Building a Preamp

[ray bronk]

Hi all,

Ok, I have read the whole thing about building a 12B4 preamp. my question is building it is it just five parts including power supply?

Now, on to my real topic. I would like to build the "GGP" that is in the Audio Reality book. So if we start from the beginning, and get a list of pparts, prices and where to get them that would be helpful. Since I was unable to borrow a "GGP" preamp, I guess I'll just build the darn thing. Now here's the challenge. Sending me pics of schematics and pics of cases and so on will not cut it in this thread. Since I am a person who is blind, I can not see those pics. So you will have to resort to describing the item in question.

I chose this preamp, because the output impedance is way down like 200 Ohms. The tubes are relatively cheap, and it's supposed to sound good. Now there are other preamps out there that also sound good, but the tubes are expensive, and the circuit more complicated. I'm not afraid of a soldering iron or gun.

I have read over and over, that the parts cost of the "GGP" is under 100 bucks, so I have the bucks, so I figured why not build it? If anyone has any ideas of chassis and layout, I'm literally all ears. I use a speech synthesizer and a Screenreader to read to me what is on the windows screen. So let the thread begin. i'm looking forward to seeing where this thread will go. I admit, I will ask some real basic questions. So I hope this won't be just for me, but also for those who like me are just starting out and trying to figure out the hows and whats to build a preamp they can be proud of.

Ray Bronk

[Bandersnatch]

Hi Ray,
What sort of circuit is the GGP?

The 12B4 can easily deliver a 1000 Ohm output impedance, and less than 100 Ohms with a cascode MOSFET CCS employing the mu follower output. I have built a few of these, and supervised several folks who've decided to build it.
cheers,
Douglas

[G]

Originally posted by Bandersnatch
Hi Ray,
What sort of circuit is the GGP?

The 12B4 can easily deliver a 1000 Ohm output impedance, and less than 100 Ohms with a cascode MOSFET CCS employing the mu follower output. I have built a few of these, and supervised several folks who've decided to build it.
cheers,
Douglas

I think he is talking about a grounded grid preamp.

[KP11520]

Hey Ray,

If you want to give it a try, I say go for it!

Here is an eBay seller from Hong Kong that has a few kits. His approval rating is only 93% so be careful but maybe that is for something different than a kit.

He has a GGP kit and the only thing you might need is a transformer and housing but you can an idea for around $200. if this all it should be.

Take a look: http://stores.ebay.com/AnalogMetric_...QQftidZ2QQtZkm

Good luck!

Regards//Keith

[Bandersnatch]

hey-Hey!!!,
I tracked it down, it is indeed the grounded grid pre.

That is a fairly complex design. It runs a cathode follower to drive the low inpedance of the grounded grid stage, and then follows up with another cathode follower to buffer the amplifier triode's plate. I have my doubts about the 12AU7 cathode follower ever reaching 200R of output impedance too.

I'd suggest a far simpler grounded cathode 12B4 circuit. B+ of ~300V, plate load of 10k Ohms, zero signal plate V of ~100V for a 20 mA idle current and about 8 red LED's to act as cathode bias. Run normal intensity red, as the high intensity is not as low impedance.
cheers,
Douglas

[ray bronk]

Hi all,

Yeah, that's Bruce's "Grounded Grid" Preamp found in the Audio reality book. That's the one I am interested in building.

Douglas, Ok, I'm game on the 12B4 with the CCS circuit. I am going to warn you, this will take some hand holding. I will ask some stupid quetstions like which tube pin is the first one. Stuff like that. How many tubes will i need. Any ideas on chassis layout would be helpful. etc. etc. etc. So I really don't care which one I build. I figure if we can keep it in this thread that might help other newbies like myself. So you will have to get granulare in your descriptions. NO PICS!

Because of the power amp, I want to keep the impedance low. That's why I am interested in the CCS side. Besides, I understand from reading the 12B4 thread, it sounds better. So where do we begin.

Keith, not seeing his pics, it almost sounds like he is using just one transformer for the GGP. Bruce uses three of them. I presume one for each tube.


Ray

[Bandersnatch]

Hi Ray,
I can answer questions as fast as you feel like asking them. Heck, you can even call me on the 'phone...

The CCS loaded 12B4 is nice to deal with on a few fronts. We don't need high B+; around 150V will do just fine. Go for 80-90 V on the plate and 20V across the CCS still leaves +/- 30V of signal excursion.

I've found big output caps to work well in these designs. 5 uF polypropylene in oil motor run caps( for example 330VAC/5uF from ASC as sold by Allied Electronics in the oval X387 can is ~$6).

I've built with DC on the heaters. It can keep some of the troubleshooting out of the way if you know it isn't *THAT* source of AC that is giving you fits...
cheers,
Douglas

[ray bronk]

Hi Douglas,

ok, so, from power supply to tubes, parts, etc. what are we looking at for total cost? What about the chassis/case?

Ray

[Bandersnatch]

Originally posted by ray bronk
Hi Douglas,

ok, so, from power supply to tubes, parts, etc. what are we looking at for total cost? What about the chassis/case?

Ray

hey-Hey!!!,
Let's look at this from a functional, but solid point of view. A voltage doubler with a little bit of inductance in the filter will do quite well.

B+ Spec's: 150V with small ripple.

60 VAC will do nicely as a power TX. root2 times this value is ~84V, times two for the doubler yeilds ~170V from ground to the 'top' of the second filter cap. Make them 330 uF/100V then connect to a 330 uF/200V cap with a 1.5 Hy inductor from Hammond. Get two inductors and make a second stage just like it.

Are you familiar with the voltage doubler circuit? In the form I am suggesting it is two half wave rectifiers with their output in series. On one half of the wave it charges the upper cap and on the other it charges the lower cap to root2 times the rms input voltage.

One takes the first two caps and arranges them in series. The AC winding gets one end attached to the node between the two. The other end gets attached to a similar arrangement of diodes, anode to cathode. The lower cap negative end gets connected to the anode end of the diode string, and the positive end of the upper cap to the free cathode end of the diode string( the banded end of the diode ).

Are you with me so far?
cheers,
Douglas

[ray bronk]

Hi Douglas,

Almost with you. Let's start by getting all of the necessary parts, tube(s) case caps, resistors, transistors inductors etc. Once I have all that stuff, then we can figure out how to build it. i never had a chance to play around with various circuits, except a 5 Tube Radio from Greymark 30 years ago. So like I said, I will be starting this from scratch, and will probably in the end be able to write a book with all of the details in how to build a 12B4 Line stage. So first, let's do all of the mechanical work, then the electrical will come later.

You can also call me at 520 444-5514, and we can discuss this from there. Sorry techys, in this game, I am merely a beginner.

Why did you choose a 5Uf cap? What about the 1Uf or 2Uf cap? just curious?

It's been years since I have looked at a power supply circuit. So you will have to get a bit more exacting than that. I almost followed your description.

Would we be making a diode ring/bridge, or how many diodes are we using here.

You will probably have to draw it out on paper, then do the verbal description. Sorry to say, but as a general statement, most sighted people don't know how to verbally describe things very well. Anyhow, you'll be an old pro before long. Thank you very much for your patience and help.

Ray