Stereomour II 45 Conversion - Anyone Do It Yet?

[Paul Birkeland]

Any chance the amp can be run with the 2K cathode resistors, or am I courting more harm?

That's not going to cause any harm, you'll just be down on available power.

[Deke609]

Thanks PB.

[Jamier]

Derek,
          when you removed the 0.13 ohm heater resistors, what value did you replace them with? I don't have the DC filament kit so I have 0.15s in my amps, but perhaps you are referring to the second set of resistors that are added to the socket with the DC filament Kit.

Jamie

[Deke609]

Hi Jamie - Yeah, the DCF calls for the addition of 0.13 ohm resistors in parallel with the 0.15's. So, as per PB's instructions, I simply removed the 0.13's and left the 0.15's in place and got lucky with the resulting DC heater voltage.  I think this is specific to SII's with the DCF.  PB's original instructions for stock SII's (or at least without the DCF upgrade) calls for swapping in 0.22 ohm 2+W resistors for the 0.15's. I would go with that. PB's instructions are quoted on page 1 of this thread.

cheers,

Derek

[Jamier]

Derek,
      Thanks for your reply. Don't beat yourself up over your mistake.Everbody in this hobby has made their share of those. I fried some $400 tweeters once upon a time.I'm going to make this conversion eventually, after I get a few work items off my plate. Thanks for being the trailblazer on this.

Jamie

[Deke609]

@PB - I installed the 8K2s in parallel with the 2Ks.

VDC between pins 1 and 4 is now 2.76 (so out of spec; spec is 2.5 +5% tolerance).  What do you suggest I sub for 0.15R's?

Other voltages:

1-  365-366
2-  386
6-  60.6
7-  191.3
14- 192
15- 60.4
19- 386
20- 365.5
21- 61
22- 62.8
24- 59.8
25- 60.8
27- 398
30- 402-404
31- 404-405
34- 398
36- 61.1
37- 60
39- 63.3
40- 61

Many thanks,

Derek

[Paul Birkeland]

VDC between pins 1 and 4 is now 2.76 (so out of spec; spec is 2.5 +5% tolerance).  What do you suggest I sub for 0.15R's?

It shouldn't change based on the 8.2K resistor installation, but either way a 0.155R or a 0.16R resistor in place of each 0.15R resistor should get you back down to the proper range.

[Deke609]

Thanks PB.

While you were posting, I tried to calculate the value myself, but got the wrong answer. Can you point out where I went wrong?

My reasoning was as follows:

If 2.76V is dropped across a 0.3 ohm resistor (0.15 + 0.15), that means a current of 9.2 amps (that seems high to me).

If I want 2.5V, and R = V/I, then I get a combined R value of 0.27ohms - or 2 * 0.135 ohms.

Many thanks,

Derek

[Paul Birkeland]

Now that I think more about it, you did those filament measurements with the 8.2 ohm cathode resistors, so everything coming out of the power transformer would be very low. 

If 2.76V is dropped across a 0.3 ohm resistor (0.15 + 0.15)

The 45 filament is in there too.

You need to drop an extra 0.26V. 45 filament current is 1.5A.  0.26V/1.5A=0.173R.  This is the total addition you need, so just increase each resistor by about half that, or 0.08R. Something like this resistor would work ok:

https://www.mouser.com/ProductDetail/Vishay-Dale/CPF1R16000GNB14?qs=sGAEpiMZZMtlubZbdhIBIN9c7v76BNFlN04yaEgXCqY%3d

Do note that when you move this far down the trail, unlike the Stereomour I, you really don't want to plug 2A3s into an Steremour 2 that has been converted for 45 use, especially if you use the 1W dropping resistors to trim the filament voltage00. 

[Deke609]

Thanks PB - yeah I not only forgot about the filament resistance, but failed to take into account that when measuring pin to pin, I'm not measuring the drop across the 0.15Rs b/c I'm measuring between them.

I have 0.33 and 0.3 ohm 3W resistors on hand that if combined in parallel will give me about 0.157 ohms. I am going to try that.

cheers and many thanks,

Derek

[Deke609]

My magic R-value for replacing the 0.15 ohm heater resistors turned out to be 0.25 ohms , for a combined value of 0.50 ohms.  With a 0.25 R swapped for each 0.15 R, I get 2.52V (avg) on one tube and 2.535V (avg) on the other.

[Edit - May 16 2019: Now that the amp has been burned in, I checked the heater voltages again and found that they had crept up a bit (about +0.02V). My new magic numbers are a combined 0.57 - 0.58 ohms per pair of resistors on each socket : 0.30 and 0.27 on one socket and 0.33 and 0.25 on the other. This gets me to a heater voltage averaging 2.5V +/- 0.005]

In case it is useful to someone, here are my DC voltages between pins 1 and 4 of the EML 45Bs for various R-values I substituted for the 0.15 ohm Rs:

0.157 ohm: (0.33 and 0.3 in parallel): 2.7V (avg)
0.18 ohm: 2.67V (avg)
0.20 ohm: 2.62V (avg)
0.22 ohm: 2.59V (avg)
0.25 ohm: 2.53 (avg)

The above data may suggest that the drop in voltage was not linear (e.g., going from 0.18 to 0.20 resulted in a 0.05V drop, while going from 0.20 to 0.22 resulted in only a 0.03V drop. This is because I tested the increasing R-values on alternating tubes - i.e., 0.18 on Tube 1, 0.20 on Tube 2, 0.22 on Tube 1, and so on.  The same 0.25 ohm value on both tubes resulted in a 0.015V difference between the filament voltages. Factoring in the 0.015V difference suggests a linear drop of approx. 0.02V per 0.01 ohm increase.

*** Note: the above values were reached after 5+ min and then became stable. I tested the VDC for the 0.25 ohms resistors by waiting 20 min just to be sure. For anyone looking to do the conversion, I would recommend waiting until the amp is well warmed up before settling on a final R-value. In my SII, 10 sec or so after power-on my reading with the 0.25 ohm resistors was only 2.4V

These measurements were taken on a fully upgraded SII after making the following mods: 3K resistors replaced with 8.2K resistors, removal of 0.13 ohm resistors that ran in parallel with the 0.15Rs, and swapping red and black wires from the plate chokes. I did not make any changes to the 130 ohm Rs.

[Paul Birkeland]

My magic R-value for replacing the 0.15 ohm heater resistors turned out to be 0.25 ohms.  With a 0.25 R swapped for each 0.15 R, I get 2.52V (avg) on one tube and 2.35V (avg) on the other.

Does this follow one particular tube if you swap it between channels?

[Deke609]

Sorry PB. Typo! I meant 2.52V on one tube and 2.535V on the other, not 2.35.

I am treating the difference as trivial.

[Paul Birkeland]

Ah, yeah, that's better. 

[Deke609]

Quick update: the SII-45B amp is up and running and sounds pretty good. I'm hoping that, at worst, I only took some lifespan off of the tubes with my resistor value fiasco. Still early days. I'll post some impressions after a month or so.

I have another SII kit in the early stages of assembly that I am building as a regular 45 to run with some used Raytheon 45's I have coming -- just to compare with the 45B amp. And then, depending on which of the 45/45B I prefer, convert one of the SII's to 2A3 to compare against my JJ 2A3-40s. Although I wonder if this is a good 2A3 comparator? I love their sound, but I've read that sound-wise they are closer to a JJ 300B than a 2A3.  If so, does that make a fully upgraded SII w/ JJ 2A3-40's a baby Kaiju?

cheers,

Derek