Does anyone have a Williamson tube amplifier?

[rongon]

Yeah, but beam tetrodes. And ultra-linear!

/

[Edit to add: This post was meant to be seen by the OP. For you experts who have graciously contributed, please disregard my comments that most likely look completely obvious to you, or possibly poorly stated. Thanks.]

The original D.T.N. Williamson amp used KT66 beam tetrode output tubes wired triode (short the screen grid to the plate and you get a triode).
Ultralinear ('distributed load' primary, first proposed by Alan Blumlein, who first proposed a whole lot of stuff!) is sort of midway between triode and pentode operation (requires a specially wired transformer primary) so it's not that far off the Williamson design idea.

If you're very interested in the original Williamson amplifier, you might want to check out the reprint of the Wireless World articles on the subject -- https://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_01/Sec_06/235_WW_Williamson_Reprints.pdf

To answer a previous question about stability:

1. The original Williamson amplifier design *required* a very special, custom designed output transformer with 10k ohm primary to 4.8.16 ohm secondaries, with very low leakage inductance and stray capacitance. The original article went so far as to provide information on how to wind the special transformer needed.

2. The Williamson amplifier circuit creates a LOT of open loop gain, which is then used to apply a relatively large amount of negative feedback around the entire circuit. It's this high amount of NFB that creates the stability issues you'll see people talking about with this design.

Since it's difficult to find a contemporary transformer with the specifications of the originals, one needs to be very careful that the high levels of NFB don't cause ringing and oscillations on the high frequency end of things.

Also, since there are 3 inverting stages that are AC coupled (resistor-capacitor coupled) to the next stage, and the output transformer has limited low frequency bandwidth too, one needs to make sure the final amplifier doesn't oscillate at subsonic frequencies ('motorboating'). With three inverting amplifier stages, if each stage has a similar rolloff in the low frequencies, and if the power supply impedance is high at those low frequencies, those low frequencies can kind of 'bleed' into other stage's power supply feeds, in opposite polarity to each other, causing the different stages to amplify very low frequency perturbations. This can cause low frequency oscillations that you may not be able to hear (not audible) but sap power from the amplifier and/or muddy the audible frequency range.

The negative feedback is boosting both the very low frequencies that can't be passed by the output transformer (and possibly the individual amplifier stages) as well as boosting the very high frequencies that can't be passed by the output transformer. These large boosts can be reversed in phase and turn the negative feedback into *positive* feedback, which will oscillate.

One has to be very careful with a Williamson amplifier to carefully check the final amp for stability both below and above the audible frequency range.

The original Williamson design was meant to show the advantages to be gained by building a large amplifier with lots of gain and then applying a relatively large amount of negative feedback to make the final output very low in THD. That large amount of negative feedback means one needs to be very careful about measuring the amplifier after building it, and then modifying various filter networks to 'compensate' the amplifier, or damp down any subsonic or ultrasonic resonances or oscillations caused by the NFB 'ringing'.

 

[MattHooper]

It is; so much goes wrong when people use any old tube and some standard tap for the screen (43% being common). Done right, it's a great way to run an output stage.

Hi SIY.

I hope you don't mind a few questions.

I'm just curious:

1. What is the most powerful tube amp that you have built?

2. Do you use the tube amps you build for your home speakers? (The modded NHTs? I think they aren't high sensitivity but a relatively uniform load for a tube amp?)

3. If you use them with your NHTs, which go quite low, was it much of a challenge to design a tube amp that would drive and maintain control of such wide frequency
speakers (low bass)?

Thanks.

 

[SIY]

Hi SIY.

I hope you don't mind a few questions.

I'm just curious:

1. What is the most powerful tube amp that you have built?

2. Do you use the tube amps you build for your home speakers? (The modded NHTs? I think they aren't high sensitivity but a relatively uniform load for a tube amp?)

3. If you use them with your NHTs, which go quite low, was it much of a challenge to design a tube amp that would drive and maintain control of such wide frequency
speakers (low bass)?

Thanks.

1. 200 watt monoblocks. Sweep tubes, screen drive, very efficient (for tubes).
2. I packed all my tube amps away when I moved to Phoenix in 2019. They never got unpacked before I moved to rural New York. When they're unearthed, I'll probably use them here in the winter.
3. My NHTs have had a LOT of changes from factory stock, so the tube amps don't need to go below 100 Hz or so. I've always used Class D on the woofers (originally Sunfire, then Purifi).

