Packard Bell project amp

[Structo]

Hate to say it but you screwed up cutting the wires off of the rectifier socket.

The two yellow are the 5v for the cathode, the two red are the HT (high voltage) wires.

Please be careful.

It doesn't sound like you have much tube amp experience.
That HT winding probably has over 350vac on it.

Having all those stripped wires now exposed just raised the danger level.

You could have just replaced the cap can then modified the input for guitar.

The tone and volume controls were obviously on the console so you have to work that out.

Try to find a schematic or just copy a design like a rocket.

[xtian]

Please be careful.

Thanks. Will do.

[Phil_S]

You have good readings on the three filament windings. You have one 5V winding that was attached to the rectifier and two 6.3V windings.

The remaining one is the high voltage secondary. It should not be zero. It should be about 500VAC give or take 50V. There are three wires involved here. The red and red/green should be the outer legs and the brown should be the center tap (CT). You'd get half the voltage from each outer leg to the CT. It is possible half is open, so you should at least find out that much. In any case, a zero reading doesn't look very good.

As for how the rectifier works, in this case a 5U4, the high voltage is sent to pins 4 & 6 as VAC. The filament and the cathode are one and the same. You need 5VAC to heat the cathode -- these are the yellow wires on pins 2 & 8. You take the rectified voltage from either pin 2 or pin 8 (not both). There is only one output, which should be ~275VDC.

Anyway, you can use one of the filament supplies to power the test for the OT's. Be careful!

[xtian]

The remaining one is the high voltage secondary. It should not be zero. It should be about 500VAC give or take 50V.

Yes! between red and center tap = 347.6 VAC
and between red+grn and center = 348.4 VAC

As for how the rectifier works, in this case a 5U4, the high voltage is sent to pins 4 & 6 as VAC. The filament and the cathode are one and the same. You need 5VAC to heat the cathode -- these are the yellow wires on pins 2 & 8. You take the rectified voltage from either pin 2 or pin 8 (not both). There is only one output, which should be ~275VDC.

Take the rectified DC voltage from pin 2 and WHAT? In order to make a circuit? Remember the PT and rect tubes are not connected to ground.

[Structo]

The wire you take from pin 2 or 8 is your B+.
I think most amps use pin 8 as the B+.

Take that on to your filtering.
Remember to not go too high with the first cap value.
Nothing over 40uF and a lot of guys say nothing over 32uF.

The other side of that will be your chassis ground.

[Structo]

Download this tool.

It is very useful and can help with questions about tube data.

http://www.duncanamps.com/tdslpe/

[xtian]

I hooked up yel, yel+blk (5.5VAC) to rectifier pins 2 and 8 (filament), and red, red+grn (350VAC) to rectifier pins 4 and 6 (plate).

Now I am reading 340-350 VDC between pin 2 OR 8 and PT's center tap (ground). Also reads 100-110 VAC.

I posted a voltage range because I just received a second multimeter, and it disagrees with the first. Which to believe?

[xtian]

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.

Now that I've got 5VAC and Hi VAC connected to the rectifier tube, the 5VAC pair (yel, yel+grn, connected to pins 2 and reads 3.08 VAC. Next, I jumpered this pair to pin 7 on the two EL84 sockets in one channel (these two pin 7s are also connected to the primary coil of the OTs, yellow and green). Then I measured on the other coil of the OTs (black and green) and only got 0.17 VAC. I repeated with all five leads from the OTs lifted, same reading. I tried with EL84 tubes in the sockets and not. No change.

I did something wrong…?

[Phil_S]

You are doing it wrong -- testing the OT. Do this with all tubes removed from the amp.

You need to isolate and insulate the OT center taps from the chassis and the circuit. Clip the CT at the connection point if you must.

Let's call the 4 EL84's, tubes A, B, C, and D. A and B are one pair. C and D are the other pair. The instructions for testing each pair and the same. Each pair is connected to one OT.

From the rectifier socket, no tube in the socket, place a jumper from pin 2 to pin 7 of tube A. From the rectifier socket, place a jumper from pin 8 to pin 7 of tube B. Turn on the power. Measure ACV across the green and black secondary wires. Measure ACV across pins 2 and 8 of the rectifier socket. These numbers will be used to calc the turns ratio.

Do the same, substituting C for A and D for B.

Do not measure any voltage to a ground or chassis reference.

You are not ready to get an accurate B+ reading. There is no circuit to load up to the rectifier. Stop fooling with that before you get hurt or worse. This is dangerous work. There is 700 volts and probably at least 300mA at the rectifier. This is more than enough to kill you.

[Phil_S]

Those meters aren't calibrated. Use only one of them throughout the amp so readings are consistent on a relative basis. They are probably both wrong, but not by enough to matter for what we are doing.

[xtian]

From the rectifier socket, no tube in the socket, place a jumper from pin 2 to pin 7 of tube A. From the rectifier socket, place a jumper from pin 8 to pin 7 of tube B. Turn on the power. Measure ACV across the green and black secondary wires. Measure ACV across pins 2 and 8 of the rectifier socket. These numbers will be used to calc the turns ratio.

OK: no tubes in sockets. 5vac running to pins 7. Center (red) OT tap lifted/isolated/insulated. Reading on ouput pair: 0.18vac. Same on second channel.

Same result when I lift the input wires of the OT off the board and apply 5vac to them directly, instead of pin 7s, which run off thru other components.

[Structo]

Just so you know if output tubes are installed you must have a speaker load connected.
Otherwise you risk burning the output transformer.

The power tubes must have a load on them if they are powered up.

[Phil_S]

OK: no tubes in sockets. 5vac running to pins 7. Center (red) OT tap lifted/isolated/insulated. Reading on ouput pair: 0.18vac. Same on second channel.

Same result when I lift the input wires of the OT off the board and apply 5vac to them directly, instead of pin 7s, which run off thru other components.

This doesn't make sense. This is a step up transformer. Output voltage should be 100-200v or so. Can you post a close up picture? Something isn't right.

[xtian]

First photo shows 5 vac output from PT jumpered with white and yellow gator clips to input winding of OT. Red center tap is not connected to anything. Red and black gators run to the multimeter, which shows 5.57 vac.

[img768]http://www.monkeymatic.com/pub/ebay/packardbell9.jpg[/img]

Now multimeter is attached to output of OT, and shows 0.182 vac. Am I reading the multimeter wrong?

[img768]http://www.monkeymatic.com/pub/ebay/packardbell10.jpg[/img]

[xtian]

Phil, isn't this correct for an OT? It's supposed to convert the high voltage, low current signal from the power tubes to low voltage, high current required to drive the speakers.

Why do you expect the voltage to rise?