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Western Electric SE 300b amp


captainbeefheart
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https://www.westernelectric.com/91e

 

Now someone is stepping up to the plate here, Class A3 getting over 20 watts with just one 300b!!!! More companies should be getting more power from their DHT this way.

 

Looks like they aren't letting any DC into the output transformer via parafeed with a constant current plate load, my guess is it's a gyrator load. I am curious how they are driving the grid current into the 300b. I also can't find what the gain stage front end tube is being used.

 

What do you all think?

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3 hours ago, Curious_George said:

 

Yes fancy term for positive grid bias like amplifiers that use 811a type tubes.

 

The highest power I see on the Western Electric data sheet for Class A1 operation is 17 watts with one 300b. Going with Class A2 operation you could relax the operating points a bit and still get 20 watts with one 300b. Surprised more companies aren't making higher powered 300b amps, highest I have ever seen is 15 watts from one. Now going positive grid bias on a 300b is something I absolutely have to try now.

 

I know people will complain about parafeed but if you want to get great bandwidth from single ended it is smart to just eliminate the DC from the output transformer in order to not need a gap which absolutely drastically increases inter-winding capacitance. A long time ago when I was between jobs I would get the itch to make an amp so I used many cheapo 70v system transformers with parafeed and the results were fantastic. The headphone amp I made with parafeed was so good and inexpensive I got a lot of requests from friends to make them one.

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11 hours ago, captainbeefheart said:

 

Yes fancy term for positive grid bias like amplifiers that use 811a type tubes.

 

The highest power I see on the Western Electric data sheet for Class A1 operation is 17 watts with one 300b. Going with Class A2 operation you could relax the operating points a bit and still get 20 watts with one 300b. Surprised more companies aren't making higher powered 300b amps, highest I have ever seen is 15 watts from one. Now going positive grid bias on a 300b is something I absolutely have to try now.

 

I know people will complain about parafeed but if you want to get great bandwidth from single ended it is smart to just eliminate the DC from the output transformer in order to not need a gap which absolutely drastically increases inter-winding capacitance. A long time ago when I was between jobs I would get the itch to make an amp so I used many cheapo 70v system transformers with parafeed and the results were fantastic. The headphone amp I made with parafeed was so good and inexpensive I got a lot of requests from friends to make them one.


I’ve never designed or built a parafeed, but they do have great potential from the lack of DC gap. The audiophool guys will cringe at the thought of a coupling cap though!

 

I have a pair of 833A tubes that I bought a long time ago. I was going to bias them at 0VDC, but never got around to building the amp. Even bought 872’s to go with them. 

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4 hours ago, grindstone said:

If it's the same animal, Broskie arrived at it being PP

 

I really do not think Western Electric would add a Mosfet to pull current in opposite direction and mix up the distinction as not being push pull and call it single ended. Straight from the data sheet from Western Electric says 17 watts from one tube Class A1. They wouldn't need to resort to a hybrid push pull trick to get to 20 watts.

 

A3 is positive grid bias

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2 hours ago, A1UC said:

That means it is A2 with no swing done from A to A2 because grid is always positive.

 

Yep exactly.

 

2 hours ago, A1UC said:

A lot dont like A2

 

Really? I would say the vast majority have no idea what A2 even is but the amps that are A2 seem to get lots of praise like the Kondo Ongaku or the Shishido designs.

 

I have tested some Class A2 amps that were driven by power tube triode plates which did not live up to the hype. I can see where if you heard one of these amps you may get a bad taste for the Class. The reason is the output impedance of the driver is not low enough to smoothly shift from A1 into A2, the abrupt transition from 100M grid impedance to 100 ohms will make for a very demanding transition from a driver. Possibly the reasoning why Western Electric is going positive grid bias so there is no large transition of grid impedance to create problems.

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I am using a 6BX7GT (in parallel) with an interstage transformer as a driver for a 211 tube and it performs great. Pushes the 211 way into A2 no problem. I am getting 25 Watts out @ 5% THD+N.  

 

Not quite like a 2A3 or 300B, but that BIG DHT triode sound. Sounds great to me. 

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2 hours ago, Curious_George said:

Not quite like a 2A3 or 300B, but that BIG DHT triode sound. Sounds great to me.

 

Yes the larger transmitting triodes have more of a sound to them vs the smaller DHT which are more transparent, the latter you end up hearing more of the driver than the actual power triode which is why the driver section is of utmost importance and accounts for the vast difference in sound between many of the different DHT amplifiers.

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15 hours ago, A1UC said:

I’m getting 25W 211 and 35W 845
211 is my favorite sound wise


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Can you switch between 211 and 845 tubes in your amp? In other words is the amp designed for both with the ability to flip a switch and use either tube?

 

I’ve been wanting to convert my 211 amp to use both,  just too many other projects. Perhaps soon.

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Yes just a switch, I listed mine for sale because I don’t have use for them right now but I’m pulling listing and keeping them now . I’d miss them very good amp


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I have used Heyboer power transformers and they are of great quality, use them with confidence for sure.

Good to hear , looks like next yr before I see them . Was ordered custom wound with multiple shields and solder terminals so higher quality wire can be used .


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On 11/24/2021 at 9:35 AM, grindstone said:

If it's the same animal, Broskie arrived at it being PP

https://tubecad.com/2019/05/blog0466.htm

 

After minor testing I believe you to be correct.

 

I have been testing the 300b with positive grid bias and not happy with the results thus far. I forget who applied for the current steering patent but I am now thinking Western Electric has licensed it from them or possibly it is in public domain by now.

 

The fact I am not running positive grid means I can actually simulate the circuit in LTSpice and have it be accurate so hang tight gang, I plan to run a simulation and we can actually see what the distortion spectra looks like with this circuit regardless of it being push pull or single ended. I don't care if it is totem pole push pull, as long as it has the same sonic signature as a single ended triode amp I am happy, and who will complain about double the power for the same tube with a little help from two transistors.

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