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Author Topic: Revox G36 High Speed  (Read 23549 times)

Offline topoxforddoc

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Revox G36 High Speed
« on: January 11, 2009, 03:40:36 PM »
Hi,

I'm a complete R2R tape newbie. However I recently purchased some R2R machines to transcribe some old tapes. Amongst my purchases is a Revox G36 high speed 7 1/2 or 15 ips machine 2 track. It works fine; I bought it from a BBC studio engineer, who was trying to thin his collection. He had 7 R2R machines in his kitchen alone! In his lounge was his recording studio with 2 B77s, 2 Fostexs, a Studer 810 and more!

Anyway I do like analogue (high end TT), but thought I might dip my toe in the water with proper tape. Any tips please would be very welcome.

Best wishes,

Charlie Chan
Cheltenham, UK

Offline ironbut

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Re: Revox G36 High Speed
« Reply #1 on: January 11, 2009, 04:41:40 PM »
Hi Charlie, your name seems a bit familiar but I just can't place it. Ever been to Stone Mountain Georgia? Just joking.
The first thing to do is to find a service manual. That will provide vital adjustment levels and procedures. For the G36, I'd like to suggest an article from Glass Audio magazine called " The Greening of the Revox G36" written by one of the members here, Charlie King. It can be found here;
http://koti.mbnet.fi/siliconf/JukkaTolonen/ga/revox/revox.html
The G36 has a storied history and is a wonderful machine but it is a little dated. Early head design lacks the frequency extension of more modern machines and the electronics tend to be on the euphonic side of neutral. That doesn't mean that you'd be better off with something else if that's what you like. I have an old tube Ampex F44 and while it has terrible spec's, it is a very pleasant sounding machine.
If I can toot my own horn, there is a sticky at the top of the General forum "Beginners Guide,.." that is meant to pick up where most owners manuals leave off and (I hope) will provide you with enough knowledge of the nuts and bolts that make up magnetic tape usage to gain a real understanding of the subject. Just browsing through the forums here will also give you lots of info.
Give a holler if you need any help and welcome to the forum.
« Last Edit: January 11, 2009, 05:07:22 PM by ironbut »
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Offline sound signal

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Re: Revox G36 High Speed
« Reply #2 on: January 11, 2009, 04:47:25 PM »
Hi Charlie,

Congratulations on your purchase of the high speed Revox G36.  I have two G36's, both normal speed.  I also overhauled an F36 which I have given to a friend.  The same friend and I also have one high-speed B77 each.  The B77 is a smoother machine in both operation and sound, but the 36 series machines have a very dynamic and lifelike sound.  We suspect it's the valve electronics.

It's very important to determine that you have an original high-speed G36 and not a modification, as a modification, made by changing the capstan shaft only, results in an incorrect tape path.  The correct way to make a modification was to change both the capstan shaft and motor, but back in the days when both parts weer available the combined cost was more than the secondhand value of the whole deck so nobody bothered, they just changed the shaft which got the speed right and ignored the change in the tape path geometry.

All original high-speed G36's were Series III machines, so they should have a photoelectric tape end switch, not the mechanical switches of the earlier Series I and Series II machines.  A mechanical tape end switch would mean it's a modification.  Also, look at the speed selection pushbutton markings on the front panel.  Are they printed directly on the aluminium insert or are they on a sticker?  I think the original factory high-speed machines had the speeds printed directly on the aluminium but I am not sure, perhaps some of the factory machines had a sticker with 7 1/2 and 15 taped on top of the normal 3 3/4 and 7 1/2 markings.

Get yourself a good demagnitiser as the tape lifters and the guide flanges are ferromagnetic and if they become magnetised they could damage your tapes.  And of course the heads will also benefit from regular demagnetisation. The Han-D-Mag by R.B. Annis is a good choice.

Also check that you have the new pattern heads as the old type heads don't last nearly as long and don't have as wide a frequency response.  The new pattern heads have integral head shields shaped like those on the heads on more modern Revoxes like the B77.  The old pattern heads have cylindrical shielding cans which are separate from the heads.  The new pattern heads are, basically, identical to the heads in the B77 and PR99 so they are as good as any heads fitted to any semi-pro machine.

