Want to build a low voltage tube buffer for car audio

Lwrs10

Lwrs10

Super Member
I want to build some tube buffers to use in my car audio setups. I don't really want to use the typical circuits used in buffer because of the high voltages required. I *could*, but would rather stick with something that is low voltage for reliability and less parts.

So far I've zero'd in on 6GM8 tubes, as these can run as low as 6 volts on the plate. Same pinout as a 6DJ8/6922. But can't seem to find a schematic tailored to the 6GM8.

Could I just use a 6DJ8 schematic? What would I have to modify to use just 12v?

Or, is there a better tube to use in this application?

Help!

Thanks in advance.
 
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probably won't find a cathode buffer schematic for one of those, its such an odd duck. Shouldn't be too hard to design though. Cathode followers don't have many parts. Plate goes to a +6.3v supply, grid to ground through an resistor, 100k ought to do. Cathode to ground resistor is the only thing to really calculate here.

From the datasheet, it says 0.4 volts and 0.9 ma. 0.4 / 0.0009 = 444 so a 430 or 470 ohm resistor. Use a coupling cap from cathode to your output and you're good. Cap needs to be big enough to not have LF roll-off problems working into whatever the load is, but if you stick a 4.7uf in there it shouldn't have any problems no matter what you connect it to. Its gonna make some thumping noises as it charges up though so you may want to consider a muting relay.

Not going to be able to drive much signal into this though, less than a half volt input, and output will be pretty nearly equal to what you put in. Whether thats enough to actually be useful you'd have to figure out.
 
gadget73 said:
probably won't find a cathode buffer schematic for one of those, its such an odd duck. Shouldn't be too hard to design though. Cathode followers don't have many parts. Plate goes to a +6.3v supply, grid to ground through an resistor, 100k ought to do. Cathode to ground resistor is the only thing to really calculate here.

From the datasheet, it says 0.4 volts and 0.9 ma. 0.4 / 0.0009 = 444 so a 430 or 470 ohm resistor. Use a coupling cap from cathode to your output and you're good. Cap needs to be big enough to not have LF roll-off problems working into whatever the load is, but if you stick a 4.7uf in there it shouldn't have any problems no matter what you connect it to. Its gonna make some thumping noises as it charges up though so you may want to consider a muting relay.

Not going to be able to drive much signal into this though, less than a half volt input, and output will be pretty nearly equal to what you put in. Whether thats enough to actually be useful you'd have to figure out.
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Well dang, that won't work then. The RCA signal is 5v RMS.

Any ideas on where to go from here?
 
What source does 5V RMS? I thought 2V was standard? Anyone know any ~12VDC circuits for a 2V in -> 2V out tube buffer?
 
I wonder about an anode follower with a 12AU7. Of course you could use space charge tubes but the AU7s have a reputation for running OK at low B+. Could just take one from the junk drawer to try out.
 
Squidward said:
What source does 5V RMS? I thought 2V was standard? Anyone know any ~12VDC circuits for a 2V in -> 2V out tube buffer?
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car audio stuff tends to run pretty high signal levels, mostly for S/N reasons. Long IC runs from front to back and dirty charging systems don't help things.

If you're not opposed to some solid state voodoo juju, you could get reasonable effective voltages with boost converters. A pair of 48v units set up to give a split rail supply nets just shy of 100 volts total, and thats plenty enough to get things done with "normal" tubes.

proper old school would be mechanical switch mode supply (a vibrator) to kick the voltage up through a transformer and rectify the output of that. Car radios ran about 250vdc.
 
Squidward said:
What source does 5V RMS? I thought 2V was standard? Anyone know any ~12VDC circuits for a 2V in -> 2V out tube buffer?
Click to expand...

Most higher end car audio head units are 5v.

Lavane said:
https://glass-ware.stores.yahoo.net/tubu.html
^^??^^
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Yep I saw those, but they are out of stock, and I want to build them myself.

gadget73 said:
car audio stuff tends to run pretty high signal levels, mostly for S/N reasons. Long IC runs from front to back and dirty charging systems don't help things.

If you're not opposed to some solid state voodoo juju, you could get reasonable effective voltages with boost converters. A pair of 48v units set up to give a split rail supply nets just shy of 100 volts total, and thats plenty enough to get things done with "normal" tubes.

proper old school would be mechanical switch mode supply (a vibrator) to kick the voltage up through a transformer and rectify the output of that. Car radios ran about 250vdc.
Click to expand...

I'm all for a DC-DC converter that would supply enough voltage to run normal tubes, but my main goal is to use some that don't need high plate voltage.
 
Don't some of these cheap little tube mic preamps and buffers run the 6DJ8s or whatever at, like 48 Vp? I'll bet if you searched the web for a schamtic you'd find one.
 
The problem with limited supply voltage is that you just end up with very limited available voltage swing. Tubes don't have the super low internal voltage drops that transistors have. I just don't know that anything exists that will give 5 volts of swing with only 12 volts on the plate.
 
I have one that I bought off an EE, it contains a single 12AU7 and operates off of 12 volts.
I have it in my rear quads circuit, but it was designed for use in vehicles.
I just rolled a triple mica, black plated 5814 in.
 
wondering what sort of internal voltage that works at. Something tells me the plate sees more than 12v.
 
Sounds like a job for a dynamotor.
Of course at that point you may as well go full tube power stage as well...
I've got a Bendix aviation dynamotor that makes half an amp at 500v
Perfect for a mobile tube power amplifier.
 
yeah that was the higher power way of making B+. Vibrators are limited by what their contacts will support. Its also fairly dirty, a dynamotor's output would be a fair bit cleaner. They are mechanically somewhat noisy though, and very inefficient.
 
trainbuftony said:
Sounds like a job for a dynamotor.
Of course at that point you may as well go full tube power stage as well...
I've got a Bendix aviation dynamotor that makes half an amp at 500v
Perfect for a mobile tube power amplifier.
Click to expand...
I would love to see you do a thread on that unit.
 
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