low/no volume in preamp

[MitchK]

Hey so I've talked about this in another thread but I've got a 5150 with volume problems. The volume is very very low, the problem is in the preamp, the preamp out into another power amp is still very quiet and when I plug directly into the power amp of the 5150 its much louder.

This all started happening after pins 3 and 4 of one of the poweramp tubes arced together.

What should I be looking for? I don't have an oscilloscope, so at this point I'm going to go through and check the value of every resistor in the preamp and then disconnected every capacitor one by one and check all of those but I mean if there is some more effective troubleshooting I can do please let me know.

[Enzo]

I'd go down the row of preamp tubes and see if B+ was on all the pins 1 and 6. And I would look to see that there was a volt or two at each cathode - pins 3 and 8. Any missing cathode voltage means a tube not conducting. And if the B+ is way high at a tube (and then the cathode is likely also at zero) that also means teh tube is likely not conducting.

No scope? Set your meter to AC volts and apply a signal to the amp. FOllow the signal with your meter.

[MitchK]

I've checked the pins, there is B+ on bothes sides of all the tubes, ranging from 170v upwards and the cathodes do have V on them too but it is anywhere from -10v to +24 v depending on which tube. is this right?

I've also tried tracing the signal but to be honest I don't really know what I'm looking for. Also I'm not sure about setting the meter to AC, I'm used to doing everything in DC like when I read the B+ for example, when i set the meter to AC everything is totally different so I really don't know what i'm looking for!

[Enzo]

On AC it ignores the DC. SO if the plate of the first stage sits at +160VDC, and there is 20 volts of signal, that means that plate is movving from +130 to +190 back and forth at the frequency of your signal. The DC average remains 160. But your meter on AC will ignore that 160VDC and only see that 20v of signal. It will read 20v. SO of course everything looks different on AC.

How much signal is there? Varies at each stage. SO apply a test signal to the input, and measure what appears at the plate of the first stage. Now remove the test signal. How much AC is there now? That would be noise and residual crap. Once the test signal is applied to the input, then the signal will rise to its active level.

One has to have a working idea of how an amp works to decide what seems normal or not.

We measure the DC to find of the tubes are even working in teh first place. Once we determine that they are, we need to go through the stages to see where the signal disappears. So for that we switch to AC. Poor man's scope.

There is no [place in the 5150 where a cathode would sit at -10v. Are you sure you were not on pins 4,5, or 9? Those heater pins ARE at anywhere from -24 to +24VDC.

[MitchK]

ahhh I see! That makes perfect sense but to confuse things even more the initial reading I had taken like that was with a sort of cheapo meter that just ends up showing the DC x2 when its set to AC.

anyways about the cathodes I was looking at the wrong pins, all is good on that front.

now, after following your directions I applied a signal to the amp. I checked the first tube the signal enters and on pin 6 I see 12v, turn the signal off and it goes to 0. I check pin 1 of this tube with the signal applied and I have 0v, the only other place I don't see voltage is pin 1 of the second tube where I see 2v and pin 6 of the third tube where I see 14v. Pins 1 & 6 everywhere else shows 0.

I'm not too sure how to read this, I expected that I would see voltage on pin 1 of the first tube, especially after seeing voltage on tubes 2 and 3?

what does this tell me? is the signal being lost on the way from pin 6 to pin 1 of the first tube?

[Enzo]

Get a real schematic and refer to actual part numbers like R101 and C54.

Peavey will email it to you - parts@peavey.com - ask politely for the 5150 schematics (unless it is really a 5150-2.)

In any case, the signal moves stage to stage through the amp. COmes in the grid of a tube, and exits the plate. From the plate, ther will be a cap and some resistors to get it to the next grid. An open cap or resistor and you got no signal.

Yes, really cheap meters are confused by AC on top of DC. All they measure is average voltage. In DC, the real voltage is the same as the average, on AC, that is close enough, but when you try to measure a signal with DC offset, then average no longer works. One trick would be to place a 0.1uf cap in series with the meter probe. Or better yet, get a better meter. A pretty nice one can be had for not a lot of $$$.

