So, some of you may remember me. I have been lurking at the forum but have been doing well with my tube experiments. Lately I have been tinkering with 1930's and 40's radios as well as the guitar amp thing. I ended up buying a lot of radio's in need of work. One was a Philco model 60 chassis which is about a 1935 vintage. In my experiments I have noticed I like the sound of older tubes in a guitar amp. To my ear the older stuff gives a more raw bluesy tone that a 12ax7/6V6 amp cannot duplicate. I got the idea I would convert this radio chassis into a guitar amp. The plan was to have a sharp cutoff pentode as V1 in this case I used a 77 which became the 6J7 and later the 6SJ7. I wanted to have a bit more gain that just coupling that with a 42 which is a 6 pin version of a 6F6g pentode. So I felt a 75 which is the forerunner to a 6Q7 and later a 6SQ7 which as many of you would know is a 100mu triode with 2 diodes in the same envelope to act as detector and 1st stage audio in a radio would be a nice addition. The model 60 had a 6A7 1st RF followed by a 78 which is a remote cutoff pentode , basically a 6K7, then a 75 and a 42 with an 80 (4 pin ST 5Y3). So my design work began. I set up the 77 kinda like a 6SJ7 in a champ into a 75 with the diodes grounded into a single ended power section using the 42(6F6g). It sounded fair when I first fired it up but then testing revealed the plate dissipation for the 42 was crazy high. Like close to 20 watts when a 6F6 is max 11watts. so I started playing with the Kathode resistor. I got up to 1.2kohms @ 10 watts resistor and at this point the voltage was up to 400volts but still the Plate dissipation was 14watts. The tone is fair at best in any plate dissipation level. It actually sounded a bit better when I was cooking the pentode but this 85 year old tube is not going to last any time under this stress. My question is how to I fix it? This radio only had one more tube I can't see how the tranny is supplying so much extra current and voltage but is there a way to tame this? Is it as simple as adding hi wattage resistors to the p.s. or somehow redirecting power to ground in the p.s.? If so is there a formula or schematic which has been used to tame down this situation? I feel like this could potentially be a blues machine if I could get the plate dissipation under control for the vintage tubes. I seem to remember there was some guitar amps that cut down voltage/ power in the power supply but I don't recall the model #'s can anyone suggest a way to tame down the plate dissipation because the higher value of resistor slows the current but the voltage creeps so high I fear these old tubes will fry like a sunny side up egg in bacon grease. Any suggestion welcome.
Topic: 1930's radio conversion (Read 2508 times)