Power transformer conversion questions

[lowbrow]

I have a old Kent Tremtone three tube radio based amp without a power transformer. I'd like to learn more about amp theory and design and have a few books on the way that address the subject.

I like the Kent amp, but the direct power to the heaters worries me and I'm more interested in modding it to have a power transformer than getting an isolation transformer. I want to use it at our practice space as a jam amp to while away the time while waiting on everyone to show up, work on songs etc. It's perfect for this as it gets a nice dirty tone at such low volumes.

The tube compliment is: 12AU6 pre-amp, 5OC5 power tube and a 12AV6 for tremolo. The heaters on these are fed direct from the wall power...so I assume the voltages across all three add up to about 110. I don't know the consumption of each tube however....which I figure would help me pick the right power transformer...??

Any online resources or tips here for helping me to figure out how to convert this to a power transformer?

[Enzo]

Are you sure there is not a 35W4 in there as well? The heater voltages on your three tubes add up to 74v. SO either there were other tubes or there is a hefty power resistor using up 46 volts.

You are not likely to find a power transformer with 12v heater windings and 50v heater windings. And if the B+ in the radio is formed by direcr rectification of the 120v mains, then your new power tranany would also have to have a 120v winding on the secondary.

An isolation transformer does the trick perfectly.

To rewire this thing for "normal" power transformer operation would mean rewiring the sockets for 6v heaters all in parallel instead of series. Then replace all the tubes with their 6v counterparts.

[Steve Conner]

Could be trickier than it looks.

It's quite likely that the amp will get its B+ from direct rectification of the mains, so it will only be 170V. That's probably what causes the dirt and early breakup that you're liking.

It could be hard to find a power transformer that gives the same low B+. If you used one meant for a Champ or the like, it would end up around 250-300V, and the amp would get louder and cleaner, not to mention maybe needing new filter capacitors rated for the higher voltage.

Unless the rectifier is a voltage doubler using a 25Z6 or the like.

[Enzo]

That is why I mentioned a transformer would need a 120v secondary winding.