6GB5 and Variants as Screen Driven Amps

[Francois G]

Originally Posted by wrenchone

I may try some 18GB5/LL500 tubes with the "Bursar" transformer coupled circuit in screen driven mode if it works out driving 63PS-E tubes. Either these, or one of the many squatty doudecar horizontal sweep tubes without plate caps. The bitty plate caps on the 6BG5s are a pain in the butt, as nothing readily available fits quite right. Somehere in my stash I have some of the old wire spring-type plate clips, but they are just sudden death waiting to happen, and should only be used with tubes that are in a cage...

Wrenchone,
This thread got side-tracked, but I for one am very interested in the 18GB5 tubes and ways to put them to good use.

An author in "Tube Audio" tested 6GB5's some years ago and found that in triode (screen grid to plate) they performed like a super 6B4 with more power and less distortion. Since then I have been collecting them and now have a good supply to do something with.

Please keep the group informed about your experiments with the xxGB5's

Francois

[wrenchone]

This is rather interesting. I just bought a bunch of 6S4S directly heated triodes (Russian version of 6B4G), with the idea of using them in a P-P class AB2 amplifier. It would amuse me to no end that triode-connected cheap sweep tubes would be the equal of these. I always cringe at using directly heated tubes in such an application due to all the extra opportunities for noise injection.

[tubelab.com]

I just bought a bunch of 6S4S directly heated triodes (Russian version of 6B4G), with the idea of using them in a P-P class AB2 amplifier. It would amuse me to no end that triode-connected cheap sweep tubes would be the equal of these.

Go directly to the 6AV5GA, do not pass go, do not collect $200........

There used to be a story about some multi grid indirectly heated 6B4G tubes being sold on Ebay. I dismissed it as an urban legend until I found one of the tubes. Upon investigation I uncovered a much bigger story.

It seems that some tube makers had contracts to fulfil as their tube production was winding down and production lines were being shut down. The manufacturers worked with the contract holders to find creative solutions to fill orders. This lead to some rather unusual glass stuffing. The manufacturers found something on hand that would work in the contract holders design and built tubes for them only. These tubes have now found their way into the surplus market. Phillips EGC and Sylvania were the biggest players here.

This led to the famous 7027A guts inside the 6BG6GA bottle (sold by SND sales as the "super 6BG6"). It also led to some triode wired 6AV5GA's stuffed inside 6B4G bottles. I have also found some wimpy RF tubes labled 6CW5's and they melt at normal 6CW5 levels.

The folklore went on to say that the 6AV5GA was a direct replacement for the 6B4G. This is obviously not true, it only means that the substitution worked in the contract holders circuit, which could have been a power supply for all we know.

I compared the 6B4G and the 6AV5GA in a Tubelab SE several years ago. That story is here:

6AV5GA testing

[wrenchone]

Well, yes and no. I have 18GB5s, but no 6AV5GAs. Guess which ones I'll try first?

[Francois G]

Originally Posted by wrenchone

Well, yes and no. I have 18GB5s, but no 6AV5GAs. Guess which ones I'll try first?

Hi Wrenchone, have you made any progress?

[wrenchone]

This one is about 5 or 6 places (I really haven't stopped to do a detailed count) down my to-do list. Too many ideas, too little time.