Amp builders across the world are suffering from their mains voltage being higher than nominal. This is very common in both the USA and UK, although I write from a UK point of view. Some days my mains suppply is 240V, sometimes you can watch the metered value slowly wobbling between 230V and 235V at a frequency similar to a wind turbine blade’s rotation.
The problem is simple – if you have a PT set at, say, 235V, and the hole in the wall supplies 250V, the 6.3V heater voltage is more than 6% higher than it should be. You have +/- 5% as the default spec on most 6.3V heaters, so 250V turns the amp into a valve-eater, as Bronsboi is my witness.
Automotive valve types can operate on a wide range of heater currents, because the heaters are designed to run in a car radio . A car battery at peak charge gives about 14.4V, whereas in the cold of winter it may only give about 10V. The heaters of the valves can allow for this variation, although shortened life may be the result if they are run below 12V for sustained periods.
So if you run them at 12V on the heater, they can handle the variations in mains current easily. The design optimum is usually 13.4V, but 12V or more is acceptable.
A lot of the automotive types can be run on AC or DC heaters, although some are specified as DC only.
There are problems finding rectifiers that run on 12V systems – and never forget that you want a separate transformer tap for the rectifier because the amp’s HT/B+ supply draws from the rectifier cathode.
The most useful one I have found is the 12BW4, which is the automotive version of the 6BW4. These are very similar to the EZ81, but slightly less poky – you can run a pair of 6V6 types, but EL84 are just a bit too much.
Overall, the most useful types for amp building are
7058 – derived from 12AX7
7059 – derived from 6U8
7060 – derived from 6AU8
7061 – noval version of 6V6, derived from (DC only) 12AB5.
6679 – 12AT7
All these can be used with AC on the heaters.
You can’t use the 6V heaters on a 7058, but otherwise it is a RCA 12AX7, and the prices are shooting up to reflect that. They are currently at about $20 from TubeDepot.
The rest of the valves I have mentioned are $3 – $4 in a lot of places.
The most obvious possibility with these is a Marshall 18W style amp with output resistor values adjusted to suit 6V6, rather than the standard EL84 output valves . Use 7058 instead of 12AX7, 7061 instead of 6V6, 12BW4 instead of EZ81.
I keep running across other 12V types with possibilities. 12CA5, 12CU5, 12ED5 are all cheap and could be used to build small amps.
Lots of possibilities!