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Post by Richard on Apr 20, 2022 10:52:37 GMT 12
I'm working on a Model 75, it had a 6K8 installed and it worked Ok. However the original was suppose to be an X65. I've come across the use of a 6K8 as a replacement for the X65 before in RCNZ radios, however for an experiment, Id look up the equivalent for an X65, which could be an ECH35. As I had a few spare ECH35's , i thought I would try one. The gain of the ECH35 was far superior than the 6K8 and the AGC voltage, for example, on 1ZB went from around -5 Volts to -15 Volts, the Audio output has obviously improved as well. I'm going to keep the ECH35 in place as the radio performs a lot better and there's no overloading on other stations.  |
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Post by Peter Walsham on Apr 20, 2022 11:52:25 GMT 12
The ECH35 is an excellent converter valve. Wish there was more of them around!
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Steve
Society Members
vintageradio.co.nz
Posts: 732
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Post by Steve on Apr 21, 2022 9:52:42 GMT 12
Actually, I believe the 6K8G was the original mixer valve designed into the model 75 - it was quickly replaced with the superior X65 though and so most service information floating around shows it with the X65.  That schematic was soon amended to, among other things, use the X65 - this is the main schematic that was used in the service booklet, and seems to be the main one floating around.  There was also a service supplement about changing the set back to a 6K8G after it became difficult to obtain the X65 and other Osram valves - because England was fully engrossed in WW2 and had just gone through the Battle of Britain.  So it should be quite common to find the 6K8 in a model 75 (its pouring with rain so I refuse to go out and check the two chassis' in my garage on the grounds that you shouldn't mix electronics and rain - thats my excuse and I'm sticking to it). Radio Corp would have been aware of the Philips valves that were available (although I think the ECH35 was pretty new at that time and might not have been available here in NZ yet, particularly in the quantities RCNZ would have needed), but there was a note in one of the letters to Fred Green who was in the UK on a buying trip in 1946 - something to the effect that Philips had yet again approached the factory with the offer to sell them valves but the prices were, as they had always been, far too high compared to what they could get elsewhere. Economics would have played a major part of their decisions on which valves to use. I do have a decent stack of service data and supplements for the model 75 that I need to scan and compile - I might try to do that today since apparently I am refusing to go outside  Cheers, Steve |
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Post by Peter Walsham on Apr 21, 2022 10:18:00 GMT 12
There also might have been a governmental 'issue' (like manufacturers not being able to get an import licence?) preventing NZ manufacturers being 'allowed' to buy valves from Europe, but could source valves from the 'mother country' (being Britain)? Material shortages were a big problem post war as well, so that may have also caused a supply problem.
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Post by Richard on Apr 21, 2022 11:03:41 GMT 12
HI Steve, if you've got an ECH35, try it in one of your Model 75 , see what you think ? obviously check that pin 1 socket is connected to earth first.
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Post by DHockey on Apr 21, 2022 14:50:34 GMT 12
Looking at the data for all three valves, the ECH35 is closer to the X65 than the 6K8. The ECH35 and X65 have grid 1 on the top cap and the triode grid is linked to grid 3 of the hexode.
The 6K8 has grid 3 of the hexode on the top cap and the triode is linked to grid 1 of the hexode.
Interestingly, the Philips substitution handbook lists the ECH35 as a direct equivalent to the 6K8 with "greatly improved performance" In my experience, the sub from 6K8 to ECH35 works in most cases, but sometimes I have run into issues with oscillation and gone back to a 6K8.
It might be worth checking the capacitor values mentioned in the RCNZ service bulletin Steve posted to find out if whoever put the 6K8 in your 75 had taken this into account.
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