Dumble - help needed urgently!

[amplifiednation]

Oh man this looks like it's turning into a fun thread. It looks like you are getting there with the amp though! Good luck!!

[martin manning]

I would not use anything smaller than 22 AWG, which has a conductor diameter of 0.64mm. What you have listed is too small, and the temperature rating is too low. The insulation technically would not need to be rated for high voltage, but if the plate circuit arcs over into the filament circuit then there could be some heavy damage. Best to use 600V 105 degrees C wire.

Yes the left relay in the layout drawing is the PAB, the center one is the OD, and the right one is the Mid Boost. The switch wiring looks correct. PAB lifts the mid control ground, thereby disabling the tone stack and giving a volume boost.

Once all of the wiring is completed per the layout, then it's time to put the power tubes in and bias them. Are you familiar with the procedure for doing that?

[drew]

Heater wiring tutorial. (The links to the larger photos are dead.)

What gauge (AWG) is the wire you're talking about?

[Mousey Dung]

When all the other wiring is done, do the heater wiring from V8 to V1. You will want to twist the wires and elevate them above the centerline of the tube sockets. Use two different colors of wire and connect the same color to the same power tube pins and preamp tube pins. For example all pins 7 on the power tubes and all pins 9 on the preamp tubes are connected with the same color, meaning they are all on the same side of the filament winding. Like this: https://tubeamparchive.com/viewtopic.ph ... 903#318903

I think I may have spoken to quickly when I said I understand. I only half-understand.
I know that I should string along pins 2 & 7 of the power tubes (while keeping the same colored wires to the same pins)... when I get to the preamp tubes, should I string the 6.3 VDC power to the filament pins, 4 and 9???

[Mousey Dung]

Once all of the wiring is completed per the layout, then it's time to put the power tubes in and bias them. Are you familiar with the procedure for doing that?

Nope!

I'm guessing I should use the back panel test points and have speaker plugged in so the power has somewhere to go?

[martin manning]

12AX7's can use either 6V or 12V heater power. To wire them for 6V you connect one lead to pins 4 and 5 (tie them together) and the other lead goes to pin 9.

[Mousey Dung]

12AX7's can use either 6V or 12V heater power. To wire them for 6V you connect one lead to pins 4 and 5 (tie them together) and the other lead goes to pin 9.

Okay, I did put in the jumpers between pins 4 & 5 of the 12AX7 sockets early on. Would've been hard to do it now with all the wires, boards and components in the way. :-p

[Mousey Dung]

I would not use anything smaller than 22 AWG, which has a conductor diameter of 0.64mm. What you have listed is too small, and the temperature rating is too low. The insulation technically would not need to be rated for high voltage, but if the plate circuit arcs over into the filament circuit then there could be some heavy damage. Best to use 600V 105 degrees C wire.

I may have used some of the wire I mentioned on the preamp board or controls. Do I need to replace them with higher-spec wire?

[martin manning]

I may have used some of the wire I mentioned on the preamp board or controls. Do I need to replace them with higher-spec wire?

It's well away from the heat of the tube sockets, and as long as you didn't use it for power supply connections or plate leads it will probably be ok for now, but I would replace it.

[martin manning]

When you're ready to install the tubes and bias them here's what to do:

Put all of the tubes in and connect a speaker to the speaker output jack. Start with the bias trimmers positioned for maximum negative voltage.

Connect a multimeter set to measure millivolts to the test jacks on the back panel. Connect the negative lead to the black (center) test jack and the positive lead to one of the red jacks. It's best if you have some test leads with banana plugs on both ends.

Determine a target reading for the meter: Estimate the loaded plate voltage at say 95% of the unloaded value, 0.95 * 440 = 420. 6V6GT are rated at 12W max plate dissipation, and we'll choose a bias point at 70% of that, or 0.7*12W/420V = 0.020 A. This is a conservative value since you will be measuring cathode current which includes the screen current. The target 0.020 A will be indicated as 0.020V (20 mV) on the meter since you are measuring the voltage drop across a 1-ohm resistor.

Turn the power and standby on and rotate the bias trim pot on the side where the red meter lead is connected until the meter reads 20 mV. Move the red lead to the other red jack on the same side and check the voltage there. If it is higher than 20 mV, use the trimmer to reduce it to 20 mV. Move the red lead back to the first red jack and note the voltage reading there. Add the two voltages together and make a note of the total.

Move the red lead to one of the red jacks on the opposite side and repeat the sequence above using the other bias trimmer. If the total voltage is higher or lower than the total recorded for the opposite side, reduce voltage on whichever side has the higher value until they match within a few millivolts.

If at any time a loud howl is herd from the speaker, it is likely that the output transformer primary leads will have to be reversed.

Allow the amp to idle while you re-measure the power tube plate voltage. If it is much different from the assumed value, recalculate the target idle current and reset the bias accordingly.

[Mousey Dung]

Thank you, thank you, thank you so much. I think can see the finish line, now. This is the kind of crucial information that should have been provided with the kit to start with. I had so many unresolved critical issues that I might have done some serious irreversible damage or killed myself, if I hadn't come here to ask for help.

It has been extremely frustrating to deal with virtually no instructions or documentation, and a gradually disappearing seller/tech support. I may have to revise my positive opinion of him. He just got less and less interested in providing any detailed support, like I used up some kind of quota of his time. He broke his promises about getting back to me with info and about his availability, and ignored most of my comments and questions.

[Mousey Dung]

When I'm done with the biasing, I suppose the next step is to mount the reverb, put together the footswitch, mount the chassis...?

[martin manning]

You're welcome. This was more fun than doing the Sunday crossword.

When you get it biased, measure the voltages again, the HV supply nodes and all of the tube pins. This time all of the tubes will be conducting current at their idle operating points, and you'll be looking for the final power tube operating voltages and reasonable operating voltages on the preamp tubes. If those look good, the only thing left is testing the signal path.

Before you box it up you can connect the reverb unit, and put some signal into the input to see if it will make some noise. You'll want to test all of controls, the relay and switch functions, and there could very well be some tweaking left to do.

[norburybrook]

Martin I'm never ceased to be amazed at your constant generosity.

As someone who has been on the receiving end it's very much appreciated.

That goes for all the very helpful people at TAG.

Marcus

[Mousey Dung]

12AX7's can use either 6V or 12V heater power. To wire them for 6V you connect one lead to pins 4 and 5 (tie them together) and the other lead goes to pin 9.

I powered up without any tubes in and tested for AC voltage at all the tube-socket pins that I daisy-chained together (pins 2 & 7 of power-tube sockets and pins 4/5 & 9 of the preamp-tube sockets). It shows 3.2 V on all of them. Am I doing okay?