Packard Bell project amp

[xtian]

Hi! Hoping I can get some advice on this project. I just pulled an amp out of a Packard Bell Space Age Stereo console. Photos below. I pulled the tubes: two 12AX7, four EL84, and one 5U4GB rectifier. The other big socket is for wires running to the colsole's tuner and controls.

I don't really want to reverse engineer and mod this thing, so I'm thinking to tear down and rebuild. I'd like a one channel guitar amp with one or two 12AX7s in the preamp and one or two EL84s in the power. There's plenty of schematics to choose from (e.g., AX84.com). My questions:

How do I evaluate the transformers? The OTs look crusty--still good? Am I doomed if I can't find a wiring diagram for the PT (I count 11 wires).

What about that big, silver, multi-capacitor can? Ditch?

Suggestions on how to approach this mess? I'm decent at soldering and reading schematics, but have no experience building a tube amp. I also don't want to spend $300 on a Champ clone kit, especially now that I have this nice chassis and transformers.

Thanks!

[img768]http://www.monkeymatic.com/pub/ebay/packardbell1.jpg[/img]
[img768]http://www.monkeymatic.com/pub/ebay/packardbell2.jpg[/img]
[img768]http://www.monkeymatic.com/pub/ebay/packardbell3.jpg[/img]
[img768]http://www.monkeymatic.com/pub/ebay/packardbell4.jpg[/img]
[img768]http://www.monkeymatic.com/pub/ebay/packardbell5.jpg[/img]
[img768]http://www.monkeymatic.com/pub/ebay/packardbell6.jpg[/img]
[img768]http://www.monkeymatic.com/pub/ebay/packardbell7.jpg[/img]
[img768]http://www.monkeymatic.com/pub/ebay/packardbell8.jpg[/img]

[Firestorm]

Pretty cool. You've got the blue molded Mallory caps inside (much prized among blackface Fender aficionados), two output transformers (which may or may not be good) and a PT in some kind of condition. You might be able to find a schematic with some research, but you should be able to sort out the PT leads just based on where they're connected now: 4 leads will go to the rectifier tube, two will come in from the wall outlet, two will go to the tube heaters, there will be a grounded center tap on the high-voltage winding, there may be a center tap for the 6.3V heaters (or not) and there may be some "extraneous" power connections because these things often powered the other system components from a single power supply. If it's still good, it should have power to spare for a 2xEL84 amp. The electrolytic caps (including the big silver one should go). Antique Electronic Supply makes a replacement like it.

Could be a fun project.

[M Fowler]

Nice find that looks like a great platform to work with. Does it have a cage top?

[xtian]

You might be able to find a schematic with some research.

Any leads? I searched for the part numbers I see on the PT and OTs, but got no hits.

Can you figure this out with a volt meter?

[xtian]

Does it have a cage top?

No. It was just bolted to the interior of the console stereo.

[Phil_S]

In addition to what Firestorm says, you can simply wring out what you've got by plugging it in. Careful, though, this isn't as simple as I just made it sound. You've got some prep work to do before you attempt to apply any electricity.

11 wires is about right for wire count. Based on what I see in the pictures, here's a guess:
primary: black - black/red
high voltage secondary: red - brown (center tap) - red/green
5V filament: yellow pair
6.3V filament: black - green/yellow (center tap) - green

I can't see it that well, so there's no saying this is all correct.

I am going to assume you have a DMM and understand the basics on how to use it. If you don't know this, stop here and read up on tube amp safety. Google will be your friend on that topic.

The very next thing to do is to disconnect the power transformer. Since you are discarding the can cap on top, you can carefully remove that and leave as much PT wire as you can, keeping track of what was connected where. It is good that you have pictures. You also need to lift any other connections. If wires run to tube sockets where tubes are pulled, then those wires can stay soldered to the socket pins, as they are not in the circuit. Any wires that are grounded need to be freed from the ground.

Next, determine if there is continuity on each of the windings and what wires are paired. Set the DMM for ohms and using the guesses above, see if there is reading. Write it down and post the results.

Do the same for the OT's. Probably all you need to do is lift the center tap on the primary side. If the secondary side is grounded, lift that, too. You can leave the primary wires soldered to the power tube pins.

