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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment. |
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18th Mar 2017, 7:56 pm | #1 |
Dekatron
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Location: Newcastle upon Tyne, Tyne & Wear, UK.
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Mystery Dynatron PA amp
Hi Folks, help with identification please.
I have a Dynatron PA amp, believed to be shipboard use and on a standard type of PA chassis. No controls other than on/off fitted, 2 pin Bulgin mains in, octal style socket to feed preamp or radio feeder. Speaker output is 50R impedance. Valve line up is 2* Z77, 2* 6V6, 5Z4 rect and a KT66 of which I am not certain of its function. Valve bases are labelled so certain of valve types . Any ideas? Thanks, Ed |
18th Mar 2017, 9:54 pm | #2 |
Nonode
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Location: Papamoa Beach, Bay of Plenty, New Zealand
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Re: Mystery Dynatron PA amp
It might be a variant of the “small” amplifier that was part of the Dynatron-Mimco shipboard broadcast receiver, for example see: http://www.vintage-radio.net/forum/s...ad.php?t=90451.
The first variant of the Mimco unit had the T99A tuner-control unit, the second (Mimco 2235A) had the T139. Both had separate power amplifier chassis of around 10 watts output, which would fit a circuit based upon a 6V6 pair. In its larger LF613 amplifier, normally used with the T139, and with a triode-connected KT66 output pair, Dynatron used a third KT66 as a cathode-follower to supply HT voltage to the associated tuner-control unit. So that may have been the case with the smaller unit. The elaborate (by domestic standards, anyway) radio circuitry of the T99 and T139 probably benefit from a reasonably constant HT voltage. The LF613 valve line-up was 2 x Z77, 3 x KT66 and 1 x 53KU. This Dynatron advertisement from Wireless World 1954 September does mention the “small” amplifier in the small print, but unfortunately does not reveal its LF-series designation. I’d guess that the industrial/marine versions of this Dynatron equipment would have had the option of output transformers suitable for constant-voltage line speaker systems. Cheers, |
18th Mar 2017, 10:25 pm | #3 |
Nonode
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Re: Mystery Dynatron PA amp
Here are a couple of downloaded pictures that show the inside of the Dynatron-Mimco T99-based unit. The separate amplifier chassis might be similar to the unit at interest.
I think that there was an "LF59" in the post-WWII Dynatron range. So this might have been that with the added KT66, in which case it might have been an "LF69". Cheers, |
19th Mar 2017, 12:38 pm | #4 |
Dekatron
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Location: Newcastle upon Tyne, Tyne & Wear, UK.
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Re: Mystery Dynatron PA amp
Hi Synchrodyne, thanks for that. I had seen the earlier thread but not managed to tie them up, this data completes the loop.
As you say the KT66 probably acts as a stabiliser for the radio feeders. It looks very close to your pictures. I'll see if I can find a circuit but I'm not very hopeful. The mains transformer has been replaced at some time and the 3 main smoothers are paper/ oil types of 4uF, quite a low value; at least one is leaking. Thanks again, Ed |
19th Mar 2017, 12:58 pm | #5 |
Heptode
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Location: Essex, UK.
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Re: Mystery Dynatron PA amp
Ed there are a couple more pictures here, the KT66 is used as a shunt stabiliser.
https://www.vintage-radio.net/forum/...ad.php?t=29662
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19th Mar 2017, 2:40 pm | #6 |
Dekatron
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Re: Mystery Dynatron PA amp
I am not sure about the KT66 being wired as a shunt regulator.
Marconi used a 6CD6G in some of those instruments with the bold picture frame front panels. It is wired as a series regulator and not parallel. |
19th Mar 2017, 3:12 pm | #7 |
Heptode
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Re: Mystery Dynatron PA amp
Refugee you could be right about this, a similar circuit for the Ether Marshal shows the KT66 used as a cathode follower just for the tuner supply.
