Completed conversion to 45

[HeyDerAudioBob]

Recently my Stereomour decided tell me it needed some attention. Thanks to Paul Birkeland and the Bottlehead team, my issue was identified and we agreed it was time to replace the C4S PCB/parts. This brought my Stereomour back to life and I have enjoyed it over the last couple of month. This past week, I decided to clean it up a bit since it was my first bottlehead kit from many years ago. I cleaned up many of my poor solder joints, tightened or replaced worn input jacks, organized the wiring a bit for that cleaner layout look.

While doing these things, I took the opportunity to convert over to 45's. This switch was very simple. About an hour of change and test. I also updated OT's to 8 ohm from 4 ohm configuration to better align with the speakers I intended to use - a pair of Forte III's.

I had recently acquired a pair of National Union tubes, NOS vintage Nov-1946 so rolled these in for my listening sessions.

My very first observation, which I have no real explanation as to why, is the reduction, almost elimination of the faint hum I had when running as a 2A3 setup. This was on all inputs - my DAC, Phono, and CD inputs. My phono input under the 2A3 could be  particularly annoying with lower listening levels, but now is gone.

There is a clear change in the presentation, vocals definitely take front stage, clarity and operation of instrument and note is improved, and finally bass is slightly improved, less muddled across the range. The reduction in amplification is definitely noticeable and requires moving from a 9-12 setting on the volume pot to a 12-pushing 3 to obtain the same listening levels depending on genre of music chosen.

So far, I love the 45's sound and will stick with them. My next step is repeat this process with my Paramount mono blocks and take them from a 300B to 2A3 fronted with my 300BeePree. This amp is running into a set of vintage Cornwalls and would like a bit of the microphonic's (ringing/sensitivity) that i currently have to be reduced or eliminated.

Thanks to the community and the Bottlehead team for designing and supporting our amps with such flexibility and longevity in mind.

Bob S

[Paul Birkeland]

Just double checking also that the blue and red plate choke leads are being used when you converted to #45 operation.  The original build manual had an error in the instructions to go to #45 operation. 

-PB

[HeyDerAudioBob]

Paul,

Thanks for the check in here -- Yes I basically swapped the Red/Blue leads such that red leads are now running to the 45 tube pin #2. I had a series of notes I had gathered from the forums (or maybe BH directly) on the actual steps i needed to perform and check to do the conversion to 45's.

It was very simple in the end as you know and it worked flawlessly. Heck the swap from 4 to 8 ohm was a bit harder simply because of the accessibility of terminal #3 on the OT once the kit is built !!!

Cheers

Bob S

[Paul Birkeland]

You need to swap the black and the red leads.  The blue leads should stay where they were and the black leads are no longer used.

[HeyDerAudioBob]

Paul,

OK -- So i follow I think. I went back to the schematic for the Stereomour - so you are saying for the choke (PC-3) that the red wire is providing 40H and the black wire is 20H?

And as a follow on, Although I will recheck - I don't recall voltages being off on A2, C2 (or at least within close tolerance) -- and the Resistance checks for A2, C2 although noted in manual as "*", with my meter they read  in the M ohm range which I interpreted as "OK".

I will go correct, but I was just curious to my questions and still trying to understand the circuit fully and components impact on the circuit and ultimately the sound that is produced.

Cheers

Bob S

[Paul Birkeland]

Yes, that is correct.  Blue always goes to the plate. 

This will not alter the voltages or resistances in a meaningful way, but the way you have things hooked up now will limit the bass response of the amp pretty severely.

[Paul Joppa]

You can check the choke's DC resistance - blue to red is about 830 ohms, blue to black is about 565 ohms. Blue to red includes more turns and has more inductance but less current capability. (The ratio is about the square root of two, and inductance is proportional to turns squared.)

[HeyDerAudioBob]

Paul and Paul,

Thank you both for these responses. Corrections have been made and all is great.

As always I appreciate your support and the products you guys offer --  although I am sad I missed out on the MonAmour amp.

Cheers

Bob S