up transformer at the input and a 4:1 step down on the output will net you about 30 mW into 250 Ohms for a 2.2V input. Whether 30 mW is enough depends upon the sensitivity of the 'phones and how loud you like it. On most new high-sensitivity 'phones 30 mW is too loud, but on your 'phones I have no idea.
The loading on the tube would be 4800 Ohms via the transformer, which might be a bit heavy as it is only 1.7 times the Rp of the tube.
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