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PostPosted: 11 Jun 2011, 18:47 
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I have been wanting to build this schematic of a single stage tube preamp for a long while. I had experiment some time back with running small power transformers back to back to develop 200 to 300V from a 24V AC wall wart. Run the 24V into the secondary of a 240V to 24V tranni to give 240V+ out of the primary. I had the PS built but no amp to go with it. The 24V you can rectified and regulated for filament voltage. This I did.

A friend from the Melbourne Audio club, who had bought some of my gear in the past, rang and asked if I would make a simple tube preamp for his center channel amplifier to warm-up the voices. The ready made PS came to mind which had enough cct. brd. extension on the end to place a 9pin tube socket in and thus a single stage tube preamp. It is well under construction and I will follow with images but for now take a look at the schematic. Two lots of NFB, low gain but hopefully it shoud do the job. Bill, whom I am building it for, also wanted a volume control. This little preamp will become an active attenuator with tube characteristics. It has a gain of about 3. Who knows it may sound better than my current BoZSE??
http://giaime.altervista.org/mypreamp.html
Attachment:
Plate_Follower.gif

NOTE: The version I am building is slightly different to this one but the basics are the same. The PS I have used in other two stage preamps I have built with great success so I know the PS is good


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PostPosted: 11 Jun 2011, 20:42 
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Cct. Brd.is complete. The pot and input cap will be mounted off the cct. brd. Once more no hook-up wire used just component-2-component connections. Yes its a mess but the signal path is short and no PCB to dull the sound. Big problem comes when you try to repair it if required. I'm about to test the cct. brd.
Attachment:
SSTAmpIIss.jpg
Attachment:
SSTAmpIIIss.jpg
.


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PostPosted: 11 Jun 2011, 22:10 
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Testing the volatge levels:

I post my mistakes so hopefully others may not be as silly as me. The heater voltage was bang on 12V. It was regulated so should be. But the HT was zero at the tube. I thought pin 9 was not used so used it as a fixing point for the HT. Yes your are right it is the center point for the filament and one side is grounded. Cutting pin 9 away brought the HT to 250V+ The specs are for 300V HT but 250V will do.

Next was to see the preamp is constipated. Can it pass music?? YES it can. See images where I have hooked it up to my main system and used a portable CD player as the source. Music came through sweet, buzz and hum free. So far so good. I ran it for 5 mins and every component ran cool. No flash of light or buff of smoke and it plays music. Now to the case.
Attachment:
SSTAmpIVss.jpg


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PostPosted: 12 Jun 2011, 01:23 
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Embarressing mistake number two:

The Cct. I have used specifies a 2uf output cap. This seems very large for a tube preamp. Usually the output caps are something like 220nf or 470nf. Anyhow I had 1uf polypropelene caps and mounted these as the output caps. They appeared to work OK then I thought about the voltage rating. On measuring the voltage across the output cap I found 165V. My polies are rated 100V. So no go.

I removed the 1uf polies and replaced with 470nf 630V polies. Hope this cap is big enough. Any comment on the cap size? The cast Al enclosure has been drilled and primed. The tube will stick-out the top. Tomorrow the top coat. I'm thinking beaten copper finish.

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PostPosted: 12 Jun 2011, 03:00 
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The wife's out tongiht so I have hooked the preamp upto my main system for a big listen. In the cct. I am using (slighly different to the one I have posted) there was no cathode bypass cap. The preamp was really down on gain. So I have added a 220uf 16V low ESR bypass cap. The gain has lifted.

The preamp sounds extremely good. Though gain is low. Bass extends down past 30hz with good audible bass down around 20hz (cones a-flap'n). Good mids and highs. Siblance controlled. The preamp is dead quiet, on or off makes no diffrence to the background quietness.

I have some nice old 12AU7s to play with. The one I have in has been in an American organ for about 50 years it has CONN in green on the tube-CONN Organ company. I would imagine it is one of the old oscillator tubes. The other nice tube I have is an Australian made Phillips again very second-hand and last a Zaerix. It is in a green and black box and has Z & I Aero Services Ltd. - London W.2, England maked on the box. I understand these tubes are actually East German made tubes. I also have read where they were re-branded Mullards??!!??

All tubes test 80 on emissions or better. All are pretty relaxed. And listening to this preamp, so am I

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PostPosted: 12 Jun 2011, 07:09 
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Hi,

I've built a few like that. They sound lovely! :D

Cheers!

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PostPosted: 12 Jun 2011, 10:08 
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What about the output cap? Is 470nf OK? I got to listen for 6 hours straight and all is good. The preamp appears to be flat across the whole spectrum of frequencies. BoZ seems to emphasise the mids a little more, not that I dislike it. I didn't tube roll but when it is in a case I will. Unfortunately I have a buzzy wall wart. I might by another to see if this is just a bad one. If not I will have to make my own wart.

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PostPosted: 12 Jun 2011, 10:39 
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Hi Mark,

With 0.47uF I am calculating a F3 of about 1Hz so you are fine with that size.

Giaime used to have a Plate Follower webpage which had a table of values for different tubes and the passive part values for the plate follower circuit, but I can't find it anymore. What changes did you make to the circuit?

Cheers

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PostPosted: 12 Jun 2011, 18:03 
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Mark, can you please post the schematic with your modifications, as well as a linear power supply?
I want to build this.

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PostPosted: 12 Jun 2011, 18:17 
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Gio: Thanks for the calcs. I can't imagine why you would want a 2.2uf cap on the output. I will have to re-draw the cct. variation I have because I can find it on the net. But originally there was no cathode bypass cap but I added a 220uf 16V low ESR and the HT is stated at 300V (I'm using 250V). With the one I'm using Rp=20K the FB R from output cap to grid is 330K and my output cap is 470nf. These are the only differences.

Do you think I should use an input cap? If so what? I'm thinking .1uf Russian PIO.

The preamp sounds very good. I'm going to put it on the CRO today. Should have some results later.

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