3A/110B into a LL1680 Preamp - diyAudio
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Old 12th February 2003, 11:51 AM   #1
stigla is offline stigla  Norway
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Default 3A/110B into a LL1680 Preamp

Hello all!

I recently scored a neat supply of some nice STC 3A / 110B DHT triodes!
http://frank.pocnet.net/short/054/0/094.pdf

This triode have a nice rp of ~5,5k and a mu of 12 and a 4V / 0,25A heater!
A perfect candidate for a Transformer coupled, battery heated preamp

So looking about in the Lundahl catalog I noticed the LL1680.
http://www.lundahl.se/pdfs/datash/1680.pdf

A prim.ind. of 210 at 5mA and a 15K/600 (or 150) ohm impedance ratio. Seems perfect for this triode! So I am strongly considering ordering a pair gapped for 8mA. But I thought I'd check with you guys first.

What will the inductance be for a 8mA gapped type? My calculations says I need at least 150H (~10Hz).

If a 8mA type gives to low inductance I will considder making a ParaFeed preamp with a Gary Pimm BBMCCCS. Should I order a "un-gapped" LL1680 then?

Any other suggestions also apprechiated

Stigla
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Old 12th February 2003, 01:54 PM   #2
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Default OPT.

Hi,

Quote:
If a 8mA type gives to low inductance I will considder making a ParaFeed preamp with a Gary Pimm BBMCCCS. Should I order a "un-gapped" LL1680 then?
I feel you could just as well order the gapped model so you can experiment with what suits you best.

Cheers,
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Old 14th February 2003, 07:58 AM   #3
stigla is offline stigla  Norway
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Hmmm, yeah, think I'll do that! thanx!

stigla
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Old 31st March 2003, 02:45 PM   #4
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hi,

if U use the un air-grab pre-amp output must use cap couple.
If not like to use the cap couple. choose a preamp output trans that can pass current. 10~15ma is safe.
too low will easy to cause saturation of core will weak the bass.

thansk

Thomas
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Old 31st March 2003, 07:56 PM   #5
EC8010 is offline EC8010  United Kingdom
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Default Is silicon heresy?

If it isn't, you could make a constant current anode load out of a semiconductor cascode using BC558B at the top, and MJE340 at the botton. Then, use the transformer in a parafeed circuit of your choice, and don't worry about gaps. The cascode would also greatly improve the stage's rejection of power supply hum.

Of course, I may have broken a taboo by suggesting semiconductors and directly heated triodes in the same breath. On the other hand, it would certainly be different.
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Old 31st March 2003, 11:09 PM   #6
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To EC8010,

do u know the air gap is physically better than the parafeel design. I agree that parafeel can slove this proble, But still use air gap is better.

thanks

Thomas
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Old 1st April 2003, 05:46 AM   #7
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Default Re: Is silicon heresy?

Quote:
Originally posted by EC8010
Of course, I may have broken a taboo by suggesting semiconductors and directly heated triodes in the same breath. On the other hand, it would certainly be different.
The tube flat earthers may object, but it's nice to have some company.
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Old 1st April 2003, 06:35 AM   #8
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I've always tried to grasp the sonic philosophy of parafeed. Other than allowing the use of puny transformers and still have bass i can't see any advantages. You have to accept the slight rounding of transients every transformer imposes plus the added flavour of a coupling cap. The whole idea of transformer coupling for me is to remove an electronic hash even the best caps impose, so why reintroduce it?
Ditto for the silicon. Maybe a FET CCS can coexist with valves but two bipolars?


cheers

peter
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Old 1st April 2003, 09:18 AM   #9
stigla is offline stigla  Norway
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Hi guys!

Nice to have an old tread back again!

Here's what I did:

I ordered the 8mA gapped types. I have them, and the tubes, and everything! I will use the Easter holiday to put them together.

As implied by fdegrove I thought it would be the wisest thing to do. I will try this normal setting first. Why use a Cap AND a transformer right?

Lundahl stated that 8mA gap will leave me with a Prim. Ind. of ~130H, so I'm a little worried that bass will be a bit weak, but we'll see (one formula I found at Steve Banch's site gave me a -3dB point at ~7 Hz and that would'v been nice if this is right)

If this is not the case, I'll try a Gary Pimm BBMCCS and not use the low impedance output in a Parafeed configuration.

Another thing that cought my attention lately is the fact that the Miller capacitance of the 3B/110B is 133pF!! Thats quite high, and now I'm worried about the HF rolloff as well, since I plan to use a 50k pot. (my CD player needs at least 50k...)

Thanks for your interest guys, I really appreciate it!

stigla
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Old 1st April 2003, 03:26 PM   #10
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Default Re: Re: Is silicon heresy?

Quote:
Originally posted by Brett


The tube flat earthers may object, but it's nice to have some company.
Don't you know that transistors are the work of the ???

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