Hi everyone.
I would like to build a copy of the Lowther LL-26 amplifier. Circuit attached.
I have a few questions, if I may. Firstly, I'm in The Philippines and finding parts can be difficult however Ali Express is a gateway to all sorts of things made in China which seem quite cheap. I have been looking at EL34 push pull output transformers and all the ones I see are stated as having 4-3 laminations. I take this to mean that the primary is split into 4 sections, and interleaved with the secondary which is split into three. My old Williamson book (I had a pair many years ago) suggested split bobbins and I think a 5-4 split.
I know there are many other aspects to transformer design but my question could a 4-3 split achieve reasonable sound quality or would this transformer be a non-starter for that reason?
Some of the valves/tubes used in that circuit are a little hard to come by now. But, a 6AS7 double triode looks close in specification to a 13D3. Its transconductance is higher at 1500 umhos vs. 1200 for the 13D3. Its plate resistance is 10K vs the 13D3's 12K. Would these differences be significant in its use as a phase splitter? The 6BJ6 is hard to find here too but the older 6K7 is very similar in electrical terms so I am assuming that it would work as a constant current source.
I have owned and restored various valve amplifiers in the past but my experience was limited to component substitution until the manufacturer's original specification was achieved so I am no expert. But I am happy to have a go at copying a schematic as long as I know that any component change I make will be ok. There is a lot to learn from building from scratch so I am keen to try.
I had thought of importing some old amps to rebuild but import duties would make it very expensive and I'd be worried about 60 plus year old transformers in this heat!
I just want to hear valves again. I'll be building some speakers with coaxial drivers, eventually. All my vinyl is in the UK, never to be heard again as it's too costly to get it here. I'm streaming music now and using a 70s Yamaha amplifier and big old Diatone 3 way loudspeakers. It's clean and pure but doesn't have the magic (or is it pleasing distortion) of valves and I wan't a project or two here.
If you got this far, thank you, and I look forward to hearing from the many people of experience here.
Yrs,
Jolyon
I would like to build a copy of the Lowther LL-26 amplifier. Circuit attached.
I have a few questions, if I may. Firstly, I'm in The Philippines and finding parts can be difficult however Ali Express is a gateway to all sorts of things made in China which seem quite cheap. I have been looking at EL34 push pull output transformers and all the ones I see are stated as having 4-3 laminations. I take this to mean that the primary is split into 4 sections, and interleaved with the secondary which is split into three. My old Williamson book (I had a pair many years ago) suggested split bobbins and I think a 5-4 split.
I know there are many other aspects to transformer design but my question could a 4-3 split achieve reasonable sound quality or would this transformer be a non-starter for that reason?
Some of the valves/tubes used in that circuit are a little hard to come by now. But, a 6AS7 double triode looks close in specification to a 13D3. Its transconductance is higher at 1500 umhos vs. 1200 for the 13D3. Its plate resistance is 10K vs the 13D3's 12K. Would these differences be significant in its use as a phase splitter? The 6BJ6 is hard to find here too but the older 6K7 is very similar in electrical terms so I am assuming that it would work as a constant current source.
I have owned and restored various valve amplifiers in the past but my experience was limited to component substitution until the manufacturer's original specification was achieved so I am no expert. But I am happy to have a go at copying a schematic as long as I know that any component change I make will be ok. There is a lot to learn from building from scratch so I am keen to try.
I had thought of importing some old amps to rebuild but import duties would make it very expensive and I'd be worried about 60 plus year old transformers in this heat!
I just want to hear valves again. I'll be building some speakers with coaxial drivers, eventually. All my vinyl is in the UK, never to be heard again as it's too costly to get it here. I'm streaming music now and using a 70s Yamaha amplifier and big old Diatone 3 way loudspeakers. It's clean and pure but doesn't have the magic (or is it pleasing distortion) of valves and I wan't a project or two here.
If you got this far, thank you, and I look forward to hearing from the many people of experience here.
Yrs,
Jolyon
Attachments
The problem with duplicating a high-feedback amplifier is that the circuit was tuned to the original output transformers. Without them the circuit may be unstable or otherwise unsatisfactory.
If you want to build a Williamson, which is a beautiful amplifier, look at the thread I posted on diyaudioprojects.com:
https://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=7557&hilit=williamson
I'd be happy to help send you the critical components, which are the beautiful Heyboer/Peerless output transformers. I would forward them to you at cost plus shipping. There are about $500US a pair. The PCBs are cheap to ship, or you can order them yourself.
If you want to build a Williamson, which is a beautiful amplifier, look at the thread I posted on diyaudioprojects.com:
https://diyaudioprojects.com/Forum/viewtopic.php?f=9&t=7557&hilit=williamson
I'd be happy to help send you the critical components, which are the beautiful Heyboer/Peerless output transformers. I would forward them to you at cost plus shipping. There are about $500US a pair. The PCBs are cheap to ship, or you can order them yourself.