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19th Mar 2017, 4:02 pm | #8 |
Dekatron
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Re: Mystery Dynatron PA amp
The Marconi is a bit more fancy with a triode feedback loop.
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19th Mar 2017, 4:24 pm | #9 |
Nonode
Join Date: Aug 2015
Location: Cambridge, Cambs. UK.
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Re: Mystery Dynatron PA amp
Ships' power supplies back in those days were notoriously variable. The majority of vessels would have had DC mains (220/110V), so that any radio/radar equipment would have been supplied via a motor-alternator.
The DC supply would have been subject to some heavy loads from numerous heavy motors started direct on line for hydraulics, ventilation etc, so the derived AC supply feeding the Dynatron would also likely have wandered all over the place in voltage and frequency. I guess that's a key reason for its supplying the receiver with stabilized HT. Until the 1990s, ships' power supplies were not very electronics-friendly, a particular problem when computer automation began to arrive! Martin
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19th Mar 2017, 8:13 pm | #10 |
Dekatron
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Re: Mystery Dynatron PA amp
Hi Gents, thanks for the additional info.
This set is definitely for AC and I believe the KT66 is a series pass stabiliser/ regulator. No sign of a ref tube though, so maybe just a way of voltage reduction. Ed |
19th Mar 2017, 10:00 pm | #11 |
Nonode
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Re: Mystery Dynatron PA amp
A close look at through the data on hand for the Dynatron Ether Marshal T139 + LF613 combination shows that it also mentioned the LF512A amplifier, with some data – including valve voltages - provided but unfortunately not a schematic. The LF512A valve line-up was 2 x Z77, 2 x 6V6, 1 x 5V4G. It evidently lacked the KT66 cathode follower voltage stabilizer that was fitted to the LF613.
So it could be that the amplifier at interest is the LF512A with the addition of the KT66 voltage stabilizer. Hetrodyne has already kindly provided the LF613 schematic in post #7. I should not be surprised if the LF512A circuit was very similar to that of the LF613, although possibly the 6V6 output valves were tetrode- rather than triode-connected. The output is quoted as 10 W, which seems higher than one might reasonably expect from a triode-connected 6V6 pair. The LF613 looks to have been a development from the LF612. And those had followed the established Dynatron form, which was triode amplifier, triode phase splitter and a triode output pair. Except that the LF613, and by inference the LF612, used triode-connected Z77s for the first two stages and triode-connected KT66s for the output. (From the mid-1950s Dynatron later adopted UL outputs using GEC and Mullard designs.) An oddity is that logically, the LF612 would have had 6 valves, but the photographic evidence available on the web shows it with 5. Maybe the extra KT66 was optional; if so, that would tie in with the fact that there were 5- and 6-valve versions of the 6V6-based amplifier. Ed, please let me know if you want a copy of the Ether Marshal data. Along another vector, perhaps look for someone who has a copy of the Mimco 2235A manual, which I think would cover the 6-valve 6V6-based amplifier. Here is a picture of the manual cover that I downloaded some time back: This item on the first version of the Dynatron-Mimco unit confirms that it was AC-powered, with the option of a DC-AC converter, which unit I imagine would have been drawn from Marconi’s established range of marine accessories. Given that the KT66 tuner supply voltage regulator was a standard fitting on the Ether Marshal, evidently it was seen as being beneficial generally, and not just for marine applications. Cheers, |
20th Mar 2017, 5:29 am | #12 |
Nonode
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Re: Mystery Dynatron PA amp
Re the KT66 cathode follower voltage stabilizer, this is from the LF613 data:
“H.T. supply for the tuner chassis is taken via the KT66 cathode follower V105. The cathode voltage is determined by the control grid voltage set by the voltage divider R119 and R120. The advantage of this arrangement is a low source impedance H.T. supply to the tuner which prevents any possibility of L.F. instability.” So stability rather than voltage regulation appears to have been the objective. The T69, T99 and T139 were all high-gain tuners, so perhaps Dynatron had seen some problems in the early days, and the cathode follower supply was seen as being the answer. Cheers, |
20th Mar 2017, 6:03 am | #13 |
Dekatron
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Re: Mystery Dynatron PA amp
Hi Ed, I have a few of the big PIO 4u 600v TCC caps if you need them.
