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Old 22nd May 2014, 12:34 am   #1
Stevie342000
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Default Varible Speed Tape Drive System

I have located in an old Tape Magazine an article on a variable speed valve tape drive system.

It's not that complicated but I am missing some detail (probably should check a later issue) there is a component value missing in the Wien Bridge oscillator section.

I could work it out or guess at about 220K but would anyone know what value should be used.

see attachment

Thanks
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Old 22nd May 2014, 7:25 am   #2
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Default Re: Variable Speed Tape Drive Oscillator

It isn't a wein bridge oscillator. The title is correct, it really is called a 'phase shift oscillator' which has always been a poor name because just about all oscillators have their frequency set by the phase shift of some network.

The valve gives 180 degrees of phase shift as it is an inverting amplifier.

A single series-R-shunt-C section will give 45 degrees of delay (lag) at its 3dB attenuation frequency, and will approach 90 degrees as the frequency gets very large. Unfortunately the attenuation also becomes huge.

The circuit needs to make 180 degrees of lag in the RC network. One RC stage can't do it. Two stages will just get there but at infinite frequency, with infinite loss, so that's not going to work. Three (or more) stages will get to 180 degrees total while there is still some signal getting through.

As printed with a single variable resistor, this circuit will only tune over a small range, so if used to drive a tape recorder capstan motor it'll give enough pitch shift to fine-tune things but won't turn the machine into a wide speed range job.

With any oscillator, the design job isn't finished with making a frequency control circuit. The problem arises of stabilising the amplitude too. Too little gain around the loop and oscillation stops or never starts in the first place. Too much gain and the amplitude grows until something bangs into its end-stops. Unfortunately, the gain required is a single exact value so you can't in theory make a circut work without some active self-adjustment of gain.

The wien bridge has feedback controlling the gain of its amplifier, so it can't use amplifier non-linearity as gentle limiting. The feedback network has to change with level, and so it uses a thermistor or else a detector and an FET as a variable resistor.

The phase shift oscillator doesn't fix the non-linearity of the valve, it uses it. The gain is set a bit on the high side, and as level builds the overall gain, averaged over a full cycle, falls and the level finds a stable equilibrium.

The grid 2 voltage of a pentode will reduce the gain if the voltage falls, and a hard driven stage will start to take more grid 2 current. That resistor hasn't got a value written on it for a reason. I suspect there is some text in the accompanying article about fiddling with its value to get the oscillator level to a chosen value.

So pick a value to start with, and be prepared to change it until it runs with a reasonable voltage and the waveform looks reasonably sinusoidal on a scope.

You can make a wide range phase shift oscillator with a triple ganged pot, and switched capacitors, but getting the gain right for a good level becomes difficult over much of a frequency range.

David
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Old 22nd May 2014, 7:41 am   #3
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Default Re: Varible Speed Tape Drive System

That is the screen grid supply resistor, with no marked value. The maximum screen current for an EF80 is 2.8 mA; maximum screen grid dissipation is 0.9 W. You could work it out but I'd just start by sticking in 100K, measuring the current and tweaking the value as appropriate. Though it'll probably oscillate anyway, whatever value you use, just because it's an EF80.

I take it the idea is for this oscillator, via a fairly hefty amplifier, to power the capstan motor and thus provide controllable tape speed?
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Old 22nd May 2014, 6:49 pm   #4
Stevie342000
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Default Re: Varible Speed Tape Drive System

Yes it does drive a power amplifier using ECC83 as voltage amplifier and phase splitter driving a pair of 807s with 450 volts on the anodes.

The output transformer is a 350-0-350 volt power transformer used in reverse, so you get 230 volts out of primary.

The only other thing is 0.25uF capacitor across the whole of the secondary winding - used to adjust so you get maximum output voltage.

But I do not need to build the power amplifier now as I just purchased a Vortexion Super 50 amplifier which has a 250R output tap which gives you the same thing.

Additionally I have a couple Ampex 375 the American version for driving an Ampex 300 or 350 or 400 usually the former. Which additionally uses 807s putting out 75 watts (American) probably more like 50 watts same as Vortexion, the Americans had a different way of measuring maximum output.

Incidentally I am looking for the Circuit diagram for the Vortexion Super 50 using 6Q7Gs and 6J5s, I have the circuit for the later 1955 version which is mark 2, the Mark 1 version was in production from about 1944, the Mark 3 version came out in 1959, yep I have the schematic for that as well but output tubes were changed to EL34s.
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Old 22nd May 2014, 7:03 pm   #5
Stevie342000
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Default Re: Variable Speed Tape Drive Oscillator

Quote:
Originally Posted by Radio Wrangler View Post
It isn't a wein bridge oscillator. The title is correct, it really is called a 'phase shift oscillator' which has always been a poor name because just about all oscillators have their frequency set by the phase shift of some network
You are absolutely right on 3 counts:
1. it has too much gain, that bit is mentioned in the blurb - the main amplifier section has a 500k log pot to set input level.
2. it does operate over a small range 35 - 80 Hz.
3. it is a phase shift oscillator.

There is no mention of the missing value in the text I have read it several times over the years, it may be mentioned in a later edition as errata item. Assuming I have that edition, I think I do but would have to look.

Thought it would be quicker to ask.

The Phase Shift Oscillator drives a standard valve power amp using 807s with 450 volts DC at 200mA on the power line. It uses the common 450v supply to the 68K resistor.

The maximum screen current for an EF80 is 2.8 mA; maximum screen grid dissipation is 0.9 W.

Last edited by AC/HL; 23rd May 2014 at 12:36 am. Reason: Threads merged
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Old 23rd May 2014, 12:03 pm   #6
Stevie342000
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Default Re: Varible Speed Tape Drive System

Thought it might be better if I attached the original article so all can see and offer any advice.
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Old 23rd May 2014, 1:46 pm   #7
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Default Re: Varible Speed Tape Drive System

Ah, it's by Fred Judd!

A man very much into electronic music, which he did for TV (Space Patrol for example) and he also wrote a book on antennae as well as originating the 'slim jim'

It's quite surprising he'd miss a value off without a comment. As said, the oscillator runs at a high level, and this id because it relies on gain compression to stablise its level.

David
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