5E3 Noise

[Direwolf]

Have a new 5E3 build from Tino Zottola's 2nd book completed. The amp works and sounds fair however, it has a good amount of hiss to it. I checked all the mechanical connections and they're all good. Pulled the preamp 12AX7 and it went away. Swapped tubes with known good ones and no change. When I checked the voltages, pins 1 & 6 of the preamp and phase inverter read very high (Read pin 1 215/183 should be 175/135, pin 6 173/170 should be 140/135). Both are 12AX7's. Pin's 3 and 8 read low, with pin 3 having 45.7/1.3 should be 55/1.5. Pin 8 having 1.29/1.4 should be 1.5/1.5.

This seems to all be supplied by the B+++ voltage. Any thoughts on how to bring these voltages back into tolerance? Could I just change the resistor value in the filter or does it seem to be something more involved? Any help would be greatly appreciated!

Thanks,

[stokes]

Addressing the "Read pin 1 215/183 should be 175/135".Are you sure the volts you say should be (175/135) are not for a 12AY7?If you put a 12AX7 in a circuit for a 12AY7 the AX7 is going to have more volts on the plates than the AY7.I am not familiar with the Tino Zottola version,but the stock 5E3 uses a 12AY7,and putting a 12AX7 in its place will show the higher volts on the plate.The differences you see in the cathodes,except the 45/55 are negligable,1.4 or 1.5 or even the 1.3 are normal you will see from tube to tube.The hiss you describe is most likely being introduced in the first preamp section.Could be a number of causes.Where do you have the heater CT or 2-100ohm resistors on either side of the heater connected?To make these more effective at cancelling hum it is a good idea to connect them to your power tube cathodes instead of ground.This puts them a couple of volts above ground and virtually eliminates the 60hz hum from the heaters.If that doesnt quiet it,check the plate resistors,faulty plate resistors are also a source of hiss.

[Direwolf]

Thanks for the reply, Stokes.

The build calls for 2 12AX7's - the preamp and phase inverter. He says the voltages should be within 10%. The center tap for the heater goes to ground. Everything seems to be connected properly. I'll have to check the plate resistors.

Thanks,

[stokes]

Try putting the heater CT on the power tube cathode,make sure it is the heater CT not the high voltage CT.This might quiet the hum.As for the voltages,if the difference between what you see and what the schem says should be there is an issue you can change the power resistors in the B+ rail can be increased to bring the voltages to your plates down.If your power tube plates and screens are okay,adjust the resistor after them.

[Direwolf]

I found the cause of the hum-a crappy star ground. After rectifying this, the noise is much better but I still have buzzing. I would imagine this is from AC ripple which repairing the ground helped but didn't alleviate. I'll poke around more and see what I can find. I don't reckon the voltages are the cause of the problem, but if all else fails...

[stokes]

Try the CT on the cathode,it works.

[Direwolf]

I put the 6v3 CT on the cathode and it helps but the hum is still there. I think I'm going to get a bigger chassis and put it in there. The one I'm using is too small and the board orientation makes it hard to to wire without crossing too many wires.