Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Stuff4bikes on April 06, 2022, 08:51:55 am
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Question
1. Why don't we use the largest filtering we can....100uf's across the rail? Tube rectifier limiting is understood...
2. Why do single coils hum on some of my builds but not others? Or is it my imagination.
3. How many stages of gain can you run. How many 12ax7's can you add. I have noticed on some older power tubes
They only recommend one gain stage before the tube....
Sorry for the weird questions, just curious..
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1 - I think it depends on how many amps the PT can generate. Something like that. Same as tube rectifier.(I may be wrong thou)
2 - Single coils are noisy, they always hum, more or less depending on the position of the guitar in relation to the amp, distance, angle, the closer the guitar get to the amp more noise it will do.
3 - I dunno :dontknow:
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3. How many stages of gain can you run. How many 12ax7's can you add. I have noticed on some older power tubes
They only recommend one gain stage before the tube....
Google: tube amp interstage attenuation
This is a real preamp :icon_biggrin:
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1. caps take time to charge, with NO-load (since tubes are cold) That could keep them above there max V rating long enough to pop
some guitarists want the supply to SAG under heavy Pick-digging, big caps won't let 'em
the cost of bigger might be a down-side.
2 outta my comfort zone :icon_biggrin:
3 you can have as many as you can afford, but once a signal is a square wave, it's just showing off. Good sound is made by good wave-shaping, not brute force. tubes have a spec for G1. exceeding that spec by large factors can have a real consequence on tube life
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1. Large caps take up space and are expensive. For a hobbyist making one-off builds for personal use, the cost of bigger caps is probably not a big issue, but you still have to fit them into the chassis.
There’s also diminishing returns for adding more capacitance.
2. Single coils always hum. If you’re close to the amp, the size, type and oriention of the transformers can affect how much hum is picked up by the single coil.
3. Old tube data sheets and manuals are written for applications where the goal was to minimize distortion. Guitar amps usually have a quite different design goal.
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1. Why don't we use the largest filtering we can....100uf's across the rail? Tube rectifier limiting is understood...
If you use a tube rectifier, there is a design limit for the first node -- for example, a GZ34 datasheet lists 60uF -- and of course it is this first node where the majority of the current flows into the OT. The second node feeds the power tube plates, and thereafter a small amount of current flows into the small bottles. Of course you want your ripple current to be low, and to have the proper amount of current available for your cranked solo (or, maybe you want some sag).
You can place higher levels of capacitance in subsequent nodes, at the added expense and need for space. Honestly, when you look at all the factors, you realize that it doesn't make much difference. Even HAD, who favored high levels of capacitance in his reservoirs (he was able to do so because he normally used SS rectification), did not go crazy in the subsequent nodes.
I'm hopeful that Merlin will cover this in his upcoming book.
How many stages of gain can you run. How many 12ax7's can you add. I have noticed on some older power tubes
They only recommend one gain stage before the tube....
As many as you want! Just remember that your signal will be distorting very quickly in the chain, and not in the way most of us want with even order harmonics. Merlin covers this nicely in his Designing Preamps book.
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A few random answers...
A, B, C, D...
Based on which quadrant the second hand is in. :icon_biggrin:
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:l2:
when I took my 1st class FCC test, I started at 3rd class, the instructors never disclosed the fact, they wouldn't let you PEE between exams!
passed the 3rd n 2nd class, but all i seen was YELLOW 1/2 way through the 1st class exam, missed it by 2 quadrants!
:icon_biggrin: