No Sound on just the NORMAL channel of a 1971 Super Reverb

[64deluxe]

I am working on a 1972 Super Reverb that has previously been "blackfaced". I solved some issues with the VIBRATO channel and it sounds perfect and the reverb and vibrato now both work, but the NORMAL channel makes no sound.

I have double-checked component values and wiring and everything "seems" to be correct. I have swapped tubes in and out with no luck. If I touch the meter probe to the middle lug of the volume pot I get a strong hum like I do with the other channel but beyond that there is no sound, not even static.

Also, I am seeing almost 400 volts on pin 1 V1.

Any suggestions as to how to proceed with troubleshooting would be much appreciated.

[AmpRX]

What are the other voltages on V1 and V2?

[64deluxe]

V1
1- 395
2- 0
3- 0 ??
4/5 6.53 a.c.
6- 281
7- 0
8- 2.0
9- 6.53 a.c.

V2
1- 260
2- 0
3- 1.93
4/5 6.53 a.c.
6- 270
7- 0
8- 2.0
9- 6.53 a.c.


I carefully cleaned and re-tensioned the sockets first.

Zero volts on V1 pin 3 kinda jumps out. The 25uf cap tied to that pin was installed by someone before me but it is a new Sprague Atom and the 1.5k resistor it's across reads right. Could that cap be the problem?

[AmpRX]

Not the cap. You would have voltage with just the resistor. Make sure the resistor is grounded well.

[pdf64]

Another possibility is that the tube socket isn't making adequate contact with the pins, or there's a dry joint somewhere in that circuit / tube section, maybe even the heater.
Try
Billm Audio » Re-tension, resolder your tube sockets
and reflowing the joints, check continuity, close inspection etc
Pete.

[MWJB]

Cathode at V1, pin 3 appears to be open. Pull V1 and check ohms from pin 3 to ground (not just accross the resistor), if it's open then you have lost the ground connection (check where the ground wire meets the brass plate on the control panel), or there is a break in the wire from socket pin to resistor & cap (don't assume the cap is good just because it is new, cathode bypass caps are cheap, if in doubt replace it, likewise the 1500 resistor may have a dodgy connection at one leg?), as Pete says, check socket tension.

[Bruce / Mission Amps]

Hmmm...if you lost the cathode ground connection, I think the cathode voltage would be very high, not zero.
If there is no voltage drop across the 100K plate load resistor, then the triode is not drawing any current so I'd suspect the tube first.
Especially it's filament section or the connection at the socket.
And check solder joints at lugs 1 or 4-5, and of course as previously mentioned, the socket itself.

Oh.... you can't have 6.53vac on lug 9 and 4+5.... you can have that voltage across those lug sets, but with respect to ground and either of those filament connections, it would be approximately 3.25vac.

[vavlehead]

check to make sure the wire the leads out of the channel to the the 2 - 220K resistors before the PI cap is attached. that is popular mod to remove it.

[64deluxe]

Many thanks, Bruce. You nailed it. I can hear all the hard work I did on this amp now and it sounds great!

I had read the AC across the heaters and never checked the individual heaters. I saw a tube lighting up and it was warm, but I never bothered to look for both filaments lighting up.

I had rewired the heaters to make them match from socket to socket and when I resoldered the wire to pins 4+5 hadn't noticed that the solder didn't flow across so only pin 5 was actually soldered. Had to look very close to see it.

So I will know in future to always measure heater pins to ground and not across and make sure the solder gets to both pins 4 and 5.