Vibro Champ from parts... B+ voltage low, 155v

[jescher356]

I have a vibro champ that I've assembled from parts. I just finished it and thought it sounded good, until I compared it to another vibro champ. I realized then that it had less volume and the distortion wasn't as nice.

So I checked voltage and found the B+ voltage is 155v instead of the 345v on the schematic.

Note: the transformer was pulled from a modded champ and not original, but seems to deliver the same voltage (350vac) to the rectifier tube.

After some reading, it seemed as the the bias might be off. So I increased the 6V6's cathode resistors value from 470 ohms to 1k ohms. The B+voltage went to 225v and the rest of the supply voltages that were low, got better. However the sound didn't improve.

Can anyone offer any suggestions as to what the problem might be?

[Chuck H]

I don't think a 1k cathode resistor is the answer. It's probably not just a bias adjustment that's needed. Whatever is causing the bias to be off needs to be corrected. At that point I think the original 470 ohm cathode resistor should be better.

Check the 6v6 grid circuit to be sure it's wired correctly and that the coupling cap isn't leaky. Also check the 6v6 bypass cap to be sure it's not shorted. Also check the 6v6 grid resistor value to see if it's too high or not grounded properly. Of course check all grounds.

The problem could also be from the trem circuit (if you have one, you did say Vibro Champ). Make sure everything is wired correctly. Perhaps disconnect the trem while troubleshooting. If this stops the problem then you will already know where to start looking.

Chuck

[Gunny]

I wonder if you've got the correct resistor value between the filter capacitors. If the transformer output is correct, the rectified DC should be in the right range.
I'd recommend measuring the B+ at the various filter caps - with the preamp and power amp tubes removed. If the voltage drops once a tube is installed, then we have a clue where any other problem may be hiding.

[Steve Conner]

Also the rectifier tube might be worn out and need replacing.

[Chuck H]

I wonder if you've got the correct resistor value between the filter capacitors. If the transformer output is correct, the rectified DC should be in the right range.
I'd recommend measuring the B+ at the various filter caps - with the preamp and power amp tubes removed. If the voltage drops once a tube is installed, then we have a clue where any other problem may be hiding.

I agree with checking the B+ series resistance, but...

There is no B+ series resistance behind the power tube plate. Which is also low. So I don't think this is the main problem.

Chuck

[jescher356]

OK,
I checked the grid circuit. It seems to be correct. Checked the Coupling and bypass caps and (they are both new) no shorts. The resistor values are correct and I checked the grounds and all is OK.

I disconnected the Trem circuit and the voltages remain low.

The resistors at B+ all check out correct.



I changed the rectifier tube with a known good one and the resulting voltage was the same.

I have checked voltage with different tubes out and the supply voltage is low all the way down the supply line until I remove the 6v6... then the voltage goes up to about 379v DC at the rectifier tube and All other supply voltages measure a bit high.

The amp functions correctly, with the exception of the low voltages, diminished volume compare to another Vibro Champ, Distortion a bit too soon 3 !/2 on the volumes knob and a bit farty bottom end.

So I think it must be in the power tube circuit somewhere... Any other ideas?

[Jazz P Bass]

Champ Clone

Try disconnecting the 6V6 screen connection.
See if the B+ pops up.
Something there is really loading it down.
Next disconnect the OPT.
How many different 6V6 tubes did you try?

[jescher356]

Try disconnecting the 6V6 screen connection.
See if the B+ pops up.
Something there is really loading it down.
Next disconnect the OPT.
How many different 6V6 tubes did you try?


Disconnected the 6V6 screen connection... Same low voltage

Next disconnect the OPT (from the power tube)...Same low voltage

Disconnected Cathode and obviously the voltage stayed at 350v DC Replaced both components, checked the board for any oddities (none)

Have tried 2 6v6's (both work in another champ at proper voltage) and 1 6L6

Thoughts?

[jescher356]

all set... I tried a third rectifier tube and the voltages are right where they should be.

Thanks all for your help!

[Chuck H]

It's not an impossible coincidence, but it seems unlikely that you had two bad rectifier tubes in a row. Unless you salvaged them from some discard bin or something. You should probably clean and retention the rectifier tube socket. Also inspect (with a meter) the leads and solder joints for open circuits and shorts. Then try the other two rectifier tubes again before you throw them out.

Chuck

[jescher356]

Hi Chuck,
They were older rectifiers salvaged from old amps. I have always read they either work or they don't... so I didn't think that the performance would degrade.

However I will check the other items you mentioned.

Thanks!