The only rubber parts a G36 could possibly ever need attention to are the pinch roller and the decoupling strips between capstan motor and flywheel.  I don't know if those are still available.  If they are I would get them to have in stock if I were you.

G36's in "as found" condition today often suffer from lack of lubrication to the capstan shaft and to the capstan motor bearings.  Especially the motor bearings were supposed to be lubricated for life, but I suspect at the time the Revox engineers designed these, they never considered some of them would still be in use over forty years later!  This sometimes causes the capstan motor not to come up to speed, or to "hunt", especially at the higher of the two speeds.  Dismantling and lubricating the capstan bearing and motor prevents this.  You might also find that old molybdenum grease has become sticky and is causing the three solenoids (pinch roller, brakes, and push button release) to stick.  Cleaning and lubricating the solenoid bores and slugs takes care of that.  With this, and a little attention to the brakes perhaps, the mechanicals of the G36 are good for another 20 years.  That really is a low maintenance transport.

You might find recordings made on the G36 sound bright, no matter what you do to the bias.  This is because the recording circuits on earlier G36's were tuned to the tape formulations available in the early 1960's.  By the end of the 1960's the frequency response of studio tapes began to approach what we are familiar with today, and they didn't need as much treble boost as earlier tapes.  Revox made a small modification to the record amplifier pre-emphasis circuit at some point, to bring the machine in line with tapes becoming available at the time. It is easy to make the same modification to those machines that haven't had it from the factory.  Indeed, the tape used by The Tape Project, which also happens to be my favourite tape, Agfa PEM468 (today's RMGI SM468) first became available in 1971, at about the same time as the G36 was going out of production.  In fact the G36 works very well with this tape when the record amps have the frequency response modification and the bias is tuned for it.  I use a reference fluxivity of 355 nanowebers per metre.  I set the VU meters to read 0dB at this level, and set bias for a 4dB drop-off at 10kHz.

Of course, if you're going to be using the machine mostly for playback, you don't care much about the bias, recoring eq, and reference level.  If that is what you are going to be doing, then choose three nice ECC83/12AX7 valves for the playback circuit and leave some old valves in the rest of the circuits.  There are another two ECC83/12AX7, two ECL82/6BM8 and one ECC82/12AU7 in the rest of the circuit and it would be a pity to fit new ones and have them just wear out while you're only using the playback circuits.  Whatever you do, though, do leave all the valves in position as removing them changes the supply voltages and that is not good news.

I hope you found the above useful.  Let me know how you get on with your G36 and if you have any questions at all, don't hesitate to ask.
George Karaolides
Nicosia, Cyprus

Offline topoxforddoc

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Re: Revox G36 High Speed
« Reply #3 on: January 11, 2009, 04:59:39 PM »
Ironbut and George,

Thank you very much for your helpful and speedy replies. I shall go and have a look at my machine to see. I've got a demag already and otherwise the  machine seems to work fine on replay. I haven't tried record yet.

What do you hear if the tape pathway is incorrect? Do you get crosstalk or something els?

thanks

Charlie

Offline sound signal

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Re: Revox G36 High Speed
« Reply #4 on: January 12, 2009, 04:04:55 PM »
Hi,

If a G36 is modified to high speed with just a change of the capstan shaft, there is less wrap of the tape around the playback head.  This will affect both the contact footprint of the tape on the head (make it smaller) and reduce the amount of pressure between tape and head.  Revoxes depend on tape wrap and tension to keep the tape on the heads, they don't use pressure pads like most high quality machines and all professional machines. Less contact area and pressure will therefore make the head harder to adjust properly for best and consistent high frequency response, and more prone to dropouts caused by contamination or any irregularities of the tape.

Another clue to the machine being a factory high-speed machine could be a factory ink stamp - probably on the electronics chassis on the back close to the valves, or on the inside.  The stamp usually reads "Geprüft 2 Spur" or "Geprüft 4 Spur" which means "Tested 2 Track" and "Tested 4 Track" in German.  In this case it might read "Geprüft 19/38 cm/s".

Yet another clue would be the equaisation resistors in the record and playback circuits.  If there are no signs of soldering done after the machine left the factory and all the resistors are the same type then it has probably not been modified.  It is very easy to spot post-factory soldering on G36's because the plastic terminal boards are thermoplastic and any attention from a soldering iron will leave signs of melting, however small.  Indeed, one should be very careful when doing soldering work on one of these.  In that way the earlier F36 is superior, the similar terminal boards in that are made of a material that was definitely not thermoplastic.

The one area where the G36 is weak is in its stationary (non-rotating) tape guides.  These do wear and need replacement, and do cause the tape to shed a bit more oxide than necessary.  They also seem to cause more scrape flutter than on, say, a B77, which can be seen on an oscilloscope screen and heard as a slightly less smooth sound - though, as I said, the valve electronics do seem to endow it with a lot of dynamics and life.  It could be possible to adapt the rotating guides from the B77 in at least the left hand guide position.

By the way, speaking of tape guides, the correct way to thread a G36 is with the tape going between the narrow guides on the outside and the wide guides on the inside - that is, with the back of the tape contacting the outside guides, and the front of the tape contacting the inside guides.  If you thread it wrong you will not get the correct tape tension.  While you're at it, check that the tape tension switch above the head block, selecting between small and large reels, is functioning properly, or once again you won't get the correct tape tension.  With no tape loaded, hold the left hand reel turntable stationary with your hand, and press the Play and Stop buttons together.   This will over-ride the automatic stop and keep the machine in Play mode without you needing to hold the Play button.  Operate the tension switch.  You should feel a greater tension with the lever retracted (large reel position) than with the lever extended (small reel position).  To return the machine to Stop, partially press the Wind or Rewind button.  Repeat the procedure while holding the right hand reel turntable stationary.  If you do not feel a change in tension, disconnect from the mains, take the head cover off and have a look at the tension switch, it's a very straightforward device and if anything is wrong with it it will be obvious.  By the way be very careful when working anywhere inside a G36, as practically everything except the pilot lamps and valve heaters operates with high DC or AC voltages and you can easily hurt or kill yourself.

With best regards,
« Last Edit: January 12, 2009, 04:16:48 PM by sound signal »
George Karaolides
Nicosia, Cyprus

Offline topoxforddoc

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Re: Revox G36 High Speed
« Reply #5 on: January 13, 2009, 04:34:36 PM »
George,

thank you for that very comprehensive reply. I shall have to have a look.

Charlie

Offline topoxforddoc

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Re: Revox G36 High Speed
« Reply #6 on: January 13, 2009, 05:08:12 PM »
George,

Just took the cover off and couldn't see anything stamped on the chassis. But then I saw the original Revox seal attached to the handle. It says 2 spur, S/N 77207, and 19/38. So presumably it's an original 15 ips factory build!
What good luck on my part. Inside the chassis, it looks pristine, much better than my standard speed G36. This little beauty only cost me GBP200 (with a standard speed serviced A77 in that cost) with NAB adaptors and a Revox 10 1/2 reel.

Charlie

Offline stellavox

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Re: Revox G36 High Speed
« Reply #7 on: April 14, 2009, 09:32:58 AM »
There is an ongoing discussion of a few folks reworking of the G36 here:

http://www.diyaudio.com/forums/showthread.php?s=53fdd62f52c17908da431a7850c29b21&threadid=127740

Offline ironbut

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Re: Revox G36 High Speed
« Reply #8 on: April 14, 2009, 10:07:06 AM »
Hi Charles, good to hear from you. Thanks for linking to that thread! That is really excellent stuff and anyone with an interest on getting past the basic adjustments on a machine would do well to read it. I know that George and a few others here have done restorations on their G36's and I'd appreciate any comments they might have on what is being discussed on the DIY Audio thread. I think this is exactly the type of info that Doc had in mind when first starting this forum.
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Offline fanriffic

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Re: Revox G36 High Speed
« Reply #9 on: June 07, 2009, 02:00:05 PM »
Hey Charlie,

I have a standard G36,but have been looking around for an original high-speed one for aaaages.

..How come this type of stuff never happens to me???...sounds like you stumbled upon a veritable Aladdins cave of RR's!!!

Anyway,if you ever fancy thinning YOUR collection down a little I'd be very happy to take the high-speed G36 off your hands for a good price.

All the best,

David