 

[MattHooper]

1. 200 watt monoblocks. Sweep tubes, screen drive, very efficient (for tubes).
2. I packed all my tube amps away when I moved to Phoenix in 2019. They never got unpacked before I moved to rural New York. When they're unearthed, I'll probably use them here in the winter.
3. My NHTs have had a LOT of changes from factory stock, so the tube amps don't need to go below 100 Hz or so. I've always used Class D on the woofers (originally Sunfire, then Purifi).

Thanks!

Wow, 200 watts! That's impressive!

Ah, the info about how you power your NHTs makes sense. Must sound spectacular. I doubt they would have to apologize for playing any type of musical genre.

 

[SIY]

Ah, the info about how you power your NHTs makes sense. Must sound spectacular. I doubt they would have to apologize for playing any type of musical genre.

I have a more complete write-up on the speakers on my website. And yes, I have had zero temptation to replace them with anything newer, but if the right ESL panels presented themselves...

 

[MattHooper]

I have a more complete write-up on the speakers on my website. And yes, I have had zero temptation to replace them with anything newer, but if the right ESL panels presented themselves...

Fantastic. I'm going to delve in to your write up on the NHTs now...

 

[MattHooper]

I have a more complete write-up on the speakers on my website. And yes, I have had zero temptation to replace them with anything newer, but if the right ESL panels presented themselves...

I read your write-up. Admittedly my eyes go a bit criss-crossed with the schematics, but you are an entertaining writer and I enjoyed the read.

Interesting that you managed to snag a rare pair from Jack Hidley himself from the warehouse. I had a similar bit of luck. I'd really fallen for a pair of Hales Transcendence 1 monitors that I found - quite rare because Hales folded not long before those became available, and it had been years since Hales was in operation. I liked the T1 speakers so much I really wanted back-ups, plus I wanted a third to try using them as L/C/R for my home theater. I searched everywhere and somehow got a tip to phone Paul Hales (Hales speaker designer). Turns out he still had an old pair, only ones left, sitting in his office that he wasn't using much, so he sold them to me. It's kind of fun owning old gear that isn't around any more, but which you personally value.

 

[SIY]

I read your write-up. Admittedly my eyes go a bit criss-crossed with the schematics, but you are an entertaining writer and I enjoyed the read.

Interesting that you managed to snag a rare pair from Jack Hidley himself from the warehouse. I had a similar bit of luck. I'd really fallen for a pair of Hales Transcendence 1 monitors that I found - quite rare because Hales folded not long before those became available, and it had been years since Hales was in operation. I liked the T1 speakers so much I really wanted back-ups, plus I wanted a third to try using them as L/C/R for my home theater. I searched everywhere and somehow got a tip to phone Paul Hales (Hales speaker designer). Turns out he still had an old pair, only ones left, sitting in his office that he wasn't using much, so he sold them to me. It's kind of fun owning old gear that isn't around any more, but which you personally value.

Thanks for the kind words. I actually got two pair, but one was disassembled for parts.

NHT was just about a twenty minute walk from my house, so I was able to get in early on a bunch of things. One thing I still have is a large pile of woofer/midrange from the Super Zero (like maybe 50 of them). Initially I wanted to do a line array with them, but now I have an alternate idea and access to some great fabrication resources.

 

[DanielT]

[Edit to add: This post was meant to be seen by the OP. For you experts who have graciously contributed, please disregard my comments that most likely look completely obvious to you, or possibly poorly stated. Thanks.]

The original D.T.N. Williamson amp used KT66 beam tetrode output tubes wired triode (short the screen grid to the plate and you get a triode).
Ultralinear ('distributed load' primary, first proposed by Alan Blumlein, who first proposed a whole lot of stuff!) is sort of midway between triode and pentode operation (requires a specially wired transformer primary) so it's not that far off the Williamson design idea.

If you're very interested in the original Williamson amplifier, you might want to check out the reprint of the Wireless World articles on the subject -- https://www.pearl-hifi.com/06_Lit_Archive/02_PEARL_Arch/Vol_01/Sec_06/235_WW_Williamson_Reprints.pdf

To answer a previous question about stability:

1. The original Williamson amplifier design *required* a very special, custom designed output transformer with 10k ohm primary to 4.8.16 ohm secondaries, with very low leakage inductance and stray capacitance. The original article went so far as to provide information on how to wind the special transformer needed.

2. The Williamson amplifier circuit creates a LOT of open loop gain, which is then used to apply a relatively large amount of negative feedback around the entire circuit. It's this high amount of NFB that creates the stability issues you'll see people talking about with this design.

