The best reel to reel Revox transport

[microstrip]

Being located in Europe I must source my machine in the Revox "Prosumer" machines, the possible candidates are either a PR99mk3 or an A700. The A700 is older, but looking at service manuals seems a better machine, with electronic tape tension sensors arms, although the PR99 seems more robust. Having spent a few nights reading about scrape flutter, I understand that friction is an enemy in the tape path, and should be reduced as much as possible. I have access to an excellent technician who can fit bearings or quartz/pyrex sleeves if needed.
I do not consider recording my tapes -  I just want a good machine to play the Tape Project tapes and will use external electronics, such as the Seduction or the Tube Repro to amplify the head signals.
Having your opinions on these two machines will be of great help for me.
Francisco

[ironbut]

The A700 was built more as a consumer machine compared to the Pr99. Perhaps when they were new, the 700 might have been a marginally better sounding transport. But when you're talking about machines that are 20+ years old, I'd go for robust every time. Things like reducing scrape flutter can be done after you get the machine working perfectly and reliably. This is 90% of the battle (especially if you're planning to use an outboard repro amp).

Another practical reason for my leaning toward the Pr99 is availability. The 700's compared to the 99 come up for sale about one for every ten PR99 so finding a minty 99 is far more likely than finding a equally good 700 (although the 700's were probably seldom seen in a professional studio/radio setting).

I can remember seeing new A700's and dreaming of owning one someday but knowing what I do now about the differences, I'd have been better off with a PR99.
If you haven't read Arian Jenson's pdf regarding his modifications of the B77/PR99 here's the thread;
http://www.tapeproject.com/smf/index.php/topic,411.0.html

[Tubes n tapes]

The PR99 would normally be the preferred way to go, especially if you carry out the few simple mods that I describe in my article.

That said, if your plan is to use the machine for playback only through external electronics, the story becomes a bit different. The audio electronics on the A700 are really bad. The stock A700 sounds about as bad as a stock RS1500. However from a mechanical point of view the A700 is really good. It shares its transport with the Studer B67. That transport was developed in the early 1970's but is remarkably modern in performance. The tape handling of the A700 is about the best I've seen except for the large studio machines A80 and A820. The A700 has tape tension control under all circumstances including during braking. You can, without exaggeration, put a plastic 3" reel on one side and a 10.5" metal reel on the other side and maneuver the tape perfectly without ever creating tape loops or over stress on the tape. This functionality all comes from an ASIC that Studer had developed especially for this transport. Must have been an enormous investment in those days.
If that ASIC breaks, you're dead in the water, but I have never seen that problem or heard about it, so it is probably a relatively low risk.

Furthermore, the A700 does have a scrape roller, but if you use 468 tape there is almost no scrape flutter in the first place.

So, if you are planning to bypass all the audio electronics, the A700 or even better the Studer B67 is a very good choice. They normally will be at least 10 years older than most of the PR99s, though.

[ironbut]

I bow to Arian's superior knowledge of all things ReVox!

I had the opportunity to listen and operate one of his PR99 conversions at CanJam2009 in LA last weekend and it was very nice. Todd the Vinyl Junkie had it demoing the 2nd Tape Project demo tape and for solid state electronics, it sounded very good (of course I'm a major tube guy so you can take that bias into account).

I hope that Arian and Phillip had an interesting time at our little gathering. It was a perfect example of an audio niche going completely out of hand.

[microstrip]

Thanks for your opinions!
I have now bought a PR99 mk3 - it needs some service, capstan shaft and heads look in excellent condition, but could not resist and also ordered an A700 from Germany. I have looked through the service manuals and the A700 tape control seems easier to service - there is even a full table with the states of the IC that controls the tape transport.
Today I decided to run some tests on the PR99, using a spectrum analyzer with .73 Hz resolution to record and analyze a 10 kHz sinusoidal signal. To my surprise I found some sidebands at 10009.6Hz and 10100 Hz - mains in Europe is 50Hz so this last one is surely manins related. To clarify this aspect I connected the PR99 to a PSAudio P300 power regenerator that can supply 60 HZ, and as expected the sideband moved to 10120Hz, although of higher amplitude. However, when I switched the the P300 to 55 Hz the side band completely disappeared and background lowered by 2dB, suggesting the capstan motor is smoother at 55Hz.
The only reason I can find for the 10009.6 Hz is the pincher roll - it runs at about 4.8 rotations per second, although the rubber looks perfect. Can you help me with any other explanation for this contribution?
 

[ironbut]

Hi Francisco,

It may be the compliance of the rubber on the roller. The rubber part is real thin (compared to some other machine) so a harder spot could be causing your problem. Try using a rubber treatment like MG Chemicals "Rubber Renew". It helps to make old rubber parts soft again. The machine could have had it's roller cleaned with something like head cleaner or alcohol and that dries it out and makes it harder (it can get tiny cracks in it too so if you examine it closely you can see them). Sometimes this won't help but there's a few places you can get replacement rollers (eBay).

