New 5f6a build, need help with voltages

[Mitch Roberts]

Hey guys, I just finished a Tweed Bassman build and just started diagnostics before loading all tubes in. I have the rectifier in, but no other tubes. Heaters look good, bias voltage good, and the power tube sockets on pin 3 look good at about 480v. What I am confused by is how high the voltages are after the dropping resisters, i.e there is very little voltage drop, like around 10 volts (I can't remember exactly, but I was expecting more like -100v drop by the first preamp tube socket). It made me think there was bad wiring shorting the last stage of the B+, but I drained the caps and did resistance checks in the B+ string and I am getting the correct resistance drop: about 98 ohms for the choke, 4.7K for the next stage, and 10K for the final stage. Do I move on to put the power tubes in for readings, or does it sound like something is not right. I know voltages are not supposed to be as low w/o tubes, but that little of a drop?? Any help would be greatly appreciated.

Thanks,

Mitch

[Bob-I]

With no tubes in you won't have any current flow, therefore no voltage drop. It all looks good, but you haven't listed the voltage on the power tube grids, should be a -40 or so.

[Mitch Roberts]

Thanks Bob. The bias is reading -48v for now. Any better number to set it before putting the power tubes in?

Mitch

[Cliff Schecht]

Set it as low as possible and move up from there. It's always better to start too low than too high. Or if you know the voltage you are going for, you can set the bias there to start but make sure to keep an eye on bias current. The bias voltage shouldn't shift much after you actually install the tubes, unless the power transformer gets heavily loaded down (i.e. it's underrated).

[Jana]

to clarify: initially set the bias voltage for greatest negative reading to give the lowest current flow through the tubes. -50 volts is "cooler" than -40 volts.

[Bob-I]

Thanks Bob. The bias is reading -48v for now. Any better number to set it before putting the power tubes in?

Mitch

You're good to go.

[Mitch Roberts]

Update: Everything was good. No major issues. Forgot to connect the presence circuit jumper wire to the speaker out (Doh!) but that was a quick fix. Great sounding amp.

MItch

[Mars Hall]

Hi Mitch, How's the Hammond PT working out for you? What plate voltage did you end up with?

[Bob-I]

Update: Everything was good. No major issues. Forgot to connect the presence circuit jumper wire to the speaker out (Doh!) but that was a quick fix. Great sounding amp.

MItch

Glad to hear it. I love this circuit. I have one that I built using EL-34's and added reverb in a 1x12 combo cab. Amazing little amp.

[Mitch Roberts]

Been tweaking it a bit. The main thing I changed was the bass cap, from a .02 to a .045. I had a nice Blue Molded in that value, too. it brought up the bass just enough for the lower volume levels. I would back it off, I'm sure, if and when I crank it. Mars, that Hammond power tranny is one impressive hunk of iron. I ended up with 450v on the plates with a pair of oldie but gooding GE 5881s biased at 35mv. I played around with the presence circuit, trying the later version with the 4.7K resistor to ground. I changed it back to the simple .1 cap off the middle lug to ground, and the 3rd lug grounded. Works very effectively. No significant scratchiness. I have no cab, so I'm running it through my early BF Concert speakers - same Oxford 10'sas on the Brown amps. SOunds amazing. If I could find my camera I'll post some pics.

Mitch