Decca Decola tone controls

[skodajag]

Not sure if this should be here or under audio.
The treble and bass controls on my Decola (the original, mono unit) have never done much. I’m in the process of checking and replacing the obvious caps and resistors, so far without much effect. Has anyone else had this problem? Interestingly, my chassis is fitted with higher value caps on the treble than specified in the manual. I’m certain these are original because all soldered joints were dabbed with red paint, and in the case of these caps this paint hasn’t been disturbed. So this suggests to me they might have had issues with these controls from the outset. Any thoughts most welcome.

[Techman]

I've got the big Dynatron version of this gram if it's the one I'm thinking of, not the same as yours, but bound to be a similar circuit and from the mid to late 1940s, and mine was always as you describe regarding the bass control, the treble control is also the radio selectivity and does make quite a difference. A few years ago I spent some time on the bass control circuitry on mine and replaced a couple or three capacitors in the end and there was a little improvement, but it was far from dramatic, so I think you'll find that it's just how it is and how they were from that era - it won't be like the way the tone controls on a modern amplifier system work. My controls are 'click' positions on switches and not pots as yours will be. I was going to re-visit mine again sometime, but it's fine as it is for something of that age, so I probably won't bother. Before I worked on mine, the base used to gradually decrease slightly over several hours of use, whereas now it remains constant, so there was actually a component fault in the circuit affecting it.

[skodajag]

Thanks for this. I think the Dynatrons from that era are very different to the Decola.

[Techman]

It sounds like a later one then.

I think there may have been a few called 'Decola', not sure, but you don't say which one or from what date, so difficult for anyone to make any suggestions really.

[skodajag]

As I said in the original post it’s “the original mono unit”. Throughout its production run this Decola used an unusual and perhaps unique “split” circuit.

[Radio Wrangler]

The effective range of tone controls, before Peter Baxandall came along with his active circuit wasn't very much. So you may just be used to the later design and experiencing period-correct performance.

David

[Techman]

Yes I gathered it was mono.

Someone who has that exact model will be able to confirm if it's just how they are or perhaps look at the circuit and see if it can be improved / modified at all - or what's likely to be at fault.

Early treble and bass circuits didn't seem to work as dramatically as later ones did - I can't say I remember ever working on any Decolas myself, so not much help from this end.

Edit: Crossed with David.

[Herald1360]

Is the circuit the one in the RTVS sheet available up top? Push pull triodes all the way through ending in a pair of PX25s? S2 bass and S3 treble? Volume control the twin gang pot setup?

If it is, I'll have a play with simulating what the controls do in the way of lift and cut or whatever.

I'll have a play anyway but if it is the right circuit, the playing will be useful!

[Hartley118]

The attached circuit portion of the input stages of the Decola shows the rather unusual form of the switched tone controls, especially unfamiliar because of the unconventional push-pull input stages. One or two points are:

1.The tone controls have no effect on the radio input. Presumably the variable selectivity on the radio unit is relied on as a treble cut.

2. The gram input includes a fixed LF boost network immediately after the input transformer.

3.Unlike the Deccalian, where the LF boost turnover frequency is automatically switched by the change of XMS pickup head from 78 to LP, the Decola relies on the bass boost 3-position switch (S3) to adjust for differences in LF recording characteristic.

4. The treble 'Brilliance' switch (S2) is for 'cut' only, presumably primarily to compensate for the HF boost characteristic inherent in Decca FFRR discs. Position 2 gives the required 6dB HF rolloff

5. Position 1 of S2 inserts a 2-stage 12dB/octave scratch filter, whilst position 3 is flat, as required for discs with the HMV/Columbia characteristic.

This particular circuit diagram, copied from the Vintage Audio Workshop Youtube site, contains an error. R14 is shown as 1M and presumably should be 100k like R13. Whether this error was carried through in production seems unlikely.

In summary, rather surprisingly, the Decola tone controls are of limited range and seem designed to supplement the built-in disc equalization rather than providing the usual +-15dB range of today's bass and treble controls.

And they don't function on radio.

Martin

[Herald1360]

I've had my play, results below.


The PU bass boost is just what you get feeding a low impedance voltage source into the R1-R5, C1-C2 network. Actual results would also be affected by the source impedance of the pickup as transformed by T1.

[skodajag]

Thank you Martin, your comments are very helpful, as ever. Interestingly, that same resistor value error is there in the Decca service manual, though not on the chassis itself. As you say, in that circuit the tone controls can’t work on the radio. And yet, I have a late copy of the owner instruction booklet, dated 1952 which says they do! See pic below. I wonder if they changed the circuit? To make matters stranger, a friend has loaned me another Decola chassis, “restored” by someone whose name might well have begun with “P”, in which the treble works brilliantly on radio and gram, while the bass works on neither. But then for some inexplicable reason the gram has been wired to the bass control....don’t ask why because I don’t know! But I am learning...slowly.

[Hartley118]

Originally Posted by Herald1360

I've had my play, results below.


The PU bass boost is just what you get feeding a low impedance voltage source into the R1-R5, C1-C2 network. Actual results would also be affected by the source impedance of the pickup as transformed by T1.

Interesting Chris - it's reassuring to see your quantitative analysis support my qualitative guesswork.

The Decola is a somewhat puzzling design. Whilst the amplifier, with its push-pull triodes throughout was clearly designed for high quality, the speaker arrangement is rather more of a 'music PA' system of more limited frequency range than the rest of the system.

The very first time I saw a Decola was in the 1950s in a restaurant, where it stood on a stage which previously had no doubt accommodated a small orchestra for tea dances and the like. I wonder whether that kind of commercial application was part of Decca's design requirement. There would have been plenty of volume from those push-pull PX25s and three high-flux 12-inch Goodmans speakers. The absence of conventional bass & treble boost tone controls would certainly have avoided misuse by unskilled operators in a commercial environment.

Martin

[Hartley118]

Originally Posted by skodajag

Thank you Martin, your comments are very helpful, as ever. Interestingly, that same resistor value error is there in the Decca service manual, though not on the chassis itself. As you say, in that circuit the tone controls can’t work on the radio. And yet, I have a late copy of the owner instruction booklet, dated 1952 which says they do! See pic below. I wonder if they changed the circuit? To make matters stranger, a friend has loaned me another Decola chassis, “restored” by someone whose name might well have begun with “P”, in which the treble works brilliantly on radio and gram, while the bass works on neither. But then for some inexplicable reason the gram has been wired to the bass control....don’t ask why because I don’t know! But I am learning...slowly.

I guess that, like any design, the Decola would have evolved over the few years of its production so that later models were probably rather different from the earlier ones. We can bear in mind that, at the start of production (1947 I believe), the only source material was the 78 record, but by 1950 the LP had arrived, with its improved quality, especially its HF pre-emphasis and wider dynamic range. That may well have prompted more comprehensive tone controls.

The original Decola seems only to have been a record player rather than a radiogram, not least to avoid luxury class purchase tax. So whilst the original design no doubt anticipated the addition of the auxiliary radio tuner, its radio input arrangement with absence of tone controls may well have been a deficiency which was rectified in later versions.


Martin

P.S. I couldn't possibly comment on your other restored version!