Peavey Classic 400 Tube Bass Amp - Preamp issue

[AtomicMassUnit]

Hi all, a little stumped here. The amp fires up, and has a good bit of ambient hiss/hum, and if I plug into the power amp in, I get a good bit of output, so the power amp works.

If I plug into the front of the amp and play, I only get a trickle of staticky sound if the preamp volume is way up.

Plugging into the front and taking a signal out of the effects send, I get a nice strong signal. Plugging into the effects return, I get nothing, so I think the problem is in the tube stage after the effects return.

All resistors around V3a measure out, and I'm getting about 350v on the plate, and it makes a hellacious loud pop when measuring. Could it be the relay? I am not familiar with them or how to diagnose/check problems with them.

schematic------> http://www.hartsafire.com/classic400..._service-1.pdf

[J M Fahey]

All resistors around V3a measure out, and I'm getting about 350v on the plate, and it makes a hellacious loud pop when measuring.

Then there's no current flowing through that plate resistor, you're measuring whatever the supply rail has (~350V) , you should have around 150 to 250V on that plate.

Probably a bad socket contact, dirty/rusty tube leg, cracked/cold solder , cracked track, look with a jeweller's loupe or a couple stacked reading glasses (no kidding) under good light, and retouch all soldering involved.

Measure at the resistor leg, the socket leg, with a thin probe tip the tube pin itself if possible, all 3 *should* be the same ... if they were in good contact, which is suspect.

The hellacious pop is you flexing the board with the probe tip and opening/closing some poor contact.

[Enzo]

What he said^^^^

My own list for triode not conducting is to first LOOK at the tube to see if BOTH heaters are lit in it. And try a different tube.

With 350v on the plate pin we know the resistor and circuit path from B+ to socket are OK. The cathode resistor may measure OK, but the real measure is from pin 3 to ground. That checks not only the resistor but the circuit as well.

[AtomicMassUnit]

I checked the solder and traces very carefully, resoldered the entire area, of course tried another tube, and I'm getting 1.5k to ground from the tube socket pin... in fact, I checked every pin from the outside of the socket through the traces.

but yes, I'm getting 350 on each side of the 100k plate resistor. Also, there's a whole lot of hiss coming out of the speakers, which goes away when I remove v3.

[Enzo]

That triode is not conducting. Did you verify visually that BOTH heaters were glowing inside the tube?

[Jazz P Bass]

No voltage drop across the 100K?
The tube is not conducting.

If the cathode does indeed measure 1.5K to ground, then all that is left is the heater. (or lack of.;()

[g1]

Do you have continuity from the plate resistor to the socket pin?

[J M Fahey]

To reinforce the correct suggestions above, turn amp on, measure voltage at tube plate and cathode pins.

In fact, I expect the loud pop to appear (which means you are opening or closing a circuit with tip pressure) and if closing you might measure normal volotages, go figure (~200V plate ; ~2V cathode) .

The suspect circuit is very simple, but is biting you every time in your *ss because the very act of measuring changes the situation (Heisenberg got a Nobel Prize for a very similar concept, go figure)

So absolute worst case, replace socket , resistors and tube with new stuff, PLUS parallel some wire bridging end to end any track involved.

It's such a simple circuit that it's not possible it does not work.

Like my Mechanic tells me every time: "if you have compression + air/gasoline mix + spark, your engine must fire, even if a muffled or agonic explosion .... if not, start checking which is missing" .

[Enzo]

Since there is no voltage drop across the plate resistor, Ohm's Law tells us zero current is flowing. And if the resistors are intact on both the cathode and plate sides, then that is why some of us are stressing that the tube heater may not be functioning.

[AtomicMassUnit]

Forgive me if my numbers are slightly off, I wrote a nice long post and it got lost when I had to re-login. Ugh.

Heaters are lit... I'm getting 12vdc on pin 4, 25 vdc on pin 5, Pin 9 is n/c. (I am unfamiliar with DC heaters)

Pin3 has no voltage, with or without a tube installed. Pin 8 has 1.7vdc.

I went ahead and undid the zillion connectors and pulled the board. Everything around that circuit checks out, and all the traces have continuity... all resistors, caps check out. Tube socket looks and measures good. I'm going to probably just shotgun the parts anyway... SO frustrated.

[Enzo]

The 12AX7 has a 12v heater. Pin 9 is a "center tap" allowing us to parallel the two halves for 6v operation if we desire. The tube doesn't car if it is AC or DC, as long as it has 12v across the heater. If you have 25v at one end and 12 at the other, then you have 12v across the tube.

Please don't shotgun a lot of parts.

Since the heaters are running, we still have no current flow. I will believe your resistors are OK. Measuring the voltages at the tube socket is one thing, but having the socket pins actually make electrical contact with the tube pins is another. Pins 1 and 3 are suspect. If you have not, pull the tube and apply your meter probe to the female socket pin up top, not underneath where the wires connect. It is possible a pin is broken in half inside the socket. In other words the circuit is soldered to the bottom half, but the top half, the female is no longer connected. Rare, but does happen. More likely is to look down the female pins 1 and 3 and see if either or both have spread a little more than the others. We may need to tighten them. Also some oxidation or corrosion may be preventing electrical connection within the female pin. One simple test for that is to clip your meter to the plate end of R140. We have the full B+ instead of something a hundred volts lower. Oor if you feel better about it, clip to the cathode end of r150. It had zero volts where we want to see a volt or two. The circuits are the same, so I'd expect the same 1.7v as pin 8.

Either way, we clip to it so we can monitor the voltage while we mess around. with the tube in the socket, verify too high on the plate or zero on the cathode, then pull the tube out just part way, so a different point on the tube pins makes contact. ANy change? No? Then twist the tube a little. Not enough to bend anything, but hard enough to press the tube pins against the side walls of the female pins. Does the voltage reading change?

Your meter probes do not touch the socket pins the same as the tube pins do, so we must check things other ways, such as I suggested.

You MUST have current flowing through the tube or it will never work.

You know, it is cheating, but here is a dirty trick. Just briefly, short pins 3 and 8 together. Does that make sound come out? or at least does that cause pin 1 voltage to drop from 350v down to whatever pin 6 has? It may cause hum or oscillation or other weird thing, but all I care about is does it make the plate voltage fall into line on V3a

[nashvillebill]

Does anybody still sell those little breakout adaptors that you plug into a tube socket, then plug the tube back into the adaptor; the breakout adaptor has test points so you can see what's actually on each tube pin in the socket. I collected mine over the years but haven't seen them for sale anywhere.

Oh, and my second question: where did you find a work bench sturdy enough to hold a Classic 400, and how did you mount your crane to lift the amp onto the bench? (I've got one--it's too dadgum heavy to take out gigging!)

[Enzo]

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[g1]

I don't think you mentioned whether or not you have voltage at pin1, and if it is the same as at the plate resistor?

[nashvillebill]

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Vector Tube Tester Socket Test Adapter New U105 U 9 Prong Pin 6 Available | eBay

Eby 9 Pin Vacuum Tube Test Socket Adapter for Radio Amplifier Tubes Tester | eBay

Tube Tester Socket Test Adapter New TVs 9 9 Prong Pin 2 Available | eBay

Didn't think about evilbay...!!! They seem mighty proud of those little jewels, though.