cosme
New Member
Thanks for sharing. I'll have to adopt a similar approach.
Thanks for sharing. I'll have to adopt a similar approach.
I asked Roy of Tubeshifi if they had any test data for the octal board mod for 6SN7's and this was the response I got.
no I don't, sorry.
The board is identical to our classic board except for the tubes used.
I did measured the freq response 30 years ago, flat from 7Hz to 70KHz but no other tests done other than listening.
I haven't heard any better board in an ST70 amplifier.
Roy www.tubes4hifi.com/ST70.htm
no I don't, sorry.
The board is identical to our classic board except for the tubes used.
I did measured the freq response 30 years ago, flat from 7Hz to 70KHz but no other tests done other than listening.
I haven't heard any better board in an ST70 amplifier.
Roy www.tubes4hifi.com/ST70.htm
That is reassuring... there is a whole lot more than frequency response when measuring amplifiers.
Thanks for reaching out though.
That's nearly the same response I got a few years ago. He also doesn't know how much feedback either. I measured it at only about 6 db, as I recall he said much higher than that. I've done a couple of these using the basic diytube schematic, attached.
I hang the compensation resistor and cap under the board. Everything else fits fine. I've used both the 12AX7 and 5751 as input tubes. I've measured feedback at about 18 db and it sounds much better than building it with all 12au7's and so very little feedback. I highly recommend taking a $30 Roy board and diytube-ing it. You should be able to do the same thing using a 6SL7 as the input tube and 6SL7's for the phase splitters if you wanna use his octal board.
AFAIK the diytube board is no longer available but I like the Roy board better as it has the 4 bias controls right on the board.
I hang the compensation resistor and cap under the board. Everything else fits fine. I've used both the 12AX7 and 5751 as input tubes. I've measured feedback at about 18 db and it sounds much better than building it with all 12au7's and so very little feedback. I highly recommend taking a $30 Roy board and diytube-ing it. You should be able to do the same thing using a 6SL7 as the input tube and 6SL7's for the phase splitters if you wanna use his octal board.
AFAIK the diytube board is no longer available but I like the Roy board better as it has the 4 bias controls right on the board.
From the schematic in the manual you uploaded, it really is not an "ST70" afterward is it? It has been transmogrified.
cosme
New Member
My plan here is to paint them a metallic (anodized) blue, specifically Dupli-Color's MC201 Blue Metalcast. I have a 3-year-old ... I've let him pick out the color.
pe
To get good results with Dupli-Color's metalcast paints, you need either a shiny or chrome finished surface. I opted for the latter and spray painted these end bells with Rust-Oleum's Metallic Silver finish (#340648):
And then re-applied the Rust-Oleum Metallic Silver:
Next up on the to-do list for these is to apply the Dupli-Color metalcast paint. Stay tuned.
Finally got around to applying the Dupli-Color Blue Metalcast. Can't say I love the color (at least right now) but my son does.
I ended up going with tubes4HiFi for the driver board. While not a perfect match, they certainly complement one another.
cosme
New Member
Thanks for linking to the C24X. Ended up going with this option after reading a few reviews. The price difference between Mouser's choke and those offered by Triode and DynakitParts made it all the more appealing.
Look forward to seeing your rebuild.
cosme
New Member
Thanks for the kind words everyone!
Pio1980
AK Member
Right, it's a Mullard variant.
I have two of the R&R builds, one T4HF with increased FB, and the DIY tube T4HF version. Both lovely amps, the DIY has somewhat more gain. Otherwise, I'd be hard pressed to say one is significantly better sounding than the other.
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That's good to know. I did not realize it was a Mullard variant.
Pio1980
AK Member
There's a progression of simplification from the Williamson thru the Mullard to the Dyna/Laurent reductive elegance. The sensitivity of the newer power tubes needing less drive had everything to do with that. Fewer phase shifting components of course greatly simplified stabilization, along with experience mass producing better OPTs.
The 6BQ5/EL84 with single envelope drivers put good sound into more listening spaces than anything else before silicon ruled.
This is for observers, you know these things.
cosme
New Member
The pair came without tubes so I will for sure be getting new [to me] tubes.
Considering trying out some Linlai or Psvane options but may ultimately just go with some old U.S.-made stock.
Related to tubes, I purchased bottom-mount Belton tube sockets but they're a hair too large and won't mount. Considering I just had it replated, I may opt to go with some smaller ceramic sockets rather than take out the Dremel to grind out a slightly larger diameter.
cosme
New Member
I should have checked but the C24X is smaller in size than the C-354 choke, so some mounting adjustments will need to be made. Sharing in case anyone isn't aware.
cosme
New Member
Thank you for the correction!
I just checked the choke that had been installed on the unit I received. While it doesn’t indicate a model number, it’s rated at 2 Henries, 200ma, 50 ohms. The C-354 on DynakitParts is 1.75 Henries, 200ma, 62 ohms.
So definitely a different choke.
cosme
New Member
Apologies for being slow to update this thread. New baby arrived in February. She's been healthy and we've all been adjusting to the larger family.
My plan was to go with new Chinese tubes from Psvane.
For the KT88s, I went with their newer KT88, i-T series:
I went with the UK series 6SN7 tubes:
Unfortunately, it didn't occur to me to measure the spacing on the new driver board. The shouldered glass means they can't sit side-by-side on the PCB. Oops! I ultimately ended up buying a lot of well maintained Sylvania. Nothing fancy, but at least functional.
For the rectifier tube, I have some new JJ 5AR4s but will ultimately go with some RCA-branded UK-made 5AR4s (Mullards?).
My plan was to go with new Chinese tubes from Psvane.
For the KT88s, I went with their newer KT88, i-T series:
I went with the UK series 6SN7 tubes:
Unfortunately, it didn't occur to me to measure the spacing on the new driver board. The shouldered glass means they can't sit side-by-side on the PCB. Oops! I ultimately ended up buying a lot of well maintained Sylvania. Nothing fancy, but at least functional.
For the rectifier tube, I have some new JJ 5AR4s but will ultimately go with some RCA-branded UK-made 5AR4s (Mullards?).
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