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RE: Max Tube voltage during warm up

Posted by Triode_Kingdom (A) on June 5, 2021 at 09:27:12

In Reply to: Max Tube voltage during warm up posted by sony6060 on June 3, 2021 at 07:40:32:

You're getting a lot of bad/useless advice here.

First, about tube voltage, what you're describing isn't an issue for most tubes in terms of anode-cathode or screen-cathode voltage. However, stages that use direct coupling can be problematic. If B+ is applied before the tubes conduct, the anode of the first tube will apply full B+ to the grid of the second stage. This can cause the second tube to arc from grid to cathode. The traditional solution is to install a NE2 across the second tube. That prevents the voltage from exceeding 80V or so. Marantz and others used this approach in their amps with solid state rectifiers.

A bigger issue in most cases regards the filter and decoupling caps. All these caps need to be able to withstand the initial turn-on surge. That includes preamp decoupling caps located in strings of dropping resistors. Bleeder resistors used in appropriate areas may be sufficient to protect low-level stages without drawing too much current from the supply.

A choke input filter obviously worsens this problem. The majority of choke input filters have been employed in industrial and broadcast/communications products, not home hi-fi. In the case of solid state rectifiers, the equipment usually employs either a B+ delay circuit of some type or a switch so the operator can apply full B+ after the tubes conduct. The latter is meant to be used by persons familiar with circuitry and electrical fundamentals, not the average consumer who buys hi-fi gear. In fact, choke input filters offer little if any benefit in this service. The combination of large current swings (AB1) and 20 Hz fidelity requires substantial energy storage in the filter cap bank. That in itself tends to aid regulation and mostly obviates the need for a critical value choke.

The most straightforward approach to resolving all this is to rely on the warmup time of the rectifier tube. In your case, it sounds as though B+ is appearing too soon (or the outputs are heavily worn and conducting late). Perhaps a different rectifier will solve the problem. Otherwise, you might need to add a delay circuit or switch, or simply up-rate all the caps and any other susceptible components to withstand the surge voltage.

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