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Prepping myself for my first st70 rebuild. I am trying to decide what driver to use. Dont know much about the circuitry options but I know I want a CCS. seems like a no brainer. I previously did 2 of my 4 MKIII's with 6DJ8 based driver boards which I purchased from VTA several years back. I could swear that those boads were an exact copy of the Curcio MKIII boards, because the instructions were pretty much copy and paste. Well it seems VTA abandoned that design in favor of 12AU7 based boards like their st70 board.

The following 3 things bother me:

1) I don't have a supply of 12AU7 tubes to work with and I would have to spend stupid money for some decent NOS.

2) I hear more bad than good things about the 12AU7 as far as musicality, linearity and lifespan.

3) I have a nice collection of Amperex Holland 6922 PQ's to work with.

Curcio has his premium mod which is CCS and based on 6DJ8 family but it costs more than what I paid for the amp.

I also noticed the new triode board supports 6992's

Can this design be modded to accept 6922's
I would really like to work with what I have and not spend a lot

Any help would be greatly appreciated.

Thanks

I thought 12AU7s used to be great for audio, but Morgan Jones says they have a bit of distortion, and recommends the 6DJ8/ECC88. Alot of people are installing 6CG7/6FQ7s or 12BH7s. But 6DJ8s should work great along with 6N1P and 6N23P. Most of them will require rewiring the socket for the different filament pinouts.

iqmike - do you already have the driver board? Do you have a schematic?

Shannon

separks wrote:iqmike - do you already have the driver board? Do you have a schematic?

Shannon

No I did not buy a board yet and the DIY board looks like the one I want to get. I'm just not sold on the 12AU7 for the previously posted reasons.

I think at some point i might want to experiment with feedback settings and I will definitely incorparate pentode/triode switches for the output tubes. I dont know if this should point me in any particular direction.

Sorry for the confusion, but which brand driver (as they're all DIY)?

Shannon

No, the 12AU7 is not a bad sounding tube, that is just one man's opinion. There are so many different variations, brands, etc of this tube and yes one will sound better in a particular application than others.

You can cut and jumper the filament traces (easy) to accept a 6v tube, then you can roll 6922, 6CG7, 6GU7 types and see which you like the best. Plus you can use a 6AU7 or 7AU7 there also, those are the same as 12AU7 except the filaments.

IMHO the Diy board is the best you can get for a number of reasons. First is the circuit is better suited for driving the EL34's. Second is that it's a high quality board that fits perfect and looks almost stock.

2) I hear more bad than good things about the 12AU7 as far as musicality, linearity and lifespan


The 12au7 is NOT a linear tube. It has its own sound kinda like syrup. In most circuits you can put a 12bh7 in its place. You'll need to do the math.

I think Amperex 6DJ8s are some of the best tubes ever made. I love them, and wouldn't hesitate using them in any driver circuit.

Definitely the 12BH7 and it's 6.3V counterpart - the 6GU7 - are pretty stellar tubes. The tubes are extremely linear, and the halves are normally better matched than 12AU7s (at least the ones in my stash that I curve traced). Maybe the larger plates enabled the halves to be more precisely matched? My next ST70 revision will probably promote the 12BH7 as the LTP phase splitter. Current rating of the PA060 be damned.

Shannon

separks wrote:I think Amperex 6DJ8s are some of the best tubes ever made. I love them, and wouldn't hesitate using them in any driver circuit.

Agreed.

The level of gain concerns me though. You may need a preamp.

Current rating of the PA060 be damned.

separks wrote:Sorry for the confusion, but which brand driver (as they're all DIY)?

Shannon

I meant the diutube driver for sale on this site.

You can cut and jumper the filament traces (easy) to accept a 6v tube, then you can roll 6922, 6CG7, 6GU7 types and see which you like the best.

you mean I dont need to change any resistor or capacitor values and use the circuit as is? Is there a tutorial on filament voltage conversion?

it's a high quality board that fits perfect and looks almost stock.

Agreed, cant stand the look of some of those oversized boards mounted on standoffs for clearance. Looks like a butcher job.

I think Amperex 6DJ8s are some of the best tubes ever made. I love them, and wouldn't hesitate using them in any driver circuit.

Love them in my MKIII's. BTW They are driving black plate tung sol 6550's. Dont know why I dont see too many circuits promoting them.

The level of gain concerns me though. You may need a preamp.

Why the concern, I thought the 6922 has a little more gain than the 12au7. If gain is an issue, couldn't I compensate with less feedback? Or will I run into issues there?

We will pick this up in a couple of days, I'm about to get destroyed here on beautiful Long Island fom this lousy hurricane.

They do! 33 compared to 19. But the overall circuit.... we don't have a schematic. Is the PI providing some gain?

Just ran the sims via John Broskie's TubeCAD. No change is necessary for subbing a 12BH7 as the long tail phase splitter. The bias voltages remain unchanged in this arrangement.

But using a 6DJ8/6922 doesn't get the right voltages as the LTP, and it's lower max voltage rating is a problem. The bias is more suited for small signals as a voltage amplifier. Unfortunately, it doesn't have suitable gain for a simple sub in the diytube ST70 driver.

Shannon

Most generally anywhere you have a 12au7 you can plug in a 12bh7. The long tail works that way as does the concertina. The Quicksilver 8417 has a 12BH7 as the phase splitter and you can plug a 12Au7 right in and everything will be perfect.

burnedfingers wrote:Most generally anywhere you have a 12au7 you can plug in a 12bh7. The long tail works that way as does the concertina. The Quicksilver 8417 has a 12BH7 as the phase splitter and you can plug a 12Au7 right in and everything will be perfect.

What about a 6CG7/6FQ7?

Same goes for the 6CG7/6FQ7 (with the filaments setup right).

Shannon