Dowser
Learning to bodge again..
Picked this up cheaply from Greece late 2020 after keeping my eyes open for years (since 2006 I've run a mildly modified Croft Micro II in preference to any other pre...and I've tried loads). My original post in Classics is here.
Glenn always varied his design's frequently, but based mainly on 3 different levels; Micro, Super Micro, Mega Micro. From the little info available on the internet, the Mega was his attempt at state of the art, and retailled at around £1500 in mid 80s I think.
I bought it with a known issue - one of the line stage transformers had failed, and so B+ was being shared by one transformer. Previous owner had also swapped the original phono stage valves from ECC807s to ECC83s (as Glenn did at some point too).
Pre-amp
PSU
Since September last year I think I have been happily living with it, it sounds much better than my lowly Micro II.
After having to pop it open recently to diagnose & fix distortion on one channel via phono stage (broken wire from transformer to stripboard, due to transformer isolation rubbers failing), I decided to dig a bit deeper with a view to bringing it back to full health.
For the PSU my Micro II used a PSU made up of ECL85, a neon tube reference and a 85A2 regulator - after research I think this was actually the PSU design of the Super version of Glenn's. Certainly when I first bought it (faulty) I sent to Glenn - he initially diagnosed a failing neon tube and suggested swapping to solid stage rectification as cheapest solution, but admitted original rebuilt would sound better, so I went with that.
The Mega has 3 tubes in the PSU too - but what looks like a 9 pin dual triode instead of the neon tube, and an adjustable pot to set the B+ rail (I assume, haven't tried yet ).
No markings on 2 of the 3 tubes, but after a little research I think they are EL86, a 12AX7 (ECC83) and the 85A2 regulator. This is based on this site, which suggests this mod to improve the supply of an ECL85 based Super Micro PSU (http://www.cma4ch.org/chemo/attivita/croftsmic.html - see towards bottom of page). Can anyone confirm? I know practically zilch about valves
For the replacement for the failed transformer, I think I need a secondary of 330v AC for B+ and another secondary of 6.8v AC for the EL86 heater. Again, can anyone confirm? This is based on AC readings of the existing PSU with the pre connected. Interestingly, the transformer that is feeding 2 B+ PSUs only has 325v AC on the secondary....I assume due to additional load or different caps used (not checked the detail of them yet).
The pre-amp heaters are today provided via a single transformer with a 17.6v AC secondary, rectified to 22.5v DC, and then dropped down by a (I assume 78x series, not checked yet) reglulator for each channel in the pre. I wonder the sound quality benefit of providing a regulated heater per valve in the pre? Seems quite easy to achieve.
So, any recommendations on a good place to get replacement transformers? I'd buy at least 2, to keep the channels matched, and potentially all 4 if transformer tech has moved on so much in recent years?
I'd appreciate any other comments on above from those in the know too please - especially if any of my assumptions are wrong! Thanks.
One last question - assuming it is a 12AX7/ECC83 in the PSU, does the quality of it matter as much as when being used for signal duties?!
Thanks again, Richard
Glenn always varied his design's frequently, but based mainly on 3 different levels; Micro, Super Micro, Mega Micro. From the little info available on the internet, the Mega was his attempt at state of the art, and retailled at around £1500 in mid 80s I think.
I bought it with a known issue - one of the line stage transformers had failed, and so B+ was being shared by one transformer. Previous owner had also swapped the original phono stage valves from ECC807s to ECC83s (as Glenn did at some point too).
Pre-amp
PSU
Since September last year I think I have been happily living with it, it sounds much better than my lowly Micro II.
After having to pop it open recently to diagnose & fix distortion on one channel via phono stage (broken wire from transformer to stripboard, due to transformer isolation rubbers failing), I decided to dig a bit deeper with a view to bringing it back to full health.
For the PSU my Micro II used a PSU made up of ECL85, a neon tube reference and a 85A2 regulator - after research I think this was actually the PSU design of the Super version of Glenn's. Certainly when I first bought it (faulty) I sent to Glenn - he initially diagnosed a failing neon tube and suggested swapping to solid stage rectification as cheapest solution, but admitted original rebuilt would sound better, so I went with that.
The Mega has 3 tubes in the PSU too - but what looks like a 9 pin dual triode instead of the neon tube, and an adjustable pot to set the B+ rail (I assume, haven't tried yet ).
No markings on 2 of the 3 tubes, but after a little research I think they are EL86, a 12AX7 (ECC83) and the 85A2 regulator. This is based on this site, which suggests this mod to improve the supply of an ECL85 based Super Micro PSU (http://www.cma4ch.org/chemo/attivita/croftsmic.html - see towards bottom of page). Can anyone confirm? I know practically zilch about valves
For the replacement for the failed transformer, I think I need a secondary of 330v AC for B+ and another secondary of 6.8v AC for the EL86 heater. Again, can anyone confirm? This is based on AC readings of the existing PSU with the pre connected. Interestingly, the transformer that is feeding 2 B+ PSUs only has 325v AC on the secondary....I assume due to additional load or different caps used (not checked the detail of them yet).
The pre-amp heaters are today provided via a single transformer with a 17.6v AC secondary, rectified to 22.5v DC, and then dropped down by a (I assume 78x series, not checked yet) reglulator for each channel in the pre. I wonder the sound quality benefit of providing a regulated heater per valve in the pre? Seems quite easy to achieve.
So, any recommendations on a good place to get replacement transformers? I'd buy at least 2, to keep the channels matched, and potentially all 4 if transformer tech has moved on so much in recent years?
I'd appreciate any other comments on above from those in the know too please - especially if any of my assumptions are wrong! Thanks.
One last question - assuming it is a 12AX7/ECC83 in the PSU, does the quality of it matter as much as when being used for signal duties?!
Thanks again, Richard