No input sound to speaker on Fender Prosonic

[Skel]

Enzo, as I said in my last post, you were right - the voltage on pin 8 was about 450 volts. With the switch in the diode rectifer position I have input signal coming out the speakers. Prior to me changing the rectifier tube out, when I would switch it to the diode rectifer, it would still blow fuses.

So naturally I tried switching back to the tube rectifier, and no output.

This if for another time, but I'm also wondering why I had to switch to the diode rectifier in order to measure the plate voltage of the rectifier tube.

I think I can tell you are systematically isolating the problem though - this is cool stuff. Thanks.

[Enzo]

That is my mantra - "Isolate the problem." That one thought is the very essence of troubleshooting.

I asked you to switch over to the solid state rectifier because it would eliminate any confusion about the tube and bypass potential damage from the bad tube. That switch setting is the simplest way. It blew the fuse before in that position because you still had the bad tube in it. The tube is still wired up and live when in the SS setting.

You SHOULD get B+ at that point regardless of which rectifier setting you use.

[Skel]

Hmmm. So now that there is 450V at pin 8 of the rectifier tube, that means this part of the problem is solved. But still there is no signal to speaker in the tube rectifier mode.

In looking at the schematic, I kind of figured out (hopefully right) that SW304, the "rectifier" switch, doesn't JUST change the rectifier used. SW304a & SW304b are just both sides of the standby switch (if that's the right terminology), common to all three rectifier positions. SW304c has something to do with the bias circuitry (?), and SW304d has something to do with how the signal is relative to pin 8 on the power tubes (?).

Since I get no output in both switch positions 1 & 2, the only thing in common these 2 positions have is SW304c, R104 & R105. R104 is 6.8k and R105 is 68k on the schematic. The measurements I got were R104 = 6.2k, R105 around 20k.

I'm probably totally off base, but ya gotta learn somehow - btw, just hearing the amp again, albeit in diode rectifier mode - it sounds SO KILLER after playing through a 50 watt Marshall the last few months. Marshalls do their thing great, but Fender clean makes me melt. I really couldn't care less about channel 2, even though it's the best Fender high gain channel I've heard.

[Enzo]

It works in SS rectifier mode, so we now know the standby switch is OK, and everything after the rectifer is OK.

So if it doesn't work in tube rectifier mode, the problem pretty much has to involve either the tube itself or the selector switch. May I assume there is no B+ when the tube recto is selected?
If the rectifier tube heater lights up, then we know the 5VAC is OK. To test the selector switch, just set it to tube rectifier and see if HV AC is getting to pins 4 and 6. If HV gets to 4 and 6 amd the tube heater lights, then you either have a bad tube or the socket is bad, or the wiring thereto.

Looking at the combo schematic, SW302 is the standby. SW304 is the selector switch, a multisection switch. 304a and 304b select which rectifier gets the HV. 304c just above selects the grid connection, either to fixed bias or to ground. ANd 304d over near the power tubes selects the cathode connection for the power tubes - ground or cathode resistor.

[Skel]

It works in SS rectifier mode, so we now know the standby switch is OK, and everything after the rectifer is OK.

So if it doesn't work in tube rectifier mode, the problem pretty much has to involve either the tube itself or the selector switch. May I assume there is no B+ when the tube recto is selected?
If the rectifier tube heater lights up, then we know the 5VAC is OK. To test the selector switch, just set it to tube rectifier and see if HV AC is getting to pins 4 and 6. If HV gets to 4 and 6 amd the tube heater lights, then you either have a bad tube or the socket is bad, or the wiring thereto.

Looking at the combo schematic, SW302 is the standby. SW304 is the selector switch, a multisection switch. 304a and 304b select which rectifier gets the HV. 304c just above selects the grid connection, either to fixed bias or to ground. ANd 304d over near the power tubes selects the cathode connection for the power tubes - ground or cathode resistor.

Correct - no B plus voltage when tube recto selected. I see the 5 volts AC and the HV AC on pins 4 & 6. I don't know what this means yet, but I also don't see the B plus at the cathodes on D103/104.

