Croft Series 7 valves?

[fiddlemaker]

Having recently bought a 7R I was surprised to discover that it seems possible to use just about any pin compatible dual triode in the input valve position. Just out of curiosity, can any valve-amp guru explain how the circuit is designed to make this possible? My (very) limited knowledge of such things had always led me to believe that in order to run say an ECC81 in an ECC83 position, you would need to change biasing resistors, operating points, etc....
So, provided that one's system can cope with the gain variations there seems to be a huge number of options here. About half a dozen (or more?) different valve options, plus the usual "rolling" of brands and designs within each particular type. So crikey, where to start?

Of the ones I've tried up to now my order of preference is: GE 5751(best), ECC99, Haltron chrome plate triple mica 5751, Sylvania 12BH7A (worst). I know that the 12BH7A is GC's preferred choice in this position, but although i find it very impressive in the short term, the very detailed airy sound gets on my nerves a bit after a while, and I much prefer the more cohesive sound of the GE5751.

would be interested to hear of other folks' experiences.......

[Jimbo]

Hi John

I would email Glenn Croft himself and put your questions to him. He is very helpful and I am sure will give you the answers your are seeking regarding amp design. You could also try Adrian at Audioflair as he is also very knowledgeable regarding Croft amps.

I run a NOS GE 5751 in the input stage of my Croft 7 and its one of the best - also recommended by Glenn too:)

[fiddlemaker]

Hi John

I would email Glenn Croft himself and put your questions to him. He is very helpful and I am sure will give you the answers your are seeking regarding amp design. You could also try Adrian at Audioflair as he is also very knowledgeable regarding Croft amps.

I run a NOS GE 5751 in the input stage of my Croft 7 and its one of the best - also recommended by Glenn too:)

I think I have probably already emailed Glenn far too often in the past couple of months (recent micro upgrades and series 7 questions). I fear if I pester him further he will have me down as a complete nutcase/pain in the arse......:)

I'm amazed how much difference the input valve makes. The two 5751s I've tried give a completely different presentation, with the GE 5751 sounding much more natural and less "in your face hifi" than the Haltron 5751 and the 12BH7A. And to think I've always been sceptical about tube rolling in the past.......

[Jimbo]

I used the GE 5751 originally to reduce the gain in the power amp but also noticed it gave a very solid cohesive natural sound as you have also found. I am planning to get another NOS GE 5751 from Watford valves - I think they even do a cryo version?:scratch: Valve rolling can indeed make such a large difference to the overall sound quality in both pre and power amps but can be frustrating as they have such different characters with some highlighting bits you like but falling down in other areas. I have tried a NOS Mullard ECC83 but had to revert back to the GE as I preferred the sound so much.