Dumble 50W 3rd gen

[Nikk]

 Hi all, my build is going well I powered up the other day and no smoke however I have a couple of issues.

I have no sound . but when I touch the meter between R21 or R22 and ground I get negative voltage ," good" and and the speaker has sound .
I have another issue which may or may not be related If you cant tell I am no expert but have fixed amps and built a champ so I am not at ground zero. This issue is the capacitor C19 has half the voltage of the one next to it C18 the rest with no tubes power up to around 450V.
I also seem to have low voltage at the filaments about 5.6 volts but I have not used the LED but a  std jewel  bulb tried removing this but no difference and the transformer is capable of 5A and I think i am around 3.2 so there should be no issue there . I know that there is a simple answer but I have reached my level of understanding on this one.
If anyone can help I would be very grateful .

Nikk

[EL34]

Welcome to the Hoffman Amps forum

There are many different boards on this forum for different topics.

The Tube Amp Building - Tweaks - Repairs board is where you can post amp questions.

That board is listed here.
https://el34world.com/Forum/index.php?board=13.0

Thanks,
Doug Hoffman

[EL34]

If anyone can help I would be very grateful .
Nikk

I posted the link above for the correct help board

[drew]

I would suggest going to the Dumble discussion forum on ampgarage.com. There is at least one active thread there on these “AA” PCB board Dumble builds.

[PRR]

That rectifier and that bias network probably do not work together properly. It will take some time to analyze. IMHO it would be more obvious to get a tiny 120V:12V transformer and work it backward from a 6VAC winding to make 60VAC, which easily adapts to the usual "50VAC tap" schemes.

It might have been wise to check for negative 40V or so at the power tube grids before putting >$100 of glass in sockets.

Gross difference C18 C19 says a very crude meter(?), resistor values not as shown, or a very sick cap. BTW, "half" is limited information, "69V on C19 and 141V on C18" might be a useful added clue.

[PRR]

The Idiot Assistant does not find any negative voltage here. The full wave bridge constrains all AC nodes to lay between DC common and DC B+. A simple rectifier has no negative swings to catch. Your 6L6es will melt very quick.

There IS a cap-coupled trick to getting negative out of a FWB but my fingers got tired.

I do see this is very neatly drawn and asks for error reporting. I'm not 100% sure it is wrong, maybe I mis-copied.

Also I do not see how poking a volt-meter at it will make it work.

[Nikk]

That rectifier and that bias network probably do not work together properly. It will take some time to analyze. IMHO it would be more obvious to get a tiny 120V:12V transformer and work it backward from a 6VAC winding to make 60VAC, which easily adapts to the usual "50VAC tap" schemes.

It might have been wise to check for negative 40V or so at the power tube grids before putting >$100 of glass in sockets.

Gross difference C18 C19 says a very crude meter(?), resistor values not as shown, or a very sick cap. BTW, "half" is limited information, "69V on C19 and 141V on C18" might be a useful added clue.

The Rectifier is not as shown on the diagram  I realised this before i built it .

Hi I have -35volts  at the grids I have sacrificial valves at the moment until i get it working but i get the point. C 18 475 volts C19 220 almost half volts. esr of Cap looks OK within 5% and nearly identical to other cap.

What do you mean by crude meter are you talking about my multimeter it is not a top end one but it is digital and seems to give reliable values on mains and other voltages unless this is a term i am unfamiliar with .

Thanks for the response .

Nikk