Need help with cathode bias ground wire that I broke off

Steve Waltz · From: USA

I was chasing a ground issue and decided to replace a few rivets to chassis with bolts and broke a terminal strip which tore a few wires off. I didn't take a picture before doing the work and I can't find a schematic for this odd late 50's sano amp. The damage makes it difficult to see where these three wires went. Could you help me out?

The 200ohm cathode bias resistor goes to the terminal strip that I broke and i think it jumpers to the tab next to it that grounds to chassis. I can't tell if the 47k resistor next to that also grounded to that same lug by a jumper wire. The problem is that tab that broke off has three ground wires on it and I'm not totally sure where they each went.

I think one goes to a ground wire that is coming from the base of the 6sl7 nearby, one goes to the 220 ohm resistor next to it and I think the third goes to the 47k resistor next to the 220ohm.

The 47k has two red wires going to two differnt lugs of the electrolytic mutisection caps. The other side od that resistor has one red wire going back to other lugs of the mutisection caps and the other red wire goes to the phase inverter close by...I think.

Sorry to be vague but I have feeling it might be obvious to others if that 47k should be grounded.

Steve Waltz · From: USA

I forgot to add some basic info if it helps. It has the main amp on the bottom of the cabinet with two EL34 tubes, one rectifier, one phase inverter and one other pentode tube. The other connections are the connections to the separate preamp on top which has three 6sn7 tubes. There is a tremolo on one stereo input and no other effects. 15" Utah speaker.

Thanks for any help,
Steve

Stephen Cowell · From: Round Rock, Texas, USA

Hard to tell from the pics... the broken wires are hidden behind the 'sand' resistor. I'd say that 220ohm block resistor is your final cathode bias... see if it's bypassed with a 25 or so uF electrolytic. If so, one end must be grounded, and the short broken wire connects to it (its neighbor). The longer broken bare wire connects to the (already bolted down!) ground lug with the broken wire. I can't tell if these are all the wires broken or not... I checked every Sano schem on Schematic Heaven (11 of them) and none had EL34's... if you're still having ground problems look to that bolted-down lug. Good luck.
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Steve Waltz · From: USA

Stephen,
Thanks for the help. I'm fairly sure the 220ohm was attached to the arm of the terminal strip next to it which was riveted to the chassis. The odd thing is that a wire from the already existing bolt to chassis was going to the 220ohm also which seems like grounding the same spot twice and to different spots. I'm not sure why they didn't just ground the 220 ohm once to that bolt rather than going to the spot next to it on the terminal strip.

Do you have an opinion if the 47k was grounded? It is part of the power supply. I can take more pictures or trace more if that helps.

Btw... The ungrounded side of the san cast resistor goes to a 20 section of the multisection can and then out to one pin of an el34 and then out to the other el34 so i assume it is the cathode bias as you stated.

Thanks for the help

Steve

Jerry Erickson · From: Atlanta,IL 61723

The cathode for the EL34 would be pin 8. Pins 3 and 4 would be seeing higher voltage from the power supply. The 47K resistor might be used to drop the voltage from a previous power supply section to another and wouldn't be grounded.

Lee Baucum · From: McAllen, Texas (Extreme South) The Final Frontier

I wish I could speak a foreign language like you guys! Laughing

Steve Waltz · From: USA

I repaired it as suggested here and it's running fine. Thanks for the help.

Lesson learned. Don't drill out rivets from the terminal strip side.

Thanks.

Stephen Cowell · From: Round Rock, Texas, USA

Glad you got it going, Steve... I didn't have a clue about the 47K. And the other ground was in order to bypass the rivet, since it doesn't make a proper ground.
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Donny Hinson · From: Glen Burnie, Md. U.S.A.

You could have just added a (temporary, or permanent) jumper wire from the suspect (bad ground) lug to another nearby chassis ground. That would have told you if it really was causing a problem, and it would also have been a lot easier. Winking

Craig Baker · From: Eatonton, Georgia, USA - R.I.P.

Steve,
Back in "the good old days", since it's a steel chassis, I would have used my dad's 100 watt American Beauty iron, scraped the steel clean and soldered the terminal strip to the chassis.

Back then, things worked by brute force. Glad you got your loudspeaker to play again.

Have a wonderful Christmas season,
Craig Baker 706-485-8792

cmbakerelectronics@gmail.com

C.M. Baker Electronics
P.O. Box 3965
Eatonton, GA 31024
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