And thanks for posting Paul's excellent and very informative writeup.  It's always good to read a well stated presentation of the tradeoffs and considerations which led a designer to choose a particular design approach.  And his writeup is certainly among the best examples of that I can recall seeing.

Enjoy!  Best,

-- Al

      

Grannyring, Paul doesn't really have his own website. 

If you PM me, I can get you in touch with him. 

Regards,

Dan

Oh, and thanks for your comments re my system!

You have a very nice system! Very nice work on your crossovers. 

I asked the designer/builder of my new amp, Paul Birkeland, how it was possible to make what would appear to be quite a low-powered amp, to deliver the beauty and impact that it provides. It just doesn't seem that it should be feasible, though it surely does do just that, in spades.

His reply is on this page.

Regards,

Dan

Well, back in the early 90s, the Japanese started resurrecting audio amplifier technology from the 30s, you will see this referred to as the "triode renaissance".  These were 45, 2A3, 50, and 300B amps that were really simple, zero feedback single ended amps, and they are the basis for a lot of what we still sell at Bottlehead to this day.  The huge downside to all of this is that these little directly heated tubes don't make a whole lot of power unless you get really big ones (845/211) and run them at really high voltages.  Even then, most of those big tubes will only give you 15 to 20 watts running off a 1kV power supply on output transformers that are very difficult to manufacture properly.  

There are some ways around this though.  The traditional directly heated single ended triode amps run the output tubes in class A1, where the grid of the output tube never draws any grid current.  If you design an amplifier to push the output tubes into class A2, you can get a little extra power.  In order to do this, the circuitry behind the output tube needs to look more like its own power amp, so the designs get a bit more involved and complicated.  The grid of a power valve when transitioning from A1 to A2 becomes a really low load that's quite difficult to drive.  To press things even further, there are many directly heated transmitting triodes like the 3C24, 811A, 801A, 810, and even the 211 that can be biased so that they draw grid current at idle, and this opens up some operating points that allow for far higher power output.  While a tube like a 300B with a plate dissipation of 40W won't deliver much over 10W, a tube like the 811A with a plate dissipation of 40W is capable of nearly 20W of output when run in class A2.  

For driving the grid positively, a coupling capacitor is no longer an adequate means of connecting the power valve to the driver stage.  Instead, such amplifiers will use an interstage transformer, a directly coupled cathode follower, or a directly coupled solid state device.  From my experience, interstage transformers perform much better when they have a predictable load impedance on the output side, which is far from what happens in this kind of amplifier.  The solid state directly coupled option is totally workable, but not all that ideal given the voltages present and the need for a monster heatsink, so I use a directly coupled cathode follower.  This technique was well developed by Michimori Hirokuni, although I opt to use choke loaded cathode followers rather than actively loaded cathode followers.  

Still, a big challenge remains with these transmitting triodes.  They have very, very high internal impedances.  To cope with this, you can use really high impedance iron (iron that is impossible to make well), or you can implement some feedback to mate these transmitting valves with a more conventional output transformer.  Within the subset of feedback amps like this, many use feedback around the entire amp, some use cathode feedback around just the output stage, and some use local feedback.  From my experiences, putting feedback around the entire amplifier results in highly conditional stability.  An amp like that might play just fine on a Magnapan speaker, but the amp would oscillate on an ESL.  While cathode feedback around the output stage is effective at dealing with the issues specific to the output valve itself, there are other issues that creep up in terms of driving the transmitting triode itself, and cathode feedback doesn't resolve these issues.  When studying Hirokuni's work, I noticed that as his projects developed, he would take all the local feedback he could get and ultimately use just enough global feedback to get the amp to work properly.  

For the 3C24 amps and other similar amps that I make, I use a local feedback loop that goes from the plate of the transmitting triode back to the plate of the first stage driver tube.  When done properly, there is no longer any requirement for global feedback.  I can set this up to get the maximum allowable power from these valves while using an output transformer that is not all that crazy to produce.  In my 811A SET amps, as an example, the plate impedance of the 811 is around 35,000 ohms at my operating point, but with the local feedback I drive this down below 1000 ohms so I can mate the 811A to a 3K output transformer.  These designs are not so sensitive to the speakers used and the exclusion of the output transformer from the feedback loop maintains more of the character of a more traditional zero feedback SET amp.  

What you end up with is an amp that's a little snappier than a 300B or 2A3 amp, but not quite as mellow and relaxing.  You also get dirt cheap old stock output tubes, as you've now noticed! 

June 8, I added to my system the new 3C24 amp Paul Birkeland built. He actually brought four different amps to try, 811 mono-blocks, shunt regulated 300B mono-blocs (I really considered buying this pair), 3C24 mono-blocks, which sounded nice but ran out of power a bit too soon to consider, and the 3C24 stereo amp.

The 3C24 stereo amp had everything I wanted in an amp. It was the best-built of all the amps, with much better parts quality, and plenty of power to be a good match for my custom-built Tannoy HPD 315 speakers.

The tonal quality was the best I've heard from any amp, anywhere. After several days and nights listening to this amp/speaker combination, it is like a wonderful gift has been given to my system, with beautiful frequency response and tone, and great ability to produce dynamic response.

