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Old 25th September 2012, 11:00 AM   #1551
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I would like to see (hear?) what you did. I am on another forum where a student wanted to analyze make a Marshall amp as a thesis for school but they only allow up to 40V. I suggested and tried a couple of 6AU6's at 40V and found they sounded much better than 12A*7 tubes in low voltage starved plate. Plan to get back to it soon.
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Old 25th September 2012, 10:54 PM   #1552
Cassiel is offline Cassiel  Libya
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Well, it works. And it's very quiet - no hum and no microphonic noises. I reckon gain is around 30. It's not enough to drive a two stage SE audio amp into heavy distortion. However, in front of my Champ this thing does Metal. I haven't played much but distortion is fuzzy, ear-piercing.... kinda fun but I wouldn't say tube-like. Warm, it is not. Anyway, early days, random values and no tweaking yet. Plate resistor is 100K, screen resistor is 1K and cathode resistor is 2.7K.

Jazbo8 - I read that but I think you'd run into problems with a high value plate resistor. The trick with pentodes is not to aim for too much gain, otherwise microphonics becomes a huge problem.

Printer - It doesn't sound like an 12AX7, that's for sure. But, as I said before, I've only played like 10 minutes. With the Champ volume knob on 2 I get very heavy overdrive. On 10, this thing is crazy, I'm not into metal anyway.

Oh, I left the heater supply floating and B+ goes directly to the plate resistor. No additional filtering. Very quiet. Hmmm.
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Old 25th September 2012, 11:21 PM   #1553
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Originally Posted by Cassiel View Post
Jazbo8 - I read that but I think you'd run into problems with a high value plate resistor. The trick with pentodes is not to aim for too much gain, otherwise microphonics becomes a huge problem.
Makes sense, with the high plate resistors, the pentode will have too much gain at low current = crazy distortion.

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Old 26th September 2012, 02:33 AM   #1554
Cassiel is offline Cassiel  Libya
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Quote:
the pentode will have too much gain at low current = crazy distortion.
It's not about crazy distortion, I like that. I like gain. The problem is that at high gain the pentode picks up all kinds of noises.


Well....I have been playing for two hours and I have to retract my comments of not sounding tube-like. I was feeling guilty for saying that. I bypassed the first stage of my Champ and played with this pentode preamp then back to the 6SL7. Just playing with a slight overdrive. Totally different tone, I can't decide which one I like best. I like both. I thought this was going to be some kind of a joke, a pastime, an experiment; but it sounds much better than it should.

I have a lot of stuff to do so this is my last comment about this preamp. Won't tweak the circuit, not anytime soon anyway. Pretty busy for the next couple of months.
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Old 26th September 2012, 08:56 AM   #1555
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My favourite pentode 6AK5 is not at all hungry for volts.
Picked up a Peavey Royal 8 at the 2nd hand shop nice and cheap. Had to "improve" it of course. One input jack is the standard input, and the other feeds a triode strapped 6AK5 (gain of 20) in front of the 12AX7. I took the bypass cap off the 2nd triode to manage the gain structure.

No more playing for now. Moving house from Australia to New Zealand.
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Old 13th October 2012, 04:22 AM   #1556
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So I am trying an alternative board material, got some eyelets from a craft store and put them in some kitchen top laminate. Seems like it will work.


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Old 13th October 2012, 12:36 PM   #1557
Trout is offline Trout  United States
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Originally Posted by Printer2 View Post
So I am trying an alternative board material, got some eyelets from a craft store and put them in some kitchen top laminate. Seems like it will work.


Click the image to open in full size.

It does work, I have done that at least 15 times.

If you check around with the local cabinet shops they often carry what we call color core laminate. It is a tiny bit thicker and comes in plain/solid colors.
It is not as hard to drill.

I got a bunch free, scrapes, and leftovers they were about to throw out.
ColorCore - Formica Group
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Old 13th October 2012, 01:09 PM   #1558
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It does work, I have done that at least 15 times.

If you check around with the local cabinet shops they often carry what we call color core laminate. It is a tiny bit thicker and comes in plain/solid colors.
It is not as hard to drill.

I got a bunch free, scrapes, and leftovers they were about to throw out.
ColorCore - Formica Group

Cool. I originally glued two pieces together and it was really stiff but my eyelets were a little short to go through them both. With the six mounting bolts I don't think I will have a problem but it is good to know there is some thicker material. Can't beat free either, I was going to ask our carpentry department if they had some scraps and before I got to their shop I saw a bunch in their garbage can. I found it drilled easy enough but you just had to take care the bit does not walk away from the intended spot. (Lack of focus on the bit's part)

I put some eyelets in the mounting bolt holes, not sure if it is necessary but I did not want to accidental crack the board. This seems to be a easy and quick way to get a circuit built up. I like it.
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Old 18th October 2012, 01:33 AM   #1559
Cassiel is offline Cassiel  Libya
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It's finished and I'm very happy with it. Thick, meaty distortion. It's called 24K for reasons I don't need to explain. I don't like jewelry looks but I lit up a joint and this was the unfortunate result. Haha. No worries, just for fun.

* Yes, photo no good. Never proud of the stuff I build so why bother.
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Old 23rd February 2013, 01:09 AM   #1560
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Originally Posted by Xmtrs View Post
"The amplifier must be reasonably safe. No line-powered (non-isolated) designs will be considered"... Why, if it's a safe line-powered design and the chassis is intended un-exposed?
Before even considering the serious safety issues surrounding non-isolated circuits, these designs are illegal in many parts of the world.

Why place your great-grandson at risk of electrocution, to save the minimal expense of an isolation transformer?
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