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Old 09-26-2012, 01:27 PM   #21
GARAGE HERMIT
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Quote:
Originally Posted by LeviMan_2001
I believe wiring the anode to the grid like that will fry some tubes in a hurry... correct me if I'm wrong, but wouldn't that be an oscillator? I've converted an AA5 radio into an amp, people said it couldn't be done/wasn't worth it but I did it anyhow. It was great for practicing when I was in a dorm. But now I'm back home and I haven't run it since...


you mean on the EL84, ? if the link from the anode to the grid is'nt needed, i wont put it in,

i'm a newbie at this tube stuff, dont forget, so i need all the help i can get, i just picked different bit's and piece's from different circuit drawing's and came up with this circuit, i'm using the two 6AT6's instead of a 12AX7 as i have them, so i've just paralled them up, will this be ok, ?? , thank's,
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Old 09-26-2012, 01:28 PM   #22
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Gotcha on the diodes. Just making sure it had a purpose and you didn't think that tube rectifiers needed diodes for some reason.

The way you have it running is not parallel. Most 12ax7s aren't run in parallel (with the exception of almost every Matchless amp). Give me a second and I'll draw up a parallel vs cascaded circuit.
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Old 09-26-2012, 01:32 PM   #23
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Originally Posted by end_citizen
Gotcha on the diodes. Just making sure it had a purpose and you didn't think that tube rectifiers needed diodes for some reason.

The way you have it running is not parallel. Most 12ax7s aren't run in parallel (with the exception of almost every Matchless amp). Give me a second and I'll draw up a parallel vs cascaded circuit.


the two 6AT6's are taking the place of one 12AX7,
a 6AT6 is half a 12AX7, that's why they are in parallel, or have i wired the 6AT6's wrong,
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Old 09-26-2012, 01:43 PM   #24
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Here is a basic comparison of cascaded vs parallel.


I'm not sure where he is saying you have a grid tied to an anode.

EDIT:
Quote:
Originally Posted by GARAGE HERMIT
the two 6AT6's are taking the place of one 12AX7,
a 6AT6 is half a 12AX7, that's why they are in parallel, or have i wired the 6AT6's wrong,


You indeed have them wrong. You have them as if you want them both cascaded and paralleled.
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Last edited by end_citizen : 09-26-2012 at 01:45 PM.
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Old 09-26-2012, 01:52 PM   #25
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Originally Posted by end_citizen

I'm not sure where he is saying you have a grid tied to an anode.



he's talking about the EL84, where i have the grid linked to the CATHODE,
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Old 09-26-2012, 01:56 PM   #26
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Quote:
Originally Posted by end_citizen

EDIT:

You indeed have them wrong. You have them as if you want them both cascaded and paralleled.



ooop's, i'll have to draw it out again, and see if i can get it right, thank's,
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Old 09-26-2012, 02:21 PM   #27
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Quote:
Originally Posted by GARAGE HERMIT
he's talking about the EL84, where i have the grid linked to the CATHODE,


That is the suppressor grid, and it is supposed to be tied to the cathode.

This is for the sake of the tube, but not necessary if you aren't too concerned about tube life: You'll probably want to put in a screen resistor (1k 5w resistor going between High Voltage and the screen grid).
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Old 09-26-2012, 02:28 PM   #28
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Originally Posted by end_citizen
That is the suppressor grid, and it is supposed to be tied to the cathode.

This is for the sake of the tube, but not necessary if you aren't too concerned about tube life: You'll probably want to put in a screen resistor (1k 5w resistor going between High Voltage and the screen grid).


so on the EL84 pinout's i'll put the 1k-5w resistor onto pin's 1 and 8, ?
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Old 09-26-2012, 02:34 PM   #29
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On an EL84, Pins 1 and 8 aren't connected to anything. Here's the pinout (NC=Not Connected):

1: NC
2:Control Grid (Input)
3: Cathode AND Suppressor Grid (You don't have an option. They are always tied together)
4: Heater
5: Heater
6: NC
7: Plate/Anode (Output)
8: NC
9: Screen Grid (Put 1K resistor here)

EDIT: It appears you are using a EL34 pinout, not EL84
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Last edited by end_citizen : 09-26-2012 at 02:35 PM.
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Old 09-26-2012, 02:54 PM   #30
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ooop's, again, you're right, i'll have to be more vigilant,
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Old 10-02-2012, 03:06 PM   #31
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drew another schematic, for paralleling up the 6AT6's, not too sure this one's right either, anyone see any mistake's, please let me know,,


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Old 10-02-2012, 05:14 PM   #32
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I'm still wondering why you want the 6AT6s in parallel. You'll get more volume and gain out of cascaded gain stages. Also, your volume control is certainly not right.

I'd do the preamp more like this (feel free to leave out the tonestack):


The output section looks fine to me. The power supply looks sufficient. The 100k dropping resistor might be a little too high, but that can be fixed later on.

If you want to keep the preamp paralleled, get rid of the capacitor going to the volume control. Move the volume control to where the 330k resistor is going between ground and the EL84 control grid.
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Old 10-03-2012, 05:47 AM   #33
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end citizen,
thank's for the schematic, i did'nt know cascad'ed would be better than parallel, i'll do another schematic and see what you think of it,
looking forward to getting the soldering iron out,
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Old 10-03-2012, 04:49 PM   #34
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here's the Mk3 drawing, hope i'm getting somewhere near to getting it right, lol,

i've read that a grid stopper resistor on the EL84 would be a good idea, would that go on pinout No9 or No2,?

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Old 10-03-2012, 06:02 PM   #35
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Cursory glance at the schematic, preamp is looking good. If you find it has too much gain when you've built it, take out one or both of the 25uF/25v caps on the cathodes. That's for you decide.

Output section: It would work, but you should put the 1K/5w resistor going to the screen back in (Screen resistor). This will improve tube life. (Pin 9)

As for grid resistor, it isn't a bad idea. It prevents unwanted interference from stray capacitance and radio signals. Although that late in the circuit, it isn't a huge problem. Anywhere from 2.2k to 22k should be sufficient, although higher resistance will equal less highs. (Pin 2)
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Old 10-03-2012, 07:00 PM   #36
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another senior moment, forgot to put the 1k/5w resistor in the schematic,

so you think i'm all ok to start soldering,?

really appreciate your help with this,
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Old 10-03-2012, 07:07 PM   #37
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Naw guys, i was talking about the 6AT6s, the old schematic has them capacitor coupled at full volume. but its all good now bein cascaded and whatnot.
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Old 10-03-2012, 07:26 PM   #38
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Originally Posted by GARAGE HERMIT
another senior moment, forgot to put the 1k/5w resistor in the schematic,

so you think i'm all ok to start soldering,?

really appreciate your help with this,


Looks like you're probably good to go. If you come across any problems, I'd be glad to help out.
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Old 10-03-2012, 09:05 PM   #39
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just ordered all the resistor's and capacitor's off ebay, but there were a few value's i could'nt find so i ordered the nearest to them,

capacitor's,
20uf/400v wanted,, ordered 22uf/400v
25uf/25v wanted,, ordered 22uf/63v

resistor's,
1k/5watt wanted,, ordered 1k/3watt,

would these value's still work ok, ?
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Old 10-03-2012, 09:12 PM   #40
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Those should all work. The 1k/5w might be a little bit stronger than needed on an EL84. So, 3W should be fine.

The tolerance on a capacitor is usually about -10%/+20%. So, there is a good chance the values you ordered are pretty close to what you designed.
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