Anyone use 46 or 47 tubes?

[Kim G]

I just read on AA about someone using the 46 tube in a 45 amp and I am curious as to whether the Paramount can be configured to run a 46 or a 47 tube and what would be needed circuit wise to to be done to make the changeover. I am running mine as a 2A3 now and it seems that at one time there was information for configuring the Paramount to run 45s.  But I've slept since then and my recollection is vague.  The OP stated that he preferred the 46 to the 2A3 and it seems that the prices for 46s are pretty reasonable for rolling so it got my curiosity up.  I haven't been using my Paramounts much lately because I've been waiting for the soft start  shunt to become available. And now this question comes up.
Thanks

Kim

[Paul Joppa]

I don't recommend it - the 46 in triode mode has a higher plate resistance than the 45, as well as a higher mu and significantly lower bias voltage. I know many people use it as if it were a 45, but it's not (in my humble opinion) close enough to take that for granted.

In a normal cap-coupled 45 amp, the 46 will bias up at about 2/3 the current of a 45. That will usually cause the plate voltage to rise a bit, and the cathode voltage will fall, about 30 volts. And the 46 rated maximum plate-to-cathode voltage in triode mode is already 25v lower than the 45. The odds are, in such an amp you would exceed the maximum voltage by 50-75 volts, which will adversely affect the longevity of the tube. The reduced current will result in an increase of second harmonic distortion which some people, in some systems, really like (I don't.) Ideally you would also have to replace the output transformer with a higher impedance one in such a case, to restore the low distortion.

The Paramount 2A3 is direct coupled which makes it even more complicated to get the voltages and currents correct.

[Kim G]

Thanks Paul for setting me straight.  I'll just wait for the soft start and maybe save up for some Shuguang 2A3-Z treasures. But is my memory correct , there is a way to run 45s on the Paramounts?
Thanks

Kim

[Paul Joppa]

Yes, the Paramount can be modified to run a 45 pretty well. I'll outline the changes here, and if anyone actually does this we can add details as needed.

The starting point is a 2A3 Paramount. This is a direct coupled amp (unlike the 300B version), and I now recommend the new C4S board as soon as it becomes available. Meanwhile I recommend building it as a 2A3 first and checking the voltages, since the 2A3 is tougher, especially the Sovteks that are shipped with the kit.

The operating current is reduced by replacing the two 2000 ohm 25 watt resistors (per amp) with 3000 ohm 25 watt resistors. The two resistors per amp are in series, so the cathode resistor becomes 6000 ohms instead of 4000 ohms for the 2A3. At this point, you should be able to plug in a 45 and it will work. (A 2A3 will also still work, but not as well as it did in the original configuration.)

For better performance, the Paramount plate choke can be replaced with a Magnequest BCP-15 or a BH-2 (the SEX amp upgrade plate choke). That will require a larger parafeed capacitor, something in the range of 8 to 12uF, but it should deliver deeper and more effortless bass. I would stick with the Paramount output transformers, either the stock unit or the Magnequest BH-5 which is used in the Paramour and Paramount. Normally Mike sells these MQ items as matched sets, so you'll have to ask him for this combination, but he's usually pretty flexible. If you are going to use the MQ iron with a 45, and can afford it, I would be most enthusiastic about the nickel core output transformer.

[Paully]

Paul,

I have a question that I don't know if it is possible to answer but I thought I would pose it anyway.  If it is too nebulous, so be it.  Changing out the resistors is easy enough.  If that is all I do to start will I get enough of the flavor of the 45 to tell if I want to proceed?  Basically, are they going to be run optimally enough to be able to tell if I want to proceed to changing the chokes and output transformers?  If there is a good chance that just changing out the resistors isn't going to leave the 45's operating well enough for me to be able to tell whether or not I like them than I would rather not spend the money on new 45's (rather get some NOS 2a3! in that case).  Anyway, tough question I think with no sure answer but your opinion is good enough for me.  Thanks,

Paul

[Paul Joppa]

Hey Paully, yes I think you will be easily able to tell. The high-inductance choke will give you better deep bass, but the heart of the music is still as always in the midrange. Just include some music with modest deep bass requirements in your listening tests.

In fact, for initial listening tests I would keep the output transformers that you are already used to, so that only the tube difference enters the equation. The stock Paramount output transformer is quite similar to the MQ upgrade; the main differences are in quality materials (some high treble improvements) and in core material, especially if you get the nickel core (cleaner throughout the spectrum). The stock unit has a few more turns on a slightly smaller core stack, so the flux is very similar.

The damping factor is smaller with the 45, so there may be some timbral shift if the speaker impedance is not fairly flat. You can try to listen past that, or add a resistor of 2 to 3 ohms in series with the speaker when listening to the 2A3 to form a baseline.

Yes, the transformer impedance of 3K is lower than usual for the 45 - that's why the damping factor is smaller. But the very best 45 amp I've ever heard was the version of Tucker's amp which he showed at the last VSAC, and it used a TF-2004Ni which is a 3K transformer.

[Paully]

Question answered.  I am going to try it out!  Thanks.

Paul

[Grainger49]

And if you want you can put in a switch and go back to the 2A3s by installing both sets of resistors.

[Paully]

Could, but you know me.  If I go down this path and I like the 45 I will end up going whole hog and putting in the choke and nickel transformers.

[Paully]

Well, I have the resistors and the tubes coming.  Will let you know next week what I think.

[tsingle999]

Paully, Can't wait to hear your results!

PJ, I decided to stick some 46's in my SR 45. The bass was incredible but seemed like too much distortion in the top end (sort of like the difference in my VSAC PDG amp going from 6FY7(45) to 6FM7(46)) and the only way it was really listenable was to disconnect my supertweeters. I do have to say however that they were better for me than the old stock 45's i tried.

[Paully]

Should be interesting actually.  Unlike last time I haven't been enjoying the 2a3 as much as I think I enjoyed the 300B.  So I will give the 45 a whirl but I have a sneaking suspicion that I might end up back with the 300B.  But I am going to try to keep an open mind and see what I think.