Since it's difficult to find a contemporary transformer with the specifications of the originals, one needs to be very careful that the high levels of NFB don't cause ringing and oscillations on the high frequency end of things.

Also, since there are 3 inverting stages that are AC coupled (resistor-capacitor coupled) to the next stage, and the output transformer has limited low frequency bandwidth too, one needs to make sure the final amplifier doesn't oscillate at subsonic frequencies ('motorboating'). With three inverting amplifier stages, if each stage has a similar rolloff in the low frequencies, and if the power supply impedance is high at those low frequencies, those low frequencies can kind of 'bleed' into other stage's power supply feeds, in opposite polarity to each other, causing the different stages to amplify very low frequency perturbations. This can cause low frequency oscillations that you may not be able to hear (not audible) but sap power from the amplifier and/or muddy the audible frequency range.

The negative feedback is boosting both the very low frequencies that can't be passed by the output transformer (and possibly the individual amplifier stages) as well as boosting the very high frequencies that can't be passed by the output transformer. These large boosts can be reversed in phase and turn the negative feedback into *positive* feedback, which will oscillate.

One has to be very careful with a Williamson amplifier to carefully check the final amp for stability both below and above the audible frequency range.

The original Williamson design was meant to show the advantages to be gained by building a large amplifier with lots of gain and then applying a relatively large amount of negative feedback to make the final output very low in THD. That large amount of negative feedback means one needs to be very careful about measuring the amplifier after building it, and then modifying various filter networks to 'compensate' the amplifier, or damp down any subsonic or ultrasonic resonances or oscillations caused by the NFB 'ringing'.

That was interesting. Seems like a fun challenge for EE, a person with electrical knowledge to tinker with.

 

[DanielT]

1. 200 watt monoblocks. Sweep tubes, screen drive, very efficient (for tubes).
2. I packed all my tube amps away when I moved to Phoenix in 2019. They never got unpacked before I moved to rural New York. When they're unearthed, I'll probably use them here in the winter.
3. My NHTs have had a LOT of changes from factory stock, so the tube amps don't need to go below 100 Hz or so. I've always used Class D on the woofers (originally Sunfire, then Purifi).

Just out of curiosity, roughly what do you think the parts for two such 200 watt monoblocks would cost?

I would guess that the output transformers that would be needed for those 200 watts must cost a small fortune, right?
(yes, I know a small fortune is a subjective term, but still)

 

[SIY]

Just out of curiosity, roughly what do you think the parts for two such 200 watt monoblocks would cost?

I would guess that the output transformers that would be needed for those 200 watts must cost a small fortune, right?
(yes, I know a small fortune is a subjective term, but still)

No idea, TBH. Not cheap to duplicate, but it was not expensive to build the Ur pair.

The output transformers I used were from Northlake, who apparently aren't around any more (they were actually made for Conrad Johnson but through some odd coincidences ended up in my hands). 1250 ohm primary, 250W rating, in case you want to price some alternatives. The output tubes are 6LF6 which have gotten ridiculously expensive, but it could probably be redesigned for a different sweep tube, maybe multiple 6JN6, which I think are still affordable. Power transformers were rescued from some Ampex mono power amps I got from a surplus shop for $10 each.

 

[DanielT]

No idea, TBH. Not cheap to duplicate, but it was not expensive to build the Ur pair.

The output transformers I used were from Northlake, who apparently aren't around any more (they were actually made for Conrad Johnson but through some odd coincidences ended up in my hands). 1250 ohm primary, 250W rating, in case you want to price some alternatives. The output tubes are 6LF6 which have gotten ridiculously expensive, but it could probably be redesigned for a different sweep tube, maybe multiple 6JN6, which I think are still affordable. Power transformers were rescued from some Ampex mono power amps I got from a surplus shop for $10 each.

Cool.

If you have knowledge and if you have been building yourself for a long time, you can construct the amplifier in the way you did. I don't know what a commercial 200 watt tube monoblock would cost? Then perhaps the term a smaller fortune would fit better.
If there are even such powerful commercial tube monoblock that is.

Better, as you mentioned in previous posts, to drive the bass modules with class D. In addition, make sure to have "tube amp friendly speakers" of at least 90-92 dB sensitivity. Then a 30 watt tube amp is probably enough.Perhaps less power if you don't listen at such a high volume. Definitely less power if it's thick compressed porridge music you listen to. It does not make such high demands on amp power. That in and of itself is good. Less fun for the ears to listen to but that's another matter.

