Croft Polestar III 2 box power amp project

[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... just... "'cos" .... and "my amp has a huge transformer so that must be why it sounds so good" 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

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 . I can only wish that it hits all the right spots for you. I'm happy that you've found something so special.