Maybe...
A 6dj8 should work with, B+ 32V, anode resistor 10k, cathode resistor 154 ohm.
Gain is about 25 (with cathode r. bypassed).
If you pretend the (triode strapped) 12AU6 is half a 6dj8, as PRR suggested...
It`s probably not the cleanest sounding pre-amp you could make... Distortion will be high, maybe too high?
I haven`t tried this, so i don`t know how well it will work...
Regards,
Peter
A 6dj8 should work with, B+ 32V, anode resistor 10k, cathode resistor 154 ohm.
Gain is about 25 (with cathode r. bypassed).
If you pretend the (triode strapped) 12AU6 is half a 6dj8, as PRR suggested...
It`s probably not the cleanest sounding pre-amp you could make... Distortion will be high, maybe too high?
I haven`t tried this, so i don`t know how well it will work...
Regards,
Peter
There is only one triode in a 14GT8, the other two sections are diodes. It was originally made as a detector/preamp combo. But yeah, use like any single section of a 12AX7. You might tweak the cathode resistor a bit, because the curves aren't an exact match.
Hm. I'm curious about this thread...
I also have an old radio with a 12AU6, and I notice in scrabbling about the Web that a number of '60's era small amps used a 12AU6 for the input pentode -- Harmony 303A and 400, Silvertone 1430, Kay 500.
Many of them seem to have used a 50C5 for the output tube, and a 35W4 rectifier tube.
Granted these are not modern high-end boutique amps...I suppose one must expect a certain amount of noise and grit out of them. But still, for a novice it seems that the simplicity of the circuits alone might make them worth investigating, especially if one can salvage parts from old Zenith radios and the like.
Thoughts?
I also have an old radio with a 12AU6, and I notice in scrabbling about the Web that a number of '60's era small amps used a 12AU6 for the input pentode -- Harmony 303A and 400, Silvertone 1430, Kay 500.
Many of them seem to have used a 50C5 for the output tube, and a 35W4 rectifier tube.
Granted these are not modern high-end boutique amps...I suppose one must expect a certain amount of noise and grit out of them. But still, for a novice it seems that the simplicity of the circuits alone might make them worth investigating, especially if one can salvage parts from old Zenith radios and the like.
Thoughts?
Granted these are not modern high-end boutique amps...I suppose one must expect a certain amount of noise and grit out of them. But still, for a novice it seems that the simplicity of the circuits alone might make them worth investigating, especially if one can salvage parts from old Zenith radios and the like.
If you're going to go that route, you'd do better with the Lil' 4X4. Here, the designer has done his best to get decent performance from the **C5 family. That's not easily done, as these types of finals were intended to get a few watts out with very low DC plate voltages. Lineraity is horrible.
If it's simplicity you want, consider any one of a large number of SET designs. These can be even simpler, yet with quality iron and some attention to detail, can outperform any **C5-based amp.
If you're going to go that route, you'd do better with the Lil' 4X4. Here, the designer has done his best to get decent performance from the **C5 family. That's not easily done, as these types of finals were intended to get a few watts out with very low DC plate voltages. Lineraity is horrible.
If it's simplicity you want, consider any one of a large number of SET designs. These can be even simpler, yet with quality iron and some attention to detail, can outperform any **C5-based amp.
The "Lil 4x4" actually uses the 4volt heater version of the 12AU6
as pre-amp. 😉
Go here;
http://www.dogstar.dantimax.dk/tubestuf/index.htm
Check out the real Mctube, especially the power supply.
Lil 4x4.
Spunky.
read it all... Then read again.
Then, look at the rest of Freds projects.
You wont go far wrong. 🙂
regards thyratron.
as pre-amp. 😉
Go here;
http://www.dogstar.dantimax.dk/tubestuf/index.htm
Check out the real Mctube, especially the power supply.
Lil 4x4.
Spunky.
read it all... Then read again.
Then, look at the rest of Freds projects.
You wont go far wrong. 🙂
regards thyratron.
Interesting you should say that. I asked the question about using 6AU6 in pentode mode to do similar duty to EF86 in another recent thread.
I didn't have time to reply earlier. Had to clean up the mess that Katrina left behind, and take down the shutters. Pray for New Orleans, this is going to be ugly.
Yes the 12AU6 can be used for a preamp. I explained this in the 6AU6 thread. Some of them are noisy and microphonic, some of them are pretty good. Most of them are probably OK for a guitar preamp. The 13.5 volt wall wart will be OK to run the filament of the 12AU6 but the plate supply of this tube will need at least 50 volts to work at all and it really wants at least 100. The trick used in the real McTube, using a small filament transformer hooked up backwards, also powered by your wall wart, will give you close to 150 volts. The preamp circuit used in the lil4X4 can be used with a 12AU6. You may have to adjust the resistor values slightly to optimize it for your particular tube.
A single triode circuit for 1/2 of a 12AX7 could be used with your 14GT8. Here again it will want at least 100 volts for the plate supply.
