Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Speakz on January 11, 2014, 04:16:18 pm
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I have been reading about the 62' Bluesbreaker and was wondering how the output was down to 35 watts? Would Marchall have used a smaller output transformer with the KT-66's or is there another way to reduce the output from a Bassman/Jtm45?
Also Researching transformers for the BFVR build the power transformers suggested for it are rated at 40 watts. The originals was around 35watts. IS it the output, power transformer or other things that effect the overall output in a valve amp?
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IS it the output, power transformer or other things that effect the overall output in a valve amp?
Yes. :laugh:
The PT can restrict the output power by not having enough B+ current and/or B+ voltage. The OT can restrict the output by not having the optimal primary Z and not being wound heavy/strong enough for a given power output.
You also need enough signal presented to the output tube grids to drive them to full output for a given design.
Speakers can make a difference too, proper Z to transfer max power.
That's a short answer, some of the other guys can show you the math and tube load lines for a much deeper explanation.
Brad :icon_biggrin:
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That is very helpful thank you. From my reading I keep getting bits of information saying it would effect output but it's good to read about an overall range of areas where it can have an impact.
When you say optimal primary Z what is that? I'm not finding it in the "Tube guitar Amplifier Essentials" by Weber.
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When you say optimal primary Z what is that?
If an amp has an OT that's secondary has an 8ohm output (tap) then if you use an 8ohm speaker you get the ~most output as apposed to using a 16ohm or 4ohm speaker. The 16ohm and 4ohm will work (and you might even like there sound better) but a (little) less output power transfer.
Brad :icon_biggrin:
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Great that makes sense. Thanks :occasion14:
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I have been reading about the 62' Bluesbreaker and was wondering how the output was down to 35 watts? Would Marchall have used a smaller output transformer with the KT-66's or is there another way to reduce the output from a Bassman/Jtm45?
According to Dr Tube (http://www.drtube.com/library/schematics/69-marshall-schemas), the model 1962 Bluesbreaker was 45w just like the JTM45's. Dr Tube also posts the same one schematic for both these models. In fact, the Bluesbreaker was the same amp chassis but bolted into a different cabinet (the head & cab versions would be a bit louder due to 2 extra speakers).
Now some of the new reissue amps (http://www.zzounds.com/item--MSHBLUESBREAKER) seem to be given a 30w rating, but that may be less significant than you think.
If you had an amp that produced a measured 35w RMS with a perfectly clean sine wave output, and drove it until the output was distorted to a perfect square wave, the measured (distorted) RMS output power would be 70w. That's due to the difference in the waveshape. So if the output was measured with some amount of distortion, it would be a larger number than the measured output with a clean sine.
Also, even if the JTM45 didn't make exactly 45w back in the day, it would be worthwhile for Marshall to market it that way. Guitar amps usually had to fill a club, and might have to have vocal mikes plugged in as well because live venues generally didn't have sound reinforcement (it wasn't too many years before that the average band was horns playing jazz, who don't need any mics). Claiming more watts might get your product sold faster (and Marshall, Hiwatt & Vox raced to build bigger 50 & 100w amp heads).