I want to build a preamp for my dad.. thoughts?

[240z4u]

I would like to build a DIY preamplifier for my dad. He was using a carver preamp that blew up and needs a replacement.

I have a pair of 12b4s, and a set of 3 6sn7 tung-sols. I have 4 octal sockets and various other stuff from equipment I have stripped.

Can someone give me a suggestion as to which way to go?

I am no genious when it comes to building. I do understand circuts better then I did. I did a pair of Dynaco MKIII monoblocks recently. Something not overly complex would be good.

I am trying to keep a parts budget around 200 bucks if possible.

Thanks guys - Evan

 

[Eli Duttman]

Evan,

If all dad uses as a source is a CDP, it's "a piece of cake". Those 12B4s WILL do the job. OTOH, if dad has (as I suspect) a collection of vinyl, you'll have to construct a phono preamp too.

I've attached a schematic for a 12B4 based line stage. You don't have to get quite that fancy with the parts to get excellent results.

BTW, the interlocked circle symbol stands for a constant current source (CCS).



Edit: added CCS remark.

 

[gerryc]

If you have access to the March 2001 issue of AudioXpress magazine (or get a back issue), there is an article by Joseph Norwood Still entitled "High-Quality ($180) Control Unit". It should do all you want it to do. It uses different tube types than you have, but if cost is true to the title, it's within your $200 budget. It uses one 12AX7 (both sections paralleled), one 12AT7, and one 5687 (both sections paralleled) per channel. It includes an RIAA phono stage, switching for various line level inputs (CD player, tuner, etc) and a "Walkman" input. It doesn't have bass and treble controls though, if that's important.

 

[240z4u]

This is going to go in my dads bedroom system so phono stage is not 100% needed.

The 12b4 line stage is exactly what I had in mind, I had bought tubes to build one about a year ago and never followed through.

The other one from audioxpress looks like something I need to build for myself!!!

Thanks fellas - Evan

 

[240z4u]

Regarding a transformer..

Most I have been seeing offer 120v out and 6.3v out.

I notice this one you specify has 120/12/12 vac out. I found one at allied but it seems too small.

Any suggestions?

Here is the allied unit

http://www.alliedelec.com/Search/Pr...4&DESC=P-8364&R=928-0018&sid=470973003AE6617F

Evan

 

[gerryc]

You can try the Hammond P-T261G6:=: $31 at Antique Electronic Supply www.tubesandmore.com

Secondary: 250 VCT, 130 mA
Filament winding: 6.3 V, 2 A
Mounting centers: 3.56" u-bracket
Weight: 2 lbs.

Should be more than you need. Just use a standard full wave rectifier using the center tap. Same as a bridge on a 125v secondary.

 

[240z4u]

Thanks Gerry.

I actually have a transformer at home I forgot about. I should check the voltages on that.

Evan

 

[gerryc]

Ya' know, I forgot you're using 12B4's with a 12 volt filament. My bad. Here's an easy way. Two back to back 12.6 volt transformers, 12.6 volt windings connected together and tap off for the filaments to the 12B4's. 120v in, 120volt out to your bridge rectifier. This is exactly the arrangement used in the AudioXpress article as well. You can get 12.6v 3A transformers from Radio Shack, Catalog #: 273-1511, $10.49 each.

 

[240z4u]

Very interesting way to go about that!

I wonder if Rat Shack has them in stock.


Thanks alot! - Evan

 

[Eli Duttman]

Evan,

Stop the head scratching about power trafos. Allied Electronics has exactly what you need.

$11.41 buys stock # 967-9502, which is a Triad N-68X 50 VA isolation trafo. That COMFORTABLY takes care of the B+ requirement.

$8.28 buys stock # 967-2438, which is a Triad VPL24-400 filament trafo with 2X 12 VAC/410 mA. windings. Energizing the 12B4 heaters is WELL taken care of.

BTW, there is GOOD reason to stick with bridge rectified "120" VAC. 600 PIV Schottky diodes are less expensive than the 1200 PIV parts a 120-0-120 FWCT arrangement would use, in order to retain a nice margin of safety.

 

[240z4u]

Thanks a ton Eli.

I have been digging through my spare parts looking for the parts I will need.

I found a suitable chassis and it is still loaded with RCA jacks which is nice.

This project shouln't be too bad!

Thanks guys! - Evan

 

[gerryc]

Let us know how the project turns out, Evan! Maybe post a few pics.

 

[240z4u]

I definitely will.

The first thing I built was my dynaco MKIII monoblocks. I learned quite a bit. Granted they were not a "scratch build".

I attempted to learn this stuff by reading books first and just never quite got it. I do better learning as I go. It can be long and painful LOL.

Thanks guys, here is a photo of my MKIIIs after completion.

Evan

 

[240z4u]

I just found the perfect chassis here at work.

