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Post by Richard on Oct 22, 2022 16:01:42 GMT 12
I've just started work on a Model 35. The set is pretty original under the chassis, but on pulling the valves out I discovered someone has changed out the valve types. A 6A8 was in place of the 6J8 and a 6Q7 in place of the 6B8. I would have thought a 6K8 would have been a better replacement or even a ECH35?. Unfortunately a rodent has chewed through the field coil and the output transformer is open circuit, so it will be a while before I can get it working and see how this set would have performed with these changes.  |
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Post by Peter Walsham on Oct 22, 2022 21:26:22 GMT 12
Hi Richard
Yes, you are right. A 6K8 can often be used in the place of a 6J8, but care needs to be taken to ensure that the triode anode voltage is kept lower than about 125 Volts, or the life expectancy of the valve will be shortened. An ECH35 is a great substitute for a 6J8. The 6A8 valve will often work OK as a mixer/oscillator, but being a pentagrid valve, it is prone to oscillator frequency drift when the AGC voltage on the control grid varies (due to the common electron stream). The 6A8 is more suitable to be used in a receiver that has an RF valve that is controlled by AGC, but not the converter valve.
As to substituting a 6B8 with a 6Q7. A 6Q7 will work fine, but has less audio gain than a 6B8. In most circumstances the gain difference may not be large enough to be a problem - just needing to advance the volume control a little more. I have actually made that substitute myself.
Peter
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Post by Richard on Oct 23, 2022 7:04:02 GMT 12
Thanks Peter, I thought the 6B8 should have more gain being a pentode.
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Post by Richard on Oct 23, 2022 9:55:06 GMT 12
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Post by Richard on Nov 22, 2022 13:40:45 GMT 12
for those that maybe interested?
Ive just finished restoring this radio, it turns out that although the schematic shows a 6J8 as the converter, it looks like Radio Corp NZ may have run out of them at the time this radio was manufactured. The 6A8 looks original, the B+ supply to the anode of the Osc plate was fed from the main B+ supply via a 25K resistor and not from the "Screen supply" Bus, as shown in the schematic. This all looked original.
I've replaced all the resistors, they were all out of spec and 90% of the capacitors, the 6K7 IF amp was faulty and replaced, the output transformer and Field coil was also open circuit, so I replaced them with my spares. One day I'll get round to rewinding them. Overall, the radio performs very well, including Short-wave reception at night with only a metre long Aerial.
I did replace the faulty volume pot with a 1Meg ohm from 500K (Diode load also 500K) to improve distortion from the .01uf coupling cap staying charged.
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Post by Peter Walsham on Nov 22, 2022 20:29:54 GMT 12
Hi Richard
My thoughts on the oscillator anode feed being changed from the screen bus feed to its own suggest that the oscillator anode voltage (being fed from the screen bus) will change as the radio is tuned to a station thus generating a -ve AGC voltage to the convertor & IF valves and increasing the screen bus voltage. I suspect this may have lead to oscillator 'pulling' or drift, and also would have likely to have caused oscillator drift due to variations in signal strength as well. Feeding the oscillator from its own relatively stable B+ supply would have made the oscillator more stable under both of those situations.
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Post by Richard on Nov 23, 2022 7:19:33 GMT 12
yes, it does seem strange that the Osc anode supply came off the screen supply, I've never seen that before.
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