Hi, i'm looking at building Peter Millets starving student amplifier,
But im interested in using a matched pair of GE 12V6GTs.
I was wondering if anyone had any idea on how to do such a swap, do i need to just conect the two grids together and lower the 19volts to 11/12 or is this swap a bad idea?
Thanx
But im interested in using a matched pair of GE 12V6GTs.
I was wondering if anyone had any idea on how to do such a swap, do i need to just conect the two grids together and lower the 19volts to 11/12 or is this swap a bad idea?
Thanx
12V6 is a completely different tube (for one thing it's a single pentode, not a double triode!) and would require a redesign of the whole amp, even triode connected - as it's gain and transconductance are completely different (about 1/5th gain, and 1/3 transconductance). In addition, the heater uses signifficantly more current, and in this design it doubles as the MOSFET's source resistor. It would be possible to design something similar around the 12V6 but pretty much every single component would have a different value, except the power supply cap!
Also note that 19J6s are cheaper than a pack of Camels. $4 a pop here: http://www.esrcvacuumtubes.com/vacuumtubes_tubelist_tubes_13CW4-90c1.html
These have gotten nearly impossible to find. Best place for info is over at head-fi. My suggestion for an alternative would be the 12au7 -- runs OK at the low voltages specified, and heater current is the same. You'll be dropping an extra 7V across the fets, so you'll need to adjust the bias resistor accordingly and be sure the heatsinks are up to snuff.
http://www.fairradio.com/ has 'em for $4.50. http://www.vacuumtubes.net/ has 'em for $4.00.
http://www.vacuumtubes.com/price8.html has 'em for $10.00
http://www.vacuumtubes.com/price8.html has 'em for $10.00
I inherited an old RCA TV Repair case that was filled with two manuals and about 1000 tubes of assorted types. i dont really see why it uses two dual triodes connected as a single instead of two triodes.
The closest things i have to 19v
(aside from some magnavox full wave rectifiers)
are:
4 12BH7A (1 matched pair)
1 12AT6
1 12AT7
3 12AX7A (matched)
1 12AX7WA
and 1 15H36
I was under the assumption that it was possible to place a capacitor parralel to ground and a resistor in series before the heater to lower the volatge to 12.
As dsavitsk said these are pretty hard to find.
Out of the handful of sites that sell them most are out of stock, as with any fear i can easily see the 19J6's price skyrocketing way out of my budget.
The closest things i have to 19v
(aside from some magnavox full wave rectifiers)
are:
4 12BH7A (1 matched pair)
1 12AT6
1 12AT7
3 12AX7A (matched)
1 12AX7WA
and 1 15H36
I was under the assumption that it was possible to place a capacitor parralel to ground and a resistor in series before the heater to lower the volatge to 12.
As dsavitsk said these are pretty hard to find.
Out of the handful of sites that sell them most are out of stock, as with any fear i can easily see the 19J6's price skyrocketing way out of my budget.
bacon665 said:
Would you care to list anything that has a more than 6V heater, including single tridoes and pentodes or BPTs? Maybe something can be found that is suitable...
That would be ebcause you don't seem tio understand how it really works
The 19J6 (6J6 with 150mA heater) has two triodes with a common cathode. In order to get decent current capability and gm at such low voltage power supply, both triodes are paralelled.
The reason why a 19V heater version is used has more to do with the heater being a 15mA type - it doubles up as a source resistor for the MOSFET source follower. In other words, the output signal is developed across the heater!
The 12BH7A has a 300mA heater, so twice the current will flow through the MOSFET, which means twice the ehat is generated, which means a bigger heatsink. But, it should be doable although low voltage performance is not s good as the *J6...
12AT7 and 12AX7 have the right heater, but can't work well at such low voltages. 12AT6 may even work at 48V but only at a fraction of a mA, insufficient to drive a MOSFET even in a source follower configuration -
You can place a resistor in series with the heater to add the extra 7V, assuming a 150mA heater, the resistor would be 47 ohms at least 3W. NO capacitor - as I said, the output voltage is developed over the heater (in this case incl. resistor).
As dsavitsk said, 12AU7 with sections in parallel may work, but the cathode and anode resistors have to be recalculated.
I've built 2 starving students and I can definitely sympathize with how difficult it is to find any 19J6 out there, I have 10, but I'm keeping those as spares. One alternative tube that I read about in the thread over at head-fi is the 12SR7/12SW7. Its an octal so the socket pin connections have to be changed and obviously some resistor values change but its all spelled out in the post there.
I ordered a handful of 12SR7 and a pair of 12SR7GT for myself to try this tube out since I already have the rest of the parts and compare it to the 19J6 version.
Also thanks dsavitsk, I didn't know the 12AU7 was a possibility, I have a bunch of those already, hmmm...
I ordered a handful of 12SR7 and a pair of 12SR7GT for myself to try this tube out since I already have the rest of the parts and compare it to the 19J6 version.
Also thanks dsavitsk, I didn't know the 12AU7 was a possibility, I have a bunch of those already, hmmm...
(not sure if this suffices as a warning but)
Do not attempt to build your own power transformer without a full safety lesson and proper training. The comments in this post are not for the general public to try.
now with that out of the way.
it doesnt seem like too much of a problem to just wind an extra voltage off the transformer and rectify it. would have to go back and look at that circuit....
not sure if that would work.
Someone said earlier the audio is coming from the heater. i dont really see how that is possible.
