Hi,

Good to hear from you again.

Current = 26.7/680 ~ 0.039ma
Power = (360-26.7) * 0.039 ~ 13 watt

You are right where you should be. You can even lower the value of the cathode resistor (at around 600 ohm), but you shouldn't bother with that. If the amp sounds good, than this is it. Congratulations!

W.S.

Hi,

Well, your voltage readings are very close to the fender schematic. It seems that your power tube is still working well beyond its specs, however. Let's do some math:

Current = 22.9/470= 0.048A
Power = (345-22.9)*0.048 = 15.4 watt

I would say it would be for the best to target 14 watt. A JJ 6V6 would stand 15.4 watt, I don't know about a Tung Sol reissue. As the plate voltage is already low, I think it would be best to increase the value of the cathode resistor. Not much. Try something between 500 and 600 ohm and redo the math. Anyway, I think your voltages look quite good. It's not mandatory to do any changes. It's just that the power tube will wear out at a faster rate. Good luck and keep us posted!

W.S.

Hi,

Yes, that's how you do it. Measure the voltage drop across the cathode resistor (with your meter set on VDC), then divide the result with the value of the cathode resistor (according to Ohm's law).

You said you have 42ma and 390V plate voltage. Well, this means over 16 watt. That's a little too high for a 6V6. The conservative rating of a 6V6 is 12 watt, but most of them do quite well at 14 watt, even higher. 16 watt, however, is too much. Things are complicated because of the voltage drop of the rectifier tube. If you replace the cathode resistor (with one of a higher value), the plate voltage will also go up a little bit. Personally, I would try to lower the plate voltage and leave the cathode resistor at the stock value. There are several ways to do that. First, if you have a Sovtek 5Y3 rectifier, try to put a NOS rectifier tube, as the Sovteks are not real 5y3s and their voltage drop is smaller. If this doesn't do it, you can always insert a dropping resistor before the first filter cap (5 watt/a few hundred ohms). The old Fender schematic shows a plate voltage of less than 350, but the real champs had well over 350V. Try to settle at 370.

Please tell us how this is going.

W.S.

Hi,

What's the plate voltage?

Hi Corey,

The photos may prove very useful. As for voltage readings, here is what you should do. First, I cannot emphasize enough the dangers of working in a tube amp that is turned on. Please take care and read the available materials on the internet. Preferably, you should use a probe with an alligator clip attached to the chassis and you should handle the other probe with only one hand. With you digital multimeter set on highest DC voltage, measure the following: the voltage from the first filter cap to ground, second filter cap to ground, third filter cap to ground, then from pin 3 of the power tube to ground (that's the plate voltage), pin 8 to ground (this is cathode voltage, which is quite low, you should set your mm to to the nearest point higher than 20VDC). Next go to the preamp tube (V1). Measure the DC from pin 1 to ground (high voltage), pin 3 to ground (very low voltage), pin 6 to ground (high voltage). Same for V2.

Please take care.

WS

Hi Corey,

While I have no idea what exactly causes your squealing problem, could you post some voltage readings? Does you amp squeal without your instrument plugged? Could you take a few photos of your build and post them in here?

WS

Hi,

I'm sorry for the delay I write back, I didn't check this thread in a long time. The mod is very simple. I added a filter stage before all the others. In other words, the DC from the rectifier tube goes to 22uf cap, then through the 100 ohm 5watt cement resistor and then to the rest of the circuit, like in the layout. It's very easy. Please let me know if you have any questions.

WS

Hi everyone,

This is my short story. I have just finished a vibro champ clone that I built according to Nik's layout. I sourced all the parts by myself (and I'm forever in doubt to Nik, who was so kind to put his fabulous layouts on him website). The amp sounds great. In addition to Nik's layout, I added a standby switch and another filtering stage (22uF with 100ohm resistor) to further reduce the hum. What I would really like to see is a voltage chart from the people who also built this little amp. It would be nice to have all these data so we can compare our results and troubleshoot our amps.

W.S.