Greetings: Yesterday I drove 30 miles north to visit my tech friend in Holland with my two most recent amp projects. These are major rebuilds I have been working on for some time and have mentioned in previous postings. I thank Hoffman Amp Forum members for their input and help. Dan was impressed with my work but as usual he connected the Sovtek first to an isolation transformer and a Variac to bring it up slowly. There was an immediate problem. He took a look at the two new filter caps first and found one of them shorted to ground due to my "custom made" cap clamp that pierced the plastic cover on the can. He refitted it using a standard clamp and that was solved. All of the new controls functioned properly as he increased power and we began to see the output on the scope. Only 20 watts at clipping. Everything looked good so he removed and tested the two 6L6 tubes. Both nearly shot. I had another matched pair so we swapped them in. Success! More than 50 watts, everything worked properly other than my new on/off switch being upside down. That was easy. It ran for a half hour with music while we got the Princeton Reverb ready to test. That amp had much more work done to it. As Doug recommends I replaced nearly every cap and resistor in the unit, added his adjustable bias and bias measurement resistors. The can cap was replaced with individual electrolytic caps that were of higher value and under the chassis. The ground switch replaced with a standby switch and a pre-out 1/4" jack added to the rear panel. The amp came up and seemed to be running when I noticed a wisp of smoke coming from a load resistor in the filter section. Dan backed down the A/C and used his laser temp gun to check the area. Once he isolated the problem he began measuring voltage drops and found that the reverb drive power feed was drawing too much. He popped it off and things were normal. The culprit was my mistake. I used a 22 nf cap instead of a 22pf cap in the circuit. He also found a missing cap in parallel with a resistor at the reverb out RCA jack. I must have missed it. With his laptop he logged on to the Hoffman site and found the schematic for my Princeton which as the pull on boost switch/volume control. That revealed the missing cap quickly. He had set the new bias pot in the middle for a start but while he was getting set to dial it in, the lead from the 23k resistor from one lug to the control can popped loose. He got out his big 250 watt Weller and after cleaning the pot with a file, he got a good connection. Now it was running properly. We measured almost 25 watts output and much less noise than I ever had before, but Dan was worried about how hard the tubes were running and the output transformer. We decided to remove my solid state rectifier and put the 5AR4 back in. That brought things into line nicely. The voltages on the 6V6 tubes are still a little stiff but the amp made a solid 20 watts and sounded wonderful. Dan pulled out his Strat and played for 20 minutes through his bench speaker cab. Both of us were impressed and very pleased. Time sure flies when you are having fun. Six hours had passed. He cut my bill by a third and I was so happy to be on my way home with two great amps in working condition. Today, they go back in the cabinets and while we watch the game in the "Party Barn" we will play them both through multiple cabinets. Cool! Jim
Topic: Sovtek and Princeton amps projects done! (Read 1729 times)