So if the tube is bad, and remember we're talking about the tube I just bought a few days ago - since I can run in diode rectifier mode now, without blowing fuses, doese this mean (if it does turn out to be the tube) that this new tube is "bad", just not AS bad as the one I replaced, which must have been shorted or something? btw I bought the GZ34 at Guitar Center, and I think they found it sitting under an old SVT; It wouldn't surprise me if this tube is bad.

Now I had to re-read all of your posts - this is free college :-)

[Enzo]

The cathodes of those diodes and the cathode of the tube are wired together, so if there is no B+ at one, there will be no B+ at the other.

You have HV AC - as opposed to HVAC, air conditioning - at pins 4 and 6 of the tube and you have heater voltage. Does the tube heater light when plugged into the socket? If not, then either the socket is not making contact with the pins or the tube is burnt out. You can always hold the tube in your hand and measure resistance between the heater pins.

EVERYTHING else works but the recto tube and its asociated socket and wiring. If the voltages are getting to the socket, then either the tube is bad or the socket is.

Plug just about any 5v recto tube you have into the socket for a test. You don't have to crank the amp, all we want is to see if the amp makes B+ through a recto tube. Even if it is the "wrong" tube, it won't hurt anything to power it up a couple minutes to see if it makes B+. Don't even have to play through it, after all we already know that if we get B+ there the amp will work. Fire it up and take a B+ voltage reading. If it appears, then your GZ34 needs replacement.

Never assume a tube is good because it is new.

[Skel]

Remember, you forgot more than I know. I measured resistance between the heater pins and it was virtually zero - I'm just using my "light bulb" common sense, thinking if the filament was burned out, I would get an "open" resistance reading. Honestly I don't even know what I'm looking for when you ask if the heater is lighted...I think it is though.

Yea, I'll never forget what you said, and won't assume a tube works just because it's new. I don't have another GZ34 around to even try. I'll just buy another one - at least I'll know then. If it turns out to be the socket, I guess I'll be "going to school" a little more - my head hurts from trying to learn so much, so fast. I have a new appreciation for amp techs!

I'll let you know tomorrow when I pick up a new tube.

Thanks!

[Enzo]

The 5v transformer winding will measure almost zero ohms, it is fairly heavy wire and not all that many turns of it. You have to remove the tube from the socket to measure its heater. Hold the meter leads agains the appropriate tube pins.

But if the tube lights up its heater when in the socket, then we already know the heater is intact AND that the transformer winding works.

WHat do I mean by lighted heater? I just mean the little orange glowing thing in the center of every tube. Or in some tubes, in the center of each section inside. If there is the little orange glowing spot in the tube, then it is "lit" and heating the tube. That doesn't tell us ANYTHING about the rest of the tube function, but without it the tube will do nothing.

If I speak over your head, please say so. I try to set my explanations at the level of the person posting.

[Skel]

Enzo, I did remove the tube when I checked the resistance between the heater pins - I listen carefully to what you say and follow your instructions as closely as I can.

You're not talking over my head at all. This doesn't mean I understand a lot of the things you refer to, but that's a good thing - I can't learn unless I'm forced to look up and study things. This is kind of my first real week of actually beginning down the road of becomming a real amp tech. I could (and will) spend hours just studying what you said about how the recto switch works in the 3 positions. When you told me the voltages I would see on the recto tube pins, the B plus voltage, etc., and then I actually saw these with my own eyes, I was like "OMG, he's right! how could he KNOW this?"

There was another person earlier in the thread who suggested I check D101 & D102, explaining that if they're good, they'll be open in one direction and have a significant resistance in the other - that made sense so I tried it and got resistance in both directions. The problem was, that I didn't know there is a special "diode" resistance reading switch position on my DMM. Now I know.

"B Plus" Voltage. I never knew what this was, and although I'm not positive, I "think" it's the raw DC voltage immediately after the rectifier converts the AC it receives from the PT.

I now know that "heater" and "fillament" is referring to the same thing.

I now know that the little notch on (at least power tubes, maybe all tubes) is between pin 8 and pin 1 (at least on octal tubes) - that's helpful when you can't read the numbers or are confused whether the number you read is referring to the lug below it or above it.

I now know how to discharge filter caps (I hope) by shorting between chassis ground and pin 3 (plate voltage) of a power tube. and I now know that pin 3 IS plate voltage. I know there are other ways as well. I didn't know any of this last week. Plus, I know the filter caps on (at least this) Fender amps are underneath a thin metal pan - I didn't know this until now.

