trinityamps.com

Hello all,
I'm new here. Just purchased and began assembling a TriWatt. Very excited to get this done. I have one question and one favour to ask, concerning my TriWatt build. The amp isn't firing up correctly, and I always like to check before I proceed. In particular, I'm confounded by the bias circuit, and I'm not seeing any readings whatsoever from the external bias test points.

Question:
There are slight discrepancies between the layout and the documentation, regarding the bias circuit wiring. How shall I decide which is accurate?

Favour:
I'd like to bypass the bias switch and hardwire for KT66. Can anyone provide a simple description and/or bias circuit layout, describing or showing the correct wiring?

In the KT66 position, the bias switch lowers the value of the resistance feeding the bias circuit by connecting R43 and R44 in parallel. It also bypasses D5 to supply more B+ voltage.

To eliminate the switch and wire the amp only for KT66's:
Remove the switch and all of its wiring.
Place a short jumper between the right ends of R43 and R44, the same way there is a jumper on their left ends.
Wire the C20/R39/Standby Switch junction directly to the D1/D4/D5 junction.

You can also remove D5 if you want as it will not be used.

I modified the layout drawing to show all of this and it's attached to this message.

For the bias circuit the layout drawing and the schematic are correct. If you are having problems, recheck all your wiring very carefully.

Best of luck getting the amp going.

Mitch,
Thanks so much. The description was great, but the layout change was beyond the call. Can't thank you enough. I'll post a pic later, with results.

Best regards,
Frank

Glad I could help, Frank. That bias switch can take a bit of head scratching to figure out.

I've never actually built a Triwatt but I've spent a lot of time studying the documentation. I'm a big Hiwatt fan and I'm pretty familiar with their circuitry. The Triwatt is very faithful to the originals.

I have built a couple of amps inspired by the originals, though, a DR504 and a 30 watt version.

A few things you could check on your amp are: Do you have a good connection from the black bias jack to pin 3 on the bias pot and to ground? Also is there a jumper running from the R45/R46/C23 junction to the R28/R29 junction? This jumper is shown as a dotted blue line on the bottom of the board in the layout drawing.

If you can measure voltage, check for about -45 volts DC between the R28/R29 junction and ground and that you can vary it using the bias pot. While you are at it, check all the other voltages, too. They should be fairly close to what's shown on the schematic.

As I said, check all your wiring and solder joints. It sounds like you missed something.

Thanks Mitch,
Much appreciation for your advice. All of my under-board jumpers are in-place. I'm going to stare at it tomorrow, with a fresh set of eyes, and implement your KT66-only changes. Here is a pic of the current board, attached for you.

mitch, thanks for stepping up to help

_________________
Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps