I have Ampex MM-1200 and would like to replace solid state amps for record / playback heads with tube amps. I do play music, but my electronic knowledge is limited; is anyone there able/willing to help in such a project.
If you have limited knowledgements, I suggest not to modify it. Tape haeads gives very few millivolts at its output, and need to be loaded with some capacitance and resistance to give good performance. So, the levels of amplifications are great, the possibility of noise and hum pick ups are high, and finally you need an equalization networks in any way, similar to the RIAA, but called NAB, and it depends of the tape velocity.
Regards.
Regards.
Appreciate reply Osvaldo. Still I am keen to do it. I understand about small signals comming out of tape heads, and equilization networks too, I can work with soldering iron and a lot more... but to design the circuit is slightly beyond me; I could try, but this is not something where I would want to "learn" by making costly mistakes.
I forgot some details. Do you have sufficient space for a new power supply, and the tubes itself, and the sufficient air flow to prevent them overheating itself and surrounding elements?
This is valid point, Osvaldo. Yes i was thinking about that, I think there is enough space, and I could surely use switch mode PS, as the current requirement will be relatively small...
SMPS spraying interference near a head amp? Lovely!
You have been given good advice. Why try to ruin a perfectly good device?
You have been given good advice. Why try to ruin a perfectly good device?
Hmm...not realy near tape head, on a safe distance, just where the main power supply and the rest of power electronic for the whole unit is situated now.
Yea - this is a huge undertaking. Do you have complete technicial information on this lovely, complecated piece of equipment? You can't even address the question properly without that in hand.
Space is not even the biggest chalenge.
Space is not even the biggest chalenge.
This is one of those projects where if you need to ask then you don't know enough to do it successfully. You may find someone local who is happy to do the job for you at commercial consultancy rates, but I suspect that their initial advice will be the same as ours.
Why do you want to make this change anyway? Are you after 'tube sound'? i.e. you want to turn a sound reproduction machine into a sound effects machine?
Why do you want to make this change anyway? Are you after 'tube sound'? i.e. you want to turn a sound reproduction machine into a sound effects machine?
Yes I understand it is a great challange; and yes, I have all service manuals, this is why I started thinking of the project. And I know if it can be done properly, reward soundwise will be equaly great...
You could start by understanding the current circuit. Study the head amp in detail, until you know exactly how it works, why each component has the value it has etc. This may take a few years, if you are starting from scratch. I suspect that once you know enough to do it, you will know enough to decide not to do it.
PS I have been studying electronics on and off for over 40 years. I still don't know enough to competently design a tape head amp.
PS I have been studying electronics on and off for over 40 years. I still don't know enough to competently design a tape head amp.
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No, I am not trying to turn it into a "sound effect machine", LOL...But I am after that tube sound, to record with all the good (and bad) atributes of tubes...I hope it will not take me 40 years to do this project...
You should check out "the tape project" and the Bottlehead tape preamp. They are focused on playback only. The tapes are mastered by Paul Stubblebine. I know Paul uses tube gear in his mastering chain but I'm doubting he uses a tube record head amp. As I recall, tape technology advanced most during it's solid state years.
Bottom line is I don't think there's a lot to be gained by building a tube driven record head, but it should not be a huge technical challenge either once you understand the nuance of magnetic record amp circuits and have played with a few prototypes.
Seems to me that a year or two is not a lot of time to get to the competent designer level in a new area like that, if you already are a competent designer of basic tube circuits and have basic EE skills. What's the hurry?
PS one thing to check out right away is whether you can successfully adapt a tube circuit to your record head impedance with the proper bias and signal levels, or if there is a head stack available which you can successfully drive with a tube circuit, e.g can you build an output transformer that properly mixes bias and signal?
Maybe you could start out with the Bottlehead kit for the tube playback preamp and go from there.
Bottom line is I don't think there's a lot to be gained by building a tube driven record head, but it should not be a huge technical challenge either once you understand the nuance of magnetic record amp circuits and have played with a few prototypes.
Seems to me that a year or two is not a lot of time to get to the competent designer level in a new area like that, if you already are a competent designer of basic tube circuits and have basic EE skills. What's the hurry?
PS one thing to check out right away is whether you can successfully adapt a tube circuit to your record head impedance with the proper bias and signal levels, or if there is a head stack available which you can successfully drive with a tube circuit, e.g can you build an output transformer that properly mixes bias and signal?
Maybe you could start out with the Bottlehead kit for the tube playback preamp and go from there.
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Time out please, guys. The machine in question has low impedance heads that work with BJTs, not tubes. A very reasonable playback setup would use a low noise, high β, BJT to drive a 6922 section that feeds a passive EQ setup and uses the 2nd 6922 section to round out the gain structure. It might be possible to use the BJT in the bottom of a cascode and the 6922 section on the top, ala Allen Wright. A CLOSELY matched pair of BJTs (1/channel) is needed, in such a no loop NFB setup.
Eli, I'm pretty sure the bottlehead kit uses step-up transformers to go from tape heads to the grid of the first amplifier stage. I see some octal plugin transformers on the back of the unit. This is also a low noise solution.
(PS Suitable BJT types are 2SC2545, 2SC3329 as I use in my ribbon mic interface circuit.)
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MK,
I haven't been following Bottlehead closely, of late. However, I remember Dan Schmalle (Doc B.) issuing warnings about low impedance heads, some time ago.
IMO, a hybrid cascode finesses the low impedance head issue quite nicely. Do you know where the OP could source a matched pair of either BJT type you mentioned?
BTW, I have an old Viking 88 R2R deck that I bought in 1964. Viking used a "sand" 1st gain block in the playback preamp, but not cascode topology. 😉
I haven't been following Bottlehead closely, of late. However, I remember Dan Schmalle (Doc B.) issuing warnings about low impedance heads, some time ago.
IMO, a hybrid cascode finesses the low impedance head issue quite nicely. Do you know where the OP could source a matched pair of either BJT type you mentioned?
BTW, I have an old Viking 88 R2R deck that I bought in 1964. Viking used a "sand" 1st gain block in the playback preamp, but not cascode topology. 😉
The best record amp I ever made worked in class D, but it caused lot of interference since unlike sinewave erase/bias oscillator it generated lots of harmonics on all AM bands. Today it may not be a problem.
Playback amp is doable, but most probably playback heads made for solid state playback amp have too low impedance. You will need some very high quality matching transformers, or different heads made for tube playback amp, otherwise noise will be too high.
You can build a stereo playback amp in a separate box and try before damaging a nice machine.
Also, you will need standard measurement tapes to set right frequency response curves, calibrate playback head.
Playback amp is doable, but most probably playback heads made for solid state playback amp have too low impedance. You will need some very high quality matching transformers, or different heads made for tube playback amp, otherwise noise will be too high.
You can build a stereo playback amp in a separate box and try before damaging a nice machine.
Also, you will need standard measurement tapes to set right frequency response curves, calibrate playback head.
Now you're talkin'. Also, many vacuum valve preamps and integrated amps had tape head playback incorporated. Even a Dyna SCA-35. Sounds like matching transformers will be needed to keep it all vacuum valve, so that's a first question:
what is the head load impedance currently?
All good fortune,
Chris
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