AU-70 tube recommendations

Discussion in 'Exclusively Sansui' started by Sansuiman, Mar 13, 2019.

  1. Sansuiman

    Sansuiman Well-Known Member

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    561
    Location:
    St. Louis, Missouri
    Now that I finally have a TU-70, I'm hoping to get my AU-70 into restoration sometime in the next few months, and was wondering what those here who have done one recently are using for output tubes. I'm talking current production, not NOS from 1965 that cost obscene money.

    Preferably something that can be readily procured as a matched quad. I've read mixed bag on the Russian tubes that are supposed to stand up to the high plate voltage on this amp, so would appreciate any first hand experience being related here on both sound quality and reliability/life span under the operating parameters of the AU-70.

    For that matter, if anyone has specific suggestions for the other tubes utilized, that would be appreciated. I went through some trial and error finding the "right stuff" with my AU-111 a while back, and hoping to avoid that this time to keep the cost from getting out of control.

    One final question, for replacing the multi-sectional cans, is there a consensus on getting repro's of these, or better to get high quality individual caps and mount them under the chassis? While I'm not that concerned about original appearance, I can leave the existing cap mounted (but gutted) to keep the stock look if the repro multi-sectionals are deemed inferior quality.
     

     

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  2. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    I don't have sufficient recent experience of valves ('tubes' to you :)) for the AU-70 to help you with that - there is at least one person who can, I'm sure he'll be along soon. ;)

    However, regarding the multi-sectional cans - I do see them being available from some sellers, 'new' I think, not NOS. But whether they have exactly the right voltage and number of sections required, with the correct diameter and height, that's another question. I especially agree with the comments for replacements, particularly the 'gutting' of the old ones, but leaving them in place, you could probably use the unused lugs for some/all of the replacement capacitors.

    This is what I was thinking of:-
    https://www.amplifiedparts.com/products/capacitor-ce-mfg-475v-20202020-f-electrolytic
     
    Last edited: Mar 14, 2019
  3. maxhifi

    maxhifi AK Subscriber Subscriber

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    Basically you have two options, if you want it to be reliable. Option 1 is the Russian tubes which everyone suggests. Option two, is NOS 7189 tubes.

    Although the more common EL84 is pin compatible, you can't expect it to be reliable, because it's a lower rated version of the 7189 - or rather, the 7189 is a super EL84.

    If this were my amp, I would seek out NOS American made 7189 tubes. I know, not what you wanted to hear, but you'll really only need to do it once - they'll last for years and years. Putting in regular EL84s, especially newly made ones, is in my opinion asking for trouble.
     
  4. Sansuiman

    Sansuiman Well-Known Member

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    Yeah, I kind of figured that. The Russian EL84 seem to get talked up as fairly solid on applications calling for 7189's, which is why in wondered if they measured up to the hype. Guess I'll start selling vital organs to fund a NOS set of 7189. I've not really gone searching yet, how hard is it to find an quad of NOS 7189's? I can't imagine there are many around for sale.
     
  5. maxhifi

    maxhifi AK Subscriber Subscriber

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    It's not all the Russian tubes which stand up, there's actually a Russian 7189 equivalent, which has higher ratings than their usual tube. Search the forums to find out what it is, it's got a pretty solid rep but I personally haven't use it. NOS tubes are surprisingly easy to find, but you need to look around for a deal. One thing I would caution you to do is only buy from a reputable dealer.

    Check it out, antique electronic supply has NOS tubes on sale right now... gotta get while the getting is good :)
    https://www.tubesandmore.com/search/node/7189
     
  6. smurfer77

    smurfer77 AK Subscriber Subscriber

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    You do not need matched quads. If you have the 2-pot version you just need matched pairs. If you have the 4-pot version you don't need any matched tubes. The 2-pot version can be easily upgraded to 4-pot as I demonstrated here

    I am still running the original tubes in my AU-70 which I rebuild back when I was a bit more novice at all of this. They seem very reliable, so if they died I might in fact seek out a NOS set of 7189A with hopes they would be equally reliable. As pointed out by maxhifi, it's either that, or the russian tubes (6p14p-ev I think), which personally I have no problem with the idea of trying but I just can't vouch for the russian tubes in this particular case I haven't tried them. The plate voltage is far to high to expect standard EL84 to be reliable.

    Here you can see that I restuffed my AU-70 caps. It's a lot of work and if doing it again I would probably be tempted to wire them under the chassis instead of restuffing the cans as the AU-70 is not so rare and sought after that you would really be doing any collectible 'damage'. Personally I trust hand picked power supply caps as they are likely better than what goes into multisection cap cans. I have bought cap cans before though, and had not problems so far but some people complain about the CE caps (I used in a ST-70 and AU-111, no problems so far).

    Here are the specs on the cans in case it helps:

    4x 20 uF 300 V RADIAL (C66-69) 35mm diam, 49mm height - RIGHT CAN
    2x 40 uF 500 V RADIAL (C70,71) 35mm diam, 50mm height - LEFT CAN

    Note also that the head-amp transistor (TR1-TR4 I think from memory) is 2SB381 Germanium. If you don't know how to modify the circuit for a modern Si transistor leave this alone; it's not that noisy in this particular case.

    There is a Ge diode too (OA-81, 90V, 25mA) but in this particular application (half wave rectifier if I recall) the 0.3V vs 0.7 V forward switch-on voltage is acceptable so you can change to Si if you want.

    My main question is, do you need new output tubes? If not, just start with the coupling caps and go from there.
     
    Last edited: Mar 14, 2019
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  7. Sansuiman

    Sansuiman Well-Known Member

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    Thanks for the feedback and suggestions thus far, gentlemen.

