Unfinished WAD 300b PP kit

[cc454]

Yes, you are correct, the rectifier tube is used as soft starter for the HT, it also being used as a hash filter and current limiter along with its voltage drop.

The filament's health concerns me more along with bias maintenance.

This is my first venture with directly heated cathodes audio tubes other then ion lasers and such. And if I apply some some of that tech to these 100 year old tech parts maybe I can get it last longer then a 1000 hour light bulb.

I posted some images from the ebay ad of the unit I bought.

[Richard]

Check and adjust the bias and you'll get good life from the 300Bs. I ran one for a while (years) for many hours a day as I worked from home and it was on as regular daytime background. (It does run mainly class A though so is like a car engine on full throttle even when minimal power is being used).

One set of valves did become, slowly, weak in the bass and less brilliant in the treble but it happens slowly, a year or more of many hours a day, maybe they last for 3-5000 hours. Really the power consumption these days is as big an issue.

The sound was glorious and you have a lot to look forward to.

Here's a later thread if you've not seen it,
http://www.world-designs.co.uk/forum...ead.php?t=7834

and my (also secondhand) example here, http://www.world-designs.co.uk/forum...5&d=1585922701
paying attention to the earthing (the original had it floated incorrectly) and phasing of the heaters etc will give the lowest hum and I found no feedback was the preferred setting.

Greg (and others here who may no longer visit regularly) has a lot of experience with these amps.

[cc454]

I'm doing a little more late night surfing/research and was wondering how are prices of Ukrainian/Russian tubes there in the UK?

I see new importers popping up in the surrounding countries but I am wary to risk too much on them.

[cc454]

The unit arrived and looks none the worse from the original ad. It looks like the original builder got as far as mounting all the iron and assembled the components to the driver tag boards. What appears to be missing is the filter caps and the filament stuffing.

Tomorrow I will verify the mains winding and start putting together a plan and a parts list!

[Greg.]

Hope it goes well

[cc454]

Lucky me, the mains are wound for 120 vac Today mine measured 122.7 vac so I'm getting 235 vac (no load) for each leg of the HT. All 5 volt heaters measure measure 5.25 vac and the 6.3 volt measures 6.75 vac.

I checked out the chokes and the measure 2.1 H (1k hz), a little off from the 3H on the print.

The inter stage transformer looks to be a 11:1) DCR 1k ohm.

Output measures ~ z 1.78k ohms per leg, DCR 65.4 ohm.

[cc454]

Originally Posted by A Stuart

This may be relevant. Have not read the whole thing.
See post #42, where the author heli001 appears to be, or to work for, Panacea Engineering.
https://www.audio-talk.co.uk/phpBB3/...=3542&start=30

Oddly enough I think this is that very same amp he started.

[cc454]

With good Iron I can build on this chassis a working amp. As with any project I want to "Start with the end in mind", make a great sounding amp that is flexible to use and modify and not cost a arm and a leg to keep running. (that includes tubes) And make it pretty enough that my wife will accept it as part of the décor in the living room.

Looking at the original circuit the big take away is the direct transformer coupled class AB triode output using 300bs. And I would bet the transformers have a very high pedigree. But to populate this thing with the very cheapest tube set I can source today is ~$400-$600. If I put in fresh WE's that's an easy $3 grand for a quartet. Not a trivial sum to justify if you know what I mean.

So given what I have the need to make the amp more flexible to other tube sets is a must, starting with the output tubes.

[cc454]

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.

[Richard]

<< With good Iron I can build on this chassis a working amp. As with any project I want to "Start with the end in mind", make a great sounding amp that is flexible to use and modify and not cost a arm and a leg to keep running. (that includes tubes) And make it pretty enough that my wife will accept it as part of the décor in the living room.

Looking at the original circuit the big take away is the direct transformer coupled class AB triode output using 300bs. And I would bet the transformers have a very high pedigree. But to populate this thing with the very cheapest tube set I can source today is ~$400-$600. If I put in fresh WE's that's an easy $3 grand for a quartet. Not a trivial sum to justify if you know what I mean.

So given what I have the need to make the amp more flexible to other tube sets is a must, starting with the output tubes.>>


That was good news about the mains TX. I dug out the first 1993 and revised 1994 articles (same OP stage revised input/driver) and one article also says a 120V mains TX would be available so your TXs may well all be from WAD.

Also mentioned (twice) is that the circuit is "pure Class A self bias". The bias is set with the cathode resistors and runs the 300Bs quite hard. Most of us removed/left out the 6k8 resistors and so used 1k5 as the cathode value instead of 1k2 to lower plate dissipation. Whatever you choose check the voltages once up and running and keep the 300Bs under 32W plate for longer life. I used 4 makes of 300Bs without any failures. Popular were the EH Gold Grids, about £140 per quad at the time off eBay or Hotrox (UK).