Stereomour II 45 Conversion - Anyone Do It Yet?

[Paul Joppa]

In the stock 2A3 build, the red wire of the PC-3 plate choke dead-ends at T2 and T19. You should be able to unsolder the red and replace it with the black, connecting the reds to 27 and 34 (where the black was originally).

The above instruction is based on the manual and my memory, so I can't be certain I have it entirely right - but that was my design intent!

[Jamier]

In my builds I cut the black wire excess off before connecting to the PS so my blacks will no longer reach 2 and 19. I guess I can just terminate them with shrink (?).

Jamie

[Deke609]

@PJ - many thanks. So just swap terminations of black and red.

@Jamie - you've done the conversion to 45? What are your impressions? You mention "builds" (plural) - have you converted two amps?  And do you have the DCF upgrade? If so, what if any tweaks did it need?

cheers,

Derek

[Jamier]

No,  I have built 3, 2A3 versions, one with the PS caps swapped for PP films. The other two are stock and series bridged to get the 7WPC. I am thinking of converting one of the stock amps to 45s and if I like it I will convert the other and series bridge again to get the 3.5 WPC, 45. The series bridged 2A3 amps are good, but I listen nearfield in a small room and I find that I really don't need the power. What I want is an even better 3.5 WPC. That is enough power in my small space at the levels that I listen at. I think this is going to be great!

Jamie

[Deke609]

Ah, gotcha, I misread your reference to cutting the black wires to mean you cut them while doing the conversion.

3 Stereomours - nice!

Re bridging the two amps: I am thinking along exactly the same lines. Please share your impression when you get the first 45 up and running.

cheers,

Derek

[Deke609]

@PB - I made the required changes (swapped 8.2Ks for the 3Ks, removed the 0.13 Ohm DC heater resistors, swapped 4.7 uf for 3.3 uF caps, and swapped  red and black PC wires).

With the 45Bs, I measure 2.53 to 2.55 V (seems to oscillate up/down between the range values) between pins 1 and 4 of both tubes.  At first, I measured 2.50V, but that value slowly climbed until settling in at the aforementioned range.

Edit: the EML 45B filament is rated for 2.5V with a 5% tolerance. So it looks like I'm within spec. But I don't know whether the filament heater volatage would have continued to slowly climb - I only had the meter leads on for about 3-5 min.

What's next? I bought a whole bunch of 3W resistors for tweaking the heater R values.

cheers and thanks,

Derek

[Paul Birkeland]

With the 45Bs, I measure 2.53 to 2.55 V (seems to oscillate up/down between the range values) between pins 1 and 4 of both tubes.

Yeah, you won't need any of those resistors.  That's a great voltage to get! 

[Deke609]

Awesome. Thanks PB. Gonna give it a listen!

[Deke609]

Well ... I screwed that up nicely.

I mistakenly ordered and installed 8R2 instead of 8K2 5W resistors!!!! So, instead of 1600 Ohms, I had 8.2 Ohms. Sweet Marie!   

I am praying I haven't fried anything (there was no smoke and no popping).

It sounded terrible .. very distorted, like the tubes were being pushed way too hard. Stupidly I gave it some time (about 20 - 30 min) and then finally turned the amp off, convinced that something wasn't right. And then I checked the mods I made to the power supply ... and there it was: "8R2" ... awh crap.

Have ordered 8K2 5W. Amp will stay off until I install them.

How likely is it that I've damaged the amp?

[Paul Birkeland]

Try running the amp with the 2K cathode resistors and do a voltage check.

The likely things that will be destroyed will be the tubes and/or the plate chokes. 

Always check your voltages after powering up.  I'm sure you would've found that all the HV voltages were extremely low.  Luckily that extra power supply resistance you added probably took the brunt of the abuse. (and your 45s)

[Deke609]

So, remove the 8R2s, and run the voltage checks with the 45Bs?

[Paul Birkeland]

Yes.

Running experimental tests with a $649 pair of tubes doesn't seem like the best choice when inexpensive used 45s are floating around all over eBay.

[Deke609]

Voltages with 2K cathode resistor and 45Bs:

1-   367.5 to 368.5
2-   391-392
6-   63.4
7-   189.5
14-  191.8
15-  63.4
19-  390
20-  371
21-  63.6
22-  65.2
24-  61.8
25-  63.1
27-  396
30-  400-402
31-  402
34-  397
36-  63.2
37-  62.3
39-  65.3
40-  63.2

Many thanks PB.

[Paul Joppa]

The error will have produced excessive cathode current, which might have damaged the filament coating of the 45Bs. However, if that happened, sometimes it can be restored, so don't panic! Fortunately, the 45B is tougher than a regular 45.

[Deke609]

Thanks PJ. I'm ok, I'm just kicking myself for making that mistake. I had lots of chances to catch it - first, when I ordered it, second when I received the resistors and checked their values, and third when I installed it.  But an "R" looks a lot like a "K" when you're looking for "8K2".

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