I had to swap out a fostec light on a machine that went bad. We don't repair these things.

http://cgi.ebay.com/FOSTEC-DCR-III-...ryZ48738QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

I might do BIN on that one anyway, we get charged an astronomical rate to get them rebuilt.

Evan

 

[240z4u]

Evan,

Stop the head scratching about power trafos. Allied Electronics has exactly what you need.

$11.41 buys stock # 967-9502, which is a Triad N-68X 50 VA isolation trafo. That COMFORTABLY takes care of the B+ requirement.

$8.28 buys stock # 967-2438, which is a Triad VPL24-400 filament trafo with 2X 12 VAC/410 mA. windings. Energizing the 12B4 heaters is WELL taken care of.

BTW, there is GOOD reason to stick with bridge rectified "120" VAC. 600 PIV Schottky diodes are less expensive than the 1200 PIV parts a 120-0-120 FWCT arrangement would use, in order to retain a nice margin of safety.

Those parts look great, I am trying to figure out how to cram all this stuff into my small-ish chassis.

I may go outboard power supply.. thoughts? I have a small case that would house both transformers nicely. Just have to make an umbilical cord.

Evan

 

[240z4u]

Another dumb question about chokes.

9H 250ma choke... where can I get one?

Sorry for the stupid newbie questions!

I did find the 5h chokes through parts express.

Thanks - Evan

 

[240z4u]

Eli, the schematic that you provided obviously has no resistor wattages listed. I used ohms law to the best of my peanut brained ability and came up with the following;

9k - 2W
680k - 1/4W
330k - 1/2W
1.5M - 1/4W
2.2K - 1W


I have no idea what to make the 500ohm one since its feeing the tube. Would 1 watt be adequate?

Also having trouble finding a 250ma 9H choke.

Thanks guys, Im trying to figure this out as best as I can through searching etc..

Evan

 

[Bandersnatch]

hey-Hey!!!,
For the 9 Hy/250 mA choke I would suggest substituting the 20 Hy/100 mA 193-series unit from Hammond. You'll run to less than half its current, and leave enough headroom for the AC flux that choke-input service creates.

Second, the CCS will allow a single LC filter. Its a very high impedance circuit, and will offer *PLENTY* of PSRR. Try a 50-80 uF/370VAC oil cap of the motor-run service type. Ebay has several sellers of this sort of thing at the $10 each range.

The coupling cap can be done otherwise too. See Mouser for the 3 uF, 735P by Sprague. It is a high current metalized polypropylene cap.

There's more to this, but it isn't all that difficult.
cheers,
Douglas

 

[Eli Duttman]

Evan,

My effing computer locked up as I was replying.

Doug is correct about a 20 H. Hammond choke being just fine. A reasonable approximation to the critical current (in mA.) required by a choke I/P filter is given by V/L. It works out that 1000 Ohms of bleeder resistance are associated with each Henry in the 1st inductor. A 20 KOhm bleeder resistor is appropriate with a 20 H. 1st inductor. Even with adequate PSRR from the CCSes, I would still retain the paired 2nd LC sections. Channel separation is BETTER that way. The arrangement is known as pseudo dual mono. However, you can save some money by using smaller 2nd inductors. 1.5 - 2 H. parts are OK.

Either 1/2 or 1/4 W. parts are appropriate for the 2.2 KOhm grid stoppers. Be certain to use Carbon composition resistors in the stopper positions. Carbon comp. resistors are both non-inductive and non-metallic, which makes them superior in the stopper role. Remember, current in the grid circuit is vanishingly small, which makes noise a non-issue.

P= I²R. CCS loading forces the current in the 500 Ohm cathode resistors to be 20 mA. 0.2 W. is dissipated in each cathode resistor. 1/2 W. rated parts should be OK, and 1 W. or higher rated parts will surely be fine. Please observe that the 12B4 grids are at -10 V., with respect to the cathodes.

The B+ rail voltage should be approx. 120 V. A pure choke I/P filter will yield an approx. 105 V. rail. The 47 nF. "fudge factor" cap. needs to be tweaked, at the bench, to get the B+ rail exactly where it should be. Refer to the 12B4 data sheet. Please observe what the anode voltage must be, when the grid is at -10 V. and I_B = 20 mA. The B+ PSU has to provide that and enough extra to operate the CCS.

Use 1/2 W. rated parts in the 330 KOhm/680 KOhm voltage dividers. A 2 W. part is appropriate for the 20 KOhm bleeder resistor.

DigiKey is the best source for the 600 PIV Schottky diodes, but there is an order minimum. This preamp is HEADS & SHOULDERS above the Carver SS unit it's replacing. Quality controls are in order. DigiKey carries the EXCELLENT PEC hot molded Carbon line. 2X single 100 KOhm log. taper pots. WILL get the job done. Between the controls and diodes, you fulfill the minimum order requirement.

 

[240z4u]

Thanks so much guys. It really means alot that your willing to hold my hand through the design aspect.

I am learning alot though.

I do have some other parts that I need to get from digikey anyway.

Evan