It could also be the bearing. If it's badly worn it will cog with each revolution.

[Tubes n tapes]

I agree with Steve. The pinch roller may look good, but if it has been treated with alcohol in the past it may have dried out unevenly. The sidebands you see are indeed a bit strange. You would normally see 4.8Hz sidebands of the pinch roller at about -30dBc and 13.9Hz sidebands of the capstan itself at about -50dBc. The second harmonic of the pinch roller is normally just visible at around -50dBc I do my measurements normally at 1kHz with a RBW of 1Hz. The mains frequency shouldn't really be visible as a mechanical effect. It may be caused by some intermodulation elsewhere in the machine, especially since you seem to see the second harmonic of the mains frequency.

[traf]

I would also recommend a new pinch roller - easily found on ebay.
PR99 is a great machine, especially after some mods. To the best of my knowledge I made one of (if not the) most extensive mods on a PR99MK3. If you are ineterested, l can give details, the results are day and night difference, both in listening terms and measurements, although evenn the stock PR99 substantially outperforms the stock RS1500. And the PR99 is really mod-friendly.

I am interested in what you mentioned about quartz/pyrex sleeves, could you provide a contact (I am also in the EU)? I am still contemplating a tape path improvement on the PR99.

Best regards,
Todor

[microstrip]

Thanks for the help of the forum members!
I have now replaced the pinch roller and the spectrum looks nicer - the 9.6 Hz is now much lower and was surely the second harmonic of the pinch roller.
I also disassembled the bearing of the aluminum roller of the counter and it seems to need some oil after cleaning, as it is a bit loose. What kind of oil do you use for these bearings?  Although they seem standard sintered bearings (2mm inner /4 mm outer) I hope they do not need replacing.

[High and Outside]

To the best of my knowledge I made one of (if not the) most extensive mods on a PR99MK3. If you are interested, l can give details, the results are day and night difference,

Todor,

We would all be interested in the mods you did. Please fill us in.

[microstrip]

Are there any disadvantages in replacing the sintered bearings of rollers by ball bearings in the tape path of of reel to reel machines?
I have easy access to  ABEC5 NMB miniature bearings.

[ironbut]

I don't think that there should be any reason why balled bearings shouldn't work per-se. I do think that if original Revox/Studer bearing are available that they would be the first choice. While ceramic bearings for example, spec out very well, they seem to have a higher rolling resistance at the slow speeds that tape moves. I've tried some and they seem to dampen the high frequencies a little. Maybe the ceramic bearings would eventually wear in but I took them off of my Technics and went with regular steel bearings which don't dampen the sound. The difference was immediately apparent.
I'm sure that a lot of time was spent on optimizing the bearings that were used on your machine. You know how proud those Northern Europeans are of their machining and metallurgy so I'd give them the benefit of the doubt. Of course there may have been advancements since your machine was made but unless someone like Arian has done the leg work already, it would be a trial and error experiment on your part.

[microstrip]

Although during playback the bearings of the PR99 right roller work at low RPM (~382 ) , during fast rewind they operate at much higher RPM and they wear relatively fast. Looking for information on Revox bearings I confirmed that sintered bearings do not need lubrication but Studer states in Technical Protocol no 174 that although expected lifetime of capstan bushings is 4000 hours, it can be extended up to 10000 hours by replenishing the lubricant. I have now located a seller of Isoflex PDP 65 in Germany, I will try it on my PR99 bearings.

[microstrip]

Thanks Steve and Arian - your were absolutely right! The A700 is really a better transport and the measurements show it, although the electronics of the PR99 sounds better. But as I will use Bottlehead  electronics it does not matter.
I have received a new pinch roller from one big Ebay supplier - it looks perfect, smoother and better finished but unhappily the performance is much worst than my old ones - the 4.7 Hz sideband is almost 10 db higher with the nice looking new pinch roller.
 

[ironbut]

One thing you should always keep in mind when introducing new parts to the transport is that even though the part is new, everything really needs to be adjusted because of it. Not just the pinch roller pressure/height, but the tape tension and tape path needs to be checked.
 
Rubber parts almost take on a life of their own after decades of use. The rubber becomes less compliant, little dents form where the tape usually is, and micro cracks form as more and more pressure is needed to keep the tape from slipping under it. If you change your roller and don't adjust the pressure, it's likely that it's exerting too much pressure on the tape/capstan.

I've mentioned it before but so many adjustments and problems with tape results in high frequency losses. If you get those right, that's 90% of the battle.
The same is true for the pinch roller/tape/capstan interface. Too little pressure and the tape slips and you loose highs. Too much pressure and the tape actually stretches as the roller deforms itself against the capstan (the rubber stretches from the center  contact point out and stretches the tape right along with it) and this also results in high frequency loses. Both of these problems result in micro speed and tension variations which prevent the tape from moving across the heads in a nice smooth, even manner. 

So, don't just figure that the pinch rollers only contribution is to keep the tape moving. Everything that touches the tape effects the final sound.