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Old 24th November 2012, 05:48 PM   #111
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The 390 VCT should yield about 260 VDC or so. If you put a FWB on it, you'd get over 500 VDC so this trans should be useful for other builds.
That transformer looks just about right for MELTING some big sweep tubes, so I bought one. I'm sure I can find something to do with it. The seller said it would be good for a guitar amp.....I think he might be right...a really LOUD guitar amp.

The 70 volt line transformer you are planning isn't listed on Ebay right now (listing expired). My experience with line transformers as OPT's has been hit or miss. Many do not heve enough inductance to handle bass. They are designed to be driven from a low impedance constant voltage source. Triode tubes with plate to plate feedback may be the best bet. Remember that the blue wire is the CT, not orange.

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Now for the big asterisk. I own Pete's other red board,
You can sky wire the 9 pin sockets just above the board. I did this to run some bigger 9 pin tubes.

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I went back to the KT88 trioded......Getting 0.06% THD @ 1 watt and clipping around 28 watts (5k load).
I remember getting about 50 WPC from the KT88 in triode, but I don't remember the load. Probably 3300 or 2500 ohms.

Got to go back to mowing the yard.....we have to do that all year round here, but I also get to jump in the pool! Sherri is in West Virginia. It is snowing there. I can't repeat the words I got when I texted her a picture from the swimming pool. May get to melt some tubes tonight.
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Old 24th November 2012, 06:40 PM   #112
pso is offline pso  Spain
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Hi Pete,

Seems not to have ruled out the KT88-trioded, I'm really interested in your PCB, not need the exact values of the resistors, only in some points measures (mA, Volts) and some specific part (LED), I understand the scheme, but neither would I go only for that configuration.

I suffice with 30W in triode mode and here seems to opt for more than 100, much more...

But it seems that your prototype sound good, if it continues to develop, I will buy your PCB's.

Even was asking coucil to order a custom transformer (EDCOR), but will wait if finally just the prototype, and like the results, to order one to your specifications.

I build a SSE of Tubelab, but I want to have your boards.

I'll be waiting for news.
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Old 24th November 2012, 06:41 PM   #113
tomlang is offline tomlang  United States
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Quote:
Originally Posted by tubelab.com View Post
The 70 volt line transformer you are planning isn't listed on Ebay right now (listing expired). My experience with line transformers as OPT's has been hit or miss. Many do not heve enough inductance to handle bass. They are designed to be driven from a low impedance constant voltage source. Triode tubes with plate to plate feedback may be the best bet. Remember that the blue wire is the CT, not orange.
Good reminder on the CT. Like I said I have a few here right now. I measured 3H across the primary p-p. I measured another OPT I had here, about twice as large and got 8H. So we will see, will be an interesting experiment for cheapskates.
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Old 25th November 2012, 02:36 AM   #114
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I want a 813 amp now. It's not about power, it's just that gorgeous looking tube.
Good thing it's not about power.....cause you aren't going to get much. I Googled a bit and everything I read said you need at least 850 volts to get these tubes going, and thats in triode, you need over 1000 in pentode.

My biggest power supply only goes to 650 volts, and I had it maxed. I was getting 35 watts in triode with all feedback removed. Clipping set in when the capacitively coupled driver attempted to drive the 813's grid positive. That big grid is like a clamp diode across the output of the driver. What we need here is a mosfet driver and some more voltage.....lots more voltage.

Well at least it looks cool, and lights up the room.
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Old 25th November 2012, 02:40 PM   #115
wicked1 is offline wicked1  United States
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Originally Posted by pmillett View Post
Don't give up yet... indecision is starting to get to me!
I'm thinking 250V for the driver and 500V for the KT88 would be a nice operating point.
Hmmm....
Need more power! (Ok, need is definitely the wrong word there).
And, does the world need another kt88 design?

Last edited by wicked1; 25th November 2012 at 02:42 PM.
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Old 25th November 2012, 04:54 PM   #116
Cassiel is offline Cassiel  Libya
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Originally Posted by tubelab.com View Post
Good thing it's not about power.....cause you aren't going to get much.

.
I'd be happy with 5 watts. Triode.

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I Googled a bit and everything I read said you need at least 850 volts to get these tubes going, and thats in triode,
Scary volts, I was thinking about 500. Too young to die.

I'm going to sim the circuit in SE CAD and report back. Thanks for the info.
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Last edited by Cassiel; 25th November 2012 at 04:59 PM. Reason: 813 curves.
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Old 25th November 2012, 06:01 PM   #117
Cassiel is offline Cassiel  Libya
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Damn, that tube is not in the library. OK, this is Pete's place anyway soooooooo..... this is my last interruption to an otherwise nice thread.

Just for giggles I did it with the 845. Look at those numbers..... lovely, aren't they? Too bad it's only one watt haha, that wouldn't stop me though what stops me is the fact that I'm not in love with the 845. 813 is my babe. I will build it even if it ends up being a super Champ. See ya.
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Old 25th November 2012, 06:54 PM   #118
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Scary volts, I was thinking about 500. Too young to die.....I'd be happy with 5 watts. Triode.
Lets see.....100 watts of heater power for 5 watts of audio.....The GREEN POLICE are watching you I was getting 30 watts on 650 volts, and about 24 when I was using the 550 volt supply. Cranking the bias up to 200 mA per tube (class A) actually lost some power since the lower bias voltage meant that the grid went positive earlier. I am sure that more power can be had with A2 or AB2, but that will have to wait until I have a mosfet driver board.

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Just for giggles I did it with the 845.
Maybe 8 years ago I got that simulator. I plugged in 845, 5K load, A2 with as much drive as the tube can eat, and 1100 volts of B+. SE cad said 42 watts of power, which nobody including me believed. I built it and the simulator was right, I get 42 watts.

Remembering the "stuff" scattered all over the house, I decided to take several boxes outside and sort tubes today. I found a pair of really BIG Eimac triodes (3-200A3) and I was tempted to bring them inside and play, but they NEED A2 and want about 2500 plate volts. A pair will make only 750 watts.

I have found lots of tasty sweep tubes to melt.....OH and a shiney Mullard GZ34.

Going back outside....later.
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Old 25th November 2012, 07:32 PM   #119
Cassiel is offline Cassiel  Libya
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Lets see.....100 watts of heater power for 5 watts of audio.....The GREEN POLICE are watching you
Haha, I know it's crazy but who cares. Not me.
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Old 25th November 2012, 08:16 PM   #120
jrenkin is offline jrenkin  United States
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Gotta say, I love it when the thread talks kilovolts and shows alligator clip set ups! You guys got nerve! And no young children, I suppose. Watch where you point your fingers...
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