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Old 7th October 2003, 08:01 PM   #11
Duo is offline Duo  Canada
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Hey Tim, never thought you'd try using a transistor!

How is it? Or did you steal some design parameters from someone
else and add it to your own tube output?

I have a few suggestions... Here goes.

First off, I would imagine you'd get a way nicer amp by
using a tube for the input stages and transistors for the output.
I can blame you for choosing any specific arrangement though, since you are the designer and it's your choice.

I must ask, how does it sound? I think the thermal stablility
will be a bit absurd with the transistors, perhaps we need
a CCS if possible to keep things a little more concentric, I'm not sure.

Also, I imagine you'd get more "t00bey" sound if you use
a tube input, since the tube will be imposing it's qualities on
the most important part of the amplification, the voltage gain.
The transistor should be relatively transparent in the output stage if used properly in class A with a CCS arrangement. Maybe I could give you some design ideas for stable transitors?

These are just my suggestions so please don't take them personally. I know you're into tube sound, so maybe we can make it more authentic in traites?

Anyway, I'm amazed that you tried a transistor design, good work!
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Old 7th October 2003, 08:30 PM   #12
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It was a cheezy breadboarded design to see how much power I could squeeze out of the relatively high-current-capable 12B4. Something like 200mA in that circuit?

I used transistors because, little as I know about them, I figured I could hack something together that'd drive the tube adequately. Obviously the 330k base resistor for instance, was selected and probably will not work again in practice.

And hmm... welcome to the tubes forum! It's about damn time

Tim
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Old 7th October 2003, 09:39 PM   #13
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Default Re: SE

Quote:
Originally posted by 316a
Hi Joe ,
If your speakers are that sensitive and you're new to power amps why not consider a small triode such as 12B4a ? They're cheap , plentiful and linear . A decent sounding 1.5watt amp can be made with one of these using a 5k output transformer and a single driver valve such as 6SL7 , 5755 or 5751 (1 section per channel) . The next step would be 2A3's but this would be far more expensive !
Yep, very nice in PP with a CCS 6N1P driving them. I threw a pair together as a breadboard experiment and was really surprised at how good they were. Not quite enough power for the Khorns when I wanted to rock out, but should do fine driving my mid/high horns if I biamp.

Joe,
the 12ax7 doesn't drive a KT88 (trioded) very well in grounded cathode. Might do better as an SRPP, but I suspect it'll still be a tone control.
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Old 8th October 2003, 12:19 AM   #14
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Hi,

Quote:
the 12ax7 doesn't drive a KT88 (trioded) very well in grounded cathode. Might do better as an SRPP, but I suspect it'll still be a tone control
A 12AX7A in SRPP does NOT have the same frequency response as a simple anode follower using the same tube.

For OPT coupled amps with a falling response from 30KHz and up, having a Mhz input is not going to help much bar, perhaps, phase response.

Other than that, there are Miller compensation tricks and topologies that can cancel it out.

Certainly, if it's a very wideband amp you're after it probaly won't cut it...

To me, wideband isn't the target, it's welcome though, but phase response at any given output power is; the proof is in the humble 2A3 PP amps I drive a two-way speaker with.
The endresult is musical and those speakers just disappear given a half decent source.

Now, tell me if that's not a good thing to have??

Oh...to add insult to injury: that PP pair of 2A3s is driven by the humble 12AX7A with aplomb and bandwidth is only limited by the OPTs.

It'll be better off with a 6SL7? Probably. Unfortunately I only have one life.

It's a grey world outside...it really is.
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Old 8th October 2003, 12:51 AM   #15
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Ok I will narrow it down to either a PP 12B4a or a PP 2A3. What the heck I need both don't I? You can never have enough amps right?

Tim

I appreciate the fine schematic you posted, I would like to rock a little harder than what that baby can offer.

Frank

Your teasing me again talking about the 2A3's. This has got to be one of my weaknesses. Nothing against the 300B but the 2A3 has a distinct sound. Point me to a good PP 2A3 design will you?

Well everyone has convinced me to trash the 12AX7 design I was thinking of.

Joe
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Old 8th October 2003, 01:17 AM   #16
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Hi,

Quote:
Nothing against the 300B but the 2A3 has a distinct sound.
Every single tube has a distinct sound.

I'd say nothing against a 2A3 but the 300B has a distinct sound...same same but different...except the price?

Now I don't want to discuss design magic but you actually can make one sound like the other...something I'd like to keep for post 8.000....

Quote:
Point me to a good PP 2A3 design will you?
And what if it uses a 12AX7A? Now I'm really teasing...just imagine: 1 12AX7 as input + splitter and than a PP 2A3.

No global feedback, no bypass caps...doesn't that sound like a wet dream?

Oh, and it does kick serious but too....

Clevor Trevor,
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Old 8th October 2003, 02:39 AM   #17
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A 12AX7? NOW I'm really confused!



Joe
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Old 8th October 2003, 03:16 AM   #18
SY is offline SY  United States
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I don't understand the prejudice against the 12AX7 used in appropriate circuits. But I don't understand a lot of things.
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Old 8th October 2003, 03:31 AM   #19
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Quote:
Originally posted by burnedfingers

A 12AX7? NOW I'm really confused!
Ditto! I thought you absolutely had to use a 6SN7 for every damned thing, from driving the hallowed 300B to even.. another 6SN7!

Tim
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Old 8th October 2003, 08:30 AM   #20
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Default 12AX7

Quote:
Originally posted by Sch3mat1c


Ditto! I thought you absolutely had to use a 6SN7 for every damned thing, from driving the hallowed 300B to even.. another 6SN7!

Tim
If you knew the prices I get for Mullard 12AX7 you wouldn't use them either ! Personally I think 12AX7's belong in non-DIY equipment such as vintage and guitar amps . I've used the things a few times and put it like this : the projects were never keepers with the things in . It's pretty simple with 6SN7 , it's a decent sounding valve ! You can't go far wrong

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