I am becoming more and more convinced that tube amps and DIY seem to be a good match. I can definitely see the allure of fiddling with tube amps, provided the knowledge in electronics is there.

OT:
For me, class D is the thing. But I want to power the tweeters with class AB amps. Some class D amps seem to have such uneven FR (depending on the load from the speaker) in the higher frequencies. In any case, it seems it can be like that with small cheap class D amps.

How FR changes even with the same speaker when small class D amps are tested. See from 6:50 into the video:

Edit:
A three-way speaker where a class D amp takes care of bass. Mid and tweeter powers by a class AB amp. That I think is a good solution.
Or for those who promptly want tube amps, replace class AB in that solution with a tube amp.Or a combo with subwoofer with HP-LP filter.Or a...there are many fun ways to assemble a HiFi solution.

 

[SIY]

Cool.

If you have knowledge and if you have been building yourself for a long time, you can construct the amplifier in the way you did. I don't know what a commercial 200 watt tube monoblock would cost? Then perhaps the term a smaller fortune would fit better.
If there are even such powerful commercial tube monoblock that is.

Better, as you mentioned in previous posts, to drive the bass modules with class D. In addition, make sure to have "tube amp friendly speakers" of at least 90-92 dB sensitivity. Then a 30 watt tube amp is probably enough.Perhaps less power if you don't listen at such a high volume. Definitely less power if it's thick compressed porridge music you listen to. It does not make such high demands on amp power. That in and of itself is good. Less fun for the ears to listen to but that's another matter.

I am becoming more and more convinced that tube amps and DIY seem to be a good match. I can definitely see the allure of fiddling with tube amps, provided the knowledge in electronics is there.

OT:
For me, class D is the thing. But I want to power the tweeters with class AB amps. Some class D amps seem to have such uneven FR (depending on the load from the speaker) in the higher frequencies. In any case, it seems it can be like that with small cheap class D amps.

How FR changes even with the same speaker when small class D amps are tested. See from 6:50 into the video:

Tubes are fun from a design/construction standpoint. Not SOTA performance, but with actual engineering, good enough for the purpose. I would never actually buy a tube amp- commercial versions are generally pretty mediocre and nosebleed expensive.

I have a new design in mind which I'll try to get finished over the summer.

 

[sergeauckland]

Tubes are fun from a design/construction standpoint. Not SOTA performance, but with actual engineering, good enough for the purpose. I would never actually buy a tube amp- commercial versions are generally pretty mediocre and nosebleed expensive.

I have a new design in mind which I'll try to get finished over the summer.

That's very much the reason I've recently gone back to tubes, for the fun of building from scratch. I'm not in the 200 watt range, 10 watts is more my level, and even then, buying the ironwork from new, (Output TF, Mains TF and smoothing choke) for a EL84 PPUL amp would come to around £600 per channel. For something more powerful, you're looking at £1000 per channel just for the ironwork.

What I did was to buy a pair of very scruffy 'For parts or not working' amps with decent transformers for less than half that, which also gave me the tubes, albeit not all were working, so I built a pair in total for about £750. I took the risk that the transformers were likely to be OK, which fortunately proved to be the case. Silly really, considering I could have a much higher power, much less distorting Class D amp for much less than that, but then it won't glow in the dark!

An OTL amp would be great, but not found one yet that didn't involve lots of parallel tubes and therefore a big cost, lots of energy consumption and heat output and still a high output impedance.

S

 

[trobbins]

If the history of the Williamson amp is your interest then I can recommend the article written by Peter Stinson https://dalmura.com.au/static/The Williamson Amplifier History.pdf.

If you are interested in the many and various articles and docs and variants and transformers made back then (late 1940's to 1960's) then this webpage provides a large compendium of items that were influenced by the Williamson amp. https://dalmura.com.au/projects/Williamson.html

 

[Valvetubehead]

My dad assembled this Heathkit W-5M back in the late ‘50’s along with a Klipsch knock-off Knight diy corner speaker loaded with an EV 12trxb.

 

[Ra1zel]

Today we an simply do so much better with a tube amp if you are willing to introducesome complexity: better loading methods (CCS, mu follower), regulation of supplies, shunt regulation, quality of parts higher than ever before. Even transformers if you are willing to splurge can be on a level that people of that era could only dream of.

Most of this involves committing the cardinal sin of audiophile purists and adding "sand" to your tube amp, not that anyone reasonable cares.