Finally finished my Clem KT88 build

[mesherm]

Finished initial power up and tweaking last night on my long overdue KT88/6L6 Clementine build. I used the Edcor power trannie and Transcendar 3.5K OTs in a chassis I got last year from Keith Kirby before his passing.
Pretty standard build except I used adjustable constant current sinks for the cathodes, TL783 3 term high voltage regulators in standard CCS configuration. I used them before in an EL34 SE amp and they worked like a charm so I used them again in this build. The part I like is that once they are adjusted for a particular current you can swap tubes and not worry about the cathode resistor value. For laughs and giggles I set the current to 55ma and swapped in SED 6L6GCs and JJ KT77s in addition to the Sovtek KT88s and in each case the current held exactly the same. I put the KT88s back in and reset the current to 80ma for the listening tests in the next few days.
The frequency response fror 20 to 20k seems good with about a 0.3 volt drop in output across the spectrum. It peaks in the 1k to 5K area so listening tests should be interesting. I was measuring about 5 watts RMS max before any waveform distortion on my scope.
I will snap some pics as soon as I remember where I put my camera.

Edit: Found camera but not the battery charger for the darn battery. Hooked the amp up and played some music. Plenty of bass from the Transcendars and Kt88s. Very little hum. Might have to add a small CPU cooling fan to exhaust inside heat.

[carpenter]

Hurry and post pictures.

[mesherm]

[carpenter]

Nice job, I especially like the volume control. Beautiful looking gadget. What make is it?

Regarding your CCS, what do you feel are the benefits? Do you hear a sonic difference?

[mesherm]

Nice job, I especially like the volume control. Beautiful looking gadget. What make is it?

Regarding your CCS, what do you feel are the benefits? Do you hear a sonic difference?

The stepped attenuator was from an ebay offering I picked up a few years ago. I'm not sure of the exact number of steps but it rotates nearly the full 360 degrees.
There is a pair of 1% resistors for each step per channel and building and soldering was quite a project all by itself. I forget where I got the black aluminum knob.
The CCS gives me adjustability and consistency in the idle current. I can adjust the idle current to 55 ma for example and it will stay 55 ma even if I switch to a different tube.
Since the CCS is bypassed by a cap the same way a cathode resistor is I'm of the opinion it is sonically neutral. Its the capacitor that passes the AC portion.
So I now can insert any KT88 (or other tubes with the same base) and not worry about what value of cathode resistor to use. I always add a 10 ohm 1% resistor to complete the circuit to ground so I can measure the current.

[carpenter]

Finished initial power up and tweaking last night on my long overdue KT88/6L6 Clementine build.....

Are there schematics to your KT88 build in this forum? I love my 1626 version, but would like to build a more powerful amp for comparison.

Thanks.

[Shannon Parks]

The KT88 build is the second batch of links, labelled the Clementine 6L6 (aka Chubby Clementine) here:
viewtopic.php?f=19&t=4893

Meanwhile, the third build - the ULtra, which is an ultralinear connected SE circuit with feedback, is here:
viewtopic.php?f=19&t=5011

Shannon

[mesherm]

In addition to the Sovtek KT88s I tried Winged C 6L6GCs and JJ KT77s although with reduced current settings. I didnt try EL34s because I hadnt strapped pin 1 and 8.
I didn't get much difference in power with the three although the KT77s seemed to be the highest. I left the KT88s in to better match the Transcendar OTs 3.5k impedance. The screens were triode strapped with a uh4007 diode in series with a 100 ohm resistor. The TL783s were configured for constant current with a 10 ohm resistor in series with a 25 ohm trimpot. The TL783/resistor/trimpot was paralleled with a 100mf 100v electrolytic cap and a 2.2mf film cap. A second 10 ohm current sense resistor was outside that loop connecting it to star ground. I didn't use the screw terminals opting to solder directly to the PCB. The large size of the stepped attenuator forced me to solder the 0.2 mf coupling caps vertically. I have to say that Shannon's board really makes the build a snap and the hum and noise is practically nil. It shames my other scratch built hand wired amps in that regard. I elected to place some solder lugs on the mounting hole nearest to the GRD hole on the PCB and connected them thereby including the chassis. The power line plug ground was connected to the chassis with a 0.02 mf ceramic cap and 100 ohm resistor in series. RCA inputs were the kind that are isolated from the chassis. There are so many ways to configure a Clementine board build. It's a DIYer's dream. (applause for Shannon).

It would be interesting to have a nine pin socket version either wired for EL84s or maybe have a pattern inside the octal pattern for 9 pin PCB sockets with holes only so jumper wires could be used to connect to the octal holes as needed. That would open up a whole new world of possible octal/9 pin tube combinations.

[Shannon Parks]

There are so many ways to configure a Clementine board build. It's a DIYer's dream. (applause for Shannon).

Thanks for the kudos! It's my great pleasure to be a tube DIYer. Hopefully the best is yet to come.

Shannon