I now know that you don't have to take the chicken head knobs and the nuts and washers off of the pots to get a Fender combo amp chassis out to work on it. I had never done this before and I do some stupid things when I'm first learning - now I know.

I now know how to tell which side of a diode is the cathode - didn't know this before.

I'm getting pretty familiar with the pins on a power tube - the heater pins, anode (plate, pin 3), cathode (pin 8). And also a rectifier tube's pins - the weird part is that I find it fascinating.

I didn't know that going from "Standby" to "On" applies plate voltage to the tubes.

I didn't know that just because a tube is glowing doesn't mean it's "good" or working, nor did I know that just because a tube is new doesn't mean it's good. Now I know.

I didn't know that properly soldering a pin should only take a couple seconds at the most. So I probably heat damaged the PCB in my DSL 100 trying to solder the fuse holder in, by holding my archaic old soldering iron to the poor board for minutes while it heated up. Dumb, but now I know. The challenge is going to be getting a new PCB since I'm not an authorized Marshall tech. I called Marshall and you have to have a store front and be in business for at least one year to become an authorized warranty repair facility - kind of a chiken and egg thing, plus I want to work out of my home.

I know that I can't read a schematic and tell what's going on worth crap.

I kind of knew this, but now I REALLY know that, for me, having the right tools, supplies, the right set up, lighting, bench, safety knowledge, little systems for where you put screws, taking pictures of how things were wired prior to taking things apart - all this will make my job much easier, and I'll do better work. (plus I didn't know a shop vac works fine for vacuuming out an amp). As I keep doing this I'll learn what measuring instruments I need (like an oscilliscope). Right now I have crap strung out all over the place in my basement family room - I'm unorganized...not good.

I could buy all the tube amp books in the world, but I can see that if I just study your posts to everybody's threads - even for one week - it's the ultimate "how to fix tube amps" book. It's not that I'm cheap, I just learn better by actually doing stuff than by JUST reading books, and it's so great to have a real pro to ask questions to - I don't know why you do this, but thanks. If I repaired one or two broken amps per week for one year, I think I'd be pretty good, and qualified to call myself an amp tech - that's my goal. The challenge might be finding amps to repair. Luckily I still have a '74 Marshall MK II Lead head that's dead (plus an extra '74 chassis I can take parts like OT and PT from if need be), an SVT 3 PRO that I have to learn how to set the "MOSFET bias" on, and an SVT II non-pro that starts making all kinds of loud crackling noise after it's been played for about 5 minutes - hopefully this will be a tube issue, but no matter what it is, it's a chance to learn more. Plus I have a "BiasProbe" but lost the manual and have never used it - I'm gonna learn how to use it now and bias the tubes in all my amps. So I have about a month or two's worth of stuff to keep me busy learning. After this, I think I could find small, non-chain music stores that buy and sell used amps, and offer to fix broken ones for a "parts only" fee until I get good enough to charge for my labor.

I know I drink way too much coffee :-)

[Skel]

Sir Enzo,

New recto tube, 450 V at pin 8, B plus voltage - what a beautiful sight!

All this #*#T because I assumed the new tube was good (but it was worth it all).

I never new the amp had to be off standby for B plus - I do now though.

Thanks for your expertise - This wouldn't have happened without your guidance.

[Enzo]

As they say, a journey of a thousand miles begins with just a step. Back about 1954 my dad bought me a little crystal radio kit. I built it, and it worked. I didn't care what was on the radio but I liked listening to it coming from my kit. Then I started adding and removing parts and changing connections to see what it might do. I have been learning ever since.

Start where you start and keep working at it, and one day you will think to yourself, "Hey, I think I'm getting the hang of this."

[Paul P]

Back about 1954 my dad bought me a little crystal radio kit.

Now that brought back a memory or two. I built one of those too, though
it was mid-sixties. Ran the wire from my bedroom window to the clothesline
pole. I think it was just one diode and a coil of wire with a wiper. Spent
hours listening on the thing.

I also made buzzers and ran wires to my sister's room down the hall so we
could talk to each other in morse code...

Paul P