Croft Polestar III 2 box power amp project

[worrasf]

I recently acquired this Croft Polestar III 2-box power amp.

Having shared some photos with Glenn he tells me that this is a true "one off" unique amplifier built in 2006 as a company development amp.

It is apparently of uber high spec featuring not only a 2-box massive external PSU but also Eichmann speaker posts and doubled up transformers and a copper chassis for the amp section. The flying power leads are a sight to behold. :eek:

Glenn reckons it would have sold for around £3K back in the day. :stalks:

It looks like it was stored in someones' shed for yonks but apart from a thin layer of dust it all looks to be OK. Glenn reckons shouldn't need anything doing to it.

However, it came sans 5965 valve and the main power fuse was missing and the fuse holder head "cheesed" where it had clearly been abused in the past. One of the other fuses was also the wrong spec so I'm not fully confident that it will be a runner :eyebrows:

I've managed to repair the fuse holder and have got the correct fuses and a NOS GE 5965 on order so hopefully will be able to test it over the next few days.

Assuming it powers up and doesn't blow my speakers then It will be very interesting to compare with my Croft 7 mono blocks.

More to follow - good or bad ;)

Steve

[The Black Adder]

Very nice find. Never heard that type but I can only think that it would be rather special. But I feel it's rather a different beast than the 7 mono's as it uses a 5965 which is a very different sounding valve than a ECC82/83.

[worrasf]

Glenn says will also take an ECC81,
There's a feedback comment I came across on one of the US valve sites that an RFT 12AT7 sounded more open and less warm than the GE 5965 that the guys Polestar 1 came with so even here there's room for tube rolling


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[The Black Adder]

Wow.. how cool.. a very flexible amp then. :)

[Firebottle]

It will be very interesting to compare with my Croft 7 mono blocks.

Keep us posted on this, very interested to hear your thoughts :)

[worrasf]

Wilco - I've also ordered an RFT 12AT7 (ECC81) so assuming I can get it running then I can also experiment with 5965/ECC81


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[DSJR]

Not my expertise, but please check the 12AT7 is a pukka drop-in valve replacement. I seem to remember the heater? current is higher, but obviously Glen will advise as he knows the current ratings of this part of the circuit.

Sorry if it's granny sucking eggs...

[worrasf]

Yep already checked with the maestro - he wanted to see photos of the valve base components and having done so told me will run 5965 and also ECC81


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[worrasf]

Very nice find. Never heard that type but I can only think that it would be rather special. But I feel it's rather a different beast than the 7 mono's as it uses a 5965 which is a very different sounding valve than a ECC82/83.

Forgot to mention earlier that Glenn fitted my 7 mono blocks with JJ ECC99 gold pins not ECC83's

Steve

[worrasf]

So I've now got this up and running - just needed new fuses and a valve. I have to say it sounds utterly stunning. It took about 30 mins to warm up/stabilise before the true character declared itself.

The soundstage is immense and very tightly focussed - bass is hugely deep and tuneful - I had to turn the gain on the REL subwoofer down a notch (I set this up until I can just hear the bass line and then up one notch so it just supports the main speakers).

It is a very dynamic presentation- PRaT are spot on for my taste. The additional power reserves over the 7 monos is evident in the iron grip on the speakers.

Tonal balance is slightly warm of neutral (maybe an ECC81 would change this a tad) - the clocks and bells on dark side have never sounded so clear and vibrant the final 2 "tangs" are awesome.

Compared to the 7's there is perhaps a very slight blurring of steel brush and cymbals which may change with a different valve type but it's of little consequence as the overall sonic picture is just so convincing and enjoyable.

Trawling the web I've not come across any references to a 2-box Polestar- all the images I've seen of the rear panels show series 1 through 3 but mine shows 1234.
Glenn was unsure whether it could be called a Polestar 4 or not.

I can not see me going back to the 7 monos anytime soon after experiencing what this 2006 amp can do so if the Polestar proves reliable they may be looking for a new home.
This has to be the best £350 (plus fuses and valve) bargain ever. :cool:

Steve


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[Arkless Electronics]

I have NOS Mullard mil spec ECC81's available ;)

[worrasf]

Thanks - I've got a couple of Siemens ECC801S's and an RFT ECC81 I might "roll" at some stage when I'm fully familiar with the new sound - it really is very impressive -i guess those 2 huge trannies really make a difference


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[Arkless Electronics]

Thanks - I've got a couple of Siemens ECC801S's and an RFT ECC81 I might "roll" at some stage when I'm fully familiar with the new sound - it really is very impressive -i guess those 2 huge trannies really make a difference


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Probably quite little IMO....