Do not attempt to build your own power transformer without a full safety lesson and proper training. The comments in this post are not for the general public to try.
now with that out of the way.
it doesnt seem like too much of a problem to just wind an extra voltage off the transformer and rectify it. would have to go back and look at that circuit....
not sure if that would work.
Someone said earlier the audio is coming from the heater. i dont really see how that is possible.
bacon665 said:
A heater is kind of a current source: the hotter it is, the higher is it's resistance. But in this particular case it is acting more as a resistor that loads a source follower. Also, it may provide kind of a thermal negative feedback that stabilizes a working point if to add one resistor.
[/quote]quote:
Originally posted by bacon665
it doesnt seem like too much of a problem to just wind an extra voltage off the transformer and rectify it. would have to go back and look at that circuit....
Pete spec'd a commonly available 48V switcher for the PS.[/quote]
Seeing as i Actually fit the title of starving unemployed highschool student, I pulled a couple bridge rectifiers (400v,200vx2 more then overkill) and an old ferrite core and wound up a power supply with 48v and 2.5volt for the LEDs. I've got two options for heatsink one from a blad server psu with 2.5 inch fins
and a zalman orb cpu cooler (copper/aluminum)
On C4 and C2 the closest thing i have is 50v im hoping since the power grid here is usually 113volts instead of 120 and that the resistor values have to be increased to allow for the 12SR7GT
that 50v caps will be enough.
My two concerns on that are
1) theyre elecrolytic and the diagram doesnt specify a + side even though it uses the elctroyltic symbol instead of the typical | |
2) if something was to happen to these would it damage anything else in the circuit?
And finally for refrence im adding the following:
NPN voltage regulater using a GE/RCA 2N6678
Bandpass filter (audio spectrum) to prevent harmonics from the tubes from reaching the headphones
Another project i worked on had a 100k ganged pot so im using that instead of the 50k
Originally posted by bacon665
it doesnt seem like too much of a problem to just wind an extra voltage off the transformer and rectify it. would have to go back and look at that circuit....
Pete spec'd a commonly available 48V switcher for the PS.[/quote]
Seeing as i Actually fit the title of starving unemployed highschool student, I pulled a couple bridge rectifiers (400v,200vx2 more then overkill) and an old ferrite core and wound up a power supply with 48v and 2.5volt for the LEDs. I've got two options for heatsink one from a blad server psu with 2.5 inch fins
and a zalman orb cpu cooler (copper/aluminum)
On C4 and C2 the closest thing i have is 50v im hoping since the power grid here is usually 113volts instead of 120 and that the resistor values have to be increased to allow for the 12SR7GT
that 50v caps will be enough.
My two concerns on that are
1) theyre elecrolytic and the diagram doesnt specify a + side even though it uses the elctroyltic symbol instead of the typical | |
2) if something was to happen to these would it damage anything else in the circuit?
And finally for refrence im adding the following:
NPN voltage regulater using a GE/RCA 2N6678
Bandpass filter (audio spectrum) to prevent harmonics from the tubes from reaching the headphones
Another project i worked on had a 100k ganged pot so im using that instead of the 50k
bacon665 said:
The caps that should be polarized are marked as such. The two interstage couplers should be non-polarized.
bacon665 said:
You could probably tinker with the circuit such that you could bias the mosfet off the tube's plate and eliminate the coupling caps all together. But short of doing that, wrong voltage could shorten the heater life. So long as the output caps are intact, you should be OK.
bacon665 said:
?
bacon665 said:
That's fine.
Ok so does the capacitance matter on C4 and C2 i have some ceremic disk caps that are non polarized but theyre in upwards of 100micro, when i go to radioshack tommorow i can grab some caps(hopefully). The 150micro electrolytics i ordered from digikey (along with all the resistors before i realized i needed to change half of them) they came today as did the depletion mosfets.
So now im just waiting on finding a box,
the sockets, tubes, and additional resistors from tube depot.
Cutting my heatsink
And Finishing the PSU
so i should have pics up by either the end of this month or withing the first couple weeks of next month.
and i would normally start another thread for this but since it appears to be a simple question i wont clutter your forum with it.
Can someone explain the "autoBIAS" technique where they place a cap connected to both ends of the BIAS resistor?
i read it in one of those tube manuals in the noob page but i didnt quite understand it
So now im just waiting on finding a box,
the sockets, tubes, and additional resistors from tube depot.
Cutting my heatsink
And Finishing the PSU
so i should have pics up by either the end of this month or withing the first couple weeks of next month.
and i would normally start another thread for this but since it appears to be a simple question i wont clutter your forum with it.
Can someone explain the "autoBIAS" technique where they place a cap connected to both ends of the BIAS resistor?
i read it in one of those tube manuals in the noob page but i didnt quite understand it
I took today to work on the voltage+current regulater,
used two 2N6698s (RCA/GE) and a 50v zenar (+/-3%)
Upon drilling the holes into the small peice of wood and using a router to burrow out the backside i began to do the wiring and componant placement as a typical point to point.
An hour later i had a harsh reminder that im still A.D.D. considering the wiring should have been a 12 minute job.
used two 2N6698s (RCA/GE) and a 50v zenar (+/-3%)
Upon drilling the holes into the small peice of wood and using a router to burrow out the backside i began to do the wiring and componant placement as a typical point to point.
An hour later i had a harsh reminder that im still A.D.D. considering the wiring should have been a 12 minute job.
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