Unfinished WAD 300b PP kit

[Richard]

Well while part hunting for another project I came across some some Tent-Labs auto-bias modules I had purchased awhile back but did not use. This would be pretty cool to get working on the WAD.

Another mod I was looking at is to run the filaments DC and looked into making regulators for the 5 volts. The WAD mains tx has the 4 isolated taps that output right at 5.25 VAC (no load) so there is not enough headroom to work with.

Looking in on my power supply shelf I have 2 industrial Sola linear 5v power supplies. What's really nice is that these are voltage and current regulated so I can lower the current limit to ~1.3A so goes into current limit when the the filament is cold, then slots into voltage regulation once it hits its target current range.

From memory those 5V TX outputs are the same colour wires. AC phase them in pairs for each valve to cancel heater hum. By all means try DC but it looks a lot of work. With careful ground connections and 10R lift applied correctly hum wasn't an issue even without feedback. The amp is very sensitive without fb though so some thought to the preamp would be useful, or even just a "passive pre" pot for line sources.

[cc454]

Thanks for the input. After reading through the Tent-Labs instructions it makes wonder if it covers the dc coupled nature of the interstage tx.

I may need some correspondence with the Tent-Labs guys to get some enlightenment.

One of the big things I want to do is to prolong the life of the tubes as long as possible by not whipping the filaments every time I turn them on and off. Included is a paper that is a good read about the subject.

I even looked into making a servo driven variac as the soft starter.

[Richard]

An interesting paper. What about an anti-surge thermistor? Some here have used them, see a couple of thread links below,

http://www.world-designs.co.uk/forum...ghlight=switch

Chris raises a point about its use, second link, but our valve amps are near or full class A and draw a fairly constant current after switch on,

http://www.world-designs.co.uk/forum...ghlight=switch

other reading,

https://www.homemade-circuits.com/us...ge-suppressor/

[cc454]

I agree the thermistor is a great little device. For the cost/performance value its works wonders!

Problems that I had to work on is to 1st get past the worse case phase inductive kick at contact, 2nd the cap bank charge cycles.

To get by the phase angle problem at turn on, the use of a SS relay with zero-cross turn on really works well along with the thermistor to take care of the caps charge cycles.

The little Sola power supply covers a lot of these areas plus I can find them all day for ~$20 US each used. I don't think I could build something for less money.

[Richard]

Great stuff, keep us updated, pics are always nice!

[cc454]

I am a big fan of John Broskie and I have been wanting to try his Aikido front end circuit and was thinking about using here. Has anybody tried something else?

[Phil Y]

Originally Posted by cc454

One of the big things I want to do is to prolong the life of the tubes as long as possible by not whipping the filaments every time I turn them on and off. Included is a paper that is a good read about the subject.

My understanding was that "receiving valves" or in other words, the ones we use, are not subject to the problems outlined in that article. That is only relevant to the high power RF valves as described.

I stand to be corrected as always

Phil

[cc454]

You are correct, the small receiving tubes that we typically use are much more robust since WWII, because a lot of them were designed to go into airplanes. After WWII they were designed to go into automobiles and then missiles. The

The 300b tube that I am trying to make last was invented in 1937 for commercial broadcasting equipment. It did not get miniaturized or ruggedized, it was replaced with more efficient multi element output tube topology.

The price of the currently available brands of 300b and their 'non-warrantable' fragile filaments make it a worth while to spend a tubes worth of time and money on it.

[cc454]

Merry Christmas! and Happy Holidays!

I finally got round to converting John Broskie's circuit image to something I can post.

[cc454]

I am thinking it would be an easy way to improve the drive signal and keep the overall sonic signature of the WAD PP 300b without going too overboard on the mods.