Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: punkykatt on May 27, 2016, 07:12:02 pm
-
Hey Guys, I built this amp kit about 10yrs ago and it has changed owners about 5yrs ago. The new owner says he had finished the 1st set, after break he flips off standby and the pilot light goes out and amps is dead. Installs new fuse, fuse blows right away. On the bench I find the PT HT secondaries shorted. The 5v and 6.3 secondaries read normal ohms. Checked rectifier tube and socket OK, no signs external shorts and the 5U4GB works well in another amp. All filter caps test good with the ESR meter, no signs of external shorts, leaking or blisters. All resistors through out the amp are within spec. except one of the two (schematic lists one 10w R for all four power tubes. I used one 5w R/pair instead) 5w 470R power tube cathode bias resistors had burnt open. I checked all four 6V6`s for shorts and tried them in my Champ type amp one at a time. They all played well and read within 1.5mA of each other. Tested for shorts in the power tubes sockets OK. Im trying to determine what blew the PT and only one power tube cathode resistor. Did they both fail at the same time? While doing tests through out the amp I did fine that the 6.3 htr circuit was shorting to ground through the pilot light. Took the bulb out and reinstalled it short was gone. You would think that short would have taken out the 6.3v secondaries??. Another thing I found when I removed the 5U4GB rectifier the guide pin was broken off the base and a white mark placed on the tube base where the tab should be. Perhaps the new owner is not telling me the whole story? Maybe he remove it and installed it incorrectly? Any ideas or thoughts are much appreciated. Thanks in advance.. Punky
-
User input is usually helpful. But not always. Sometimes you have to just set aside what the user tells you and just fix what is broken based on your own observations. It's not always necessary to know what may have caused this or that.
-
Thanks for the reply Sluckey. I have had a couple of Egnator amps come in with blown PT`s for no reason found. Replace the PT and the amp is fine. What would blow the power tube cathode bias resistor if the power tube/s did not short out?
-
Why did you use a pair of 470Ω cathode resistors? Schematic shows 150Ω for all 4 tubes.
-
The bias was a bit hot with 150 ohm 10w so I cooled it down with the 470 ohm 5w, one for each pair if 6V6s.