[sunnydaze]

Hi Roger and Music Reference afficianados......

Apolgies upfront if this has been covered already, I can't find it.

I own the above amp and wanted to know the correct tube placement when one halves the output tubes.  Is it the front pair, the rear, or a function of the power tubes used?

I am most interested in:

el-34
kt-66
kt-88
kt-120

Also, what is the bias procedure with half the tubes?  Just until barely lit, as in normal biasing?

How is the power output effected with half the tubes?

Thanks!
John

[Ericus Rex]

IIRC, you must use all the 9 pin miniatures.  You then use either the front row or back row of power tubes, doesn't matter which.  I think I remember the biasing procedure to be as follows; bias with all 8 power tubes as described in the manual.  Then remove either row.  The bias should remain good for the remaining 4.  However, I did this and found that the bias did not stay the same in  my amp.  The plates got red.  But my amp had been monkeyed with prior to my getting it.  I then used a guitar bias too from Amp-Head and my dmm to get the bias right.  But, according to Roger, the way I just described should work.  Keep an eye on those plates!  I'm pretty sure you can do this for all the tube types you list, I don't see why not.  It does roughly 1/2 the power.  I bet that amp will sound great with KT120s!  Keep us informed.

[sunnydaze]

Thanks!

I did try my KT-120's, put a pair on each side front position only.  Promptly blew a pair of fuses on one side, so I didn't get to listen.  Inserted my last two fuses, so I don't wanna risk it again.  Matter of fact, blown fuses are fairly common for me lately, regardless of power tubes used.  Anyone know what causes this?  Hopefully it's a sign of tired tubes, and not a needed amp repair.

Of possible interest to RM9 owners re: the fuses....

They are extremely difficult to find.  When I needed some 6 mos ago I scoured the internet and came up empty.  Had to buy them from RAM -- not cheap!  Yesterday I found an E-bay vendor.  You are forced to buy in bulk, but at 13 cents each (including shipping), who cares?       And at the rate they're blowing, I'll probably need them!   

http://www.ebay.com/itm/120909765447?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

[Chazro]

"They are extremely difficult to find"

?????  I've owned my RM9 for close to 10 yrs and have always found fuses easily and on the cheap from my local hardware store.  In Georgia AND Florida!  Unless yr one of those gents who buy 'audiophile' fuses I'm not understanding the difficulty.     

[sunnydaze]

Depends on which RM9 you have.  The factory made mk2 (not mk1 upgraded to 2)  takes 8 fuses on the top plate, one for each power tube.  The correct value is 160 mA, slo-blo, ceramic high break point.  IIRC, other versions of the amp use different  (maybe more common?) fuses.

Not available at all my local shops.  Google above fuse and try to find a vendor.   Good luck.

PS:  Sal at RAM labs told me 200 mA is also fine, so I bought 'em.

PPS:    and no,  I'm not a gent, and we surely don't need no steeeeeeeekin' audio-phool fuses!         

[rbwalt]

roger likes the ceramic fuses better because when the blow, no glass particles. my SE takes the same fuses.

[rbwalt]

also try Wickmann fuses.

[Roger A. Modjeski]

http://www.ebay.com/itm/120909765447?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

These are not necessarily high breaking fuses unless they say so. We charge $3 each or $20 for 10 which is pretty cheap I think. I have done a lot of research on fuses and my comments on premium fuses are found here. I buy only Littlefuse and would not recommend anything else.

If an amp is blowing fuses often it is likely an output tube. If you have unmatched tubes one can be hogging most of the current and will glow red and fail. I recently tested 200 pcs of KT120 and they have a very large range from top to bottom. So large that one can be almost off while the other it taking all the current. This is not good for reliability and distortion will be rather high and bass muddy. Tubes need to match within 10% so that the output transformer will not saturate. That is the way of all push-pull amps.

One can put KT-120s (preferably from RAM TUBES) in the front or back row of an RM-9 MK I or II and adjust the bias for a very dim LED or a bit below and have no trouble. The power output is about 60+ watts.

[sunnydaze]

Thanks for chiming in Roger.

My bad then.   They are ceramic, and I always thought ceramic was same as high breaking.       

Can all other output tubes be placed the same as KT-120's?  That is,  a pair in either front or back row, with bias set to dim glow.

[hoppy08520]

John, it looks like Roger and others answered, but I just wanted to let you know that there's an older thread that talks about KT-120s in the RM-9 that might be of interest to you: KT120 tubes for RM 200 mkII?. If you're a RM-9 owner, it's easy to overlook this thread because the subject line is about the RM-200, but the thread discusses the RM-9 as well.

[sunnydaze]

Thanks for the link!

Roger addressed KT-120's specifically, but I also asked about halving with other output tubes.  See my original post.  Unless I'm blind, I don't see an answer to that.

[Ericus Rex]

I tried it with KT88s.  I don't see why it wouldn't work the same with any power tube.

[Roger A. Modjeski]

Thanks for chiming in Roger.

My bad then.   They are ceramic, and I always thought ceramic was same as high breaking.       

Can all other output tubes be placed the same as KT-120's?  That is,  a pair in either front or back row, with bias set to dim glow.

Ceramic fuses are often high breaking but a particular example that is not is TUNING FUSES where they are ceramic but not sand filled. They are actually the lowest breaking (worst) ceramics ever made. The idiots put the silver wire in a Teflon tube which then contains the plasma of the vaporizing element. In a proper high breaking fuse the sand prevents the plasma from forming. I find it amusing that the people who make the most expensive fuses know so little about fuses.

Anyone tuning in late to my thoughts on fuses can check the original post which I have made sticky.

For the 2 tube/channel configuration any tube with a dissipation rating of 35 watts or more is suitable. That eliminates the EL-34 which is 25 watts. The LED comes on at 120 mA which makes the total dissipation 55 watts per channel for a MK-1 and 62 watts for  MK-II as it has a bit higher B+. Divide that number by the number of tubes you are using (either 2 or 4). The RM-9 gets unusually long (10,000 hour) tube life because with 4 EL-34 they are run at about 14 watts each.

For comparison purposes many makers such as Rogue, ARC, CJ etc. run their tubes close to max dissipation which I think is a foolish thing. The owners of these amplifiers seem to be OK with the factory recommendation of replacing output tubes every 1500 to 2000 hours. I got it from the plant manager at Sylvania in Altoona, PA that the maximum dissipation of a tube is the maximum. We don't run our tires at their maximum inflation pressure and neither should we run our tubes at their maximum pressure. He told me that used properly 10,000 hours was their goal and we who own Music Reference amplifiers have found this to be so. This is achieved by running tubes at about half of their maximum dissipation.

It is also interesting to note that in the mid 20th century the rating of tubes changed from "design maximum" to "design center" and many of the numbers (dissipation, max voltage, max current) went down. Some tubes are still rated "design maximum" and some "design center". This new rating system came in because engineers were using the maximum numbers without consideration of high line voltage or higher than normal bias currents in circuits that were usually not adjustable by the user. The earlier "smart" engineers would account for high line voltage etc and put this margin into their design. The not so smart ones pretended like the line voltage is always the same everywhere and all tubes biased up the same. So the tube makers did the "de-rating" for them.

The danger of running tubes to hot can be seen in the Dynaco ST-70 and some early CJ amps where as the line voltage rises 5% the output tube current also rises at about the same or higher rate. However coupled with the fact that the B+ is also rising  5% now the dissipation has risen 10%. If its already at the limit this is enough to push things over the edge. In my amplifiers the tube current actually goes down with rising B+ keeping the dissipation constant.

Sorry for the long answer, I hope you find this interesting.