845 tube question

[franksmith]

I need to find a pair of backup 845 for my diy amplifier.

I'm running them close to 92 watts on the plate.

My old valve art tube can handle this dissipation and did so for years but I've read that the modern 845 are often limited to 60-80 watts.

I don't want to buy expensive vintage tubes so I'm looking to find a modern 845 able to take that 92watts plate dissipation.

Any recommendation ?

[danFrank]

The way I see it, you have 2 options...
1) Spend the mony and get NOS American (RCA, NU)

or

2) Change the socket and filament transformer and use the GM70 which will easily take 92 watt dissapation.

Current 845s will not do it as they are rated for 75 watts dis.

[franksmith]

Even the shuguang 845b ?

There is a pair for sale on ebay and it state this :

The 845B is a recent development by Shuguang, and structurally it is a very close copy of the 1940's vintage Amperex brass base 845. It has a graphite plate (like all 845s since the 1930s), and a beautiful copper base. Pins appear to be nickel-plated brass. Like the Amperex, it has no top mica connecting the plate structure to the glass envelope. (current production 845s from China, Kron and Cetron all have top micas which couple the plate structure to the glass). These 845B have full 845 dissipation ratings and 1250v plate rating and as far as I can tell are compatible with any 845 amplifier application.

The "as far as I can tell" doesn't help me much.

I've tried at 70w plate and didn't like the sound as much as my current operating point.

The conversion to gm70 could be a solution but I really like the amps sound right now.

[Rob11966]

Hi Franksmith,

I have played around with the Shuguang 845b and 845c tubes and there was a discussion concerning these a little while back.

Each run or batch of these tubes have different specifications to the point that it is probably better to think of each production run as different tubes!

Each pair of Shuguang tubes come with a data sheet and whilst this sheet does not quote the dissipation rating it is possible to work it out from the voltage and current figures. The earlier 845b's had a dissipation rating of around 75W, However the later ones are significantly higher than this. For example a pair of 845b's that I bought earlier this year had a current rating of 112mA @ 1250v. This equals a power dissipation of 140W. I am unclear as to whether this is the operating point or maximum but even if it is the maximum rating and you ran the tubes at 75% of this, the tube should handle over 100W. My 845c's purchased a couple of months ago are better with a rating or 120mA @ 1250v.

Shuguang 845 tubes are (relatively) cheap. Why not target a pair on eBay, email the seller and ask him what the specs are. If they are not up to scratch then ask if he/she has a better pair. If not, go to another seller until you find a pair with ratings that you are happy with. I believe that you should be OK to run with a 90+ watts with a decent pair, but if not you have only smoked a hundred dollars or so.

If you want to be more certain than this, you can still get Cetron tubes from Richardson Electronics. I believe that they are made in China (not by Shuguang) but have a reputation for high quality. I got a quote from them a while back and they were close to AUD 600/tube. I couldn't really justify the cost.

As an aside, the Shuguang tubes are really well made. One arrived broken (845c) a few months ago (smashed in transit) which gave me a chance to have a look at the plate structure up close. The plates in this particular tube are made by Toshiba (bottled by Shuguang) and are precise and solid looking units.

Danfrank, I have seen the old data sheets for the Shuguang tubes listed at 75W but I have seen nothing recent to confirm that this is still the case and the data sheets that come with the newer tubes suggest otherwise (as above).

Hope this helps.

Rob

[franksmith]

Thanks for your answer Rob,

Can the rating given with the tubes be interpreted as a maximum plate dissipation under normal use or more am short time absolute maximum ?

If I compare with my tj mesh plate 300b, those rating seem to be rater optimistic if I compare with real life span of the tubes under 75-80% of these ratings. I have two tubes that failed under 1000 hours of use and the replacement took months (more than 6 months and many emails actually) to get here.

Still your suggestion is probably my best bet. My valve art did last long and still go strong and I would not be surprised that the shuguang are of similar construction if not better.

[Rob11966]

Originally Posted by franksmith

Thanks for your answer Rob,

Can the rating given with the tubes be interpreted as a maximum plate dissipation under normal use or more am short time absolute maximum ?

.......

Still your suggestion is probably my best bet. My valve art did last long and still go strong and I would not be surprised that the shuguang are of similar construction if not better.

Well, I am not sure whether this is normal use or maximum tolerances. No one seems to be able to tell me including the supplier. I emailed Shuguang some time ago but no response. Personally, I suspect it is maximum and that's why I suggested it may be advisable to run the tubes at 75% of this rating.

As for your valve art tubes, I believe that they are re-labeled Shuguang 845's (to the best of my knowledge, valve art has never manufactured 845's) so the new Shuguang 845b's should be just as good and probably better.

Cheers,

Rob

[franksmith]

I've found informations about a 845w premium which would take 90watts but it seem to be out of stock and not produce anymore. I would stress them at 92-96 watts anyway.

I've found only information about the 845b to be 75 max, nothing about the new production and the it's quite risky to get old production when buying these tubes.

Anyone tried the kr audio 845 which are rated to 100watts ?

1400-1600$ for a pair of nos rca tubes ... I haven't had good experience with nos tube so far and it's a crazy amount of money to gamble.

I usually switch between my drd 300b amps and the DC 845 during winter. The 845 can easily heat the living room considering they are ON all day long but it may be an expensive heater this year ...

[franksmith]

My amplifier use a el34 triode configured in directly coupled cathode follower for the 845.

I've replace the almost dead ge el34 with a pair of new EH tubes just to confirm they where the problem. The old valve art 845 seem to still go strong after hours and hours of abuse still I need replacement tubes in case something happen.

The EH seem to have a slightly dark sound which I would prefer a little bit more vivid.

Anyway, similar question for choosing a pair of el34.

Thought of a pair of winged C el34 or maybe tube amp doctor (had mixed experience with two pair of tad el84-str, one pair if very good). Is there very good el34 still produced ?