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Old 9th November 2009, 09:20 PM   #121
Arnulf is offline Arnulf  Europe
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Quote:
Originally Posted by andyjevans View Post
Hi Rod,

Can you explain your circuit for the electronically faint hearted. I'm only used to the LM1084 with adjust to the positive voltage, and I don't understand why R2 isn't 1.2R. I don't understand R1 at all!
I'm not Rod but R1 is the (simulated) tube filament, it even says so in the comment, ADJ is connected properly to the ground but R2 doesn't make any sense to me either (should be 1.2R IMO).
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Old 9th November 2009, 10:45 PM   #122
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hi Andy - Yes, the circuit is an LTspice schematic - modelling the currents & voltages through R1, which represents the filament. And yes, R2 should be 1.2 ohm - I was trying diffeent values to get low voltage operation & forgot to switch it back!

The filament does not behave exactly like a resistor, of course... we have to keep this in mind when using simulation tools. But I thoroughly recommend LTspice for all of these kind of circuits - so quick to try out things you'd never get round to doing with the soldering iron. And it's free, and is designed for Linux use (via WINE)... DIYer's dream.
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Old 9th November 2009, 11:20 PM   #123
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I was reading V1 on the right as "Valve 1" so thought it WAS the filament. This is the DC input presumably (like 6v or something), and R1 is the filament? I'm really crap at solid state. I haven't seen the output of the LM1084 taken from the input pin - just unfamiliar with all this solid state stuff!

andy
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Old 9th November 2009, 11:29 PM   #124
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I hadn't thought that V1 does look a bit like a valve!

But yes, V1 is the dc supply, and R1 is the 26's filament.

The LT1084 is trying to get 1.2V across the out-adj pins, so long as its input is positive.
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Old 10th November 2009, 12:14 AM   #125
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Where would you connect the cathode resistor - LM1084 side or the other side or does it matter?

andy
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Old 10th November 2009, 04:22 AM   #126
Richard is offline Richard  Australia
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Rod,

Thank you for sharing that modified simpler circuit, I just have a few of queries.
Can one use a LM 350 or LT1086 instead?
Also can your circuit be placed in an outboard filament power supply or does it need to be placed near the tube? Finally would a 1.5A 7806 pre-regulator be sufficient.

Cheers,

Richard
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Old 10th November 2009, 08:17 AM   #127
CFT is offline CFT  United States
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Fantastic Rod, thanks so much! This will help make a lot of us a happy camper
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Old 10th November 2009, 08:44 AM   #128
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Quote:
Originally Posted by andyjevans View Post
Where would you connect the cathode resistor - LM1084 side or the other side or does it matter?

andy
Western Electric and recent designers on this forum have all used the negative side for the cathode resistor. The sound may suffer if you get it wrong.

I use two resistors for my 300B design (see 'new DHT heater' link above). They are calculated to give a voltage drop across each that is equal to the filament voltage.

Probably easiest to stick to the single R mounted on the -ve.
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Old 10th November 2009, 08:57 AM   #129
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Quote:
Originally Posted by Richard View Post
Rod,

Thank you for sharing that modified simpler circuit, I just have a few of queries.
Can one use a LM 350 or LT1086 instead?
Also can your circuit be placed in an outboard filament power supply or does it need to be placed near the tube? Finally would a 1.5A 7806 pre-regulator be sufficient.

Cheers,

Richard
hello Richard

The first thing to look at on the data sheet of these 3-terminal regs is the Noise - usually expressed as % of Vout.

LM350 offers 0.001 % vs LT1086 0.003% (typ) 10Hz..10kHz (bandwidth must be same for comparisons).

So it may well be a better part for the job... but only maybe. The dynamic behaviour of the feedback loop (internal to the chip) is very important too.

So LT1084/6 or LM350 meet the functional requirements, provided your heatsink is big enough, but you have to listen to them to be sure.
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Old 10th November 2009, 09:00 AM   #130
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O, and I mount the power transformer and filter caps >2m from the amp (this is a low impedance circuit). Mount the CCS/gyrator right near the valve since it's a high impedance circuit, and may suffer pickup.

I think Andy has done some work on location of the modules for this kind of design
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