[rlw3]

removed half of the power tubes in order to reduce gain so that my sub could keep up volume-wise. noticed a "sweeter" sound as i lost a little control which i like as it was formerly a little too tight fisted in controling my ref 3a grand veenas. Roger what do i give up if amp is not pushed too hard? is there a difference in the interaction between the beefy transformers and the power tubes with only 4 rather than 8 of them?? i would guess maybe not as accurate this way but more of a classic tube sound which i like but that might change with time.

[GMuffley]

An impedance mismatch with the output transformer will result.  The plate to plate impedance of the output tubes will be twice as high and well outside the parameters of Roger's design.  You can partially compensate by using the 4 ohm tap for an 8 ohm speaker, buit it is far from optimum operating the amplifier in the manner you describe.

[rlw3]

thank you for your reply. does this imbalance stress the amp in a bad way? the only sound difference i am hearing is a little less volume and a more classic 80's tube sound. my feedback damping switch has a no feedback setting which i use for minimum constriction, if that matters. i like this change soundwise but was wondering if anyone else had lowered the power of their amps tried it for a while any came to any conclusions soundwise.i wonder if roger might be able to say whether liking this 4 tube situation was reasonable or if i was missing some serious sound chacteristics. thanks again

[GMuffley]

The B+ on the output tubes might increase a little (not much since I expect the regulation, meaning load to no-load variation, of the power transformer is pretty good) because their is less current flowing through the output tubes, but it won't hurt anything.

[K.C.]

Yikes, not a good idea. 

[rlw3]

KC your reply was reeeeeal useful. thanks for nothing.

[jon_010101]

He may be right though.  If you removed the pushing or pulling half of the push-pull circuit, rather than one push-pull pair, it could be especially detrimental.  Your OPT then has a huge current imbalance to deal with, which only increases distortion, dissipation, and the chances that it will become temporarily magnetized.  I don't know if Roger uses any clever bias circuits - I'm hoping it's just standard fixed bias - but if it were shared cathode bias (unlikely) or some type of servo it puts your tubes and/or output transformers in jeopardy. 

If the amp was designed for this - fixed bias, fairly high primary ratio, and appropriate feedback compensation - and you pulled the right pair of tubes - it could be fine - but only Roger can confirm.

[GMuffley]

Jon, 

You made a good point, which I failed to note.  If you pull the "wrong" pair of tubes you create a, for lack of a better term, a pseudo single-ended amp with a transformer designed for push pull operation.  There would be no current flowing through half of the output transformer's primary winding.  Another reason not to operate with 2 tubes per channel. 

Truthfully, rlw3, if you don't need the power and you would like to use less output tubes, consider the RM-10.  I have an RM-10, in addition to my Rm-9 MK II, and it is a real joy.

[rlw3]

actually i checked with roger who advised me to pull all of the front power tubes or all of the back tubes. i have a hunch i could not do this if Roger's designs were not so damn good. one time the bottom nub which gives the tube its orientation to the pins moved. the tube blew up noisily, i quickly turned the amp off and thought it would be expensive repair time when i realized what had happened. put in a spare el34 and i was up and running. my rm9 has been with me for over 21 years now. i run at the 4 ohm tap because my speakers are 5.5 ohm; should i run at 2 ohms? the sound is great as is, i don't want to wear the piece out in any way/ or have anything blow ( my main concern  ) thanks

[jon_010101]

actually i checked with roger who advised me to pull all of the front power tubes or all of the back tubes. i have a hunch i could not do this if Roger's designs were not so damn good. one time the bottom nub which gives the tube its orientation to the pins moved. the tube blew up noisily, i quickly turned the amp off and thought it would be expensive repair time when i realized what had happened. put in a spare el34 and i was up and running. my rm9 has been with me for over 21 years now. i run at the 4 ohm tap because my speakers are 5.5 ohm; should i run at 2 ohms? the sound is great as is, i don't want to wear the piece out in any way/ or have anything blow ( my main concern  ) thanks

If you pulled the correct pair, then you shouldn't worry.  If you want to try at 2 ohms, go for it, and pick whichever sounds better in this case.  I'm guessing either tap should provide a decent impedance match with a 5.5 ohm speaker, but one might be subjectively preferable. 

[GMuffley]

I agree with Jon.  It might work OK on the 2 ohm tap, but I really think you are giving up sound quality.

[Roger A. Modjeski]

rlw3 is correct. In any RM-9 (except the SE version where the pairs are connected differently) you can pull either the front row or back row of tubes with no problem. The currents will be balanced and the amp will run just fine. It is advisable to use the lower impedance taps as always whether all or half the tubes are used. It is always better to use the lowest tap that will provide ample volume without clipping.

If the bias is set for the full tube compliment I recommend it be left in that setting when half the tubes are out. You can re-adjust the bias to light the LED but that will reduce the tube life to the typical life of 1000-2000 hours rather than the extended life of 10,000 hours.