Yes, we expect to see signal all the way through the amp, but when the amp is broke, we only will find signal up to the point the signal path breaks. Imagine a backed up highway. You fly down the highway overhead on a plane, and every mile you check for a lot of cars down below. At some point you see NO cars. WHatever is blocking traffic is between the last point with cars and the first point without.

[MitchK]

alright so its been a little bit but I've gotten my hands on an oscilloscope finally so thats kept me busy.

I'll cut right to the chase and describe what I'm seeing.

basically I can see the signal on the plate of V1A but then is diminishes on V1B stays that way on V2A then is back even stronger on V2B, bigger still on V5A and then on the way to V5B the signal goes back down again. Now, after this point it passes through R86 and R89 and after either of these resistors the signal diminishes yet again. I follow R89 to the tone stack and this is where the signal gets lost but i can't really tell where, its not on the EQ pots, but when I look at the other route that goes through R24 this is where I get lost. There is no signal on the other side of R24, however, it connects to ground here so am I right in thinking that I'm not going to be able to read this point because why would the signal flow into my scope instead of directly into ground? when I measure R24's resistance with the amp off it seems fine?

I don't know where to measure from here is the problem. Its pretty hard to trace lines on the pcb a lot of the time, but it really seems to be somewhere in the tone stack or its just gradually decreasing through the circuit?

Can someone give me a little shove in the right direction?

[Enzo]

As a rule I don't trace the lines on teh pcb, unless I know there is a break between two circuit nodes. Then I might look for the break to repair it, but only after verifying the open with a meter. Mostly looking at the drawigns I follow the parts. If it moves from C1 to R2, then I don't care about the trace unless the signal turns up missing.

Do you know what a voltage divider is? This amp is full of them. A basic volume control is a divider. If you turn it half way up, you could replace it with two resistors - one from each end to the center terminals.

From the plate of V1A, through C6, the signal heads towards teh gain controls VR1, VR2. Each has a resistor in series - R21, R29. The resistor and the pot make a voltage divider in each place. If you have a 1M pot and a 1M resistor in series, then the most signal there can be at the top of the pot is 1/2 what it was coming in, due to voltage division. SO yes, the signal will diminish after V1A.

Note between V1B and V2A there is a divider - R6 over R9. SO the increases of V1B and then reduced before feeding through V2A. SO the net result is the level is about the same. (Tone shaping occurred though)

Note the signal passes through V5B first, then V5A.

In the schematic, R86 is the plate load for V5A, so one end of it will have the signal coming out of V5A, the other end if it is just the B+ supply. We would not expect to see hardly any signal on the B+. R89 is the path out to the tone stack.

R24 does go to ground. it terminates the output of V5A. The signal at V5A should be pretty hot. After R89, it will head through the tone stack and prtty much needs to be at the level of the FX loops coming up very soon. The loops want to see a signal level of about a volt, a lot less level than the plate signal of V5A.

Back to voltage dividers. From the plate of V5A, (C58 is invisible to the signal) the signal runs through R89 and R24. They form a voltage divider. Voltage dividers work on rations, just like pots. The signal at the junction between the two resistors will be proportional to their values. SInce R89 is 470k and R24 is 33k, we should expect some serious level reduction. Starting with XX volts at the plate, the volts at the junction between the resistors (what goes on into the tone stack) will be: 33k/(33k+470k) times XX. 33k/503k = 33/503 = .06 So what is at the non ground end of R24 will be only .06 pf what is at the plate of V5A. So if the signal there is to be about a volt or two, then the signal at the plate ought to have been about 16v - 32v. Did you find a similar ratio? Did 20v of signal drop down to a volt or so? That would be normal there.

Without worrying about the tone stack for the moment, the signal exits teh stack to teh two post controls. See they are in parallel? Whatever is on top of one is on top of the other. What signal level is there? Remember we should now be running more or less line level - a volt or two..

After the post controls, one or the other will be selected by K2B, the signal then flows to V3B. V3B is a cathode follower driving the FX loop. The sifnal at is grid will be about the same at its cathode. Plate of V3B is direct connection to B+ and will have no signal.

