5E3 clone voltage reality check

[Randall]

I finished my first 5E3 today. Dim bulb tested and all seems good. O-scope tested the pre section without output tubes, looks good, and quiet, too. Put in the JJ 6V6's and went to work measuring. Here's what I got:

PT secondary 335 VAC to grd each side
B+ first cap 320v, 2nd cap 215v, third cap 156v
6V6 plates 295v
cathode 36.8 v

By my calculations, and please correct me if I'm wrong, I am idling at 19 watts. This how I got to this conclusion:

36.8v/250 ohms = 73.6 milliamps per tube, seems high?
plate voltage - cathode voltage = dissipation voltage 258v
73.6 mA x 258 v = 19 watts

So..... am I doing this right, what do think of my voltages, and why am I drawing so much at the cathode? And is 19 watts too high? Thanks for your help.

[martin manning]

Your calcs seem correct. Nominal 100% for 6V6's is 14W (plate + screen), but JJ 6V6's are pretty tough. Look for red-plating, and maybe consider increasing the cathode resistor value.

[Randall]

No red plating. Does the plate voltage and B+ seem low considering it is a Mercury PT that I expected would be high.

[martin manning]

You are right, with 335VAC at the rectifier anodes I would think you should be seeing closer to 400V at the first filter. Recheck the wiring, but maybe lowering the idle current will bring the voltage back up?

[Randall]

Man, I have been OCD meticulous in wiring this. I will measure the 250 cathode R and get back.

[Randall]

What and how does the 25/25 bypass cap do what it does?"

[Structo]

Pretty much allows the full audio spectrum to pass through.

[Randall]

Got it. I missed running the under board ground wire from the 220K resistors. Looks good to me now, seems like it's running a bit hot like an original would with today's 120 vac. I'm getting 382v at first filter cap, 371v at plates, and 21v cathode. Puts it at 15 watts I believe. I'm going to run it like this for a time and then later experiment with dropping the v's to get it running like it would have in the 1950's. I'm running JJ 6V6s at the moment, which I understand can handle the higher specs.

One thing that still makes me wonder is, how is it I can have a slight hum at the speaker with the standby switched off? Goes away when standby is switched on. How can the OT/tubes make hum while in switched off mode? Weird.

[rock_mumbles]

Got it. I missed running the under board ground wire from the 220K resistors. Looks good to me now, seems like it's running a bit hot like an original would with today's 120 vac. I'm getting 382v at first filter cap, 371v at plates, and 21v cathode. Puts it at 15 watts I believe. I'm going to run it like this for a time and then later experiment with dropping the v's to get it running like it would have in the 1950's. I'm running JJ 6V6s at the moment, which I understand can handle the higher specs.

One thing that still makes me wonder is, how is it I can have a slight hum at the speaker with the standby switched off? Goes away when standby is switched on. How can the OT/tubes make hum while in switched off mode? Weird.

Glad you got things worked out ...

The hum in standby mode is probably the OT picking up magnetic field noise from the PT ...

Your 371V on the plate is measured to ground correct? Once you subtract the cathode voltage that puts your actual plate voltage at 350V right in the good tone target zone ...

Even though jj 6V6's can handle high voltages a it's still a 14 w dissipation tube ... so keep an eye on them and look for red plating.
I wouldn't lower voltages I would swap the 250 ohm cathode resistor for a 270 ohm then your dissipation should be OK.

[Randall]

.

The hum in standby mode is probably the OT picking up magnetic field noise from the PT ...

Can anything be done about this, and should I be concerned?

And yes, my plate to cathode is 351v.

[Randall]

Figured out the hum and noise when I switched standby off. I had the switch breaking the B+ to the board, but still had the OT on the 5Y3, so it was still energized. Moved the OT CT to the secondary of the standby switch, and there it is! And quieter than I expected, too.

[vibratoking]

Congrats! How does it sound? I love that amp.