Mk3 clones

I am having problems with a mk3 build. Using hammond iron triode driver board kt88's 12au7's 5ar4 rectifier. The amps both work but I can't get the bias down to 1 volt which the triode build sheet says I should get. One output tube after about 1 minute of use will flash. I assume the voltage is too high. The bias will not go beneath 3 volts. I am waiting for the cap board triode supplies I think its an sds lab. I tried using 2 double cap cans. 500v 50uf 50uf. Could the 500v be the problem? Should I just wait for the cap boards to show up? I am new to this so any help would be appreciated.

Hi -- some time back I purchased a pair of the Triode PCB kits for an old pair of original Mk3s that I had. I am using 12AU7's on the PCBs, 6550 power tubes and a 5AR4/GZ23 Rectifier tube, also built a pair of SDS Cap board clones instead of the Can Capacitor. My configuration was biased at 1.25v instead of the old 1.56v (less wear and tear on the tubes).

I compared the Triode Documentation that came with my PCB kits with the Triode MK3 Build Manual that is available at their website and the PCB part of the documetation looks the same.

But, I did not see a schematic in the website build instructions so I can't tell what the differences are in wiring between what you built and the stock Mk3 + their PCBs that I have.

Was there any sort of schematic and/or voltage chart in your documentation? How did your readings compare with these?

Is there any way for you to take some pictures of the underside of the chassis and post them here for comparision? If all else fails, I found the Triode Tech helpful when I asked him questions via email directly.

I don't think your two 500v can caps are the problem, but you might ask him also.

Hope some of this helps! Please keep us posted with what you find out.

I found that the problem was the connection from pad 4 on the driver board to pin 4 had to be changed from one output tube to the other. It solved the problem completely. I would love to know why this must be. The build instructions say nothing about which output tube to attach this wire. Love the sound from these amps.

Hi Again --

On a Stock Mark III with the Dynaco Driver Card and also on a Stock Mark III with the Triode Driver Card:

(1)the GREEN Transformer Wire goes to Pin #4 of the LOWER Power Tube (that is the tube alongside the Rectifier Tube) and you also run a wire from Pin #4 back to Pad #4 on the DRIVER Card.

(2)the GREEN_WHITE Transformer Wire goes to Pin #4 of the UPPER Power Tube (the tube that is alongside the QUAD CAP). There is no corresponding wire from the UPPER Pin #4 back to the Driver Card because the UPPER and LOWER Power Tubes are sort of "in parallel" on a "stock amp."

(3)As far as Biasing the Power Tubes on the Stock Amps, you bias the pair of Power Tubes and not an individual Power Tube.

If you go to the TUBES4HIFI Website, you can download the Stock Mark III Manual and look at its wiring info and diagrams. I think the Triode Website also had build instructions available for its version of the driver card. If nothing is there for their Driver Card, send me a Personal Message with your email address and I will send you their documentation for the Driver card.

Glad you got it working.
Stew

Thank you for your reply. As I said I got it working and absolutely love the performance of these amps. One more question. I have kt88 winged c's which by all accounts are decent tubes. My driver board tubes are cheap 12au7's. Do these make a noticable difference in your opinion.

Hi -- different brand 12AU7's can sound very different. I prefer NOS tubes to the modern versions available, so I tried some NOS TungSol, NOS Sylvania and NOS RCA Clear Tops and was pleased with all of them. Mullards are also very good, but usually expensive. If you Google "12au7 tube comparison" or variations of that text string you will see what other people prefer. Then based upon price you can make choices. "Tube Rolling" is fun, but can be expensive. But, it is not unreasonable to try 2 or 3 different brands and see which sounds better to you. The Google comparisons just get you started.
Have fun, Stew