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With my Tweed Bassman reissue and another clone circuit tweed bassman I own I've found that some new manufacter 12ax7 's don't last very long or at all in that circuit position. Heater to cathode voltage is to high. I've been told most modern preamp tube use a spiral filament and those types have a lower rating for that voltage difference. The sovtek WA's were recommended but I've seen some branded as such but the spiral filament wire is visible. ? Any thoughts or suggestions for new manufacter tubes that work well in that part of the circuit. Also the sovteks with the straight wire filament work fine the EH "tung-sols" may only last 10 minutes to an hour and their toast. Thanks.

With Any Valve that only lasts for an Hour,, then I'd be looking at the circuit Not Valves.
Phil.

I admit my post isn't as articulate and to the point as it should be. My vintage RCA's 12AX7's have lasted darn near forever in the cathode follower position of the the tone stack. The Sovek WA's last a long time as well but I have a hard time getting them. When I had tube life issues I called EHX customer service and they told me their reissue Tung Sols would be a bad choice for the that circuit position. The Sovtek 12AX7'S WA's with the straight wire filament work well too. I brought this up because some but not all Sovtek WA's I have seen lately have a spiral heater filament and those do not last. All amp B+ voltages are normal, all filters good and the amp runs and sounds good. I don't use the solid state rectifier that came with the amp. Running a GZ34 to lower voltages down to "vintage" levels and also installed and bias pot and that is correctly set.

Hi onebuzz,
Better minds here maybe of more help but here's something to google;
The Value of the cathode resistor (the one right before the tone stack) is important.

If it's very low and the plate voltage is excessive then it's possible that that triode draws too much current and I believe you can needlessly waste magic smoke.
You may need to use an AT7 which has much higher current rating.

Also check your heater voltage at pins,, if over 10% high then that may not help.

I have no faith in brand names these days and just because it'sa XYZ name plate does not mean they are well designed so there maybe issues hiding within the circuit and not the fault of Valves.
Phil.

Like you say its probably a branding issue on or with the tubes. This isn't a hard failure (smoke) certain new tubes just degrade quickly. Older fender amp designs were known to push the limits of tubes back in the olden days. So some of todays new tubes can't take strain. Blackface Deluxe Reverb output tube B+ and reverb driver circuit B+ as examples of "pushing it". I appreciate your reply phatt. Thanks.

I seem to remember reading somewhere that if you connect the centre-tap of the 6.3V AC heater-supply to a resistive voltage-divider from +HT to ground, that can reduce the heater-cathode voltage strain, I think I've seen this done in some Hi-Fi Tube/Valve amps.....

Thanks for the tip Dr. Nomis. I'll look into it.