Stereomour II 45 Conversion - Anyone Do It Yet?

[Deke609]

Yeah, that was a blast of information without enough context. Apologies.

No apologies necessary! After your post I did a little reading and thought I'd take a stab at estimating "Bmax" only to find that you need to know the # of turns and cross-sectional area of the core - neither of which I know, and can't see a way to estimate them based on what I do. So how you are able to make estimations is a total mystery to me! And no need to try to explain - even were I to apply myself rigorously to learning this stuff, I am likely a very long way from being able to understand (but making slow and steady "progress": I am getting more accurate in identifying what I don't understand) 

cheers and thanks, Derek

[Tom-s]

There is no optimum capacitance. Bigger is better, limited only by practical considerations such as the start-up voltage sequencing, physical size of the cap, or high-frequency cap problems (generally worse the larger the cap). There is a minimum size, set mostly by the low frequency limit and the margin you choose. The analysis for a minimum size is pretty complicated because it involves the output circuit (plate choke, parafeed cap, OPT, and power supply) as well as the load (speaker impedance function). In practice, 47uF is good. I've seen lower values down to 20uF but I would not try it myself unless I could audition alternatives in the target system.

I've searched the forum and this thread in particular to find out how the cathode bypass cap of the 45 would need to change.
This is because i'm planning on using a 30uf or 60uf oil bypass in the 2A3 SII and might want to change to 45's later on.
How would going from 2A3 to 45 change this equation? How do i calculate this myself?

[Paul Joppa]

I've never done the full analysis, or set criteria which might be used with such an analysis to select a "minimum" cathode bypass capacitance. As in other cases, there is no optimum; larger is better but with diminishing returns - where do you draw the line?

That said, the 45 conversion goes lower in frequency than stock, so I would not go below the 47uF previously mentioned. Use the 60uF in preference to the 30uF.

[Doc B.]

PJ's experience makes him pretty damned good at predictions. But at some point you have just dive in there, try some different cap values using his predictions as a starting point, take measurements and listen.

[Paul Birkeland]

I'll present my two lazy methods of looking at this...

#1.  Find a capacitor with a reactance that is 1/100th the value of the cathode resistor at 20Hz.  For a 1.6K cathode resistor, that's 50uF.  I don't have any analysis to back this up, but it's quick and dirty.   

#2.  Use this calculator.  Since we are interested in low frequency response, we can figure out the numbers for Rload and Rp based on the circuit itself.  A 40H choke (SMR wired for #45) has a reactance of about 5K at 20Hz, and the reflected load of the parallel feed output transformer is close to 4K.  The 5uF parallel feed capacitor will have an impedance of 1.6K at 20Hz, and this will be in series with the OT, so the Rload is 5600 ohms and the Rp is 5000 ohms.   I use 20Hz for the -0.1db box. For most of the other calculators on this website, there are formulas explaining how they created their calculators, but they are notably absent from this page.  When I put all these numbers in, that calculator spit out 44uF. 

I would imagine the Radiotron handbook has the appropriate formula in it, and that formula could be programmed into Excel to spit out cap values.  A part of me hopes that's what's going on with the MH-Audio calculator!

[Raymond P.]

I am in the process of converting an SII to 45's and have some questions. But first, thanks to everyone here for the invaluable information to help me get started.

Ok, here's my situation. I'm trying to fine tune the filament resistor values, I have the DCF supply installed, and I have standard 45 tubes. Base on data in this thread, I started with a pair of 0.25 ohm resistors, then took measurements: DCF supply voltage = 3.4Vdc,  voltage across each 0.25 ohm resistor = 0.375V,  voltage across filament = 2.66V,  current through filament = 1.5A. This data was recorded after voltages stabilized.

So the filament current is at spec, but the voltage is 6.4% too high. To reduce the filament voltage, I can play around with increasing the resistor values, but if I did that, the current would go down (right?). So is there an achievable solution where the current is 1.5A and the filament voltage is 2.5V? Is hitting one target more important than the other? Any suggestions on what resistor value(s) to try next based on this data point?

Just to see if the filament is "behaving", I gave it 2.5Vdc with a power supply and it consumed 1.5A, so it seems ok.

[Paul Birkeland]

The filament is not purely resistive, so reducing the voltage won't necessarily reduce the current. 

You need to drop an extra 0.16V at 1.5A, which calls for 0.11 ohms of extra resistance, which would mean 0.3 ohm resistors.


If you want a different way to achieve the same goal, you could draw another 0.3A through the filament supply, which you could do with an 8.2 ohm resistor across the filament (pins 1 and 4 on the socket).  It would need to be a 3W resistor.  This is totally OK since everything is rated for 2A3 use.


You don't have to be dead nuts on; the pickiest tube manufacturers would want 2.4-2.6V.

-PB

[Raymond P.]

The filament is not purely resistive, so reducing the voltage won't necessarily reduce the current. 

That's interesting. Thanks, PB. I'll try 0.3 ohms next.

[Deke609]

The filament is not purely resistive, so reducing the voltage won't necessarily reduce the current. 

@PB - can you say a bit more about this? Since Raymond has the DC filament supply, that knocks out anything frequency dependent - so no impedance, right? So what would allow the filament to present a variable effective resistance?

cheers and thanks, Derek

[Paul Joppa]

Resistance is a strong function of temperature. When the temperature goes up, so does the resistance - so the current does not increase nearly as rapidly as the voltage alone would suggest.

[Deke609]

Many thanks PJ. That makes sense. Guessing that explains why superconductors are super-cooled.

[Raymond P.]

That's interesting. Thanks, PB. I'll try 0.3 ohms next.

Quick update: 0.3 ohms filament resistors work very well. The voltage and current are comfortably within 5% of spec.

[Thermioniclife]

Holy smokes this topic is way beyond my understanding but I will chime in regarding Lundahl opt's.
In the past 20 years I have used 3 sets of Lundahl 1664 opt's.
I had previously used Hammond 1600 series for hifi  and the 125 series lower end trans for single ended guitar amps and Edcore.
In my opinion the Lundahl opt's have been Fantastic!!!
They are Pretty expensive, Ugly as sin, but i have had nothing but excellent results using them.
The 1664 is a 3k primary but I'm sure what ever you need for a primary impedance if Lundahl makes it I feel you will be happy.
Give them a try, they offer different current versions that may match your requirements.
Just my unsolicited 2 cents worth.
Happy Holidays all
Peace, Love and Happiness
Lee

[debk]

Quick update: 0.3 ohms filament resistors work very well. The voltage and current are comfortably within 5% of spec.

I just converted my Stereomour II to 45 tubes.  Love the sound of the 45 tube.  I too used 0.3R resistors and got voltages of around 2.45V with EML 45 mesh plate tubes.  I have the shunt regulator, DC Filament, and morequiet upgrades installed.  Not that difficult to do, but had to work in some fairly cramped areas with everything already installed

[Tom-s]

For those interested in going 45 and contemplating on the DC supply.

In my setup the SII is quiet on the speakers (102db/w) without any DC supply and EML 45 globe mesh tubes.
EML's are definitely more-quiet than old stock 45's in my collection on AC heaters.