[Bhmartin]

Roger,

I am currently using your "upgrade" KT-88 tube set in my RM-200 and am running it at your recommended bias settings per the owner's manual.  I would like to try other tube types.  I have set of your RAM branded Siemens EL-34 tubes that I would particularly like to try.  The manual says that I can use these, but it does not recommend a bias setting for them.  I assume that the setting for these would be less than for the KT-88s?  What range would your recommend.  Also, I recall that the RM-9 series could use many types of octal tubes.  What types can I try in the RM-200 besides the KT-88, 6550, and EL-34??

Many thanks

[Roger A. Modjeski]

You can also try 6L6s and any of the KT-88/6550 family. You can use any output tube that has the same pin-out. The EL-34 and 6L6 have lower dissipation levels so you should run the bias at 30 mA (30 mV at the test point). I purposely made the bias pot have enough range to use all standard based octal tubes. Amplifier owners and some manufacturers think that the output transformer ratio has to be the one listed in the RCA tube manual. David Manley and I argued this point more than 20 years ago in the pages of Stereophile. The load impedance is more a factor of the power desired and the B+ voltage, not so much the tube.

Please let us know your findings.

[bdp24]

Roger, what is your current thinking on the use of the Tung Sol KT-120 tube in the RM200 amp?

[Roger A. Modjeski]

Roger, what is your current thinking on the use of the Tung Sol KT-120 tube in the RM200 amp?

There are several RM-200 owners using them and like them. They work just fine.

[bdp24]

Do they provide anything the other tube choices don't, increased linearity, output power, etc?

[Roger A. Modjeski]

Since you can run the idle current higher the distortion will be reduced, damping improved, power will be about the same. I am not one to comment on the different sound of this tube and that. I leave that up to the listener.