Champ not working, low hum.

[Oddvar]

I am trying to get som advise for this great forum.  I am building a Champ, for fun so to speak, believing it would be easy.
But now everything is hookup up, voltages on the different B's are ok, plate on preamptubes are about 290 v.  Tubes light up, I have changed tubes, but only a tiny, low hum is heard in the speaker. 
I am using the impediance selector from Hoffman for the first time and I am wondering if the OT is hooked yp wrong.  It has a Blue and a Red lead out, the red is wiered to the 16uf cap and the blue one to the third pin on the 6V6 tube. 
Could they be reversed?

Any hints would pe appriciated.

[tubeswell]

What are the DC idle voltages throughout the amp (i.e. at each B+ supply rail filter cap node, and at each plate, screen and cathode)? Take your measurements carefully.

[Oddvar]

I am not familiar with the term idel, but without the pramp and outputtube the three B's are 486, 484, 484.  The plate on the preamp was 484 both.  The plate on the 6v6 was 480, the sreen 480,

[Oddvar]

and when I scratch the OT pin on the 6V6, i can hear sound in the speaker.

[sluckey]

Check your wiring. Especially the input and output jacks.

Click on the link in my signature line at the bottom of this post.

[eleventeen]

"It has a Blue and a Red lead out, the red is wiered to the 16uf cap and the blue one to the third pin on the 6V6 tube.  Could they be reversed?"


This would not matter for this amp. The red is the B+ supply, the blue goes to the 6V6 plate. You have it right. But it wouldn't matter if you had it wrong.


"....without the pramp and outputtube the three B's are 486, 484, 484. "


With no tubes installed, the amp is not doing any work and nothing can draw current, so all these voltages will be "the same" AND they wil be too high. You want to be careful about doing this, because if you have 450 volt caps, theoretically you could blow them with the too-high voltage.


You don't specify whether you are building a Tweed Champ or a blackface...a blackface should have about 50 volts more on the output tube plate.


EITHER type of Champ should have about 180-200 volts on the preamp tube plates.


And we want to see about 1.2 - 2 volts on the preamp tube cathodes. Either type.

[sluckey]

This would not matter for this amp. The red is the B+ supply, the blue goes to the 6V6 plate. You have it right. But it wouldn't matter if you had it wrong.

The champs have a NFB loop from the speaker, so the red and blue wires could be reversed. But it makes no difference at this point because you have bigger issues.

Since this is your first time dealing with the impedance selector there's a good chance that's the problem. I suggest you take it out for now and just connect the 4Ω tap to the speaker jack. You can just leave the NFB wire disconnected at this time also.

[Oddvar]

It's the 5f1  build.

[sluckey]

Have you tried any of the suggestions offered? It would be nice to see your actual schematic, layout, and some high rez pics of the actual amp. Hard to be specific without that info.

[Oddvar]

I have now changed the input jacks.  There is now a very low, audible sound, almost silent.  I wonder if it could be the volume pot, being a on/off switch as well?  There is no reaction when I turn it up.

I will measure all parts tomorrow. (It's midnight here now)

[sluckey]

I wonder if it could be that impedance switch or the output jack?

[tubeswell]

I am not familiar with the term idel, but without the pramp and outputtube the three B's are 486, 484, 484.  The plate on the preamp was 484 both.  The plate on the 6v6 was 480, the sreen 480,

So are you saying that when you put the tubes back in the plate voltages stayed at 484? If so, that would indicate that the tubes aren't conducting. (In which case, check that the cathodes are referenced to the ground return and also that you have heater voltage (VAC) on your heater winding and across the heater pins on eth tube sockets. Also check that the pre-amp tube is wired with pins 4 and 5 tied together going to one side of the heater winding, and pin 9 going to the other side of the heater winding - in order to get the proper 300mA @ 6.3VAC feeding that tube's heater.)


When tubes are conducting, the current through the load for each stage pulls the plate voltage down to the level determined by the bias point. Typically, you would see a plate voltage in a normal (inverting) gain stage sitting at about 2/3 of the B+ supply voltage for that stage.

[sluckey]

plate on preamptubes are about 290 v

[Willabe]

Do you have more than 1 speaker jack?

Are your speaker jacks the shorting kind, with a switch contact for the jack tip?

[Oddvar]

One speaker jack, without the tip...

[tubeswell]

Can you post some hi-res (in-focus) gutshot pics?