Looking for some help

[dude]

I can't seem to figure out why such a high a voltage is passing through a 68K resistor.


The power tube is a 7591, pins 8 and 4 (grids) are getting voltage from the power rail's last e-cap (orange wire), they are tied together as in pic. A 68K goes from pin 8 (grid)  on the 7591 to pin 6 (plate) of the 12AX7 and a 100K goes from pin 4 of the 7591 (grid) to pin 1 of the 12AX7.


With all tubes in, the voltage on the 7591 grids is 281v, the 100K drops the voltage to 110v (pin 1 12AX7).  But pin 8 (7591)with also 281v (they are the grids tied together) goes through a 68K to pin 6 of the 12AX7 but only 3 volts are dropped...?


I replaced both resistors as you can see the old ones hanging loose, same values. With all tubes out, still the same same drop on the 68K, higher voltages but still the 68K is not reducing the voltage as it should.


What gives? the voltage on the one plate of the 12AX7 is way too high and I'm getting a ratty amplified, overdriven sound. Sorry for the bad pic's .


If someone can help much appreciated. This amp is old, from mid sixties, SE 9591, one 12AX and one tremolo 6AV6 (tremolo work )


EDIT: 12AX7, pin 8 cathode is grounded through a 2.2K resistor and pin 3 grounded with a 2.2meg resistor from that ground to pin 2 the gris.....?


al

[sluckey]

No voltage dropped across that 68K plate resistor means the 12AX7 tube ain't conducting. No current means no voltage drop.

Measure resistance from pin 8 to ground. Is it really 2.2K? Then measure the voltage on pin 8. And measure voltage on pin 7 too.

[dude]

OK, first half of 12Ax7 pins, 1,2,3, the voltage is dropping across the 100K plate R (dropping 150v). That half has one leg of the 2.2 meg R, on pin 3 and ground then the 2.2 meg R goes to pin 2. No voltage on either pin 2 or 3. But that half is dropping at least 150V. see pic.


It's the other half that isn't dropping but 3 volts across the plate R of 68K. Pins 6, 7, 8 - small voltage drop on plate of 12AX7 of 3 volts, no voltage on pin 7 but 3 volts on pin 8. Pin 8 (cathode) has a 2.2K R, to ground and is not tied to the grid, pin7 like the other half.


Resistance: pin 8 to ground, 2.2K. And 3 volts on pin 8.


No voltage on pin 7.
 

[sluckey]

I'm not concerned about pins 1, 2, 3.

Resistance: pin 8 to ground, 2.2K. And 3 volts on pin 8.

That means the tube IS conducting. 1.36mA is flowing thru that 2.2K, the tube, and must also flow through that 68K. That means there should be 92.7V dropped across that 68K.

Did you actually measure the resistance from pin 8 to ground or did you just assume it was 2.2K because the resistor is red-red-red?

[dude]

I measured the resistance of that cathode R, it's 2.2K, I didn't take it out and measure but without a tube in it measured 2.2K and 3 volts.


I tried a few different 68K R's on that plate, still doesn't drop any voltage.

[kagliostro]

Did you tried one other tube ?


Franco

[dude]

I've tried several 12AX7s, all the same no drop in voltage more than 3 volts. I tried several 68K resistors, same, no drop...?


Without tube, just as Sluckey says, no dropping either, put tube in, V2 plate no drop. But when I'm checking for drop that pin 6 plate as a lot of noise or signal on it...?


The only thing I did was add a tone control but the plate of the 12AX7 to the vol is on the side of the 12AX7 that works.

[dude]

I'm not concerned about pins 1, 2, 3.

Resistance: pin 8 to ground, 2.2K. And 3 volts on pin 8.

That means the tube IS conducting. 1.36mA is flowing thru that 2.2K, the tube, and must also flow through that 68K. That means there should be 92.7V dropped across that 68K.

Did you actually measure the resistance from pin 8 to ground or did you just assume it was 2.2K because the resistor is red-red-red?

Yes, it's 2.2K


OK, I take it that the 12AX7 is wire properly and conducting, but for some reason no voltage is dropping on half of the tube. There is only one thing I can think of, the socket. This amp is old and all the sockets are dirty, I tried retensioning the socket but I don't think it tighten up the all the pin contacts. Perhaps one of the second half of that 12AX7 pins are not seated, bad contact. I can remember once during start up, that 12AX wasn't working and I tapped on the tube and it started working.