More to follow after we get this far.

[Phazor]

Nice find. I would build a Rocket preamp to put in front of the power amp. The poweramp could be like an AC15 vox. The Output Transformers are a little small but for guitar might be fine. I have an AC30 with 5ar4 and just played it. Its a great design derived from the AC 15. There is a big Choke in the AC 30's and 15's that feeds the power tubes and you have a Resistor for that, not a choke. The power Transformer is rusty on the top which could cause eddy currents in the transformer. Also an AC 30 is cathode bias and it looks like yours is fixed bias. This could have an FX loop added to feed a stereo out to FX unit and then to a power amp. So many ways to go. but I love a Vox with Tube Rectifier. Plus its a simple preamp.

Michael

[Phazor]

Yours is Cathode bias. I see the 120 ohm resistor. from the cathode's on the pwr tubes. missed that location.

Michael

[xtian]

Next, determine if there is continuity on each of the windings and what wires are paired. Set the DMM for ohms and using the guesses above, see if there is reading. Write it down and post the results.

Phil, awesome! I was just staring at this spaghetti monster. Thanks for kicking it off!

Power Trans:
orange, yel+grn pair = 0.2 ohms (this pair feeds the brown wires that feed the heaters on all the tubes)
red, red+grn = 80.0 ohms (to rect. pins 4 and 6)
yel, yel+blk = 0.2 ohms (to rect. pins 2 and
grn, grn+blk = 0.2 ohms (these head out the console bus…not sure where they return)
yel+red (this goes to chassis ground)
blk, red+blk = 2.2 ohms (mains 110v AC)

Output Trans:
three wires on input side: yellow, red, blue
red, yel = 232.2 ohms
red, blu = 256.4 ohms
yel, blu = 490 ohms
two wires on output side: black, green
blk, grn = 0.2 ohms

Now that the PT is isolated, should I plug it in a check DC voltages?

[Phil_S]

You're doing very well. I'll try to guide you for a while. I should disclose that I am an experience amateur, not a tech.

PT, conclusions based on colors and ohms:
orange, yellow/green are clearly the 6.3V filament supply.

red, red/green are clearly the high voltage secondary; the grounded red/yellow wire must be the center tap. I'm surpised you got a decent reading, did you lift the ground from the chassis? If you didn't you should do so. You'll need to lift the ground before you plug it in.

green, green/black -- probably another 6.3V supply, this may have supplied another chassis

black, red/black, good colors for the mains. 2.2 ohms suggests this PT has a high VA rating, maybe 200V. It appears to have 3 filament windings; allowing 15VA for each, it probably has 150VA for the high voltage winding. Once we know the voltage, you'll have a pretty good idea of the mA capacity. These are rough estimates and a bit low for 4 EL84's. Let's say this is a very conservative estimate. The only way to find out for sure is to test it until you let the smoke out, after which it makes an ugly door stop.

You need to be double sure all secondary wires are isolated from the chassis and the circuit. If there is no fuse and no plug, what I do is to use a wire with a plug on one end and pigtails on the other. I use wire nuts to connect the cord to the primary wires. I plug it into a power strip that will trip if there is excessive current draw and I use the switch on the power strip as the on/off switch. Do this, and find the voltage on each pair of wires. Your amp ran 4x EL84. I'd expect something like 500VAC across the high voltage legs. You place the meter leads so one lead is connected to each end of the winding. Do not measure any voltage from wire to chassis, as that's a dead short and you don't want to find out what happens.

Use clip on leads with the meter. Keep your hands in your pockets and turn it off when you move the meter leads. Don't be a big shot and try to lock onto hot leads. It isn't worth the risk to YOU. Post what you find. Unloaded voltages are typically ~5% higher than loaded, but it can vary from that.

On to the OT's. the primary appears to be yellow- red CT -blue. Ohms are reasonable. Was the yellow connected to the power rail (if you know what that is)? You need to lift the center tap from the circuit.

The speaker side is green-black. This is a typical combination, often for 8 ohms but no saying any convention was followed.