    The link to tubes and more shows NOS 7189 on hand, but no 7189A in stock. My understanding has been that the AU-70 specs 7189A, which has a higher plate voltage than 7189 (non-A), which in turn is higher than the EL84. So, I guess I need to keep digging around to find 7189A specifically.

    I think I read somewhere a while back now that the AU-70 ran the plate voltage on the hot side even for the 7189A (near design limits) so I figure standard 7189's will get smoked pretty quickly. There is also a matter of internal connection differences to pins 1 and 6 on the A variant, I believe.

    FWIW, the current tubes are shot, one has the getter visibly chalky, and another looks highly suspect. I'll not power this amp up with these tubes for fear of doing OPT damage, even after I do the re-capping. So, new tubes it is, regardless. Maybe the Russian stand in version will get a shot at the job.

    As far as the multi-sectional caps, I'm not uncomfortable with putting individual high quality parts under the deck, and leaving the old multi-sectional in place - but gutted - so it can never leak out and do damage. I'm not sure I trust the current production mukti-sectionals, they strike me a an inexpensively made compomise. It's a nice sentiment, but I don't see much point in restuffing the one in there, as pointed out already the AU-70 isn't into super-collectibility status and I'm not that worried about keeping it looking bone stock. If I ever do sell it, if someone wants to ding me on valuation for that approach, well, I'm not going to loose sleep over it. So, the search for tubes gets underway.
     
  8. maxhifi

    maxhifi AK Subscriber Subscriber

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    The manual says 7189, not 7189A, and the amp doesn't use the higher screen rating of the 7189A. It may be the conservative choice to use 7189A, but I don't see a reason why you cannot use a normal 7189. Yes the voltages are on the high side, but that's common across all vintage tube amps for ratings to be pushed - usually nos tubes took it no problem.

    To preserve tube life I would simply bias it a tad on the cold side.
     
  9. Sansuiman

    Sansuiman Well-Known Member

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    Maxhifi, thanks. Guess I had some bogus information regarding the correct tube type. Will give the 7189 a go.
     
  10. maxhifi

    maxhifi AK Subscriber Subscriber

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    The Russian tubes will no doubt work just fine too, I'm just a believer in NOS being the best. American tubes will be the easiest to locate, and will last ages if you bias them properly, and sound great. What I would do if that was my amp though, is blow way too much money and time seeking out these - original matched pairs of Toshiba "for hi-fi" 7189A tubes. You do see them come up from time to time on Japanese auction sites. I would do this simply because they have a cool logo and to have Japanese tubes in a Japanese amp though, there's no performance related reason to seek these out.

    a.jpg
     
  11. Sansuiman

    Sansuiman Well-Known Member

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    I do like that old Toshiba logo, very cool. Actually have a line on some NOS of that exact type but now I'm wondering, can I sub 7189A in the AU-70 directly without any wiring changes to the sockets? There is a difference in the pin 1 and 6 internal connections for 7189 versus 7189A, and I don't want to screw anything up, nor do I really want to change the wiring if that would be needed. Any feedback on that would be appreciated before I committed money to buying these which it appears I can get for slightly less than NOS US made 7189's (not that I'm pinching the pennies that tightly).
     

     

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  12. maxhifi

    maxhifi AK Subscriber Subscriber

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    Take some photos of the underside of your chassis, showing how the wires connect to the output tube sockets.
     
  13. Sansuiman

    Sansuiman Well-Known Member

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    561
    Location:
    St. Louis, Missouri
    Ok, will get it dug out of storage here ASAP and update with pics.
     
  14. maxhifi

    maxhifi AK Subscriber Subscriber

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    Cool, take some really clear photos of all four output sockets, and we can check if there's any issues. My guess is it'll be fine, but it's worth it to look.

    Where'd you find the toshiba tubes? (maybe pm that lol)
     
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  15. Sansuiman

    Sansuiman Well-Known Member

    Messages:
    561
    Location:
    St. Louis, Missouri
    Source of the Toshiba tubes is a rather eccentric gentleman here quasi-locally I've dealt with a few times before for various odds and ends. He apparently had a habit for some time of buying up entire lots of assorted tubes (cheaply, I surmise) from liquidations and such from repair shops that closed up. Seems like more receiving tubes for old amateur radio gear and TV's than audio specific types, but I did once get some nice 2A3 from him for an old Hammond organ amplifier, and some NOS 6L6GC a while back as well, that went in my AU-111.

    He is sitting on a lot of strange stuff, much of it is probably junk, but some diamonds in the rough for sure. For example, he has 5 or 6 shelves of old phones. I restored a pink rotary dial Bell System 500 desk set he had for someone I know who wanted one badly. Also many other oddities and miscellanea that struck his fancy for whatever reason, like what remains of a long decomissioned Digital PDP-11/20 with the pink and purple toggles on the front panel.

    I like hanging out there sometimes to talk shop (he's a very knowledgeable sort, and seems to enjoy imparting his vast expertise and many off-color humorous stories on those few interested) but try not to impose too much on him. He is kind of reclusive, but has a small fan club of electronics gurus and other nerdy sorts - of which I suppose I'm a member - that he tolerates and will sell things to periodically. Otherwise, he is entirely off the radar near as I can tell. I figure you probably get the picture from all that.

    Anyway, I'd been talking to him Saturday morning about something else entirely and asked if he might have any 7189's. He looked, and voila, 2 NOS matched pairs of Toshiba 7189A. If they'll work, I'm going to pick them up. Pictures to come probably Sunday evening, the amp is on a shelf in a rather cramped corner of my basement presently. Not looking forward to digging it out, but guess it's as good a time as any.
     

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