From the cathode of V3B we head through a couple caps to teh FX send jack. SHould have a volt of so signal there.

You did already connect the send and return jacks together with a spare guiitar cord didn't you?

From there on it is the powr amp, and I believe you already tested that.

[MitchK]

ah thanks for the info on the voltage divider, I knew what one was I guess I just didn't really recognize it in the circuit or its purpose there. So with an amp like the 5150 where is has so many preamp tubes so many gain stages, its not really to boost the size of the signal its more to color it then? The pre-tubes build it up and the voltage dividers break it down along the way coloring it?

I have connected the effects jacks together this doesn't do anything, however my guitar directly into the effects return does boost the volume which leads me to believe the power amp is fine. I've tried the pre out into a seperate power amp and even with the power amp dimed I just get some hum, the effects send to the seperate power amp gives a tiny bit of signal if dimed.

The signal at the middle of VR5 which connects to VR6&7 the post controls is about 0.4 volts which is the same as after C14 and C60 (the other side of each of these caps is about 1 volt) and the the centers of VR6&7 are like 0.05 volts. this goes to K2B and then nothing is coming out of K2b and so there is nothign on the cathode of V3B. So is the problem one of these pots? or K2b?

[Enzo]

Just because there are a lot of parts hanging off them does not mean most circuuits are not built on the basics.

Yes, they use a bunch of stages to get the gain instead of just two so they can massage the tone into what they wanted. Some guys have reqwired it so they just have like two triodes. All they did was make it into a MArshall.

OK, I belive the power amp is OK as well, then. Jumpering the loop was to detect potential problems in the loop jack cutout contacts. Seems that was not your problem.

From the drawing: Preamp out is a signal from the power amp. So if no signal comes through the loop, it won't be on the preamp out either.

Nothing coming out the send jack tells us the preamp is the bad guy.

If one of the post pots were bad, then the other channel would work. Well, unless the top of one was grounded somehow. Since they are in parallel, grounding the top of one would kill both. A quicjk ohm meter test will tell you that.

The "other side" of C60,14 is also the top of R24, right? A volt there sounds reasonable. COming through C14, will drop the level, because now you have lost the low end which is a large amaplitude generally.

Forget the centers of the post controls, what is at the tops of them? Yes the wiper of VR5.

Let us establish some terms. The center leg of a pot is its wiper. WHen you day center, I thing you mean that, but to me, I think center means the wiper set halfway up. I have to stop and think to see it in yout terms. Call it the wiper, I am old and can't adapt. Snort.

Whatever - and 0.4 seems OK - is on the wiper of VR5 should also be on the top of the posts, so turn the posts all the way up and their wipers should then also have that 0.4v. Might as well max VR5 wwhile we are at it. We need to verify VR5 is not open anyway. If the signal is not on the posts, then there is an open between them and VR5.

If there is something on the wipers of the posts, but not after K2B, then K2B is very suspect. K2 is hard to get to, so R81 is a handy place to check. Me, I'd just go right to the grid of V3B.

At this point signal injection would be a good tool. Just touching a point in the signal path with a meter probe - leave the other meter probe not connected to anything - should inject some hum. You could use a signal generator, but we have a go/no go situation here, either sound comes through or it doesn't. We don't care what it sounds like yet. Worry about that we we hear something. One meter probe and its wire acts like a hum antenna, so touching it to the circuit adds that hum. A small screwdriver works as well. Be wary B+ volts of course. Stay insulated.

So far, we have signal up to the posts or close to them anyway. And we know that from the return jack on, it works. Touch the grid of V3B. Hum comes out? Circuit is working from V3B onwards. No hum and the problem is V3B or after. Touch the cathode of V3B. Then the two caps after is, hell even K3B could be bad.

If before V3, then it is a break in the path from the posts to V3. AN ohm meter will tell yyou if K2 is OK. Yes, ohms reading while the amp is powered, that way you can switch channels to move the relay.

Ooh ooh, here is a wild guess, if it pays off you will think I am from MArs.

