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Old 10th November 2009, 11:48 AM   #141
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Quote:
Originally Posted by Richard View Post
Hi Rod,

Thank you once again for the excellent advice, I should be able to squeeze it into the preamp chassis.

Do you think it would be beneficial to mount all the parts for this circuit (except the big cap) in a small sealed aluminium diecast box (also using it as a heatsink) and have leads out for the tube filament and voltage in?

As for cathode resistor position, I’m using the centre tap of 2, 20ohm resistors. Looking at a lot of the data sheets I’m at a loss as to which filament pin is the +and the -. Can anyone enlighten me?

Arnulf, It’s a pity you may have a bad batch of LM350’s. I just measured the mounting tabs thickness of my Fairchild LM 350T at 1.34mm and the LT1086 at 1.26mm. Perhaps the T version is a bit more robust.

Cheers,

Rich
Rich, sometimes the valve data sheet indicates a +ve/-ve filament terminal, others not.

If not indication given, the negative is the one that returns to the filament -ve supply. connect this one to the cathode resistor (and capacitor, if used).

I would try both terminals and see if the sound changes!

Don't use a centre-tapped resistor for CCS heated amps. The resistor will rob current, and may degrade the sound too.
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Old 10th November 2009, 11:51 AM   #142
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I would be careful of sealing a 1A transformer in a box. may overheat

I prefer the transformer AND rectifier/caps away from the whole amp, since there are wide-bandwidth magnetic field pulses around from the cap charges and recovery pulses. These are not easy to shield without thick (12mm Alu) chunks of metal.
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Old 10th November 2009, 11:59 AM   #143
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but LTspice won't simulate without a ground
Rod,

I am fully aware of that. Was more thinking of the others in this thread, not being LTspice users.

Isnīt 5,5V a little lean?

We did a std heater-CCS for 6B4G a while ago and found that good regulation didnīt, for some reason, occur for the 1084 until it had in the ballpark of 5V including resistor over it.

Also simmed the regulatorwith added ripple and it pointed in the same direction.

Donīt know how much your MOSFET needs over it, but this points to a total of maybe 10V?.

Must sim when I get home.
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Old 10th November 2009, 12:15 PM   #144
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Originally Posted by revintage View Post
Rod,


Isnīt 5,5V a little lean?

We did a std heater-CCS for 6B4G a while ago and found that good regulation didnīt, for some reason, occur for the 1084 until it had in the ballpark of 5V including resistor over it.

Also simmed the regulatorwith added ripple and it pointed in the same direction.

Donīt know how much your MOSFET needs over it, but this points to a total of maybe 10V?.

Must sim when I get home.
Lars, You may be right. If the chip is not getting adequate voltage headrooom, its current source characteristic will be degraded.

At 1A, ordinary FETs will lose 4 to 5V, "logic level" types 2,5 to 3V


10V is probably a good idea.
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Old 10th November 2009, 12:43 PM   #145
Richard is offline Richard  Australia
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Thanks Rod, I'll take those resistors out.

Quote:
I would be careful of sealing a 1A transformer in a box. may overheat
Sorry, I may not have been clear with my question, I was referring to just the parts of the gyrator (the 1086, fet, resistors and 5uF cap). I have the transformer and big caps about a meter away.

Rich out
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Old 10th November 2009, 12:50 PM   #146
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Rod,
Home at my lunch-hour simming when the rice is getting cooked.

Have tried some randomly chosen MOSFETs from the LTspice library. Most of them are over 5.5V!

So if this is correct probably 10-12V is needed.

Will try to find a model of the one you recommended.

Anyway great idea with the gyrator and current sink!
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Old 10th November 2009, 01:19 PM   #147
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Rod,
Found the STP55..... It sims with just below 2V over it. Got the model from STM and checking their datasheet it seems right. Can you confirm this is correct IRL? Then only 8V would be needed.
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Old 10th November 2009, 02:20 PM   #148
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STP55NF06L should give 2V drop typical. Allow 2,5V to cover variations between samples!
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Old 10th November 2009, 02:21 PM   #149
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Originally Posted by Richard View Post
Thanks Rod, I'll take those resistors out.



Sorry, I may not have been clear with my question, I was referring to just the parts of the gyrator (the 1086, fet, resistors and 5uF cap). I have the transformer and big caps about a meter away.

Rich out
Rich, can box the Gyr/CCS if preferred. I haven't tried this.

1m clearance for the trafo/rectifier/caps should be perfect.
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Old 10th November 2009, 02:43 PM   #150
Arnulf is offline Arnulf  Europe
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Arnulf, It’s a pity you may have a bad batch of LM350’s. I just measured the mounting tabs thickness of my Fairchild LM 350T at 1.34mm and the LT1086 at 1.26mm. Perhaps the T version is a bit more robust.
I checked the part once again just to make sure: it is a LM350T with F logo and J43 on the right hand side of the logo. I believe this is Farnell item #1417673 although I don't have the receipt anywhere handy. I'm beginning to wonder if this is really a Fairchild part, regardless of what Farnell claims - Fairchild's datasheet specifies tab width to be more than twice of what I've just measured.

Tab thickness is 0.6 mm (in comparison to 1.36 mm of a Motorola-branded LM337 I had lying around). 1.3-1.4 mm seems the norm for any other TO-220 component I can find.

Anyway, stay clear of the parts with thin tab if you don't want to end up like me
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