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Old 16th Jan 2016, 9:21 pm   #1
RojDW48
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Default Decca Deccalian 6V6G amp data

This is a Decca Deccalian 6V6G single ended amp with a 78rpm only deck. I have stopped it smoking (!) but when I connect a CD player I get what sounds like distortion from incorrect equalization. There are lots of Deccalians on the service data site - how do I know which one to pick?
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Old 16th Jan 2016, 9:28 pm   #2
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Default Re: Decca Deccalian 6V6G amp data

I expect you are over loading it, this set has two stages of amplification before the output stage. Have you replaced that cap, there are two of them as there are three stages in all

Last edited by electrogram; 16th Jan 2016 at 9:43 pm. Reason: Missed a bit
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Old 16th Jan 2016, 10:07 pm   #3
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Default Re: Decca Deccalian 6V6G amp data

I had one of these and it needed "that capacitor" changing it then sounded really nice with an ipod poked in the back so they can in fact take a modern input level. Mine was borderline output transformer destroying current when first tested the diagram came off of the vintage radio cd so should be available here I think it was this one
http://www.service-data.com/product....94/1618/r11194
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Old 16th Jan 2016, 11:41 pm   #4
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Default Re: Decca Deccalian 6V6G amp data

What an obscure combination?! When it comes to your Deccalian, do not think "equalisation". This use of technology does not apply to such early analogue technology. Edward
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Old 17th Jan 2016, 12:04 am   #5
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Default Re: Decca Deccalian 6V6G amp data

Thanks Nigel - that might be the same amp but it's a different item. The pic I have posted is a dead ringer but Decca put a lot of different models in that box. If I can get the diagram I should be able to identify 'that' cap
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Old 17th Jan 2016, 12:31 am   #6
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Default Re: Decca Deccalian 6V6G amp data

Deccalian 81 in RTVS data has 6SL7, 6V6, 5Z4. 33/78
Deccalion in manufacturer's has EF37, 6V6, 5Z4. 78 only
Auto Deccalian in ERT data has 6SL7, 6V6, metal rectifier, Garrard RC70

That cap will be connected either direct or via lowish value resistor to pin 5 g1 of 6V6. Likely to be 50nF (0.05uF) or similar.

HTH
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Old 17th Jan 2016, 1:16 am   #7
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Default Re: Decca Deccalian 6V6G amp data

Thank you very much Chris. The 78 only is the very fellow! Lots of useful info - hope I can get my head around it! - Roger
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Old 17th Jan 2016, 2:39 pm   #8
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Default Re: Decca Deccalian 6V6G amp data

Quote:
Originally Posted by Edward Huggins View Post
What an obscure combination?! When it comes to your Deccalian, do not think "equalisation". This use of technology does not apply to such early analogue technology. Edward
Interestingly, you may be pleasantly surprised about the technology here Edward. This Deccalian model is a piece of high fidelity history.

Harvey Schwartz, the engineer credited with leading the Deccalian design team, took a great deal of trouble in the design of the Decca type D 'lightweight' pickup head and the associated amplifier circuit to achieve a sensibly flat response from 25Hz to 15kHz. The type D pickup later evolved into the XMS plug-in heads, of very similar construction and 'needle armature' sapphire stylus assembly.

The type D was designed to reproduce the then new Decca 'Full Frequency Range Recording' (FFRR) 78s which claimed an HF range up to 15kHz instead of the previous 8kHz limit. There's internal LF curve compensation, and the switched HF cut looks after FFRR HF de-emphasis and scratch filtering. Of course, the internal 6-inch speaker can't do justice to this range - later models were thoughtfully fitted with an external speaker jack.

On my own '78' Deccalian, from around 1948, the 'D' head is low impedance (around 100 ohms) and feeds the EF37 through a quite substantial step-up transformer which looks rather like a rectangular IF can. So, if yours is similar to that and you're feeding line-level signal directly into the pickup input socket, the transformer will be stepping it up and likely to be causing overload.

So, once you've got the amp sorted and replaced that coupling capacitor to the 6V6, I suggest you feed your input signal in parallel with the input transformer secondary winding. If you want a flat response, disconnect the LF boost circuit, though that will increase the gain further.

If you can find some reasonably recent 78 records to play on the machine, you may be rather pleased with the sound.

By the way, you say you caught it smoking! Have you worked out just which component was emitting the smoke, and whether there's any item looking a bit worse for wear as a result?