To test the OT (one at a time), you'll need a low voltage AC source. Finish testing the PT and then you can use the 5V supply for the rectifier. Use a pair of jumpers with 'gator clips on both ends. One pair of EL84's is mated to each OT, so do one pair at a time. Run one jumper from pin 2 on the rectifier socket to pin 7 (plate) on one EL84 socket. Run the other jumper from pin 8 on the rectifier socket to pin 7 on the other EL84 socket. Turn on the PT, as above. Then measure the voltage between pins 2 and 8 at the rectifier socket. (You'll get a lower reading because the OT presents a load.) Write this down. Then take a voltage reading between the black and green secondary wires. Assuming a 5VAC input on the primary, I'd expect something between approximately 110 and 220VAC on the secondary.

Once you have the voltage readings, you know the turns ratio. The square of the turns ratio tells you the impedance ratio. Since we don't really know the primary impedance, let's assume the standard 8000 ohms for the primary. If you get 160VAC on the secondary, 160/5=31.5. 31.5 * 31.5 =~1000. 8000/1000=8. Your secondary is 8 ohms. With good readings, you can be more precise.

Post results...please don't leave me hanging.

[xtian]

Really appreciate your expert amateur advice. Disappointment follows:

PT: You need to be double sure all secondary wires are isolated from the chassis and the circuit.

Just to make certain, I cut ALL 11 leads and stripped the ends. Meter shows 118 VAC on the mains (connected to power strip as you suggested). However, none of the pairs shows any significant DC voltage. Flutters around 30 mA, but that's it. The transformer is definitely drawing some current, because I can hear it humming quietly, and I get a pop in my stereo system when I flip the switch (it's attached to the same circuit). Seems fried, huh? (I tested some batteries to make sure my meter is working.)

On to the OT's. the primary appears to be yellow- red CT -blue. Ohms are reasonable. Was the yellow connected to the power rail (if you know what that is)?

Blue and yellow wires are attached to pin 7 on the four power tubes (my pinout diagram says Anode). Reds are both connected to the yel+blk from the PT.

To test the OT (one at a time), you'll need a low voltage AC source.

Well, the PT is fried, but I have a wall wart that gives 29 VAC. Too much? Probably time to invest in a new transformer pair?

[M Fowler]

It will give you AC voltage not DC since it isn't rectified yet through your pair of 5U4G tube rectifiers.

Did you get any AC voltage on the secondary wires?

[Phil_S]

<shaking head>Like Mark says, it's all ACV until you build a circuit and rectify the voltage. I skipped that part...see above...I assumed you were familiar. If it hums, and you've got continuity, I'm expecting it to work. Try again!

You didn't need to snip all the leads, though no harm is done. Leave them as is for now. For the next time, leaving them soldered to the sockets solves the problem of having 11 flying leads, which can easily short at the least convenient time. When I have all those flying leads, I use a screw down terminal strip on a decent sized block of wood. This is for safety and to keep it organized. If you have the wires on a terminal block, then you can move your probes easily while the PT is hot. This is the sort of terminal block, two kinds:
http://www.action-electronics.com/grc/irc3140.jpg
http://www.connex-electronics.com/html/ ... 02hds.html

You are working in an old chassis and seem to have the intention of leaving some of the existing stuff there. Try not to disturb more than is necessary. "Slow and steady wins the race," said the turtle to the hare Reworking an old amp like this takes patience. I have done this sort of thing before.

29VAC is too much for an OT. With 30x turns or so, that will put 1000v on the secondary. Think before you act! If you must use 29V, apply it to the secondary and see what comes out the primary -- you'll get about 1v, which may not give a reading that is sufficiently accurate to calculate turns reliably.

[xtian]

* blush* I knew that.

orange, yel+grn = 6.76 VAC
red, red+grn = 0 (is that what you'd expect?)
yel, yel+blk = 5.59 VAC
grn, grn+blk = 7.17 VAC

I'll investigate the OTs next.

[xtian]

Here's my guess; please confirm before I try.

PT wires yel, yel+blk (5.59 VAC) were connected to pins 2 and 8 of the rect tube (pinout says Filament). PT wires red, red+grn (which read 0 now) were connected to rect pins 4 and 6 (plates).

So my guess is the rect tube will deliver two different DC voltages to pins 4 and 6, which when paired with the yel+red PT wire (center tap) will make two DC circuits.

Doesn't sound quite right to me, because nothing is grounded. Let me know.