I know it's a remote possibility that maybe one of the pins 6,7,8 isn't making good enough contact...? Have you ever heard of this? Maybe I should replace that socket at this point...? 


What else could it be...? Only three pins to that half and the heaters work...? Maybe I bent the sockets out of contact trying to retention them.


Is replacing the socket seem worthy, at this point?


I appreciate you advice, I've got many amps up and running with your help.


Thanks, al

[sluckey]

I didn't take it out and measure but without a tube in it measured 2.2K and 3 volts.

Surely you measured 3 volts with the tube plugged in.

At this point I'm thinking bad connection. Maybe tube socket, or bad solder joint on pin 6. Measure resistance directly from pin 6 of the 12AX7 to pin 8 of the 7591.

[dude]

Yeah, tube in, 3volts on cathode pin 8.


67.5K pin 6 to pin 8 (7591), all solder joints on pins are good, I'm sure.  No resistance readings measuring from top of pins to bottom of sockets, continuity beeps on all pins bottoms to top of pins.


But solder has gotten down inside a pin, possibly preventing a good connection. The more I fool with the socket the worst it gets. I'll replace it, if that doesn't do it, I'll look for a demon in  the house

[2deaf]

A 12AX7 will not drop 3 volts across a 2.2K cathode resistor until the plate gets up over 350V or so. 

[2deaf]

A 12AX7 can drop 3 volts or so across a 220K cathode resistor with the plate at 278 volts with the minuscule current that continues to flow when the tube is in cut-off.  The current is so small that the voltage drop across the 68K plate resistor is going to be minimal.

That cathode resistor looks to me like a 220K in the pictures, but the pixels don't allow me a really good look at it.  Maybe you could quickly clip a for sure resistor in parallel with the cathode resistor just to see what happens.

[dude]

The cathode R for that half of the 12AX7, is 2.2K, not 220, I measured it and the bands are red, red, red.


I put the new socket in, same thing, no voltage drop on the 68K. But something is off when I measure the drop across that 68K. On the side toward the 7591, I get no signal noise when I touch the resistor with the meter but on the other side of the 68K (12AX7 side) I get a lot of static signal noise....? This may be a clue.


I did add a tone pot that wasn't there before. Wired it like a 5F2A Princeton. Since I'm getting a signal static noise on that leg, could the wiring I did for the tone pot have anything to do with no voltage dropping?  2deaf, I don't understand exactly what you said, this amp was made with the 2.2K cathode R on pin 8...


http://el34world.com/charts/Schematics/files/fender/Fender_princeton_5f2a_schem.pdf


al

[dude]

Sluckey,


If I take out the 2.2 meg R, and 2.2K on the cathodes of the 12AX7, old way to bias them and use a 1K5 with a 10uf cap on both cathodes, like a more modern amp would that help?


There is a another shielded wire attached to pin 7, side that's not dropping, that goes to the 6AV6 tremolo tube, pin 2 (has the 10uf also). Could this be causing the problem?

[2deaf]

Is there a wire connected to pin 8 in addition to the 2.2K resistor?

[2deaf]

Now that I have downloaded the other pictures, I can see the red-red-red of the 2.2K and a wire going to pin 8.  If this wire is the source of the 3 volts, that would fit the symptoms described.  I would try disconnecting that wire from pin 8 and see what happens.

[sluckey]

On the side toward the 7591, I get no signal noise when I touch the resistor with the meter but on the other side of the 68K (12AX7 side) I get a lot of static signal noise....? This may be a clue.

That's normal. The side toward the 7591 is just a B+ point. Pin 4 and 8 of the 7591 tie directly to a B+ point. It is just convenient to connect that 68K to the 7591 socket. There should be no noise at that B+ point due to the big filter cap that's also connected. The 12AX7 side of that resistor is the plate and that is directly in the signal path. You should get noise there when you probe it.

I did add a tone pot that wasn't there before. Wired it like a 5F2A Princeton. Since I'm getting a signal static noise on that leg, could the wiring I did for the tone pot have anything to do with no voltage dropping?

Sure it could, especially if it was working before. Why don't you remove it?