[worrasf]

Interesting- but isn't the reason why the 7R is a step up from the 7 because of the bigger/better regulated PSU?
Going back to the ecc81's given their higher gain over the 5965 I suspect it might be "too much". Glenn modified / increased the gain on my 25RS to cater for a coupe of low output MC's I'm running


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[Arkless Electronics]

Interesting- but isn't the reason why the 7R is a step up from the 7 because of the bigger/better regulated PSU?
Going back to the ecc81's given their higher gain over the 5965 I suspect it might be "too much". Glenn modified / increased the gain on my 25RS to cater for a coupe of low output MC's I'm running


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The regulation is for the gain stages not the output stage... IMO the "mains transformer 10 times bigger than required" thing is another bit of marketing foo. It may add say £150+ to the parts cost but the "wow! Look at the size of the transformer/s!" and "feel the weight of that!" can add say £1K to the perceived value of a unit.... whilst usually giving only a small increase in SQ, which could be bettered by spending the money elsewhere. I'll say "IMO" as no doubt I'll end up having to give a long technical explanation as to why... after which several people who probably didn't understand the explanation will tell me I'm wrong...:D just... "'cos" .... and "my amp has a huge transformer so that must be why it sounds so good":D So... YMMV and all that...

Well I hope I just killed the circular argument that I may have been a silly boy and started there:lol:

As to the valves well if Glenn modified it to take the 5965 rather than ECC81 then the 5965 is the one to use. The values of the resistors used to set the operating point etc will only be correct for one particular valve type;) One sometimes hears of "I took the ECC83's out and replaced them with ECC82's and...." People sometimes even report favourable results from this.... but often you will have increased distortion from say 0.05% to 4% or worse because the resistors in the amp are chosen to set the voltages on the electrodes of the valve to the correct ones for the ECC83... not ECC82.

Some types are so similar that they can be interchanged without problem of course. There is often all sorts of versions of a particular valve, mil spec, extra long life, newer revisions, equivalents from another country etc that have different type numbers but are built to a very similar spec. and intended to be interchangeable.
In the case of eg the ECC83 and 82 though they have the same pin-outs and look very similar but they have very different characteristics!

Probably teaching me granny to suck eggs on that one but it could be useful for some people:)

[worrasf]

All good info and lots of reasons for me to stick to my day job 😷


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[Wakefield Turntables]

Jez, I think your correct about the Extra large transformers being Foo. I gave Nick Gorham complete freedom to mod my Croft Epoch he concluded that the transformer did a perfectly good job thanks very much, I did ask about the two box mod and he advised against it. Who am I to argue against people that design/build and mod such things? ;);) My Epoch is still a one box affair and if anything else needs modding it's probably the valve regulation.

[Arkless Electronics]

Jez, I think your correct about the Extra large transformers being Foo. I gave Nick Gorham complete freedom to mod my Croft Epoch he concluded that the transformer did a perfectly good job thanks very much, I did ask about the two box mod and he advised against it. Who am I to argue against people that design/build and mod such things? ;);) My Epoch is still a one box affair and if anything else needs modding it's probably the valve regulation.

;)

[worrasf]

Mmm! If Glenn as the designer and builder (not the aftermarket modder) sees fit to build 2 boxes that's good enough for me - and I've heard the results!


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[Wakefield Turntables]

Mmm! If Glenn as the designer and builder (not the aftermarket modder) sees fit to build 2 boxes that's good enough for me - and I've heard the results!

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Yep, makes perfect sense to me as well, but I'm very happy with what I own and quite often what Glenn does dosen't seem to make sense but works!
Additionally I didn't have any more rack space for another box :lol:. I can only wish that it hits all the right spots for you. I'm happy that you've found something so special.

[The Black Adder]

Mmm! If Glenn as the designer and builder (not the aftermarket modder) sees fit to build 2 boxes that's good enough for me - and I've heard the results!

+1

Andrew, I certainly wouldn't go back to the internal stock type. :nono: - No way, not a chance, not even if you paid me.