Martin
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Old 17th Jan 2016, 6:04 pm   #9
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Default Re: Decca Deccalian 6V6G amp data

Thanks for this detail. Yes, those FFRR 78s do sound good. I have been encouraging David Mellor to do a feature on them on Classic FM. WRT my use of "equalisation", I was thinking more about the way this is used in EQ circuits and DSP in modern digital devices RojDW48 might use as signal sources. Edward PS Martin, I think the "PANATROPE" was designed by HS' team.
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Old 17th Jan 2016, 7:22 pm   #10
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Default Re: Decca Deccalian 6V6G amp data

Yes, FFRR was an important development and it's interesting to search out the reproducers designed to exploit the wider frequency range. My regret is that it's difficult to find FFRR 78s with reasonably low surface noise: for some reason, Decca must have had difficulty getting a reasonable quality of pressing material in those immediate post-war years. Those additional high frequencies are often obscured by noise. It was only with the advent of vinyl that the improved recording standards shone through.

AFAIK, the Panatrope started life as a relatively powerful twin-turntable record player for theatres. It was originally a Brunswick product, so, like the Deccalian, may well have been designed by Harvey Schwartz, a Brunswick man. I believe Decca got both the Panatrope and HS when they acquired the UK end of the US Brunswick record company in 1932. HS, a man of many parts, is probably best known for designing the Decca Navigator system during WW2, but that's way off-topic!

Panatrope became a by-word for grams in theatre language - as in 'Hoover' for vacuum cleaner. I did hear once when I was in the audio business that, when modern comprehensive mixing desks were introduced into theatres, one sound engineer, on seeing an unfamiliar pan pot control, asked whether the label 'PAN' stood for 'Panatrope' !

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Old 18th Jan 2016, 12:19 am   #11
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Default Re: Decca Deccalian 6V6G amp data

Thanks you Martin, that is fascinating (and gives me hope!). Having acquired the schematic, it occurred to me that the little step up transformer might need to be bypassed for higher level inputs. I intend to fit this player with a multi speed deck to, hopefully, make best use of the rather nice amp. The CD player wasn't that obscure (!) rather it was a known reliable input. I am FAIRLY sure that the smoking component was the 8uF smoothing cap, which was looking a bit tired and emotional. I have replaced it and run the amp for much longer than it took to start smoking before - so, fingers crossed. I am a big Decca fan. I think I have had at some time or another nearly all the cartridges they ever made and recently restored (with a lot of help from Richard Smith of Studio 7 Music Repairs) a magnificent Decca Stereo Decola from 1959. - Roger
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Old 18th Jan 2016, 11:39 am   #12
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Default Re: Decca Deccalian 6V6G amp data

A good alternative "multi-speed" single player deck would be either the Garrard 4SP or the Collaro AC354 (I think?) single player which came in 3 and 4 speed versions. These had strong steel 10" turntables and good motors. Be aware, their headshells are rather cramped for the fitting of a new stereo or stereo-compatible cartridge. Also, currently available ceramic cartridges tend to give a lower output than the old crystal types (even the old medium-output crytal types would give a healthy 250mV) but with 3 stages of amplification on the Decca amplifier, there should be no problem. Edward
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Old 18th Jan 2016, 12:59 pm   #13
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Default Re: Decca Deccalian 6V6G amp data

Something I am not clear about is the function of L1. When I first looked at the amp I thought it had the traditional power supply transformer and output transformer. But then it appeared to have a line transformer next to the speaker (not much of a 'line' - I thought). I now know that is the output transformer and the little T on the chassis is designated L1 and is part of the power supply - what does it do? Keep it simple please - I have a rudimentary grasp of full wave rectification and smoothing - but that's it!
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Old 18th Jan 2016, 1:19 pm   #14
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Default Re: Decca Deccalian 6V6G amp data

In simple terms an inductor in series with the HT supply impedes the passage of ripple current whilst having little resistance to dc. This makes it better than a resistor in the same place. Trade off is it's more expensive than a resistor. Historically it made sense when large value capacitors were expensive too, but nowadays they're not.
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Old 18th Jan 2016, 2:24 pm   #15
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Default Re: Decca Deccalian 6V6G amp data

As Chris says, L1 is a smoothing choke - it works with those (smoky?) electrolytic capacitors that you've replaced to smooth the DC output of the 5Z4 rectifier valve. Otherwise you'd hear a loud hum from the speaker because the HT would be very rough..