There are tube distortion boxes that run the tube on 48 volts. These do not do clean sounds wery well on such a low voltage. I have not been impressed with the "fuzz" sounds of these boxes either. There are tube distortion boxes that run the tube on higher voltages that work pretty good.
There are also special tubes that were originally designed for car radios in the late 1950's that did indeed run the filament and plate on 12 volts. These were called space charge tubes. They didn't work particularly good, and were replaced by transistors in a few years. Since they were only around for 2 or 3 years, they are hard to find now. I have experimented with a few of these, 12CN5, 12CX6, and 12DK7. The sound quality was not great, but usable. These tubes have about half of the gain of a conventional tube.
Yes the 12AU6 can be used for a preamp. I explained this in the 6AU6 thread. Some of them are noisy and microphonic, some of them are pretty good. Most of them are probably OK for a guitar preamp. The 13.5 volt wall wart will be OK to run the filament of the 12AU6 but the plate supply of this tube will need at least 50 volts to work at all and it really wants at least 100. The trick used in the real McTube, using a small filament transformer hooked up backwards, also powered by your wall wart, will give you close to 150 volts. The preamp circuit used in the lil4X4 can be used with a 12AU6. You may have to adjust the resistor values slightly to optimize it for your particular tube.
A single triode circuit for 1/2 of a 12AX7 could be used with your 14GT8. Here again it will want at least 100 volts for the plate supply.
There are tube distortion boxes that run the tube on 48 volts. These do not do clean sounds wery well on such a low voltage. I have not been impressed with the "fuzz" sounds of these boxes either. There are tube distortion boxes that run the tube on higher voltages that work pretty good.
There are also special tubes that were originally designed for car radios in the late 1950's that did indeed run the filament and plate on 12 volts. These were called space charge tubes. They didn't work particularly good, and were replaced by transistors in a few years. Since they were only around for 2 or 3 years, they are hard to find now. I have experimented with a few of these, 12CN5, 12CX6, and 12DK7. The sound quality was not great, but usable. These tubes have about half of the gain of a conventional tube.
More to the point why would you want to? The 12AU6 is not particularly linear and is extremely noisy, hummy and microphonic compared to say an EF86.
Just because someone says it, that doesn't make it so. See for yourself; here are the 6AU6A characteristics. These look pretty linear, wouldn't you agree? As for "noisy", "hummy", and "microphonic", well, there were a lot of these produced "on the cheap". Buy inferior tubes, expect inferior performance. There are also good ones out there. Now triodes can be better than pentodes in terms of linearity, noise, and distortion. They are also easier to use and design for. However, you shouldn't be afraid of small signal pentodes if you believe that these will get the job done better.
Just because someone says it, that doesn't make it so. See for yourself; here are the 6AU6A characteristics. These look pretty linear, wouldn't you agree? As for "noisy", "hummy", and "microphonic", well, there were a lot of these produced "on the cheap". Buy inferior tubes, expect inferior performance. There are also good ones out there. Now triodes can be better than pentodes in terms of linearity, noise, and distortion. They are also easier to use and design for. However, you shouldn't be afraid of small signal pentodes if you believe that these will get the job done better.
Attachments
For comparison purposes, here's another small signal pentode: the 6EW6. Now that's what I would call a nonlinear characteristic. Of course, the 6EEEEWWWW!6 was never intended for audio amplification. This is a high gain RF voltage amplifier, such as you'd find in an IF amp. In that case, the nonlinearity and distortion is of no consequence since an IF amp will include at least double tuned xfmr's, and perhaps narrow BP filters as well, to clean up the messes this tube will make.
Attachments
Most tubes that were originally designed for IF amplifier use had characteristics such that the tubes gain changed with the operating point. This was done so that the receivers AGC (automatic gain control) function would work. The gain of the receiver was reduced on strong signal, and increased on weak signal. This was done by adjusting the grid bias. Often these types are called "remote cuttoff pentodes". These tubes are designed to be non-linear on purpose. if you take one apart you will see that the control grid is not evenly spaced. These tubes (6BA6, 6SK7 etc) are generally not well suited for audio. Other IF tubes like the 6AU6 or 12AU6 will work for audio but they are not well suited for large signals since the sweet spot may be small. They should be fine for a guitar preamp. It was my experience from my limited testing that there is a pretty large variation from tube to tube. I have seen the 12AU6 used for the input stage in low cost guitar amplifiers from the 1950's that ran off the AC line without a transformer. I have an old Electrolab amplifier that was made this way. A truly shocking experience.
Thanks much for the info, all.
I would love to get good iron and accoutrements, but I have to stick to a garage-sale level budget. I simply can't shell out $60 for a project...but I do enjoy the scavenging game.
I'll probably play around with a preamp unit, maybe with a silicon power stage added later.
I would love to get good iron and accoutrements, but I have to stick to a garage-sale level budget. I simply can't shell out $60 for a project...but I do enjoy the scavenging game.
I'll probably play around with a preamp unit, maybe with a silicon power stage added later.
- Status
- Not open for further replies.