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Old 12th June 2010, 05:57 AM   #1
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Default College-proofing an ST-70 (aka autobias+ST-70)

Hey guys,

I'm about to go off to Auburn this fall, and I want to take my ST-70 with me. However, when I built my driver board, I used two pots with two large knobs on them so I could have 4 bias pots total. I didn't think about just how much these look like tone/volume controls! I haven't had any accidents yet, but maybe I'm just lucky.

I would like to fix this problem, not by replacing the pots (that'd be too easy, you see...! ) but rather by making an autobias circuit. Either I could use the op-amp approach or the constant current fixed bias method (any others?). I would really love to find a way to use the op-amp approach...but I fear it may be more costly in the long run. I'm ready to order about $30 worth of parts to make the CC fixed bias circuit for each cathode, but I want to see what ya'll think first.

The fixed bias would take away almost 40 volts from the tube...but how much power would I really be losing? Would this add much distortion? I'd be using a TL783 and a 1000F 100V audio cap for each cathode. I don't run my amps at full blast. I'm quite conservative, really. I also keep the bias at 40mA.

Has anyone implemented an op-amp autobias circuit for an EL34/6CA7/6L6C? More specifically, on the ST-70?

I'd love to see some schematics if anyone has any. Thanks in advance!

Kyle
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Old 12th June 2010, 12:15 PM   #2
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Autobias on an AB amp is tricky (remember, the tubes do NOT run constant current!). I'm still not sure why you'd put knobs on bias pots- once they're set, they hardly need readjustment. If it were me (and once upon a time, it WAS me), I'd just put trimpots in there, preferably in a spot where you have to know where to stick the screwdriver.
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Old 12th June 2010, 03:28 PM   #3
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I wouldn't take an amp that's worth anything to college because it's apt to get stolen, get wrecked or get a beer dumped in it. Survival odds 50%. Find or build a SS design. As for the pots, I'd just change them to locking screwdriver adjust types. Problem solved.

CH
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Old 12th June 2010, 06:05 PM   #4
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hey-Hey!!!,
second Conrad's solution; change to screw-driver pots. The environment would deserve a risk assesment, and unless it is nutsy, I'd give it far higher than 50%. If it is that nutsy, more than your amp will suffer.
cheers,
Douglas
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Old 12th June 2010, 08:42 PM   #5
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Alright, before I even THINK about taking this amp to college (well, I guess that might be too late...!), I will check out my dorm situation. I'm in the honor's college, so I hope things are a little less rambunctious over there.

This isn't JUST about making my amp college-proof. It's obviously a learning experience, a new experiment, and a great way to never have to get out my voltmeter again to check the bias (until something goes wrong).

I would still love to implement an autobias circuit in the amp. If I wanted to go with the constant-current fixed bias method (with a bypass cap), what would that do in class AB operation? Is there a better way?

Kyle
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Old 12th June 2010, 08:50 PM   #6
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The first and VITAL step, before you fuss with minor problems, is to beer proof it with a water tight cover.

2nd, if you don't want to take the great advice of lockable screwdriver bias pots, just use plastic shaft pots and attach the B+ to your metal knobs. This will keep fingers clear, at least after the initial attempt to fiddle...

Then #1 really has to be in place!

Regards, Allen
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Old 12th June 2010, 09:13 PM   #7
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Quote:
Originally Posted by antiquekid3 View Post
If I wanted to go with the constant-current fixed bias method (with a bypass cap), what would that do in class AB operation? Is there a better way?
Distort horribly after a few watts.

There's an autobias circuit based on opamps in Morgan Jones's "Valve Amplifiers" that would probably work. But really, you've got about a hundred years of tube amp experience in this thread all saying the same thing- use screwdriver-adjust (and even locked!) trimpots.
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Old 12th June 2010, 09:29 PM   #8
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I'd knock up a nice new mini-amp for college, perhaps basing it on a cheap chinese model. An ST70 seems a bit much in value, size, weight and power for college TBH.
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Old 12th June 2010, 09:34 PM   #9
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There are some circuits discussed by Broskie for AB autobias.
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Old 12th June 2010, 11:25 PM   #10
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I have some pictures of my ST-70, which might help explain the challenge of installing another pot. My ST-70 has a driver board that I made (the circuit is a 6SN7 x3 Blackburn Audio driver) with two extra pots on it. I refuse to drill in my new chassis. Period. Any modifications will be done with existing holes on the chassis or modifying my driver board. Heck, I might even make a whole new driver board and install good pots on it this time.

IMG_0330.jpg

IMG_0331.jpg

IMG_0326.jpg

They preferably need to be 1/8W pots (like I have in there right now). Can I get 1/8W pots that have a screwdriver adjust that mount by means of a nut? The lockdown part is optional, I suppose. I figure most people won't be touching something with a screwdriver adjust anyways, especially if it's right next to a hot 6SN7! In truth, the more flush it is with the chassis, the better. Any suggestions?

Thanks!

Kyle
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