Dynatron mid 30's 'tuner'

[PJL]

Originally Posted by Herald1360

But an IO base would not fit a HL41..... unless it's actually a MO instead.

I suggested the HL41 as it is another Mazda on an MO base. It has heaters connected to 1&8 whilst IO mostly have them on 2&7. The cathode connection also looks correct as it goes to a decoupler. Needless to say I don't have one but I do have an HL41DD so I could fiddle with the grid wiring. It is probably the pickup amplifier but not sure where the diodes come into play.

[Techman]

Originally Posted by PJL

It is probably the pickup amplifier but not sure where the diodes come into play.

No, it's both. Remember, there's a single volume control for both radio and gram (or as far as I can tell), so the triode is an amplifier for both and the two diodes are detector and AGC for the radio. There's unlikely to be any sensible circuit information out there for this unit.

It's all falling into place now regarding the apparent rarity of this tuner and the radiogram that it would have originally been fitted in. The HL41 valve dates from the very late thirties, 1938 in fact, so that tuner would have originally been designed for a different and much earlier valve. There would have been a last minute production change to take advantage of this new, all singing, all dancing HL41 triode, then, within just a few months, world war two would have put an abrupt end to production, with Dynatron only producing gear for the military. After the war, that tuner would have been regarded as 'old hat' and the 'all new' T69 with its military 'leanings' was the order of the day...well that's my take on what happened and why so few of them exist now - anyone else agree?

[Techman]

As an additional note to the above post, in the link posted by Nick in #8, it states that it wasn't until as late as 1937 that the Hacker brothers/Dynatron brought a superhet receiver to the domestic market (or similar wording), so that would indicate that the first incarnation was as late as 1937 with a late production change probably a few months to a year later to incorporate the new valve type HL41 in 1938.

[PJL]

My detective work identifying it as an HL41 is correct although brushing the dust off the top of the chassis to reveal markings would have been quicker! I will have a go at powering her up next week.

[Techman]

Originally Posted by PJL

I will have a go at powering her up next week.

Sounds like a good plan!

Dynatron nearly always mark the chassis with the valve types and I was surprised that you hadn't said that this was the case here - I certainly couldn't see any markings in the photo, so it must have been more dusty than we all thought.

I think we'd all still like to know what the other valves in the line-up are.

So although originally thought to be mid 30s, it's actually pretty much now proven to be (very) late 30s. WW11 began in September 1939 and we know that the HL41 didn't come out until 1938, but we don't know when in '38, so it gives us a very tight window of manufacture between that date and the start of the war.

[PJL]

HL41, 2D4A, 2D4A, DD41, AC/VP2, AC/VP2, AC/TH1, AC/VP2, AC/VP2, AC/VP2, MH41

Triode x2
Pentode x5
Triode/Hexode x1
Diode x6

I would say a 1936 design but with MO valve substitutions (the sockets don't fit well in the chassis) either as a build change or quite possibly as an upgrade. Expensive radiograms like this would continue to be serviced by the manufacturer for many years.

[60136 Alcazar]

3 double diode valves - that's a lot of diodes! As previous contributors have stated, this valve series originates in 1936 (there are no earlier types except the MH41), except for the 2 MO types (DD41, HL41) which cannot be earlier than 1938. Strange that a solitary MOV MH41 should appear instead of the Mazda AC2/HL - maybe something specific about the circuit and GEC valves made this essential.
The solitary MO DD41 against the 2 B5 Mullard 2D4A's is slightly odd as well - these are all general purpose signal double diodes - you'd have expected 3 2D4A's or 3 DD41's, so could this lonely DD41 be a wartime modification on the grounds of availability?

[PJL]

The 2D4A shares the same cathode but the DD41 contains two separate diodes.

I have extracted the paper dial and it shows LW+MW+3xSW. It has 'Television Sound' but is designed to be replaceable so this may not be the original.

[60136 Alcazar]

The MW stations on the station list include BBC Welsh on 373m and BBC West of England on 285m - both these commenced transmission on 4th July 1937. Also significant is the absence of both Start Point (14th June 1939) and Stagshaw (19th October 1937). It would seem then that the dial dates from 1937.

[Techman]

An interesting valve line-up. All those diodes! I thought those two small valves together were probably diodes and the fact that they were B5 bases looked like they could be double diodes, but a DD41 double diode in there as well?

The AC/VP2 seems to be an earlier version of the VP41 that I mentioned earlier, but has a B7 base rather than a MO. The VP41 is a dreadful valve, developing internal electrode shorts when operated on its side for long periods - I've had this with not just one, but a few. I wonder if the earlier AC/VP2 happens to be any better in this respect, as it looks like they were operated on their side in this particular tuner too.

I still think this tuner dates from late 38 to 39 and that the valve base changes were factory, rather than dealer mods. I agree that the tuner would have been some time in design, which would likely have begun at least as far back as 1936, but from the information we have was only first released to the public in 1937.

The dial paper is interesting. I wonder whether they all had TV sound marked, or whether this indicates that the tuner came out of one of the large TV, radio, gramophone units such as the Ether Emperor of 1939, although the Dynatron Museum only has an artists impression, or possibly a photograph print of one with the doors all closed up, taken from literature of the day, I guess. The tuning range is similar to my 1947 dated T69 tuner, but mine doesn't go much beyond 10 metres, whereas the one in question here goes well into 8 metres. Mine goes well beyond 80 metres, probably to 85 metres, whereas the one in question looks to go just as far as 80 metres and no more, so a slight frequency shift in the short waves between the two, perhaps to cater for the TV sound of the time, seeing as it's marked on there.

[Synchrodyne]

There was a brief mention of Dynatron’s adoption of superhet circuitry (actually combined superhet/TRF circuitry) in the WW 1937 September 03 report on trends seen at the 1937 Radio Show. The pertinent pages are attached.

It was noted that the larger Dynatron models had fourteen tuned circuits. That would seem to correspond with the T1114 model at interest, 11 valves and 14 tuned circuits.

The combination of a four-gang tuning capacitor and an apparent single RF stage might indicate the use of bandpass RF tuning, either at the input or at the interstage. At the input would be more appropriate for MF, whilst at the interstage might be preferred for upper HF. Possibly the use of bandpass in the RF section was driven primarily by the need to obtain better skirt selectivity on MF in the TRF mode.

The post-WWII Dynatron radio units were superhet only. The T69 had a three-gang tuning capacitor, but in a four-gang casing, with one unfilled slot. As far as I know, the T69B and T69C were the same, but the T69D had a four-gang capacitor, as did the following T99 series (including at least the T99A and T99B.) The T139 reverted to three-gang, but that may have been for component availability reasons (it might have used the standard Weyrad bandspread front end). But at upper HF it had a grounded grid aperiodic RF pre-stage, and that might have allowed some tightening up of the first RF tuned circuit, as it was fed from a known impedance rather than a random aerial impedance, and as the noise/gain tradeoff would not be so critical.


Cheers,