Pre-amplifer for magnetic phono pick-up.

[Geordie McBoyne]

I fully understand Jeffrey's point of view that that a solid state solution is best approach for a phono pre-amp. However, I must abide the rule which is to use the technologies employed when the equipment was made and as the RGD 1046 was made in the late forties it has to be valves. Instills a sort of discipline you could say. The Garrard SP25 was introduced in the 1960s so we are still in the valve era, although transistors were appearing in audio equipment during that decade.
So it's Bob's valve pre-amplifier that going to be installed in the RGD, mildly modified of course.
Watch out for the use of many adaptor plates and brackets.

Geordie McBoyne.

[Mike Watterson]

The SP25 is cheat especially with magnetic pickup. Stick a ceramic cartridge if you want more authentic!

[Geordie McBoyne]

Ah yes, but surely you have to allow some engineering licence. When I acquired 1046G the original Garrard RC60 had been replaced in a very rough and ready manner with a BSR record deck. I'm keeping in the old RGD tradition by installing another Garrard record deck. The original up-market high quality equipment RGD company was taken over in 1952 by the mass market firm Regentone. Actually, in 1953 and under Regentone ownership a very RGD like over engineered radiogram was made, the model ARG5000.
Information for the ARG5000 can be found in the 1953/54 Radio and Television servicing book in pages 238 to 240.

Geordie McBoyne.

[Valvebloke]

...  The design of the pre-amplifier demands that the valve heaters are supplied from a 12volt DC source , however, it is a simple matter to rewire the circuit for a 6.3volt AC supply ...

Sorry, I've just got back to this so the above quote is from a post which is all the way back on p2 of this thread.

The designer will be 'demanding' a DC heater supply for a reason, which will be to keep the hum under control. Back in the day people tolerated a lot more hum than they do now (my late father-in-law used to say "The hum is how you know it's switched on !"). But now people just want to hear the music. I'd definitely stick with DC.

VB

[Synchrodyne]

Even back in the valve era, 50+ years ago, it was the “conventional wisdom” that a triode pair with series feedback used as a disc input amplifier required DC heating in order to avoid undue hum levels, given that the input stage cathode, to which the feedback was returned, was not at signal ground.  The inconvenience of having to use a metal rectifier for the heater supply probably mitigated against widespread use of this approach.  An unlikely example was the Jason J2-10 Mk III stereo amplifier of 1959.  This used an ECC83 with DC heating and series feedback for its input stages.  A more sophisticated example was the Revox G36 tape recorder of the mid-1960s; this used DC-heated half-ECC83s for the input and head amplifiers, although I think that their respective feedback pairs were made up by AC-heated half-ECC81s.

So it would appear that the 1s37 has simply followed established good practice in using DC heating.  The feedback pair would probably suffer from a relatively high hum level if AC heated, and the situation would likely be worse for the grounded grid head amplifier, presumably provided for very low output cartridges, perhaps including some moving coil types.

An aspect of series feedback RIAA-equalized amplifiers is that they have an inherent HF error, with the actual curves having another HF pole after which they flatten out, the position of this being dependent on the open-loop gain relative to the amount of feedback.  For the most part this seems to have been treated as negligible and ignored, even in the H.P. Walker design, where the departure from the RIAA curve is visible in the published curve.  Sometimes it was corrected by an appropriate passive RC circuit following the RIAA-equalized stage, as in the Quad 33.  Apart from the primary objective of obtaining an immaculate RIAA curve, possibly there was some advantage in  continuing the 20 dB/8ve downslope well into the supersonic range, in that any noise, clicks, etc., in this range coming off the disc would be attenuated before they got very far into the amplifier.

Cheers,

Steve

[Kalee20]

An aspect of series feedback RIAA-equalized amplifiers is that they have an inherent HF error, with the actual curves having another HF pole after which they flatten out, the position of this being dependent on the open-loop gain relative to the amount of feedback.

True, but isn't that an HF zero, not a pole? The gain stops falling and levels out at unity.

Good point that adding a simple RC stage can cancel it, and moreover if placed early on, can stop ultrasonic signals getting in and overloading things, without upsetting the RIAA characteristic!

[Murphyv310]

So where would you expect the ultrasonic signals to come from?

[Murphyv310]

This article makes very interesting reading
http://wiki.hydrogenaud.io/index.php?tit...hs_(Vinyl)

[Valvebloke]

This article makes very interesting reading
http://wiki.hydrogenaud.io/index.php?tit...hs_(Vinyl)

There's a great deal of truth there. The first point, where he discusses what 'soiunds better' means, is particularly important.

VB

[Mike Watterson]

So where would you expect the ultrasonic signals to come from?

No idea, but the cartridge might be microphonic and someone might have an "Ultrasonic Pest Repeller", which actually don't repel pests, though may annoy dogs, guinea pigs etc?
I've heard RF pickup, Taxis, Police etc in 1970s on record players.