Studying the schematics of the famous Ampeg B15N, I noticed the bias arrangement of the '62 model is completely different from the '68 model: cathode vs fixed bias respectively.
Can anyone shed some light as to why this has changed, what differs in tone and what is considered the 'industry standard' B15?? Anything really 😀
Edit: link to the different schematics:
http://www.drtube.com/en/library/schematics/58-ampeg-schemas
Can anyone shed some light as to why this has changed, what differs in tone and what is considered the 'industry standard' B15?? Anything really 😀
Edit: link to the different schematics:
http://www.drtube.com/en/library/schematics/58-ampeg-schemas
The B15 with the fixed bias has approximately 35 extra volts headroom. Without any details, voltage and transformer wise, I cannot comment further.
Also an interesting drawing error on the input jacks.
Also an interesting drawing error on the input jacks.
In case you have not seen it, here is a good article on the B-15. From which, it seems that the key of the B-15 sound is the speaker cabinet, and not the amp which was basically lifted directly from the RCA datasheet. Fixed-bias was also used on the Heritage re-issue, so I speculate it provides a bit more power and better stablity over the cathode bias version.
Nice article! Missed it completely.
Still, the amp itself is a nice build I guess. I now have to decide wether I'll use fixed or cathide bias and where the tonestack will go (after the first or second stage).
One thing I can't figure out, is why the heater's artificial center tap goes to the bias supply. The opposite of raising the heaters? 😀
Still, the amp itself is a nice build I guess. I now have to decide wether I'll use fixed or cathide bias and where the tonestack will go (after the first or second stage).
One thing I can't figure out, is why the heater's artificial center tap goes to the bias supply. The opposite of raising the heaters? 😀
Thai is to reduce the potential difference between the cathode and heater.
I looked at the http://www.drtube.com/schematics/ampeg/b15n68-jp.gif schematic and saw drawing mistakes.
I looked at the http://www.drtube.com/schematics/ampeg/b15n68-jp.gif schematic and saw drawing mistakes.
Ah I see. In the C version, the heaters are referenced to the output tubes' cathode.
Yes! That would make for some severe attenuation 😀.
Yes! That would make for some severe attenuation 😀.
This is a very good article about fixed vs. cathode bias:
http://ken-gilbert.com/images/pdf/fixbias.pdf
http://ken-gilbert.com/images/pdf/fixbias.pdf
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The first model was just a RCA manual rip-off. According to RCA (and most other tube manufacturers) cathode bias is prefered over fixed bias due to risk of unstable bias, even thermal run away with fixed bias.
Turns out business is business, and an amp boasting 50-60watts sells better than one with 'only' 30watts. So like all other instr-amp makers fixed bias is king, not b/c it sounds better or is more stable, but b/c it sells better.
That doesnt mean cathode bias necessarily sounds better, that is all up to preffered tone, personal taste etc. Many guitarists like fixed over cathode bias perhaps due to the slightly grittier sound.
As for the heater going to neg voltage, that must be a mistake asking for hum. Either connect it to gnd, or better 40-80V above cathode.
Turns out business is business, and an amp boasting 50-60watts sells better than one with 'only' 30watts. So like all other instr-amp makers fixed bias is king, not b/c it sounds better or is more stable, but b/c it sells better.
That doesnt mean cathode bias necessarily sounds better, that is all up to preffered tone, personal taste etc. Many guitarists like fixed over cathode bias perhaps due to the slightly grittier sound.
As for the heater going to neg voltage, that must be a mistake asking for hum. Either connect it to gnd, or better 40-80V above cathode.
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