A couple of comments I would offer:
1. Regarding the .01 Cap before the choke: It is very likely that the increased PIV rating of the GZ34 precluded the need for the cap, but if it were mine, I'd surely install it as their use is considered best design practice for any true choke input filter design, providing protection for both the choke, and the rectifier tube(s). The cap does nothing during operation of the amplifier, but at turn off, the instant loss of current through the choke allows it to produce a very healthy kick -- just like an ignition coil for an engine does, jumping the spark plug gap, and producing a spark to ignite the fuel mixture. Without the cap, that kick can break down the insulation inside the choke, and potentially exceed the PIV rating of the rectifier tube. The original cap was a 1600 volt device, but with the GZ34's 1700 volt PIV rating, it likely deemed no longer necessary. And, the insulation material in the choke was possibly improved by then as well. But those who lived during that day and worked on cars know that ignition coils were invariably prone to failure at some point -- precisely because they would break down internally due to the constant kick it was configured to produce. With the addition of the cap in a choke input filter design, it absorbs the choke kick at shut down, to prevent any damage from occurring. Maybe the cap is technically not needed with GZ34 rectifiers, and maybe the choke can withstand the kick internally well enough. But it's such cheap and easy insurance that falls into the category of "why not" include it? As long as a quality cap of appropriate voltage rating is used, there is zero downside to its installation, and everything to gain in the way of tube and choke protection.
2. Use of damper tubes as rectifiers: Dampers do have a slow ramp up, which is invariably aided by your operating their 6.3 volt heaters at 5.0 vac. And while this doesn't affect their super high PIV rating of 5500 volts (meaning any cap before the choke would only be protecting the choke when using these tubes), it does have a significant impact on their current handling capability.
The 6DE4 is rated for an maximum average plate current of 180 mA, which when used with a second tube 6DE4 in a conventional full wave rectifier circuit, means a pair of these tubes are capable of passing 360 mA. And, since this is a choke input filter design, virtually all of that current handling ability is available for the load to consume (unlike cap input filter designs, where significant de-rating is required to account for the charging current of the first filter cap). But, that 360 mA capability is only available with a full 6.3 vac applied to the heaters. There is only a very loosely defined correlation established between a reduction in heater voltage and the reduction in maximum current that a tube can then safely pass, but using the drop in heater voltage as a guide (operating at only 79% of design center voltage), it means that now the full wave configuration of your 6DE4s is capable of handling only 285 mA. Even if 100% of this value is available for the load to consume, it is now of a value that is less than the amplifier will attempt to draw at a full boil, which since not possible, will either damage the damper tubes, reduce full power output, or (likely), both.
I know the old argument -- you'll never be operating the amplifier at a continuous full boil, so the dual damper approach will no doubt likely work just fine for typical use in a high fidelity installation. There's another AKer who staunchly holds to using a single to GZ37 to power his 120 watt post Dynaco amplifier. He's been roundly told that the tube is nowhere near capable of handling the full needs of the amplifier, but he doesn't care because he likes the looks of the tube -- and the tube is in fact just capable of handling the quiescent needs of the amplifier, or its needs with typical home based use. My question is, what if Fisher had designed the 50AZ so that it could work fine in everyday use, but couldn't stand up to the full capability of the amplifier? They'd be roundly called out for it as poor design. Dynaco basically was for the use of a single GZ34 in the ST-70, as the tube is clearly at its limits if not overly so in that design -- but got by because the original Mullard tube was such a workhorse that was hard to kill. It is hopelessly so in a design capable of 120 watts however -- even as a tube rated higher than the GZ37.
As long as you know these things, then all is good. My background from the 60s is in a band, and I always go after everything with a "band rated" approach. It's got to be able to take a lickin and keep on tickin -- even under continuous full power use. But that's just me. The original 50AZ could do that very well. As you say, you're having some fun with the amplifiers, and that's great. Just be aware of the limitations of using dampers at less than rated heater voltage is that's what the installation is providing.
I hope this helps!