Andy.
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22nd Mar 2017, 11:48 pm | #14 |
Nonode
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Re: Mystery Dynatron PA amp
There is another concurrent thread asking about the Dynatron LF512 amplifier, namely: http://www.vintage-radio.net/forum/s...513#post930513.
If nothing else this is additional evidence that the LF512 was the designation for the amplifier associated with the Dynatron-Mimco marine broadcast receiver. Cheers, |
23rd Mar 2017, 10:40 am | #15 |
Hexode
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Re: Mystery Dynatron PA amp
I have a T139/LF512A combo in the workshop which I am slowly refurbing. The LF512A definitely uses a KT66 as a series regulator for the oscillator in the T139 and the LF512A is a 50V line out for marine use.
A previous owner changed the X79 to an ECH81, so that has to be changed back, it also has a selection of black Hunts capacitors to be changed and also the usual ceramic bodied Rs to measure/change. I have the schematics for both which I would be happy to scan/copy for people (don't all rush at once!), they are not brilliant but are useable. John |
23rd Mar 2017, 12:32 pm | #16 |
Dekatron
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
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Re: Mystery Dynatron PA amp
Hi Synchrodyne, thanks for those details; it is a fairly simple circuit so I'll trace it manually, it should not be too difficult but it has been slightly got at.
Earlier post was also concerning this amp, which has now wound up with me to look at. Thanks Andy, I'll be in touch if I need any caps, Ed |
24th Mar 2017, 4:34 am | #17 |
Nonode
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Re: Mystery Dynatron PA amp
I had another look at the T139 service instructions. As mentioned, these focus on the LF613 amplifier with some mention of the LF512A, albeit without a schematic.
Whilst nothing explicit is said about a 50-volt line output, the performance specification is written around a 10 W output into a 250 ohm load, which of course corresponds to 50 volts. I had missed that previously. A plausible chronology then is that Dynatron developed the LF512(A) specifically for the marine/industrial market, and even more specifically for the initial version of the Mimco-Dynatron 1155 (?) marine broadcast receiver, in which it was paired with the T99, and which was announced in Wireless World (WW) for 1953 August. The T99 itself dates to around 1952 third quarter, as does its companion LF612 amplifier, the combination being used in the “Ether Conqueror Series V” and other “1512” series products, including an updated version of the “Merlin” console receiver. The above-mentioned WW article noted that the basic Mimco-Dynatron receiver with 10 watt amplifier could supply up to 10 speakers, which I think is consistent with a 50-volt line system. It was also said that larger power amplifiers could be supplied, and the inference is that such would be external to the receiver. Whether Dynatron was involved with the supply of such amplifiers is unknown. But Mimco could draw from large range of marine audio products, some from third parties. (See WW 1956 January, p.42.) It is known that for the T139 iteration (Dynatron 1712 series) at least, there was an industrial version with a 500-ohm line output rather than a power amplifier, and one imagines that this was intended to couple into PA systems with larger power amplifiers. 1155 seems to be a plausible Mimco number for the initial (T99) iteration of the Mimco-Dynatron marine broadcast receiver. Around that time Mimco had been using 1000-series numbers, such as 1017 and 1018 respectively for the Mercury and Electra marine main receivers. Then it flipped to the 2000-series, such as 2207 for the Atalanta marine main receiver, which as best I can determine dated from 1954. The second (T139) iteration of the Mimco-Dynatron receiver, the 2235A, probably dates from late 1954 or 1955. There was certainly a potentially confusing multiplicity of designations. For example a Mimco 2235A comprised a Dynatron T139 tuner/control unit and a Dynatron LF512A amplifier, and this pairing had a separate Dynatron number, possibly R1712AA, although that might not have appeared on any nameplate. Cheers, |