Dumb mistake firing up DR clone - need tips

[jlatrace]

Well here is one for the record. I fired up a new DR clone with a 5Y3 in the first Power Tube socket. Now I have a 60 cycle hum and an unstable bias circuit and am looking for guidance.

I cannot get voltage readings with the power tubes installed because of the bias issue and concerns about blowing tubes. Preamp through vibrato voltages look high, but normal, with the power tubes pulled. Power section voltages look high, but normal except for the pin 5 voltages.

Without the power tubes = B+ 454 vdc but pin 5 voltages only range -12 to -24. Also the pin 5 voltage on the socket that formally had the 5y3 is unstable and varies from -18 to -24 (regardless whether the vibrato circuit connected or not).

I know I have an issue in my bias circuit. Does the hum indicate that I have power filter cap issues too? Anything else I should be aware of running a 5y3 in a 6v6 socket? I am a hobby builder, not a technician or engineer, and I truly appreciate your help.
Thanks,
Les

[xtian]

Yes, focus on bias. Don't even power up, in fact don't even put power tubes in until you can get a steady -40 or -50 vdc on your grids, period.

Pictures?

[JazzGuitarGimp]

Are you saying the amp hums without the output tubes installed? If yes, is the hum coming from the speaker, or is it a mechanical hum being amplified by the chassis?

[jlatrace]

Yes, focus on bias. Don't even power up, in fact don't even put power tubes in until you can get a steady -40 or -50 vdc on your grids, period.

Pictures?

Thanks!

[jlatrace]

Are you saying the amp hums without the output tubes installed? If yes, is the hum coming from the speaker, or is it a mechanical hum being amplified by the chassis?

No the hum is present only when the power tubes are in their sockets. Thanks for the help.

[martin manning]

On the 5Y3, pins 4 and 6 are the anodes and pins 8 and 2 are the directly heated cathode.

On the 6V6 socket, pin 4 is the screen and pin 6 is a tie point for the screen resistor and screen supply. Pin 8 is grounded, and pin 2 is connected to the PT filament winding,

Half of the 6.3V filament voltage was across pins 2 and 8 so the 5Y3 was able to conduct some current. The screen supply was connected directly to one anode at pin 6, and through a 470 ohm resistor to the other anode at pin 4.

So, the screen node of the power supply was (almost) shorted to ground, and the filament circuit was possibly hit with some high voltage.

I would do the following:

With no tubes installed, check all PT secondary AC voltages to ground, and the bias supply DC voltage and range (say -30 to -40V) at pin 5 on both power tube sockets. I think you found bias voltage at one socket to be erratic. Look for a bad solder joint, perhaps, and get that issue resolved.

Install the rectifier and check the DC at pin 8, then switch the standby on and check the voltages at all power supply nodes.

Install the power tubes and set bias.

I don't see how the bias supply would be damaged by having a 5Y3 in one socket since neither pin 5 nor pin 1 (in case you have the grid stoppers tied to pin 1) are used. If this event ocurred on the initial power up, there is no guarantee that there aren't some other errors.

[jlatrace]

Martin,
As always your analysis is extremely helpful. I couldn't figure out how the bias circuit was even in the mix either. Sounds like at least one of my problems are in the bias circuit - but not because of my dumb mistake.
I a really appreciate your help.
Les

[jlatrace]

Solution found, I forgot to make the bias filter cap to ground connection under the bias board.

Everything but the vibrato is working fine. I figured I'd have some troubleshooting on the vibrato since this is my first vibrato amp.

Thanks to everyone for the help.
Les

[martin manning]

Great, almost there then! Seems like there was no real damage from the mishap.

[Randall]

Except for pride perhaps?

[Structo]

This is why we seem to stress the use of a light bulb current limiter.
It will immediately show if there is a bad current demand (short circuit) when you power up a new amp.
It only costs a few bucks to make and the benefit can number in the $100's.
I got turned on to it when I built my Brown Note D'lite amp.

Here is the thread about it and it's use.
For a first time power up I recommend using a 25 watt light bulb which will limit the current to the amp to about .25 amps or 250 ma.
That will limit the current low enough where it most likely won't harm the amp.
http://ampgarage.com/forum/viewtopic.php?t=20341

This is a very worth while build if you don't want to fry your first or 100th build.

[jlatrace]

Thanks Tom.

I highly recommend a current limiter as Tom suggested. I used one for this amp which is probably why I didn't blow up something. After all the mistakes I only fried one of my test 6v6's (an old 90's EH).

An AB763 amp was much harder than I expected. My next project is a SSS 150w and I did this to educate me on reverb and vibrato circuits.

My vibrato - I incorrectly hooked the B+ directly to the plate instead of before, the dropping resistor. I moved the wire to the right place and everything worked perfectly.

Thanks to everyone for the help.