I agree that it's worth replacing the record deck if you want multi-speed operation. Later models of Deccalian from around 1953 were fitted with the Garrard model T 3-speed player. One of those, or a similar unit from Collaro could probably be fitted with a bit of craftsmanship. The cabinet isn't tall enough though to accommodate an autochanger.

A turntable with a turnover ceramic cartridge is probably the best bet. If you can find a Collaro unit with their 'Studio' crystal cartridge, you might find that cartridge still works. Unfortunately, it's very tricky to fit a more recent stereo-compatible cartridge in the compact Collaro head. The Garrard model T with its larger plug-in head might be a better bet. As Edward says, the Garrard 4SP would also be a good choice. If it comes with a GC2 crystal cartridge, it might still be working, but if it's a GC8, my experience is that it'll probably be dead. Either way, you'll probably want to fit a more recent stereo compatible ceramic cartridge, and need to find one that fits. Other forum members do. I'm sure, have useful experience of fitting these.

As Edward says, the Deccalian amplifier has plenty of gain. If you're going to use a ceramic cartridge, I'd disconnect that input transformer altogether and connect a 1 megohm resistor from the EF37 grid to earth (chassis). Then connect the cartridge direct to the EF37 grid. I'm not sure how the tone control switch is connected in your amplifier. If it's like the later models, it was right at the input and won't work well with a ceramic cartridge, so might need to be disconnected or moved. But that's for another day!

Best of luck!

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Old 18th Jan 2016, 3:26 pm   #16
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Default Re: Decca Deccalian 6V6G amp data

There is bass boost for the magnetic cartridge which consists of a capacitor and resistor in series from the grid of the first valve to chassis which you should remove for a crystal or ceramic cartridge. It is only a standard 2 stage amp so will be ok.
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Old 18th Jan 2016, 5:23 pm   #17
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Default Re: Decca Deccalian 6V6G amp data

Thanks gents. It certainly works - I needed to put my ear right up to the speaker to detect any hum. I like the look of the Garrard 4SP, Edward, looks similar to my favourite deck of the era - the Collaro AT100, but I suspect there might be a problem fitting either of them in. The old stereo Decola has the AT100 (in all but name) and it's a beauty - I shall take some measurements. Yes - so far as I can tell - the cartridge input goes via the step up T directly to the volume control. Bass boost is achieved with an 80K R and a 0.0045uF cap in series to ground and treble attenuation by 0.0008uF and 0.002uF caps successively off a 3 position switched tone control. The spec says that the input from the old 78 cart was 30mV at 100cps (so much nicer than these new fangled 'Hz' I always feel). So, if I substitute a ceramic cart of, say, 100mV output after the step up T, won't that result in severely diminished gain?
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Old 18th Jan 2016, 5:32 pm   #18
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Default Re: Decca Deccalian 6V6G amp data

OOps - that's 1000cps!
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Old 18th Jan 2016, 7:08 pm   #19
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Default Re: Decca Deccalian 6V6G amp data

Best to remove the input tranny and go for the 1 meg resistor as above, noting the characteristics of a ceramic/crystal cartridge are very different to the magnetic the original input circuitry was designed for. Also ceramics need no input equalisation. If you use the Collaro 3/4 speed turntable (Decca also used Collaro decks in Deccalians) and it has an original working "Studio" crystal cartridge, be aware these were some of the best crystal cartridges ever made. If you find a "Studio" O already fitted, this gives 250mV, the extended HF response "Studio" P gives 50mV. If you find a "Studio" T already fitted and it works, it's worth a small fortune as it delivers a massive 1 Volt, but will overload the amp. Also a stereo stylus can be fitted to a "Studio" cartridge, but it will only have a limited vertical compliance. Edward

Last edited by Edward Huggins; 18th Jan 2016 at 7:11 pm. Reason: typos
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Old 18th Jan 2016, 7:10 pm   #20
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Default Re: Decca Deccalian 6V6G amp data

I've never checked the ratio of the step-up transformer, but the impedances involved suggest that it'll be around 1:10. From memory, I think you'll find that the bass boost network then attenuates by about 10 times at mid-frequencies in order to give 20dB boost at the bottom end. So the two more or less cancel each other out on voltage gain

If you remove both the transformer and the bass boost network, feeding a ceramic cartridge straight into the volume control, you should get enough volume.

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