Thank you for that, grovergardner. I should have thought more about the relationship between the transformers and the rest of the circuit. Thanks, too, for the kind offer. It would be nice to have some modern, high quality iron of good provenance. I did like my Williamsons. I swapped my Leak TL50s for them. In value terms it was the worst deal I have ever done, I was seen off, but, sonically, despite being way down on power compared with the Leaks they were far superior. Recently on UK eBay a pair of leak TL25+ went cheaply. I was going to bid but I remembered how my TL50s sounded and was mindful that the more powerful Leak valve amps became the worse they sounded so I wondered just how good the 25s would be. I did love my 12+ and Stereo 20s though. I noticed that the TL25+ were on the same size chassis as the 12+ and that the output transformers had the same frame size so that's why i didn't try to get them. maybe I have made another bad decision!
I find that it's only if you dig really deep online do you find honest and thoughtful comments on old gear. It's either wonderful or terrible! I am therefore thinking about the Williamson circuit. There is a lot of love and hate for it and I am too ignorant to see through either. It's because I have already had a pair that I am tempted by something different even though I liked them.
Could the excellent transformers you mention be used in other circuits like the Mullard 5-20 or something else? Or, could you recommend a more "forgiving" schematic/circuit that these Chinese output transformers are likely to work in?
Jolyon
I find that it's only if you dig really deep online do you find honest and thoughtful comments on old gear. It's either wonderful or terrible! I am therefore thinking about the Williamson circuit. There is a lot of love and hate for it and I am too ignorant to see through either. It's because I have already had a pair that I am tempted by something different even though I liked them.
Could the excellent transformers you mention be used in other circuits like the Mullard 5-20 or something else? Or, could you recommend a more "forgiving" schematic/circuit that these Chinese output transformers are likely to work in?
Jolyon
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There shouldn't be any trouble in substituting slightly different valves in the phase splitter and its CCS. As pointed out, an amplifier's compensation networks are tailored to the particular output transformer, but it's not difficult to optimise values by observing a 10kHz square wave on an oscilloscope and tweaking values for cleanest square wave with minimal overshoot. Obviously, if you don't have access to square wave and oscilloscope, life is a little harder, but there's not as much difference between output transformers as you might think. In short, go for it!
The only people who "hate" the Williamson are people who've never heard it. ;-) And I've employed some tweaks to the circuit to improve stability. But, no, those Heyboer transformers wouldn't be appropriate for a Mullard-style amp. The load is too high. Here's a fully tested copy of an Eico HF-87 (warning, the ads are awful if you are not a member):
https://audiokarma.org/forums/index...dified-eico-hf-87.847158/page-6#post-15999768
It uses off-the-shelf transformers that you should be able to source. A very nice little amp. Of course, with Lowthers you could always build a single-ended amp with no feedback, which makes things simpler. But the LL26 is a challenge because of the adjustable compensation network which has to be tuned with a scope.
https://audiokarma.org/forums/index...dified-eico-hf-87.847158/page-6#post-15999768
It uses off-the-shelf transformers that you should be able to source. A very nice little amp. Of course, with Lowthers you could always build a single-ended amp with no feedback, which makes things simpler. But the LL26 is a challenge because of the adjustable compensation network which has to be tuned with a scope.
Thank you agsin for the helpful comments. I get too easily diverted and I've seen all sorts of nice output transformers on ebay, including Dynaco and Tango. I just need to select a particular solution and stick to it. I do like the idea of coaxial sensitive drivers so I don't need many Watts so now I'm thinking about single ended as well but I know even less about them and the hifi press' generous prose about them rather put me off, maybe unfairly.
I shall not be returning to Lowther drive units, thiugh. That much I do know.
Jolyon.
I shall not be returning to Lowther drive units, thiugh. That much I do know.
Jolyon.
JDef: The 6AS7 (mu=2) is not a subsitute for the 13d3. You can use an ECC82 (mu=17) there or an ECC 40 (which has a mu close to that of the 13D3);
An other possibility might be to use the schematic of the LL18 (which I have). A cascode ECC82 is directly coupled with a Schmidt fase splitter and the following EL34's are hooked up the same way as in the LL26. Lowther uses the same outputs as the Leak (ultralineair). You can find the schematic on the net. Actually I don't like to steer you in a different direction but the ECC82's are easier to find (if you don't already have those) and it's still Lowther what we are talking about here.
An other possibility might be to use the schematic of the LL18 (which I have). A cascode ECC82 is directly coupled with a Schmidt fase splitter and the following EL34's are hooked up the same way as in the LL26. Lowther uses the same outputs as the Leak (ultralineair). You can find the schematic on the net. Actually I don't like to steer you in a different direction but the ECC82's are easier to find (if you don't already have those) and it's still Lowther what we are talking about here.
There's nothing wrong with SET amplifiers, provided that you understand their strengths and weaknesses, that you pick a trustworthy design, and use high-quality components. My first amplifier build 25 years ago was a Western Electric 91A clone touted by Sound Practices magazine. It used Magnequest output transformers, which sadly are no longer made. It was a lovely amplifier. My wife often asks me why I didn't stop there. ;-)
There are some winders of very good OPT and PS iron in Philippines. To avoid a comment that could be construed as commercial, PM me for some contact details. Some quality tube amp builders as well.
My 2 cents, ymmv etc.
I only have a hand drawn copy. As it's incomplete (the power supply voltages are missing). Tomorrow I will take the amp from the shelf and power it up to see what the voltages are. If you mail me your address through direct mail I'll draw a better copy and send it to you.