Changes to SR-45 for 5k OPTs?

[Jim R.]

I'm planning on installing some 5k opts (EXO-145 nickel) in my SR-45s at some point and wonder what changes have to be made.  I'm guessing nothing more than the cathode resistor, and parafeed cap value, but beyond that I don't know, just want to make sure I'm not missing anything.

And yes, I know output power will be a bit lower, but that's ok and expected.

[2wo]

Hi Jim, I put a set of nickel EX0-45 in mine with no other changes at all. Sounds great...John

[Grainger49]

John,

Are you running 45s as the output tubes? 

[2wo]

It wouldn't be the SR-45 without them  ...John

[Jim R.]

John,

Well, that makes it pretty easy then :-).

Today or tomorrow I should be able to finally fire them up for the first time.

[Grainger49]

IIRC, the Paramour II came with some instructions for substituting 45 output tubes.  I thought ... well, I'll check the manual.  I wasn't going to drop 45s into my Paramour (ones) without a cathode resistor change.

John, are your Paramours original or IIs?

[Paul Joppa]

It will work with no other change but the transformer. You'll get less power, at lower distortion. It's possible you might even hear the difference if you listen carefully (!)  :^)

My original notes show an optimal 28mA current with a 5K load; that's an 1800 ohm cathode resistor. (Stock was 36mA,  1300 ohm cathode resistor.) This would however increase the shunt regulator dissipation to 7 watts, and the 6CM7 is rated 6 watts maximum. Ideally I would re-design around the 6DN7, but in practice just reduce the C4S current  to 51mA (R1 16.5 ohms) to get the dissipation right at 6 watts.

The reduced plate current from this change will improve the deep-bass regulator current margin; however keeping a higher current will reduce the distortion. Both are very small effects, and it's not obvious which is more important - probably depends on your personal preferences and music. So in the end I'd suggest you split the difference and use a 1500 ohm cathode resistor, which conveniently requires no other changes.

[Jim R.]

Paul,

Thanks for the notes.  I'm actually not doing this for any major sonic reason, just that I want to use the nickel tfa-2004 jrs in my 2a3 paramounts.


I was under the impression that I could just sub a 6dn7 except for the socket and socket wiring.  If there's more to it than that, what else would I need to do?  My plan all along was to use 6dn7s in the rebuilt amp.

[Paul Joppa]

Yes, the 6CM7 is basically a 6DN7 in a smaller envelope, with a lower dissipation rating. Notice the sequential letters in the type designation ... presumably not an accident!

OK, so the 6EM7 has similarly adjacent letters, but not sequential ...

[Jim R.]

Ok, but following the sequential logic for a moment, wI would have guessed the octals to come before the novals, no?  After all, there were big TVs before there were small ones.  Unless it had something to do with needing the higher dissapation for use in color TVs where all the voltages were generally higher.

Is there actually a rhyme of reson for the tube letter designations?  I always thought they were only assigned to a basic category and then more or less sequential, or maybe each designer/manufacturer was assigned a range to use?  I get the numbers, just not the letters.  Sorry for the O/T, but tube lore fascinates me.

-- Jim

[Paul Joppa]

If you ever find out, I want to know too! I figure the odds were 1 in 338, so maybe not completely accidental - but it's certainly not a 4-sigma outlier.

BTW, I love the Pirsig quote.

[Jim R.]

I'll see what I can come up with regarding the tube numbering.  Need something to do today anyway -- knees and hands out of commision today due to a gout attack, so resting reading is about all I can manage.  Which BTW, is killing me as I am just dying to get that preamp hooked up!

Anyway, yes, ZAMM is a very worthwhile read for anybody interested in a pursuit such as DIY audio, at least if you like to take a philosophical interest in why we do and enjoy things like this very specialized hobby.  I dug up the quote when I downloaded iBooks on my iPad a couple of weeks ago and discovered how many books I now have access to and can read without having to resort to digital audio formats and the like, and which aside from classics that are available myriad places in text form, the iBooks library stays pretty current with bestsellers, magazines, some professional journals, catalogs, and even textbooks.  All that in one place, accessible except for graphics (still a huge part of technical reading) and with no additional add-ons or specialized, often not very robust accessibility interfaces, is just huge for me.

[2wo]

I got around to measuring my SR-45 yesterday and with the EXO-45, I am running about 35ma at 300V on the plate. This is about the upper limit for the 45 and is higher then I want. So I plan to raise the value of the cathode resistor.

Also I was looking at the reg circuit, if I am looking at it right. The 6CM7 drops 50 volts at about 13ma. for 6.5 watts?   

[Natural Sound]

Hi Jim, I put a set of nickel EX0-45 in mine with no other changes at all. Sounds great...John

Which plate chokes are you using? Did you increase the value of the parafeed cap?