Vibro Champ build advice for a first timer?

[BartVV]

the ground connection of the resistor was disconnected. Now the measuremnt v3 pin 5 is 564k and not 220k
and indeed the resistor is 564k but should be 220k according schematics
on another scheme there is a 1.5k gridstopper resistor which is not present on the board

[Stevem]

For the1.5k grid stopper to be effective it needs to be right on the tube socket.

[sluckey]

the ground connection of the resistor was disconnected. Now the measuremnt v3 pin 5 is 564k and not 220k
and indeed the resistor is 564k but should be 220k according schematics
on another scheme there is a 1.5k gridstopper resistor which is not present on the board

564K will work but I would replace with 220K. BTW, 564K is a very odd value. Could you post a pic of that 564K resistor?

I've never seen a grid stopper on any original Fender Vibro Champ schematics but adding a 1.5K stopper is a good idea and easy to do. Just install the resistor directly on the tube socket between pins 1 and 5. Now move the wire from pin 5 to pin 1.

[BartVV]

Afbeelding van WhatsApp op 2025-01-28 om 10.46.14_6d901828.jpg

Afbeelding van WhatsApp op 2025-01-28 om 10.46.09_d95f4d4b.jpg

[sluckey]

OK. The resistor is really a standard 560K 5% (green-blue-yellow-gold). Your meter measured 564K and that is fine. You should still replace with a 220K 1/2W.

[BartVV]

thank you
the 1.5 k is also 1/2w?

[sluckey]

yes.

I also noticed another incorrect resistor value in your pic. That 330K in the lower right quadrant of your pic should be 100K. I suggest comparing your board with a Fender layout and replacing any other components that are wrong. Post some high rez pics of the entire board and we can help you get it right.

[BartVV]

thanks
we didnt check any resistor value (except cathode bias) as we presumed it was correct but apparently not , our bad.
i have schematics and will go through them all
i will place a pic when done , for double check

[BartVV]

hi again
checked all resistor values and changed 3
2x 100k and 1x the 220k
fixed also one loose ground connection
kept the 560ohm cathode resistor
now i got other values
cathode - plate voltage is now 408v
and 25.5v over the cathode resistor
much better but still power dissipation about 120%
guess need to put higher resistor for cathode

[B Ingram]

kept the 560ohm cathode resistor
now i got other values
cathode - plate voltage is now 408v
and 25.5v over the cathode resistor
much better but still power dissipation about 120%
guess need to put higher resistor for cathode

You can do that if you want.

I prefer to leave the cathode resistor alone & instead change the 1kΩ resistor in the power supply (between the filter caps feeding the output transformer & the 6V6 screen) to 18-22kΩ (2-3 watt is likely ideal).

- The higher resistance here lowers screen voltage, which lowers the plate current.