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January 15, 2021, 03:30:45 AM

Author Topic: stereomour 45 plate voltage  (Read 190 times)

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Offline bqc

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stereomour 45 plate voltage
« on: January 06, 2021, 01:48:14 PM »
I reconfigured my Stereomour to use #45 tubes. I noticed that the power output is about the same as what the 2A3 output was and it sounded about the same as the 2A3 was ie no noticeable improvement like what I have read on various forums.  Is that because of the high plate voltage? I measured supply voltage HV+ to be about 360V and plate voltage A2 and C2 is about 350V. Is that too high for the #45? the #45 datasheets spec plate voltage to be max 300V and nominal 275V.  What would it take to reduce supply voltage, and any other changes so that  plate voltage is between 275V and 300V

Thanks

Offline Paul Birkeland

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Re: stereomour 45 plate voltage
« Reply #1 on: January 06, 2021, 01:57:43 PM »
How did you determine that the power output is the same?

While the B+ is 360V, the voltage at the plate of each 45 will be more like 340V, and the bias voltage at the filament would be roughly 57V, so the voltage across the tube will be roughly 280V, which is well below 300V.
Paul "PB" Birkeland

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Offline bqc

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Re: stereomour 45 plate voltage
« Reply #2 on: January 06, 2021, 02:20:11 PM »
When I listen to some familiar music, the volume setting on my PC is the same and the perceive loudness out of the same speakers are the same, so I thought the power output is about the same and not 1.75 Watt vs 3.5 W as before

Offline Paul Birkeland

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Re: stereomour 45 plate voltage
« Reply #3 on: January 06, 2021, 04:15:45 PM »
That's gain, not power output.  Power would be how loudly you can play the amp before it clips and starts distorting like crazy, gain is how much signal comes out with a certain amount of signal coming in.
Paul "PB" Birkeland

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Offline Paul Joppa

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Re: stereomour 45 plate voltage
« Reply #4 on: January 06, 2021, 07:48:56 PM »
I looked up my original design notes from 2012, and the 45 Stereomour should produce around 2.3 watts. That's only 2dB less than 3.5 watts, which is a barely audible difference. Most audiophiles can probably hear the difference in an A-B comparison, but not in a yesterday/today comparison.  The gain is also similar since they use the same OPT.

There are some more technical reasons for what you hear:

1. Using the same 3.3uF for both versions provides better protection from overload for the 45 in the deep bass. These were not accidents; at the time they seemed to me the best compromise for each situation. But it does give them more similar bass extensions

2. Since I was bending both to use the same output transformer, I did choose to make them sound as much alike as I could in the midband, by choosing operating points. If you don't hear much difference in tonality, that may be a reason. You might get closer to the more classic 45 sound if you substituted a 5K transformer for the 4K OT-2.
Paul Joppa

Offline bqc

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Re: stereomour 45 plate voltage
« Reply #5 on: January 06, 2021, 09:08:26 PM »
Can I replace the OT-2 with any off the shelve 5K output transformer or is the selection of OT more complicated because of the plate choke. Could you recommend some 5K drop in replacement OT?

Thanks

Offline Paul Birkeland

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Re: stereomour 45 plate voltage
« Reply #6 on: January 07, 2021, 05:52:18 AM »
A Sowter 8983 would be a good transformer to try.  You'll need to drill some different holes, but it should fit in the space available.
Paul "PB" Birkeland

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Offline Paul Joppa

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Re: stereomour 45 plate voltage
« Reply #7 on: January 07, 2021, 11:02:50 AM »
Can I replace the OT-2 with any off the shelve 5K output transformer or is the selection of OT more complicated because of the plate choke. Could you recommend some 5K drop in replacement OT?

Thanks
Yes, parafeed outputs are different from series feed, and there are not many of them. In addition to PB's suggestion of Sowter, you might find a pair on Magnequest EXO-45 or 46 transformers on the used market.

A better experiment would use a more similar OPT, and as it happens, I have a couple pairs of early prototype OT-2s with the wrong number of turns, giving them around 5K impedance. I can loan you a pair if you like, just promise to post your results! Or I can arrange a sale through Bottlehead if you want to keep them.
Paul Joppa

Offline Paul Birkeland

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Re: stereomour 45 plate voltage
« Reply #8 on: January 07, 2021, 11:50:11 AM »
as it happens, I have a couple pairs of early prototype OT-2s with the wrong number of turns, giving them around 5K impedance.
You gave those to me years ago (unless you have more?) and they went into some 45 amps (in wood bases/stands that Dan gave me years ago, also out of his basement).
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Paul Joppa

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Re: stereomour 45 plate voltage
« Reply #9 on: January 07, 2021, 12:22:41 PM »
PB, you got version 3.1 I believe; I still have v2.0 and v3.0 sets  :^)
Paul Joppa

Offline Paul Birkeland

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Re: stereomour 45 plate voltage
« Reply #10 on: January 07, 2021, 12:24:58 PM »
I thought the probability that you forgot you gave them to me vs. the probability that you had more was roughly equal ;)
Paul "PB" Birkeland

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Offline Paul Joppa

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Re: stereomour 45 plate voltage
« Reply #11 on: January 10, 2021, 05:56:40 AM »
I thought the probability that you forgot you gave them to me vs. the probability that you had more was roughly equal ;)
You're not wrong  :^)
Paul Joppa

Offline Doc B.

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Re: stereomour 45 plate voltage
« Reply #12 on: January 10, 2021, 08:11:08 AM »
I have no idea how you guys keep track of that stuff.
Dan "Doc B." Schmalle
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Bottlehead Corp.