DIY Audio Amp Project.

Pulling an old amp and preamp from a Webcor Console Stereo. The tuner and phonograph crapped out. So I decided to use the amp for my Dual 506 phonograph. I plan on changing the tubes and filter caps. I will also attempted to rehouse it into a cage like enclosure. My only unsure things are, what Ohm rating it has for the output and can the right and left speaker outputs share the same ground wire?

Well here are some pix. Help or suggestions are welcome.

(1) I see a meter laying on the floor, so read the existing speakers and see what they are. If they are 8 ohm, it will read somewhere around 6.8 ohms, it 16 ohms it will read around 13-14 ohms and if 4 ohms it will read around 3 ohms on your meter.

Is this STEREO? If not, you can also just read the total load where the speakers come together at the OT, with the OT unattached.

(2) YES

I would replace ALL the electrolytic caps in that old piece of equipment. Clean the pots. Have the tubes tested, don't be too quick to replace those old US or European tubes with the crap in production now unless you have a stash of NOS tubes. What tubes are in that amp and preamp?

Good luck...

EDIT: Some of those tubes look like Mullards and are made in England, they are expensive quality tubes.

I am trying to figure out how the speakers are wired up.

it is stereo. The speaker configuration is a tweeter and mid on both sides and one woofer in the middle. I keep trying to trace the wires and make sense of it but keep coming up with different readings. I get either 6.8 or 14.1 or so.

I really hope it takes an 8 ohm load so I can use my existing home speakers.

The tubes are british made. A few give me a little hiss and static.

Thanks for the help and suggestions. I am sure I will need more along the way:^)

the tubes are

12ax7 for the pre and phase.

12ab5 for the power ( if someone can give a little info on these that would be great)

5u4 (i think) for the rectifier

Yes, the 5U4 is your rectifier.

The 12AB5's are 12 watt Beam power tubes, you might be hard pressed to find replacemants for those.

I found these, at least thay are not very expensive.

http://search.ebay.com/search/search.dll?from=R40&_trksid=m37&satitle=12AB5&category0=

If you still can't figure out the speaker wiring, you may need to determine what impedance the OT was designed for.

Read this and see if it helps you out any with this :

http://us.f13.yahoofs.com/bc/43c71b6em9c650a2b/bc/Output+Transformer+Impedance++Text+file/OT+Impedance+HTML.htm?BCuMrnHBibt2f4e8

I have found that the tubestore and tubedepot sell them. Not sure ont the brand though. They don't seem to be pricey at all and that is nice.

the page is not working

Thats typical Yahoo...

OK, I copied and pasted it here>>>--->

Finding Unknown Values for Output Transformers

(A little Math is required)

If you have an Output Transformer with unknown values and want to find the intended speaker impedance for a given tube type there are several things to consider. An OT reflects the impedance load (the speaker) on the secondary back onto the primary winding.

There are a few abbreviations I am going to use for the purpose of this explanation.

VR = Voltage Ratio

IR = Impedance Ratio

TR = Turns Ratio

IL = Impedance Load (The speaker)

ELR = Effective Load Resistance (from tube manual)

ELR Examples for two tubes in PP AB1 or AB2

6L6 ELR = 3,800 ohms

6V6 ELR = 8,000 ohms

EL84 ELR = 8,000 ohms

EL34 ELR = 6,500 ohms

The Turns Ratio (TR) can be found by applying a small AC voltage to the secondary and measuring the result on the primary. If you put 1v AC on the secondary and measure say, 31.6v on the primary you have a Voltage Ratio (VR) of 31.6 which means that the turns ratio is 31.6:1.

Finding a 1v AC source is difficult unless you have access to a Variac, small transformer or similar device. You can use a 5v rectifier winding from an existing PT or the 6.3v heater winding. You must then divide the answer by the number of volts you applied to the secondary. For example the 31.6:1 TR would measure 158v with 5v AC applied to the secondary.

The Impedance Ratio (IR) is the square of the Turns Ratio (TR), in this case:

31.6 (sq) is 998.6

To determine the expected Impedance Load (IL) or speaker impedance you will divide the known Effective Load Resistance (ELR) by the Impedance Ratio (IR)

So: ELR / IR = IL

In this example we want to use a pair of 6V6 tubes that have an ELR of 8,000 ohms with this transformer and want to determine the proper speaker impedance (or IL).

8,000 / 998.6 = 8.0 ohms

So you would use an 8-ohm speaker with this transformer and a pair of 6V6 or EL84 tubes in AB1 or AB2.

What about a pair of 6L6 tubes? RCA says the 6L6s have an ELR of 3,800 so:

3,800 / 998.6 = 3.80 ohms

So you would use a 4-ohm speaker with this transformer and a pair of 6L6 tubes in AB1 or AB2.

Casey4s

02/04/01

REV. 07/08/01

I

Alright. I have the amp and pre out and on a board for tinkering. I have located and swapped out the on of switch. I also swapped out the 12ax7s. The British originals are noisy in the amp. So I put them in my Laney AOR and they sound pretty darn good in there. So I will keep messing with them. I would like to change the power cord. The original is cracking and fading. Would a # prong be a good idea? I have the terminal and cord for one. If so is it just attaching the ground to the chassis? or am I missing something?

Oh ya. I found found on the schem. that when biasing (or whatever) it says to hook up to a 16 ohm load. So I think that is what it will drive. That is a bummer cause how many home speakers are 16 ohm? Not many.

I will get another pic up in a bit.

Glad to heare you're makin some progress. I would definitly get a 3 prong cord and ground it to a good solid ground to your chassis and and tie that point to your transformer grounds and CT's if any.

But yes you can ground it just to the chassis if you want.

Here are some pix.

It is now separated from the cab. I am in the process of rerouting and shortening some wires.

I added a temp switch and a new cord.

Here is one with most of the wires shortened.

The empty space next to the preamp will house 2 new lcr type filter caps. The can has uncommon values and the lcr types are more cost effective.

That is it for now.

Makin good progress..

I'd suggest taking a reading on those old carbon comp resistors. You may need to replace a few.

Thanks!

Thanks for your help and suggestions also:^)

Resistor ?

I am having trouble finding a 135 ohm 5 watt resistor.

How much wiggle room does one have with these? I can find a 150 ohm and a 120 (or so) but not the exact tolerence of 135.