Interesting fault on a Rock Ola amp - UK Vintage Radio Repair and Restoration Discussion Forum

vinrads · 19th Jul 2020, 4:13 pm

These amp's use 6973's output valves , the reported fault with this one was int cracking sound and a flash in one of the output valves ,whilst checking the connections around the valve I noticed that the G1 which is pin6 had the coupling capacitor going to it but no - 25 volts they had used pin 3 which is internally connected in the valve to pin 6, to supply the bias volts ,the trouble was pin 6 to pin 3 was reading high a few hundred ohms and varying , we have a duff valve , so re connected the bias resistor to pin 6 as it should be ,all was ok the other channel was ok all connections to pin 6 , Mick.

vidjoman · 19th Jul 2020, 6:36 pm

Could just be dirty or loose holder contact. Did you tighten the contacts, they are invariably loose. Most of the jukebox amps have the bias on one pin and the signal on the other, probably makes it easier to wire.

vinrads · 19th Jul 2020, 6:46 pm

I thought that ! but the readings were taken on the pins of the valve , having the sig on one pin and the bias on the other is ok as long as the bias is on pin6 , which in this case it was on pin 3 . Mick.

19th Jul 2020, 4:13 pm	#1
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,762	Interesting fault on a Rock Ola amp These amp's use 6973's output valves , the reported fault with this one was int cracking sound and a flash in one of the output valves ,whilst checking the connections around the valve I noticed that the G1 which is pin6 had the coupling capacitor going to it but no - 25 volts they had used pin 3 which is internally connected in the valve to pin 6, to supply the bias volts ,the trouble was pin 6 to pin 3 was reading high a few hundred ohms and varying , we have a duff valve , so re connected the bias resistor to pin 6 as it should be ,all was ok the other channel was ok all connections to pin 6 , Mick.

19th Jul 2020, 6:36 pm	#2
vidjoman Dekatron Join Date: Jun 2013 Location: East Sussex, UK. Posts: 3,326	Re: Interesting fault on a Rock Ola amp Could just be dirty or loose holder contact. Did you tighten the contacts, they are invariably loose. Most of the jukebox amps have the bias on one pin and the signal on the other, probably makes it easier to wire.

19th Jul 2020, 6:46 pm	#3
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,762	Re: Interesting fault on a Rock Ola amp I thought that ! but the readings were taken on the pins of the valve , having the sig on one pin and the bias on the other is ok as long as the bias is on pin6 , which in this case it was on pin 3 . Mick.