[Arkless Electronics]

As far as the 2 box bit goes, and above a certain "hi-end-ness" quite a bit of gear is 2 box, the erm.. gospel according to Jez on that is

MC phono stages - Very desirable! It's almost impossible to completely ban the last remnants of mains hum with the transformer in the same box, and with all that gain any mains vibration is not good...

MM phono stages - Desirable but not nearly so important as with an MC stage due to lower gain.

Line stages - No real advantage.

Power amps - Not a good idea unless size and weight dictate it. You are using huge transformers etc and then putting the resistance of the cable, and any plugs and sockets used, between this humongous PSU and the bit needing the power.... Not a game ender or anything if the cable and connectors are sufficiently heavy duty and the cable as short as practicable but, technically, not ideal....

YMMV :)

[Wakefield Turntables]

Yeah, happy for you as well Joe. I'm perfectly happy plodding on with what I have sitting in front of me.

[Wakefield Turntables]

Suppose I better declare my 4-box phonostage then... :rfl:, I'm not a hater of the two box pre or power amp, and better still if it sounds better in an individuals system.

[The Black Adder]

One of those things to do at a later day maybe.

[Wakefield Turntables]

One of those things to do at a later day maybe.

Me? Nope. Valve regulations for me mate if anything else.

[The Black Adder]

Good move... :)

[Jimbo]

A question for Jez, would you recommend valve regulation of a power amp to be more important than a large PSU and do you think valve regulation is necessary/ better than non regulated?

[Arkless Electronics]

Suppose I better declare my 4-box phonostage then... :rfl:, I'm not a hater of the two box pre or power amp, and better still if it sounds better in an individuals system.

It's an MC phono stage so it fits in with the technical philosophy I outlined:) I certainly don't hate or want to preach about any amount of boxes as a solution! Just saying where, technically, there is an obvious case(sic) for the use of either one box or more than one....

IMHO some hi-end gear is offered as a two box solution purely to allow a suitably hi-end price tag to be applied to it... a second equally bling box with maybe a digital read out of PSU status or something may increase the parts cost by say £350 but allow the asking price to be £12K when the one box version is "only" £7K, and without any genuine technical reason... it's just what "the market" expects and perceives as the next step up from a single box solution maybe?

[Arkless Electronics]

A question for Jez, would you recommend valve regulation of a power amp to be more important than a large PSU and do you think valve regulation is necessary/ better than non regulated?

A can of worms...

I wouldn't recommend valve regulation of a solid state or hybrid power amp under any circumstances! Disaster! I think you mean regulation of the gain stages in the case of the Croft....? A valve regulated supply couldn't supply the sort of current needed by the output stages;) As to whether regulated or non regulated is better well that could bring forth a range of opinions! Mine is that regulated is definitely better, and can be a lot better, but there is lots of scope for getting it wrong!
As to valve regulator V SS regulator there is simply no contest technically... A good SS regulated supply is better by an order of magnitude or several than a valve one in every respect. To the point that there can be no technical justification for the use of valve regulation under any circumstances! Now as to whether it has any subjective advantages due to some valve magic being imbued to the actual voltage is something that I'm sure could be debated..... I can't see it myself but admit that due to the technical superiority of SS being crushing when it comes to regulators... I've never tried it. Maybe I'm missing out... who knows? It has occurred to me that a valve regulated supply is still better than an unregulated supply and that the obvious reason they are used in valve equipment is to stick to the notion of all valve, "no SS", "no sand" etc for marketing/brand reasons... call me cynical:D

I did design and build a bench PSU for testing valve prototypes etc in which I used valve regulation purely for its ability to withstand a dogs abuse electrically! Which it did for many years. For powering really sensitive stuff I sometimes had to put a SS regulator in line after the valve one though, which should tell you quite a lot ;)

[Jimbo]

A can of worms...

