Amp design based on Ampeg R12RT

[chickenPlucker]

I recently purchased a '65 Ampeg R-12-Rt on ebay. I love the sound, but when I opened the chassis I found wires loose and unconnected (I have no idea what they are for yet) and sloppy repair jobs. Almost all of the components are original, including the caps (excluding the can cap only), and I have considered replacing the electrolytics. The layout is so tight it would be a nightmare to revamp. Most of the tubes also are obsolete: 6BK11, 6U10, 12AX7 x2, and 7868 x2.

So I started thinking, why not build a new amp based on this design, replacing the obsolete tubes with tubes more readily available, preferably ones currently manufactured. The 6BK11 and 6U10 are triple triodes. I should be able to match the specs of each triode to a current tube, perhaps, just off the top of my head and without looking at the tube data sheet (later), replace the two triple triodes with 3 dual triodes. Naturally the caps and resistors would have to be adjusted to meet the demands of the new tubes.

Why? Cause I wanna'. Everbody and their brother, and sometimes even their dog, make Fender and Marshall clones. I want to do something different.

Any thoughts on feasibility of redesigning the circuit around new tubes?

[J Martin]

I have done the dual triode/compactron replacement in a few Ampegs. You don't have to adjust anything if you use the equivalent dual triode sections. Both the 7868 and its equivalent, the 7591 are in production again. Good luck with your project.

[chickenPlucker]

Who is producing these tubes? I checked with all the major manufacturers. And which triode did you use?

[Enzo]

EH makes 7868, contact New Sensor for those, or any EH outlet.

The triodes depend on the compactron in question. Look up your tube in RCA or some other data sheet. Look at amplification factor. The 6C10 for example is 100 - same as 12AX7. The 6U10 has two at 17 and one at 90, so a 12AU7 and half of something high gain. In the case of each triode, select the dual triode section most appropriate. All the 12A_7 types of various gain ought to set you up, but don't forget tubes like the 12DW7/7247/ECC832 with its high and low gain sides. That moght save you an extra tube sometime.

[chickenPlucker]

I can't find data on the 6BK11 anywhere, but the 6C10 is suppose to be an appropriate substitute: a previous owner of my amp crossed out the 6BK11 and wrote in 6C10. In the absence of documentation I'll have to assume that's acceptable. In addition, I guess the difference in amp factors between the 6U10 and the 12AU7 (17.5 vs 17/19.5) is nominal with no modifications required?

It seems all I have to change is the heater voltage to the tubes - from 6.3 to 12.6.

[chickenPlucker]

Here are schematics. The photos are missing the middle. I have photos of my amp schematic, but it's missing the top. Between the two I came up with this redraw. One thing I don't quite understand is the tremolo portion of the circuit.

Notice where it goes into the first 6BK11 (V1), where two sections are tied together. Usually the signal input goes into the grid, but here the grid input is generated by the third section of the 6BK11. Furthermore, where is the output and how does it return to the signal.

I guess I have a fundamental lack of understanding of how the tremolo circuit works and how it affects the signal. I'll work on that this weekend...

[Mark Ingram]

I can't find data on the 6BK11 anywhere

Try this: http://www.nj7p.org/Tube4.php?tube=6bk11

[chickenPlucker]

Thanks, but I have that one. Antique Electronic Supply says the GE Tube Data book has the info I need. If I can find a copy of it online I won't have to buy it, but if I so it's only $15.