mikeyB
Well-known member
Any experience?
Me matey has been having an ebay buying frenzy! - managed to get 2 pristine units along with 2 dodgy(tell you later) units and one that looks older than my grandma! - who`s been dead for a long while!
Dodgy units had psu`s supplied! - oh my !- did i learn fast! - no earthing and no ht discharge - i`m just getting over the (actual) shock and my fear of working on them !!!
Dodgy units - supplier went to a lot of trouble removing the input transformers from the cans and re-assembling - wired the empty cans unbalanced - 1 unit with caps to grid, the other direct to grid! Outputs on both dodgy units had 470k pot strapped to earth - wiper out.
Earthed the supply and added bleeder resistors to the ht rail (345V ht!). Noticed on output that many 10`s of volts before heaters warm up and ht current is drawn! Thinking of using Carnhill 9600:600 output tx to isolate this voltage - any thought`s? There is about 40db of gain when using a SM57 (unbalananced) mic.
"Grandma`s" unit and the pristine units have the input transformers intact - they roughly have input dc res of 27R CT and 1k2 out res to the valve stage - ratio measures 1CT:14.5 - i`m guessing that`s well over 20dB of gain! I`m guessing low in z for ribbon mic`s.
Any guru advice as to what the pot network around the first valve is? - Matey reckons it,s a high freq rolloff - but i`m a deaf old drummer so i don`t hear it!?!
Looking at lifting the CT ground so as to phantom power for condensers and probably 20db pad(based on 200R impedance)
Hopefully looking to bypass the mixer pentode(looks like there is plenty of gain) and replace with duplicate output stage(is this a withe cathode follower?) for 2nd input - thus ending up with a 2 channel per rack system.
FOOTNOTE - this rather large output voltage is a bit of a worry - do all our valve projects need a delayed or slow start HT supply at turn on?
Me matey has been having an ebay buying frenzy! - managed to get 2 pristine units along with 2 dodgy(tell you later) units and one that looks older than my grandma! - who`s been dead for a long while!
Dodgy units had psu`s supplied! - oh my !- did i learn fast! - no earthing and no ht discharge - i`m just getting over the (actual) shock and my fear of working on them !!!
Dodgy units - supplier went to a lot of trouble removing the input transformers from the cans and re-assembling - wired the empty cans unbalanced - 1 unit with caps to grid, the other direct to grid! Outputs on both dodgy units had 470k pot strapped to earth - wiper out.
Earthed the supply and added bleeder resistors to the ht rail (345V ht!). Noticed on output that many 10`s of volts before heaters warm up and ht current is drawn! Thinking of using Carnhill 9600:600 output tx to isolate this voltage - any thought`s? There is about 40db of gain when using a SM57 (unbalananced) mic.
"Grandma`s" unit and the pristine units have the input transformers intact - they roughly have input dc res of 27R CT and 1k2 out res to the valve stage - ratio measures 1CT:14.5 - i`m guessing that`s well over 20dB of gain! I`m guessing low in z for ribbon mic`s.
Any guru advice as to what the pot network around the first valve is? - Matey reckons it,s a high freq rolloff - but i`m a deaf old drummer so i don`t hear it!?!
Looking at lifting the CT ground so as to phantom power for condensers and probably 20db pad(based on 200R impedance)
Hopefully looking to bypass the mixer pentode(looks like there is plenty of gain) and replace with duplicate output stage(is this a withe cathode follower?) for 2nd input - thus ending up with a 2 channel per rack system.
FOOTNOTE - this rather large output voltage is a bit of a worry - do all our valve projects need a delayed or slow start HT supply at turn on?