Arkless Electronics
22-08-2017, 19:46
Well I saw this old TF1370A on the evil bay and put a daft bid in for it, thinking it may fetch £100+.... I won it for £7:) Then saw it was "collection in person only"... In that London:doh:
That is where AOS came into play:eyebrows: "Edwardlon" kindly collected it for me and took it as far as Sheffield to Si (SQ) who brought it up to me with an amp he needs sorting on Saturday (he even cleaned it first!). Thanks guys!:)
It's about the size of a microwave oven, weighs in at 37 Lbs and contains 13 valves! (rectifiers are SS so that's 12 "active" valves) My biggest fear was that the crop of Mullard and Brimar valves had been filched but this proved not to be the case:)
I wound it up on a variac over a couple of hours and it worked... after a fashion... The regulated HT, from a full on complicated all valve voltage regulator with two ECC83's, 85A2 voltage reference and 6080 series pass valve, was found to be at 166V rather than the correct 285V! A 1M resistor open circuit and 0.5uF cap leaky were both replaced. This had the HT restored:)
The heater supply is also regulated... using them new fangled transistor thingies... 3 of them with the output series pass on a nice sized heatsink. This seems to be spot on as delivered:) These are the only transistors in it but it uses several germanium diodes.
The 10 and 30V outputs come via a 20dB amplifier and these were found to be giving a VERY distorted output... Also the square wave section isn't working....
The output stage is a clever variant of the White cathode follower using two EL84's, one pentode connected as the follower and one triode connected as the active current sink. The pentode, being floated up at 200V above ground, has it's own floating heater supply to avoid cathode/heater shorts;) It can give 30V RMS into a 2K load @1MHz and 10V RMS to 10MHz!! A similar topology could make a stonking headphone amp!
This was I guess around the time of the "last hurrah" for valve gear (manual dated 1965 and this one built late 1968) and uses some of the last and best valves to be made such as frame grid EF184's, E810F's and E88CC (yes gold pin Mullard!).
The components used back then are crap compared to what we have today and I'm expecting to find more open circuit or 50%+ high resistors and several leaky caps yet before these sections are working correctly.
One thing I did just notice in the schematics is the presence (not for much longer!) of a couple of 500V rated 1nF paper caps, one each from live and neutral to chassis!!:eek: They hadn't heard of "X" and "Y" rated caps back then and I don't fancy my chances of the one from live to chassis remaining a non conductor for long!
Interesting to note that, as is often the case, the valves have outlived the resistors and capacitors!
That is where AOS came into play:eyebrows: "Edwardlon" kindly collected it for me and took it as far as Sheffield to Si (SQ) who brought it up to me with an amp he needs sorting on Saturday (he even cleaned it first!). Thanks guys!:)
It's about the size of a microwave oven, weighs in at 37 Lbs and contains 13 valves! (rectifiers are SS so that's 12 "active" valves) My biggest fear was that the crop of Mullard and Brimar valves had been filched but this proved not to be the case:)
I wound it up on a variac over a couple of hours and it worked... after a fashion... The regulated HT, from a full on complicated all valve voltage regulator with two ECC83's, 85A2 voltage reference and 6080 series pass valve, was found to be at 166V rather than the correct 285V! A 1M resistor open circuit and 0.5uF cap leaky were both replaced. This had the HT restored:)
The heater supply is also regulated... using them new fangled transistor thingies... 3 of them with the output series pass on a nice sized heatsink. This seems to be spot on as delivered:) These are the only transistors in it but it uses several germanium diodes.
The 10 and 30V outputs come via a 20dB amplifier and these were found to be giving a VERY distorted output... Also the square wave section isn't working....
The output stage is a clever variant of the White cathode follower using two EL84's, one pentode connected as the follower and one triode connected as the active current sink. The pentode, being floated up at 200V above ground, has it's own floating heater supply to avoid cathode/heater shorts;) It can give 30V RMS into a 2K load @1MHz and 10V RMS to 10MHz!! A similar topology could make a stonking headphone amp!
This was I guess around the time of the "last hurrah" for valve gear (manual dated 1965 and this one built late 1968) and uses some of the last and best valves to be made such as frame grid EF184's, E810F's and E88CC (yes gold pin Mullard!).
The components used back then are crap compared to what we have today and I'm expecting to find more open circuit or 50%+ high resistors and several leaky caps yet before these sections are working correctly.
One thing I did just notice in the schematics is the presence (not for much longer!) of a couple of 500V rated 1nF paper caps, one each from live and neutral to chassis!!:eek: They hadn't heard of "X" and "Y" rated caps back then and I don't fancy my chances of the one from live to chassis remaining a non conductor for long!
Interesting to note that, as is often the case, the valves have outlived the resistors and capacitors!