12AY7 as a DC-coupled cathode follower

[jamesmafyew]

In my Fender-made Sunn Model T I wanted to get cleaner cleans and more manageable gain on the clean channel, which does not have a master volume. After the volume pot, there's a DC-coupled cathode follower (V2 in this amp) that drives the TMB tone stack for the clean channel, after which the signal goes to the FX loop and PI.

The V2 CF stage puts about 160VDC on the cathode of the 2nd triode in V2 (typical 100k cathode resistor, 284V supply, 100k plate load/820Rk in the preceding gain stage). I put a 12AY7 in there (an EH new production) and it sounds great to my ears, and keeps things really usable on that channel. Problem is, 12AY7s are only spec'ed to handle 90V heater-to-cathode voltage. The thing seems to be working, but am I just asking for all kinds of trouble by doing this? Will it fail in two weeks?

I've considered elevating the heaters in the amp, but it will require some serious surgery and I'd rather not if I can help it. :/

[MWJB]

Ever heard of a 5E6A bassman, 12AY7 equipped CF stage, harp players run 12AY7 in 5F6A CF stages all the time. Safe as houses.

[km6xz]

Is the heater winding grounded anywhere? If not, where is the return path for the cathode?......there is only the breakdown voltage of the transformer that would provide that return which might be in the 1500volt range or higher. Voltage is measured as potential difference between two points that which have a common path. In the heater versus cathode, they are different paths and a voltage measurement between them is meaningless. Ground the heater somewhere and sure, you have now a common path and the 90volt limit becomes an issue.

[kg]

james: the answer to your question is very dependent on quality of tube.

my advice is to just have a spare ay7 handy in case the sound gets wonky on you, and replace as needed.