Dave
1. Regarding the .01 Cap before the choke: It is very likely that the increased PIV rating of the GZ34 precluded the need for the cap, but if it were mine, I'd surely install it as their use is considered best design practice for any true choke input filter design, providing protection for both the choke, and the rectifier tube(s). The cap does nothing during operation of the amplifier, but at turn off, the instant loss of current through the choke allows it to produce a very healthy kick -- just like an ignition coil for an engine does, jumping the spark plug gap, and producing a spark to ignite the fuel mixture. Without the cap, that kick can break down the insulation inside the choke, and potentially exceed the PIV rating of the rectifier tube. The original cap was a 1600 volt device, but with the GZ34's 1700 volt PIV rating, it likely deemed no longer necessary. And, the insulation material in the choke was possibly improved by then as well. But those who lived during that day and worked on cars know that ignition coils were invariably prone to failure at some point -- precisely because they would break down internally due to the constant kick it was configured to produce. With the addition of the cap in a choke input filter design, it absorbs the choke kick at shut down, to prevent any damage from occurring. Maybe the cap is technically not needed with GZ34 rectifiers, and maybe the choke can withstand the kick internally well enough. But it's such cheap and easy insurance that falls into the category of "why not" include it? As long as a quality cap of appropriate voltage rating is used, there is zero downside to its installation, and everything to gain in the way of tube and choke protection.
2. Use of damper tubes as rectifiers: Dampers do have a slow ramp up, which is invariably aided by your operating their 6.3 volt heaters at 5.0 vac. And while this doesn't affect their super high PIV rating of 5500 volts (meaning any cap before the choke would only be protecting the choke when using these tubes), it does have a significant impact on their current handling capability.
The 6DE4 is rated for an maximum average plate current of 180 mA, which when used with a second tube 6DE4 in a conventional full wave rectifier circuit, means a pair of these tubes are capable of passing 360 mA. And, since this is a choke input filter design, virtually all of that current handling ability is available for the load to consume (unlike cap input filter designs, where significant de-rating is required to account for the charging current of the first filter cap). But, that 360 mA capability is only available with a full 6.3 vac applied to the heaters. There is only a very loosely defined correlation established between a reduction in heater voltage and the reduction in maximum current that a tube can then safely pass, but using the drop in heater voltage as a guide (operating at only 79% of design center voltage), it means that now the full wave configuration of your 6DE4s is capable of handling only 285 mA. Even if 100% of this value is available for the load to consume, it is now of a value that is less than the amplifier will attempt to draw at a full boil, which since not possible, will either damage the damper tubes, reduce full power output, or (likely), both.
I know the old argument -- you'll never be operating the amplifier at a continuous full boil, so the dual damper approach will no doubt likely work just fine for typical use in a high fidelity installation. There's another AKer who staunchly holds to using a single to GZ37 to power his 120 watt post Dynaco amplifier. He's been roundly told that the tube is nowhere near capable of handling the full needs of the amplifier, but he doesn't care because he likes the looks of the tube -- and the tube is in fact just capable of handling the quiescent needs of the amplifier, or its needs with typical home based use. My question is, what if Fisher had designed the 50AZ so that it could work fine in everyday use, but couldn't stand up to the full capability of the amplifier? They'd be roundly called out for it as poor design. Dynaco basically was for the use of a single GZ34 in the ST-70, as the tube is clearly at its limits if not overly so in that design -- but got by because the original Mullard tube was such a workhorse that was hard to kill. It is hopelessly so in a design capable of 120 watts however -- even as a tube rated higher than the GZ37.
As long as you know these things, then all is good. My background from the 60s is in a band, and I always go after everything with a "band rated" approach. It's got to be able to take a lickin and keep on tickin -- even under continuous full power use. But that's just me. The original 50AZ could do that very well. As you say, you're having some fun with the amplifiers, and that's great. Just be aware of the limitations of using dampers at less than rated heater voltage is that's what the installation is providing.
I hope this helps!
Dave
