Amp start up not to good.....

[playonit]

I finished my 5e7 Bandmaster build, turned it on took it off standby and a loud powerful hum comes out of the speaker with the volume off.. Voltages are good and went over the layout with a fine tooth comb and all checks out... I added a impedance selector and wonder if wired wrong if this could be the culprit?? this is my second 5e7 build and the first sounds great... could it be a bad tube?

Thanks

[Structo]

Sure, anything is possible at this point.

Double check your output transformer hookup and phasing at the phase inverter.

If your amp has negative feedback, disconnect that to test the amp.
If the problem goes away when you unhook the feedback then your problem is most likely the phasing or polarity of the OT or PI.

[Colossal]

Voltages are good and went over the layout with a fine tooth comb and all checks out... I added a impedance selector and wonder if wired wrong if this could be the culprit??

Hey playonit, the first thing that came to mind is checking the ground on the speaker jack(s)! In my haste and excitement to finish a build I once mistakenly didn't ground my jacks once (!) and got a loud hum. Whoops!

[playonit]

[quote="Structo"]Sure, anything is possible at this point.

Double check your output transformer hookup and phasing at the phase inverter.

If your amp has negative feedback, disconnect that to test the amp.
If the problem goes away when you unhook the feedback then your problem is most likely the phasing or polarity of the OT or PI.[/quote

Since I am still in the newcomer stage can you determine from the layout if the amp has negative feedback?? Also, wiring out of OT matches the layout??

Thanks Tom.....

[Deric]

Yes you have negative feed back. On the layout, it's the yellow wire from the 8ohm speaker jack to the 56k resistor on the board.

[playonit]

Sure, anything is possible at this point.

Double check your output transformer hookup and phasing at the phase inverter.

If your amp has negative feedback, disconnect that to test the amp.
If the problem goes away when you unhook the feedback then your problem is most likely the phasing or polarity of the OT or PI.

Dis-connected the Negative feedback and the loud hum subsided substantially.. I had the NF hooked to the 8ohm tab on the selector switch is this right or should it go to the jack??

[Structo]

Yes, you want it connected to the 8 ohm tap or wire from the output transformer.

If you have a selector switch, the NFB wire goes to the 8 ohm wire.
In the case of the layout you have, the yellow wire.

Can you take a close up picture of your ohm selector switch so we can tell you if you have it wired correctly?

[playonit]

Yes, you want it connected to the 8 ohm tap or wire from the output transformer.

If you have a selector switch, the NFB wire goes to the 8 ohm wire.
In the case of the layout you have, the yellow wire.

Can you take a close up picture of your ohm selector switch so we can tell you if you have it wired correctly?

Here is the pic.... on my previous 5e7 build the 8ohm tap is yellow from the OT.. on this build the 8ohm tap is green??? at least on the Magnetic Components OT drawing it is?

[Deric]

Looks like you have the NFB wired correctly. If it's causing the problem you need to swap the blue and brown brown wires from the OT.

[Bob-I]

The NFB wire seem to have 2 minor faults. 1) There's too much stripped off the end. The insulation should almost touch the terminal and B) the wire runs very close to the tube socket. Lift it up so there's max clearance between the wire and socket.

Second problem I see is that the speaker jacks are wired backwards. You have the hot from the impedance switch going to the ground on the jacks, ground from OT going to the hot.

Post some pictures of the entire amp. These little minor issues can add up.

[Bob-I]

Looks like you have the NFB wired correctly. If it's causing the problem you need to swap the blue and brown brown wires from the OT.

If it's just a hum, not likely. Reversing the OT leads would cause a SCREAM, not a hum.

[playonit]

The NFB wire seem to have 2 minor faults. 1) There's too much stripped off the end. The insulation should almost touch the terminal and B) the wire runs very close to the tube socket. Lift it up so there's max clearance between the wire and socket.

Second problem I see is that the speaker jacks are wired backwards. You have the hot from the impedance switch going to the ground on the jacks, ground from OT going to the hot.

Post some pictures of the entire amp. These little minor issues can add up.

Here is some more pics.......... Thanks!

[sliberty]

Looks like you have the NFB wired correctly. If it's causing the problem you need to swap the blue and brown brown wires from the OT.

If it's just a hum, not likely. Reversing the OT leads would cause a SCREAM, not a hum.

Especially on Halloween!

[Bob-I]

Nice clean build...

But it looks like you also have the bias cap backwards. The last cap on the end is for the - bias voltage and therefore has to have the positive to ground, not the negative like the rest.

Unfortunately this means the cap may also be blown. Try reversing it but if you still have a hum, it may be from this cap as well as the other issues listed before.

[Structo]

Yes indeedy!

Bob has nailed about all I see.

Those Cliff jacks can screw you up if you are used to the open Switchcraft jacks.

The lugs nearest the chassis wall are the sleeve lugs, those get the black ground wire from the OT.
You also will need to run a ground wire from those lugs to the grounding point you have near the input jack. It can be a small wire, not much current flows there.
I'll bet that is what was humming.

Also eagle eyed Bob has spotted the backwards bias cap.
The diode and bias cap both need to be hooked up "backwards" in order to get a negative bias voltage to the power tube grids.
You are lucky that cap did not explode.

I would replace it no matter what since it surely has been compromised.

If there are no other faults, the amp should work fine after fixing those items.

Sometimes the prettiest wiring schemes are not the quietest.
For example, I would not run the heater wires from the transformer right next to the bias pot.
That is feeding the power tube grids their bias current, so no need to inject any 60 Hz noise right there. I would run those green wires close to the transformer.

Just curious, why did you remove the bell cover of the power transformer?
That also helps shield the amp from stray hum.

Also, watch your color code on your power wiring. You have a green wire going to the fuse holder.
Traditionally, green is for earth ground only in that spot.
You should use black wire for the hot AC wire path and white for the neutral path.

It also appears that you are grounding your buss wire down near the pilot light fixture.
I would terminate the buss wire at the other end, near the input jack since that is a much "quieter" area of the amp.

I would also not run that standby wire to and from the standby switch with the other AC carrying wires.
I had an amp where I had a nasty hum that I finally traced to having that wire too close to the AC wires in the power supply.
Run that wire discretely to the switch and back away from the power wires.

Hope you don't think we are picking on you. Just trying to help you make a quiet amp.

Good luck!