My SSS completed!

[David Root]

Timely update of this thread, I just did some work on my SSS#001, no rvb or trem. Marked up schem attached.

Because I took the PS nodes out in parallel for the screens, driver, and PI (with V2 & V1 nodes in series after the PI), I have 435 B+ on the PI and 303 and 307V on the plates. I am thinking this might be a tad high, might increase the dropping resistor from 5K6 to 10K to drop the plates to about 296/300V.

Any opinions on that?

It sounds great with the current tube complement. I can attest to needing the DC cathode drivers to get the SSS tone.

Anyone else used or considered using the 12BZ7 as the driver, like I have? The 12BZ7 seems to be holding up to the 400V plate to cathode OK. Getting just under 2.5mA each side on it. For those who may be unfamiliar with the 12BZ7, it's two 12AX7 in parallel in the same glass, with considerably bigger plates as you might expect.

I might try putting in a 12BH7 instead but I think it would draw a lot more current. Maybe too much for my 3W 51K cathode resistors

I started this rework because the gain and master pots seemed to be the wrong taper. Sure enough I had managed to put a linear pot in the gain slot. Duh!

Fixed that OK, then tried to fix a moderate hum problem by elevating the heaters about 70V--didn't make any difference.

So I changed out the master vol. pot, it was audio taper but an out of spec 1M AB Type J that measured 1.2M. Didn't have another PEC 1M pot so I put in a 500K PEC. Presto Change-o, big drop in hum level. I guess the 1.2M was attracting too much interference from its location.

[Structo]

I tried to improve the readability of the schematic, probably didn't help that much but it seems easier to read.

[David Root]

Thanx Tom, that is better.

[Structo]

Thanks David.

You may want to draw the power supply a bit clearer, as it is kind of congested on that drawing.

[David Root]

I can redraw it and post it separately if you like. In order to get the V1 and V2 voltages right I had to drop 6mA off the end of the V1 node to ground, just like in an ODS without the FET board in it.

[Aaron]

Thanks Dave.

Newbie question though, if you went from the master volume straight into a standard P.I, would that be reverse phased?
My way of reasoning is, if the signal is taken from the cathodes in the mixer circuit (flipped 180), then the cathode driver would flip it back?

Thanks,
Aaron

[10thTx]

Reposted schematic

[David Root]

Aaron, cathode follower does not invert phase, so the CF drivers don't change the phase relationships at the power tube grids. That is the same with or without the CF drivers.

10thTX, awesome stuff! Thank you so much! I can sort of edit a .sch but I can't yet create one. I'll go thru your schems and edit the .sch as best I can.

Very quickly, V1a and the tonestack are correct except the treble pot defeat switch is not there. The gain pot is 1MA, not 100k as shown.
V1b, the 1k to ground should be 100R, same as V1a.
Filter values are quite different I'll do those later, plus there is a dropout position on each filter pot that bypasses the filter pot entirely.

The famous inductor was .9H, not 9H. The one I actually used, a yellow Fadel, is 0.7 to 0.6H between 120Hz and 1kHz, respectively. V2a and V2b and the LNFB between V2a/V1b and V1b/V1a are correct. The MV pot is now 500kA.

The PI is correct as per Bill Dunham's schem. It would be easier to balance with a 10k trimpot and/or 100k/110k plates rather than 110k/110k/5k trimpot.
The GNFB values are correct ie 4k7 to a 100R. That is very low GNFB, probably because of the two LNFB lines further back.
The drivers are correct except the tube is a 12BZ7, not a 12AX7, and the lower triode B+4 should be labeled B+3. The two 51K driver cathode resistors are 3W. The 12BZ7 grids are at minus 52.3V and the cathodes at minus 51.0V.
The drivers bias setup is correct. Not shown is the raw bias value which is minus -176V, obtained from a Hammond 187A120 PT thru a 400V rated FWB. Total bias current is 2.0 mA.
The power tubes section is correct except the heater winding CT is not connected as shown. It is connected to a 270K 2W/51K 2W voltage divider & there is a 10uF 160V cap across the 51K. Top of 270k connected to the screen node and bottom of 51K to ground. This is a 70V heater elevation.

The power supply schem is off a bit. The rectifiers are two 1500V IXYS, IIRC, I'll check that. There are no 100R resistors balancing the heater supply, there is the elevated CT instead.
There are no 50uF reservoir caps. The B+1 caps are two 270uF 400V in series (actually measure about 240uF each).
The parallel PS arrg't is basically correct as you show it. The 13K is actually a 5W 10K and a 3K 2W. B+2, 3 and 4 caps are actually a 20+20+20uf 525V FP can and B+5 and 6 are a 20+20/450 ditto. The 50K 4W to ground is actually a pair of paralleled 100K 2W.

Voltages: All normalized to 120VAC wall.

B+1 451
B+2 447
B+3 350
B+4 435
B+2 305
B+1 298

Plates etc.
6L6GC 446 Screens 445
PI 303/307 Cathode 57
V2a 181, cathode 1.25, V2b cathode 182
V1A 199, V1B 200 Cathodes 1.57, 1.55

[10thTx]

David,

Hopefully, this is closer. Feel free to edit the SCH schematic and repost any more corrections. Thanks for sharing the schematic.

Remember you can also just use PAINT to edit sometimes.

I am not sure what you're doing with the elevated heaters, so I left that out. Sorry.

With respect, 10thtx

[David Root]

Thanx again 10thTX. I tried to edit the .sch PS file but I just cannot do it, it's too difficult.

I did manage to change the 1.5K screen resistors on the pre/power .sch to 1k 5W rather than 1.5k as you had them. The original schem has them as "1K5W" (no space), instead of 1K 5W, so I can see that would be misleading.

[jface]

Regarding Sergio's SSS 002 build:

Are the 470K grid leak resistors in the original schematic functionally incorrect?

What is the preferred bias method using a standard Twin power transformer?