First Id like to say hello. Ive been lurking for a while and am very thankful to find this forum full of fellow Sunn worshipers.
I have a 1975 Model T (mid select ver) that Im having problems with. Ive done a few mods, first was removal of the blue/white wire. Also a cap added to the mid select to swamp it out at zero. Then a few days ago I swapped the 68k resistor for 100k on pin 1 of V2.
This morning I was finally able to test it out. After about 45 mins I started hearing this sputtery oscillation while I was playing some distorted chords. It would seep in with feedback. When I looked inside I noticed a burned out resistor. R43 a 330 ohm according to the schematic. I measured the fried resistor for s and gs and it read 333 ohm. Surprisingly I had a 5watt resistor that measured 324 ohms that I replaced it with. I figured that small of a difference wouldnt hurt anything.
I checked the bias before I turned it on and it read -55 V. However when I struck my first chord I think I saw a white flash from one of the power tubes. When I looked at the power tubes I saw the pair on the left red plating. I killed the power immediately.
Is my bias off? I should note I have some old kt88s in there. When I bought the amp the bias was set to something like -30V. I ordered 6550s and biased to -55v. Recently I thought Id give the kt88s another try and found I liked the increased headroom and bass. Also it appears all other components (other than the mods Ive done) are original. Up until today Ive had no major problems.