There is a mute circuit. Every time you switch chanels, it silences the amp a brief moment so no loud noises come out. On the drawing it is called CLAMP. See the CLAMP arrow right above K2B? Between K2 and R81 there? The clamp circuit is on the other page, right above the bios supply. If CR21,22 are shorted, or Q7 is shorted or stuck on, that would kill the sound.

Try this. At the non-ground end of R81 and the amp runnig, measure for DC volts. 24VDC either + or - means a shorted diode. If zero DC< then flip to ohms. SHorted to ground? If not, the clamp is OK. If shorted to ground, then Q7 is shorted - pull it and see - or it is stuck on. SHorted triac CR23 or shorted C55 would cause that. Also open R84 I guess. In the grand scheme of things these are unlikely. FAr more likely Q7 or one of the diodes is bad.

And if all that is OK, oh well.

[MitchK]

Enzo, I hate to tell you this, but you really are from mars.

You were dead on, it was Q7. It was shorting to ground.

I tried injecting a signal on the grid of v3b and I could hear the hum and where as the signal died right after k2b I though that was it so I short the "wiper" of the ultra post just after it but before C52 and everything in the clamp but still nothing here, so I checked out Q7 like you said and sure enough only approx 60 ohm on the first leg, so I pulled up Q7 and it works great now. Haven't played it other than at very low volumes but I'll crank it up tomorrow and make sure it maintains.

So I guess all I need to know now is what kind of transistor is Q7? What should it be replaced with?

Also I just wanted to say thanks a lot Enzo, for years I've wanted to learn about guitar amps and be able to repair and eventually build them and I'll tell you just learning it and picking it up from 0 background in it, just by reading stuff on the internet, is really hard. No one rewards you for coming on here and helping people like me out, so I hope its a certain amount of pride or duty to your trade to come on here and help us out, I wish more people were like that these days. I've learned a TON in the last year and its mostly thanks to you and the other folks on this board. This was major accomplishment for me, thanks very very much for helping!

(don't worry I'll be back, that jcm800 is still sitting on my floor hahah)

[Enzo]

Listen when you change channels. Does it make a squawk sound or loud pop or other unwanted noise? If not, then forget Q7. It is not in the signal path and will not affect tone.

However, to be thorough, as noted on the schematic, Q7 is a J174. That is a P-channel JFET. Most any P-channel JFET would work, but the J174 is very common. Note that there is no standard pin arrangement for JFETs. All the J174s will be alike, but a different type, no matter how well suited electrically might have the legs in different order. That means mounting the transistor facing a different direction more or less.

By the way, JFETs are weird. They are "on" until a voltage at their gate turns them off. There is a relatively low resistance between the source and drain pins until the gate voltage opens that path. This makes it easy to tell which pin is the gate though. Out of circuit, sitting in your hand, measure resistance between the three leg combinations. The two legs that have a low resistance between them are the source adn drain. They can essentially be used interchangably. The remaining leg is the gate. Flip the meter over to diode test, and it will check like a diode from gate to source or drain. SO if you are unsure which leg is which, that is how to tell.


And thanks. I hope that you, and those who only watch silently, learn something of HOW to troubleshoot from my writings.

[madkatb]

Kudos to my favorite Martian. I know that this is an old thread but it got me out of problems.
5150 ii with very low volume and really harsh distortion. I went through the usual preamp tube substitutions, checked the voltages, bias, etc. No luck. Then the channel switching quit. Replaced K4 and C6 (bad solder joint on pcb). Now the channel switching worked but the sound was still the same. So it must be something common to both channels.Tried tracing the signal with an o'scope (still pretty much a noobie at that so it was good practice). Found out that signal was getting to VR5 (very low level). Jumpered from VR5 to V3 pin7 with a capacitor. Increase in volume. Much head scratching, looking at schematic, head scratching...
Went looking for Enzo's words of wisdom. (Should have done that first.) Followed through this thread. Replaced Q7. Happy amp...happy me.
So kudos to Enzo. And now I understand a lot more about relays and channel switching circuits. May (will) come in useful some day.
Now I need a critique on my troubleshooting from Enzo. Did I do this right or was I wasting time with the easy to do preamp swap? Should I have just gone to the scope first?