Interview: Boutique Amp maker Daytona Dave – Tube Bias and Speaker Selection
By Max Milne
Adjusting tube bias and picking the right speakers are vital when amp building. Daytona Dave of Boot Hill Amps gives Max some tips in part two of his interview.
- Part One: Getting Started
- Part Two: Tube Bias and Speaker Selection
- Part Three: Soldering and Circuit Building Safety
Tube Bias: Pots and Plates
Tell me about tube bias…
There are two types of bias in a tube amplifier; Your simpler and smaller amplifiers use cathode bias, which is self-biasing. So if you have one of these, a 5F1 or a 5E3, you don’t have to worry about the biasing because it’s self-biasing. No matter what tubes you put in there, as long as it’s a 6V6 type tube, it’ll sound good.
The other type, which the 5F6A has, is called fixed bias. Now fixed bias requires adjustment when you change your tubes. This can be quite a complicated thing, but it can be remedied by the installation of an adjustable bias pot.
“No matter what tubes you put in there, as long as it’s a 6V6 type tube, it’ll sound good.”
Many builders who sell ready-made amps or kits provide a pot to adjust the bias and you can use a meter to establish the correct milliamps or bias. But, you can also just use your ears and eyes, to adjust it to the sound that you like, as long as the tubes aren’t glowing red on the plate (the plate is the big surface area that you see on the tube). If the bias is too high, the plates will glow red, it’s pretty unmistakable, and if the plates are glowing red, turn that bias back down until the red goes away.
So how does the bias affect the sound of the amp?
It affects it in a huge way, because the bias controls the idle of the amplifier. Kind of like a carburettor in a car. So, if the bias is too low, the amp will sound weak and too lifeless, and the higher you go, the more cranked it sounds. You can adjust the bias to however you like, but the higher the bias, the faster your tubes will burn out. You can also have such a low bias that the sound will be almost anaemic.
How would a speaker influence the sound of an amp?
Well, there are a couple of things that stand out. One is sensitivity rating, which is the efficiency of the speaker. A high efficiency speaker will be louder than a lower efficiency speaker. So, it affects the sound because the amplifier is driving the speaker harder.Once you get over 100dB sensitivity – some are around 105/110dB – they start to become significantly more expensive.
The other thing is the wattage rating of the speaker. And that pertains to the thickness of the wire in the voice-coil. So if you’ve got heavy wire in the voice-coil, you can handle more wattage. Sometimes a lower powered speaker that’s being driven harder will sound better than a more powerful speaker, that’s not working so hard. So musicians tend to experiment, to find which they like better.
“If you can get those loosened up, you’ll change the dynamics of the speaker right away. You can’t really judge a speaker until you’ve put some miles on it.”
The cone also plays a part: material, density, type and thickness of the material. Even the age of the speaker affects the sound: Older speakers are more dynamic because they’re looser. That’s why vintage speakers can sell for a lot of money. So if you get a new speaker, you should plug it in to a music source or a low-voltage source and let it play for a good 24 hours so you loosen up the spider, and the cone and the area around the cone, known as the diaphragm. So if you can get those loosened up, you’ll change the dynamics of the speaker right away. You can’t really judge a speaker until you’ve put some miles on it.
Now if you have a small amp, and you’re running it at or near it’s breaking point, it may have a really nice, warm, dirty tone, but if you have a large amplifier, and to get a good tone you have to play it so stinking loud before you get that distortion that you want…You have to figure out what you need the amplifier for, and what kind of wattage you need. Bigger isn’t always better, it’s just louder.
Is there a specific type of speaker that you would suggest for your kits?
I’m an Original Equipment Manufacturer for Jensen Speakers. Jensen has a lot of speakers in their range. You can check them out at jensentone.com. They’ve got really good speakers for all of the amp kits that I sell. They have speakers at different price levels, and if you go to the site and look at the speakers, they tell you what it’s supposed to sound like, how it compares to others, and all kinds of details about them. So, they’re a good quality product.
Read The Rest of The Interview with Amp Builder Daytona Dave
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