RM200II tube matching and biasing questions

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nnck

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RM200II tube matching and biasing questions
« on: 10 May 2014, 03:25 pm »
If anyone hasnt noticed by the numerous comments and questions I have made recently, I now own a Music Reference RM200II-T that I am extremely excited about. I've had it for a couple of weeks now with the original supplied Gold Lion Genelex KT88 tubes and have been thoroughly enjoying it with my Modwright LS100 pre and Deadalus DA1.1AP speakers.

First let me note, that I am completely new to tube amps, so forgive my newbie questions. I have a slightly nit-picky, hair-splitting question about the tube matching and biasing that I am hoping Roger or someone very knowledgeable can chime in on. I got hold of a multimeter to do some reading and very minor adjustments to the bias to match the 2 sides as well as possible according to the well-laid out instructions in the manual. Here's what I got for my readings (the average of the 2 sides it what we are trying to match on this amp):

Left side: V5=37.6; V3=43.7; avg=40.6
Right side: V4=40.7; V6=39.7; avg=40.2

Avgs are close enough is what I am thinking. But my questions are in regards to the 2 tubes on the left side: 37.6mV for one and 43.7mV for the other. The individual readings for the 2 tubes differ from one another by 6mV, whereas the right side tubes differ by only 1mV. Keep in mind that with this amp, when you adjust bias, you are adjusting both tubes on a side simultaneously (ie. you are not adjusting each tube individually). So if I make adjustments up or down on the left side, both tubes tend to move up or down by roughly the same amounts.

Since you dont adjust bias on each tube individually, my question is whether the 6mV difference with the left side tubes is acceptable, or if I should make any other changes or adjustments? Is it really just the average that matters, or should the 2 tubes on each side be close to one another as well?

It has been suggested to me that I take the 2 higher tubes from either side (so V3 and V4) and make a 'new' left side using those tubes and do the same with the 2 lower reading tubes from either side. This seems logical and may result in tubes that have readings that dont differ by as much as 6mV on the one side. But I hesitate to do this because I know that Roger is matching tubes for things other than simply the overall output. Plus I'm a newbie and dont want to make newbie mistakes. Would you feel free to swap the 4 KT88 output tubes around on this amp? Thoughts?

Ericus Rex

Re: RM200II tube matching and biasing questions
« Reply #1 on: 10 May 2014, 03:48 pm »
Have you read Roger's paper on power tube matching?

http://www.ramlabs-musicreference.com/virtues.html

nnck

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Re: RM200II tube matching and biasing questions
« Reply #2 on: 10 May 2014, 04:00 pm »
Have you read Roger's paper on power tube matching?

http://www.ramlabs-musicreference.com/virtues.html

Thank you, and yes I have. I won't pretend to understand the entire thing. I will have to re-read this again. But this is what I was refering to when I write 'I hesitate to do this because I know that Roger is matching tubes for things other than simply the overall output.'

pehare

Re: RM200II tube matching and biasing questions
« Reply #3 on: 10 May 2014, 06:40 pm »
I wouldn't go swapping around output tube positions, eg V3, V5, etc. until you get Roger's input.  The output tubes are numbered for a reason for their corresponding positions. 

I'm not a tube roller anymore - I've learned to leave well enough alone & my RM10 MKII has proven to be the most reliable & easiest on tubes I've ever owned.

Enjoy your new amp.

nnck

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  • Music Collector, Audiophile, in that order :)
Re: RM200II tube matching and biasing questions
« Reply #4 on: 10 May 2014, 07:17 pm »
I wouldn't go swapping around output tube positions, eg V3, V5, etc. until you get Roger's input.  The output tubes are numbered for a reason for their corresponding positions.

Exactly. And that's why I am hoping for Roger to chime in and address my question.

FWIW, it is actually the people at Roger's tube audio store- http://www.tubeaudiostore.com/ (the ones that supply replacement tubes for the Music Reference amps) that made the suggestion. So I guess I am being even more cautious by asking for Roger's opinion himself.