I love this amp.

Regards,

Dan

Lalitk, thank you for your comments, they are much appreciated.

Regards,

Dan

Just replaced my old Kef Rederence 100 center channel with Kef Q 650c. Much better for multi-channel and home theater. Been wanting this for many years, along with upgrading my surround speakers, which was also done recently with Andrew Jones designed, Kef built Pioneer S-Z9 speakers.

Multi-channel and HT listening is much improved.

Regards,

Dan

Purchased a pair of Andrew Jones designed bookshelves, built by KEF for Pioneer, to be used to upgrade my old surround speakers, KEF Q 15.2.

They look and sound good (along with new 32" tall stands)  The speakers have a concentric tweeter/midrange and a bass driver. This purchase has made a nice upgrade to my multi-channel system.

Regards,

Dan 

Jet, I still love the sound of my SET 300B, it's a bit more refined in how it sounds compared to the SET 845. The 845 just wakes up my Tannoys and makes them kick butt in a way the 300B just can't do.

I was listening to Reference Recording's "Mephisto & Co;" last night, and at a pretty high SPL, and it's hard to describe, but there is a "wholeness" to the sound that comes with the 845 that my HPD 315's really like.

My tech I found last year (Paul Birkeland, designer and builder for Bottlehead, gives me a hard time about it, since it's what he calls a $500 Chinese amp, I affectionately call my 100lb. boat anchor, really sounds spectacular. I would love to have a $30,000 845 amp, but that's not going to happen in my lifetime. Paul has done a great job, at very reasonable prices, of keeping it up and running. He's replaced all the caps, his last job was to replace the undersized diodes in the amp, using circuit boards he designed, and had built. 

I feel very lucky to have found Paul, and he is within driving distance, so I hope any other problems that come up he will be able to rescue me once again. I've had it on a lot lately, but I enjoy it so much I can't help myself. 845 tubes rule!  

Regards,

Dan  

Have you thought about using the MyMat under your copper mat?

Check it out.

Steve

Hi, the 845 amp is a Renaissance Audio I bought from a friend, I haven't had it very long. I had it recapped, and It came with two sets of 845 tubes, both Psvane 845 that are identical, so I haven't tried changing output tubes.

It also came with two sets of Psvane 300B tubes. I am amazed at how good this amp sounds, I think it's now my favorite, the Tannoys just love the SET 845 amp.

Thanks for your nice comments, it's appreciated. Regards,

Dan

Uber, 

Thanks much for your kind words. The copper mat Is something I used bare for years, but I heard (read) you guys talking about Steve's MY MAT, and decided to try it. This mat is the only one in all these years I've heard a difference with. It's not a great improvement but discernable.

It seems to have reduced the amount of static electricity that builds up on the LP surface, so I guess it stays. I think I'd like it better if it were clear, though, as I like how the TTW mat looks.

I finally discovered how great a cartridge the Acutex is (M320 STR III). I don't know why, but it took me years to finally be wowed by it, and am I ever! I must not have had it set up right, but what a difference. My favorite cart right now, and it will probably remain that way, until something better comes along.

Regards,

Dan

Great looking system, I love that plinth on your 401!.

Now the ttw copper mat?

Best you have tried?

I have a hybrid cork and lead mat on mine right now but I do keep going between it and Slaws MyMat.

I also have an Acutex, the str312.

For the money quite something!

Keep enjoying the music brother!

Lately, I've been getting a great deal of pleasure from my system. Last time I tried my Acutex LPM M320 cartridge, it was making some horrible sounds. Looking at the stylus, I found some fuzzy-looking material (under magnification), which I removed, and was rewarded with great, and enjoyable, sound. Sometimes we forget how good something is, until we give ourselves a reminder.

This cartridge, up 'til now, hadn't been used with the new Modwright 9.0 SE   Line Stage and the SET 845 amp, which also is sounding wonderful since it's recent rebuild by Paul Birkeland.

Lots of fun with my system and music these days. Regards,

Dan

Noromance, tell me more about this bearing, if you please. It might be time to replace mine, after around 50 years, although it still sounds quite good to my old ears.

Thanks,

Dan 

The Modwright SWL 9.0 Line Stage has worked out very well. It's turned out my assumption of my preamp being the weak link was correct. The new line stage has taken my system to the state I've always been searching for.

I believe I really am "Done For Now". I think I may be "Done Forever"!

Regards,

Dan

I'm making a change in preamps, going from my kit-built pre, to a newly upgraded Modwright SPL 9.0 preamp, with upgraded power supply, new tube board with the new MWI cap upgrades. This will match my phono stage, my still great-sounding SWP 9.0 Signature.

Probably have it early next week, I'm looking forward to it.

Regards,

Dan

I just added a Benchmark DAC 1 Pre to the system, that I picked up on US Audiomart. It just might be the best $500 I've ever spent in this hobby. I know there are supposed to be much better DAC's available, but this thing is so good, it's almost scary. I also found out (after twenty years!) my Sennheiser HD 600 headphones are much better than I thought they were. The old Musical Fidelity X-CANS just wasn't doing justice to the HD 600's.

My digital front end has taken a substantial upturn in sound quality. I'm happy.

Regards,

Dan