Tube Bias Oscillator for Tape Recording?

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Hyld

Member
Joined
Nov 3, 2015
Messages
8
Hello,

Can anyone point me in the direction of a simple tube oscillator, preferably with a 12AU7, for mixing into the signal for recording onto audio tape? Something in the 150kHz range AND preferably made with components that aren't unobtainable. The only one I've found uses a choke or transformer that has no specs.

I'm not sure of what voltage it needs to be printing at, is that crucial?

Sounds pretty specific now that I see it written down, but this has to have been common practice at one point.

Thank for any info you might have.
 
> The only one I've found uses a choke or transformer that has no specs.

That is the way it was done.

150KHz was very high for those days.

Your best bet may be to buy a sick tape recorder off eBay, and salvage the bias.

The voltage (and current) depend on the head(s) used.

Or admit that the quality of recording depends on bias cleanliness, NOT whether it comes from a vacuum or a crystal, and solid-state it. (Even so, mystery oscillator transformers are the rule in solid-state tape.)

And Welcome.
 
All the tube bias oscillator circuits I have seen use an LC resonant circuit. You need a low distortion sine wave - any distortion appears as noise. For this you need a high Q resonant circuit - the higher the Q the purer the sine wave. In the better quality products, a push pull circuit was commonly used for its distortion cancelling properties. 100KH is the highest frequency I have seen in semi pro gear (Brenell).

The only other method I can think of is to use a Wien bridge oscillator with a thermistor stabilised feedback loop.

Cheers

Ian
 
If you want tube, there's no way around winding your own bias transformer. See e.g. G36 for reference - although you'll want to up the frequency in that one (details found elsewhere on the interwebs).

For solid-state see e.g. "Audio Electronics", by John Linsley Hood - p.38ff:

https://books.google.dk/books?id=EOE1tx3CJ98C&printsec=frontcover&hl=da#v=onepage&q&f=false

Jakob E.
 
Wow, you all are fast.

Thanks for the welcome, PRR. That was what I was afraid of - everyone doing their own thing with their own chokes, not much of a standard. I'm guessing the transformers were more abundant in the tubes hayday. Canabalizing an old deck seems like a good way to go, but I'm sort of leaning more towards solid state now. I'd rather do something with current parts and have it be reproducible for future fixes.

Ruffrecords - I will look into the push pull type configuration you mentioned. I did read that some decks used a power amp as a bias oscillator, now with your comment, that makes more sense.

gryaf - thank you for a link to that book. Some of the pages are restricted, but I'm going to have to read through what there is. That first schematic on p38 looks really simple, I will have to plug it in to LTSpice and see what's up.

abbey road d enfer - are we looking at something like 10V or 100V for bias oscillator? There are tape circuits running off of 12V transistors and others with tubes pushing 100V. I don't quite understand this aspect.

 
Hyld said:
Wow, you all are fast.

Thanks for the welcome, PRR. That was what I was afraid of - everyone doing their own thing with their own chokes, not much of a standard. I'm guessing the transformers were more abundant in the tubes hayday. Canabalizing an old deck seems like a good way to go, but I'm sort of leaning more towards solid state now. I'd rather do something with current parts and have it be reproducible for future fixes.

Ruffrecords - I will look into the push pull type configuration you mentioned. I did read that some decks used a power amp as a bias oscillator, now with your comment, that makes more sense.

gryaf - thank you for a link to that book. Some of the pages are restricted, but I'm going to have to read through what there is. That first schematic on p38 looks really simple, I will have to plug it in to LTSpice and see what's up.

abbey road d enfer - are we looking at something like 10V or 100V for bias oscillator? There are tape circuits running off of 12V transistors and others with tubes pushing 100V. I don't quite understand this aspect.
I'm having bad flashbacks from my work on a 4 track tape recorder design I inherited back in the '80s.  IIRC the bias oscillators made a pretty healthy output voltage (60-90V?), while running from modest DC supplies (no doubt step-up windings in the resonant circuits).

The voltage kind of depends on the head impedance at the bias frequency. IIRC my old project ran the bias oscillators up at 85kHz. You will also want high Q traps to remove bias leakage from the audio electronics. I had to keep the bias leakage out of Dolby C playback processing level detectors.

I can imagine more fun things to do.  +1 to buying some old scrapped deck to steal circuitry from... A very old wheel to reinvent, then if lucky you get results as good as the old circuitry.

JR
 
Hyld said:
abbey road d enfer - are we looking at something like 10V or 100V for bias oscillator? There are tape circuits running off of 12V transistors and others with tubes pushing 100V. I don't quite understand this aspect.
Since most bias oscillators are transformer-based, it's easy to produce any voltage from any power source (within reason).
Combined record/repro heads are high impedance for tube gear, low impedance for solid state; bias voltage may be 100v on tube gear and 10V on SS gear.
 
In german broadcast audio the tape amplifiers were often individual units for separate functions. You might be able to get hold of a tube amplifier with bias oscillator for a few bucks on german ebay if you get lucky since nearly noone has a use for them.

I did a modification of a solid state tape machine into tube rec / playback amps, but I'm not sure if I'd use a tube oscillator if I'd do that again. It seems rather difficult and I'm not sure whether there's any benefit vs a solid state design. In my case it turned out that the tube amp doesn't deliver sufficient energy to really totally erase the tape. So now I have to erase first before I can record again if I want to be sure that there's absolutely nothing left of the previous recording.

Michael
 

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