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Old 30th January 2003, 01:39 AM   #1
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Default No grid connection

To all,

Ok, I am opening up this subject again because... I found something very intetresting.

On the subject of not needing to connect the grid of a tube in order for it to work, I got some supporters, some who were tactfully saying I was right but wrong, or perhaps just plain old full of it, and one who adamantly said I was an ignoramus, and knew little about anything tubes and electronics.

OK, I recently had the pleasure of building/repairing/modifying two amplifiers from two separate modern kit and amp manufacturers/competitors, Bottlehead and Sun Audio.

Now, I am not promoting or denegrating their products. They are both very good. They are both 2A3 single ended.

They are both direct coupled inputs! That is, the connection from the RCA jack to the tube is a straight wire. The amps worked very well when my CD player was directly connected... pure AC coupled! No DC connection to the cathode except that which anyone who really knows tube technology would tell you, the internal connection.

SO, I guess Doc Bottlehead and the engineers at Sun Audio are also ignoramuses.

Gabe
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Old 30th January 2003, 01:55 AM   #2
SY is offline SY  United States
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I don't know if those guys are ignorami or not, never having heard of these companies, but if there's no provision for some sort of grid-leak resistance, it's a design which is inherently unreliable (not the first time that latter-day tube "designers" have paid no attention to the lessons of the past). My guess is that, despite the ac coupling of your CD player, there's an output resistor to ground following the output cap- otherwise, the player would have horrible turn-on and turn-off bangs. And that resistor is acting as a remote grid leak.

Direct wire connection to the grid, even with a grid-leak resistor to ground, is poor design practice anyway, an invitation to RFI and oscillation. But I've seen highly regarded (not by me) amps selling for 5 figures that had horrific slewing distortion, blew up output tubes on a regular basis, and had power bandwidths orders of magnitude below spec. I guess that's what their "designers" called "musical." So merely making an amp vulnerable to external noise is not the worst error a "designer" can commit.
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Old 30th January 2003, 02:16 AM   #3
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Ok....so....let's go back to the top and tell me exactly WTF we're talking about here?

If you mean no DC resistance to the grid, I've done a little experimentation with that. Not much, mind you.
The jist of it is, the tube will self-bias at a few volts below cathode. But, if the tube's been working hard, it's possible that grid emission may take over and it climbs positive! This makes for a runaway situation, of course.

Tim
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Old 30th January 2003, 02:26 AM   #4
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SY,

Right, I agree. All my amps have grid resistors.

My contention, though, was that it was not necessary to have it for the tube to work. And if you recall, in my argument I did say the amp would be unstable (I guess that is what is considered "musical", since many also desire the 5Y3 rectifier, a tube that changes the B+ by merely looking at it).

BTW, those amps sound very good, FWIW. Also, there are quite a few other "engineers", if you like, who also design their amps with no input grid resistor or coupling cap.

As for the CD player... I will look in it physically. It does have a mute to disconnect output when not playing. But I am pretty sure there is no resistor after the cap. I mean, why put one there if the next stage is usually a preamp with an input resistor or a stepped attenuator anyway (think "bean counters")?

At any rate, this prompted me to use a coupling cap after the preamp I was using, since its output was designed to mate with a power amp that had a coupling cap in it, so there was none in the output. So it was changing the input tube's bias in the power amp. I isolated it with caps.

Sun Audio is not cheap. They are a Japanese company, if that means anything. They use Tamura iron.

Bottlehead is home of the Audio Asylum forum. They tout Magnequest iron.

Gabe
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Old 30th January 2003, 02:33 AM   #5
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Yeah, I took a look at Bottlehead's website after I read your post. 2.5 watts at 5% distortion for $1200. Barnum was right.
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Old 30th January 2003, 04:15 AM   #6
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Quote:
Originally posted by SY
2.5 watts at 5% distortion for $1200. Barnum was right.
This is all you need to know about an amp to judge its sonic merits? You're good, really good!
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Old 30th January 2003, 04:33 AM   #7
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Yeah but at 30mW into your $5,000 107dB horns it only gets .1% THD and it's almost all harmonious 2nd H.! ....

Tim
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Old 30th January 2003, 12:15 PM   #8
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Default LOOK MA,NO GRID LEAKS...

Hi,

Quote:
My contention, though, was that it was not necessary to have it for the tube to work.
Yes...it will work.

If I read you correctly the input valve on the amps you worked on had no gridleak resistors and had cathode bias.
The tube will still bias itself through its' internal capacitance form grid to cathode,no problem there.

But in real life as soon as you connect a CDP or preamp to that it will effectively see the bleeder R at the output of these devices.
This may be anything from a few K to a M ohm.

So your gridleak will be that resistor and will always vary anyway you look at it.
Imagine the manufacturers would have used a 100K gridleak R and you hook up a CDP with a 10K bleed R at the output.
Mow you will have two resistors in // and the end value is going to be smaller than the 10 K,right?

Naturally,if you have another fool at the other end of the stick following the same reasoning and he doesn't use any bleeder at the output of his gear you're going to be in for a few surprises too.

The way I see it: it will work alright,another costfactor is reduced,but it sure is not good practice not to terminate the transmission lines and leave it to the inherently nonlinearity of the tube to determine this.

Cheers,

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Old 30th January 2003, 01:31 PM   #9
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Quote:
Originally posted by jeff mai


This is all you need to know about an amp to judge its sonic merits? You're good, really good!
Not all, but enough. I don't buy homeopathic drugs, wear magnetic insoles, or have my auras read, either.
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Old 30th January 2003, 01:43 PM   #10
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Quote:
I don't buy homeopathic drugs, wear magnetic insoles, or have my auras read, either.
He he...me neither..

I don't believe in luck either. There's no such thing! It's a method of diverting blame.
I told this to a colleague who was a bit depressed due to his "bad luck". Unfortunately I never saw him again, because he died sitting in his car in the carpark.

Life's odd, but not irrational.

And, UFO's are "unidentified" Otherwise they're something else!

Cheers,
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