5F2-a problem

[brian newman]

I"m building one of Steve's tweed princetons.
I've got it together but I just get a hum for the speakers.
The Voltages are
6V6 pin 3-345V
4 323V
1&8 43VAC

12AX7
pin 1 289V
3 2V
4-5 6.9Vac
6 293V
8 3 V

Can anyone see any problems with these voltages.

[stokes]

To me,the voltages look to be "in the ballpark" and dont point to any reason you get no signal to the speakers.Can you do some signal tracing?Find out where you are losing the signal.You should put a signal generator in the input and then trace the circuit,starting at the input and proceed to the output cap of the first preamp tube and so on, to see where the signal is not passing.

[brian newman]

I checked the circuit three times,everything seems to be in order.
I'm curious if I've got the wrong output and input jacks.I will post a picture of my circuit tomorrow
To trace a signal in a circuit I'm assuming I need a scope?

Brian

[tubeswell]

Have you got the OT primaries the right way round? If all you're getting is a hum you might have positive feedback (instead of negative feedback) so you could try swapping them and see.

[brian newman]

I swapped the OT primaries around but to no avail.
I'm posting these pics to see if they help.

Thanks Brian

[stokes]

I checked the circuit three times,everything seems to be in order.
I'm curious if I've got the wrong output and input jacks.I will post a picture of my circuit tomorrow
To trace a signal in a circuit I'm assuming I need a scope?

Brian

A scope would be best,but you could make a probe using a .1uf 600v cap.Probe with one lead of the cap and connect the other lead to a 1/4" jack plugged into another amp.

[tubeswell]

You could have a ground loop problem the way you have done the ground.

Try changing your grounding scheme by separating the pre-amp grounds from the other grounds in the amp. Ground the pre-amp filter cap (together with all the rest of the preamp grounds) to a terminal post wire going to the input socket ground lug.

Then have the other filter cap grounds and the output tube cathode ground going to one of the PT bolts along with the PT ground and the AC mains ground etc.

You could also be getting unwanted coupling from all those wires everywhere.

Shorten some of your leads (e.g. coupling cap to vol pot could be shorter)

Cover up the big hole in the chassis where the OT tranny wires are coming through. (use a rubber grommet through a 10mm dia hole to put the OT tranny wires through.

Have the speaker output socket wires going through a separate hole near the speaker out.

Is your HT wired correctly off the rectifier? Not sure about it the way it appears in your photo.

[stokes]

"You could have a ground loop problem the way you have done the ground."This wouldnt keep the signal from getting to the speaker,it would just have a lot of hum along with the signal.The way I read his first post he is getting only hum,and no signal to the speaker.Is that the case,Brian?If so you gotta find where you are losing the signal.

[brian newman]

Thanks Guys
I will neaten up the wires and change the ground loops when I Figure out where the signal disappears.
Thanks for the idea Stokes as I don't have a scope.

Brian

[tubeswell]

Taking a closer look at your pics now I've got more time - Its hard to tell from teh angle of the photos but those two x 100 Ohms resistors grounding the heater to the cathode pin node of the 6V6GT aren't touching the ground/Chassis are they? (That would ground the cathode on the wrong side of the resistor, or at least possibly ground the cathode through 50 Ohms and 470 Ohms in parallel, which would be next to nothing)

[brian newman]

Talk about an exercise in futility.After another hour of searching for the problem I dismounted the input jack only to find a small scrap of plastic stuck to the positive tabThese jacks came from a local store and where individually
packaged,you know the type that have a paper back and shrink wrapped plastic on the front.
After removing the plastic the signal came through,still has a hum to it but I'll
work on the grounding today to see if I can reduce it.


Thanks again
Brian