diytube.com

Folks-

This is just the proto, but a 1/2 unit has been in a Mark III for almost a year performing flawlessly, so I'll be able to buy boards quickly. The production board will have solder mask and silk screen with all the original eyelet numbering. This makes modification a snap. I sat the ST70 on my work bench, put in a CD, and was done as the final song stopped playing - seriously. That includes using my wiring wraps

Here's the schematic.

Shannon

Excellent! This is the circuit a '70 should have. What is the projected cost?

TerrySmith wrote:Excellent! This is the circuit a '70 should have. What is the projected cost?

PCB and manual will be $49. Parts from Mouser ring up at $23 via the BOM.

Initial testing last night went very well. Though we are limited by the saturation characteristic of the A-470s for max output power, the frequency response and performance is much more uniform than the original circuit. I should be able to have it tweaked up this weekend and post some numbers.

Maybe the 7199 was a much better match for the ST70 than the 6AN8 was for the MKIII.

Shannon

If I remember right, the 7199 was originally intended as an audio VA and PI tube, while the 6AN8 was more of a general purpose and TV "plinker" variety.

A step better than 7199 but not as good as going all-triode, is to use two EF86s and a 12AU7 for the ST70. EF86s are one of the very few good audio pentodes, and better than that in the 7199, and definitely a lot beter than 6AN8, or using a 6AU6 or 6BA6 (uggghhhhh)

I've already worked up my own BOM for this based on the schematic, and have everything but the 51K resistors on hand.(unless I cheat and use 2 47K, and a 10K "balance" pot).

/ed B in NH

Looks good Shannon. I am in the process of collecting parts for another ST70 type. I haven't selected a driver board yet so let me know when you are finished the tweaking and in the selling phase. I will buy one.

Hi Shannon, am very interested in your board. What similarities, if any, to Ned's triode board? What is your projected release date?

Thanks

Shannon,

Looks like you have been busy. The board looks great. I can't wait to stuff one and install it in my ST70!!

separks wrote:Folks-

This is just the proto, but a 1/2 unit has been in a Mark III for almost a year performing flawlessly,

When I first met Shannon a couple of years ago, I was just finishing (troubleshooting) a homebrew ST70 driver board project using Kevin Kennedy's schematic. http://www.kta-hifi.net/ This amp sounds pretty good, but in order to DC couple the 12AX7 to the 12AU7 phase splitter, neither one is operating optimally. So when I was researching driver boards for my MKIII's, Shannon suggested AC coupling to get the 12AX7 operating point better and using the LM334 on the cathode of the phase splitter. He already had some proto boards with a more traditional circuit and I offered to to mod them and install them in the MKIII's. I cut the board traces to add the AC coupling cap and drilled holes to install the LM334. The result in the MKIII's was stunning!!! I thought they were pretty good amps with my upgraded 6AN8 driver (when I rebuilt them the first time, I wanted to keep David Haflers design intact). But with the diytube proto driver board, they have become exceptional amps. In fact, I am enjoying a little REO "157 Riverside Avenue" playing through them right now. This new ST70 driver board promises to work the same sonic magic with an ST70. Looks like Shannon came up with another winner!!!

JT

Thermion wrote: This new ST70 driver board promises to work the same sonic magic with an ST70. Looks like Shannon came up with another winner!!!

Ok, ok, I'll get that 'transconductance standard' article from the latest AudioXpress to you. 8^)

Shannon

Could this board be used in other applications? For example, I have 3 sets of Heathkit AA-100 iron waiting for projects. The OPT's are 6600 ohm, no ul taps, designed for 7591's.

TerrySmith wrote:Could this board be used in other applications? For example, I have 3 sets of Heathkit AA-100 iron waiting for projects. The OPT's are 6600 ohm, no ul taps, designed for 7591's.

Terry,
This board could be used as the driver board in many amplifiers. The modified Mullard circuit (a grounded cathode voltage gain followed by a long-tailed-pair phase splitter) is a standard in many classic amps including the Eico amps. As long as it fits in the chassis, this board could be used to upgrade many old amps. This is true for many output tube types and independent of UL, pentode, or triode output mode. Of course, some of the component values may need to be tweaked somewhat in other applications.

JT

I've already got PCBs and getting ready to supply kits for this project if anyone is interested
Roy www.tubes4hifi.com

Hi shannon,

Very much impressed with the diver board! Yellow_Light_Colorz_PDT_15 Would point-to-point wiring possible to implement using the above diagram?

Thanks.

s2kov wrote:Hi shannon,

Very much impressed with the diver board! Yellow_Light_Colorz_PDT_15 Would point-to-point wiring possible to implement using the above diagram?

Thanks.

Hi s2kov,

Do you mean recreating my circuit using point-to-point? Sure, anything is possible. Or do you mean using the board for a custom amp? That would be a good application, too.

Shannon

Hi shannon, thanks for the response. Yellow_Light_Colorz_PDT_15 Actually, i had the stock ST70 and would like to try the other driver board unfortunately, i am not located in the US to order your board and custom tax is pretty high!

separks wrote:

s2kov wrote:Hi shannon,

Very much impressed with the diver board! Yellow_Light_Colorz_PDT_15 Would point-to-point wiring possible to implement using the above diagram?

Thanks.

Hi s2kov,

Do you mean recreating my circuit using point-to-point? Sure, anything is possible. Or do you mean using the board for a custom amp? That would be a good application, too.

Shannon