I wouldn't recommend valve regulation of a solid state or hybrid power amp under any circumstances! Disaster! I think you mean regulation of the gain stages in the case of the Croft....? A valve regulated supply couldn't supply the sort of current needed by the output stages;) As to whether regulated or non regulated is better well that could bring forth a range of opinions! Mine is that regulated is definitely better, and can be a lot better, but there is lots of scope for getting it wrong!
As to valve regulator V SS regulator there is simply no contest technically... A good SS regulated supply is better by an order of magnitude or several than a valve one in every respect. To the point that there can be no technical justification for the use of valve regulation under any circumstances! Now as to whether it has any subjective advantages due to some valve magic being imbued to the actual voltage is something that I'm sure could be debated..... I can't see it myself but admit that due to the technical superiority of SS being crushing when it comes to regulators... I've never tried it. Maybe I'm missing out... who knows? It has occurred to me that a valve regulated supply is still better than an unregulated supply and that the obvious reason they are used in valve equipment is to stick to the notion of all valve, "no SS", "no sand" etc for marketing/brand reasons... call me cynical:D

I did design and build a bench PSU for testing valve prototypes etc in which I used valve regulation purely for its ability to withstand a dogs abuse electrically! Which it did for many years. For powering really sensitive stuff I sometimes had to put a SS regulator in line after the valve one though, which should tell you quite a lot ;)

Thanks for the comprehensive reply Jez, I think I would agree with you regarding SS regulation vs valve. I believe regulation of a valve power amp helps to control the speakers a tad more?

[worrasf]

Well I'm no electronics designer and have zero knowledge - I spend my hard earned dollar on Croft kit because it is superb value - I don't think anyone would accuse Glenn of building "unnecessary" kit simply to increase profits ( have you seen his wreck of a Porsche?)
All I know is the Croft valve head amp had a massive improvement in SQ when he made me a custom external PSU (supports the Arlkless theory for this kit) but so did the 25 when it went to 25RS spec. I can't directly compare a 7 to 7R but I am in no doubt that the SQ of the 2 box Polestar with all the flying leads etc is a significant increase in all areas over the single box 7 mono. I only paid £350 for the Polestar (somewhat less than the £1400 I paid for the 7 monos) so I don't even have to justify it on cost grounds ( ie. I didn't pay £3K and as such I'm absolutely going to like it).

TBH I have little or no interest in the esoterica of circuit design (I wouldn't expect folk to be overly interested in the design parameters of the titanium plates and screws I use to fix their busted faces) I just go by my ears and buy what I find enjoyable at a price I think is fair which was the original aim of my post.


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[Arkless Electronics]

Thanks for the comprehensive reply Jez, I think I would agree with you regarding SS regulation vs valve. I believe regulation of a valve power amp helps to control the speakers a tad more?

I'm not sure if you mean regulation of the main HT supply which powers the output valves and therefore, indirectly, the speakers or, as in the case of the Croft amps, a regulated supply which powers the stages before the output stage, ie the gain stage....

I can say that I once added 3 separate regulated supplies to a friends Leak Stereo 20 in a cost no object upgrade so that all supplies were regulated. Other mods were also carried out. It was one of the best sounding amps I've ever heard. Period. And driving ESL57's definitely the best!

[Arkless Electronics]

Well I'm no electronics designer and have zero knowledge - I spend my hard earned dollar on Croft kit because it is superb value - I don't think anyone would accuse Glenn of building "unnecessary" kit simply to increase profits ( have you seen his wreck of a Porsche?)
All I know is the Croft valve head amp had a massive improvement in SQ when he made me a custom external PSU (supports the Arlkless theory for this kit) but so did the 25 when it went to 25RS spec. I can't directly compare a 7 to 7R but I am in no doubt that the SQ of the 2 box Polestar with all the flying leads etc is a significant increase in all areas over the single box 7 mono. I only paid £350 for the Polestar (somewhat less than the £1400 I paid for the 7 monos) so I don't even have to justify it on cost grounds ( ie. I didn't pay £3K and as such I'm absolutely going to like it).

TBH I have little or no interest in the esoterica of circuit design (I wouldn't expect folk to be overly interested in the design parameters of the titanium plates and screws I use to fix their busted faces) I just go by my ears and buy what I find enjoyable at a price I think is fair which was the original aim of my post.


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All absolutely fine by me of course. Please don't take an overview of pros and cons of 2 box V 1 box etc as personal in any way :)

[worrasf]

De nada [emoji41]
I've just replaced the 2 main power switches as one was sticking on intermittently. So for the £350 I paid for the amp plus about £35 on fuses/switches/valve/case screws I am the proud owner of a very fine sounding and likely unique power amp.
Even Glenn declared it a good result - praise indeed [emoji106]


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[Macca]

Does sound like you got a huge bargain there. That's what it's all about. :)

[worrasf]

Thanks - what is so good (and makes projects like this possible) is that Glenn is always happy to give advice and help even at the weekend for a product I didn't even buy from him - Legend [emoji56]


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