If I take out the 2.2 meg R, and 2.2K on the cathodes of the 12AX7, old way to bias them and use a 1K5 with a 10uf cap on both cathodes, like a more modern amp would that help?

The cathode (pin 3) is connected directly to ground. The grid connects thru a 2.2M to ground. That 2.2M has nothing to do with the cathode. The cathode just happens to be a convenient ground point for the 2.2M. You could have just as well connected the 2.2M directly to those ground lugs you installed when you changed the socket. The pin 1,2,3 triode is in a grid leak bias circuit. I would not modify it.

There is a another shielded wire attached to pin 7, side that's not dropping, that goes to the 6AV6 tremolo tube, pin 2 (has the 10uf also). Could this be causing the problem?

I see a shielded cable connected to pin 8. Is that what you mean? You can disconnect that cable and leave it disconnected while troubleshooting.

It would be helpful to see a bigger pic that shows how the rest of the circuit connects to that 12AX7. Also, a schematic, even a hand drawn one, would be very helpful. And tell us more about the amp. So far your only complain is that theres no voltage dropped across that 68K resistor. Does the amp work at all?  Is this some commercial amp that you are modifying for guitar? Did it work before you modified it? Tell us the story.

[2deaf]

2deaf, I don't understand exactly what you said, this amp was made with the 2.2K cathode R on pin 8...

The graphs for a 12AX7 indicate that it would take about 355V on the plate to get 1.36ma to flow across the 2.2K cathode resistor for the observed 3V at the cathode.  In order to have 355V at the plate with a 68K resistor, you would have to have 448V for the B+.  This just isn't even close to what is going on, so we can rule out a circuit with only a 12AX7 unit, a 68K plate resistor, and a 2.2K cathode resistor.

One way to get 3V across the cathode resistor would be to have a very large value resistor with a small current flowing through it.  Another way would be to have an external source of 3V across a 2.2K resistor.  Either way will produce the same numbers and be consistent with the graphs.

[dude]

Thanks for the advice and explanation, 2deaf, knowledge is helpful.


I'll try the suggestions that you mentioned, Sluckey and hopefully I'll get this little amp up and running, as of now the amp has too much unwanted distortion, even at mid to low volume. Not sure if it's blocking distortion but sounds like a bad speaker, very rattley. The 68K plate R on the 12AX7 is not working right, too much voltage, 281V on pin 6, amp at this point lost all sound, I think that half of the 12AX& is not conducting. That's the problem now.


The amp was bought at a flea market for $15. It played but had that ratty distortion. I never checked all the voltages down the rail as I figured replace 50 year old e-caps, check all caps and drifted resistors and the like. After I replaced almost all the R's and half of the caps. I added a tone knob as the amp was very bassy and weak on volume. I should have checked all the voltages down the rail and got the amp running before I started mods, live and learn. So, I have no idea if that 68K was dropping voltage or not before the tone was added.


I'll disconnect the tone pot and wire the vol pot as it was without the pot, like you say that could be the problem. That grid wire off pin 7 to the tremolo tube was always there from when the amp was made, I can tell that so that's hopefully not an issue.


If I can't figure out the problem, I'll take pictures in better detail of the whole amp and try to draw at least a preamp schematic, it was drawing the tremolo circuit that became difficult for me.


It's alway something simple. If I do figure out the problem, I'll post what it was. I appreciate all your help, I wasn't familiar on grid bias.


al 

[2deaf]

No need for destructive measures.  How about shorting pin 8 to the chassis with a screwdriver while watching the plate voltage.  Have you tried removing the 6AV6?

[dude]

ok, I'll try that 2deaf.


Here are some more updated pic's of complete amp

[dude]

Few more pic's

[dude]

Is there a wire connected to pin 8 in addition to the 2.2K resistor?

Yes, a shielded wire goes to pin 2 of the tremolo tube, 6AV6 pin 2.

[dude]

No need for destructive measures.  How about shorting pin 8 to the chassis with a screwdriver while watching the plate voltage.  Have you tried removing the 6AV6?

Now we're getting somewhere, by passing that 2.2K cathode R (pin 8 directly to ground), gives me sound and the 68K is dropping about 90 volts, just like Sluckey said it was suppose too....?


What next? Looks like the problem is something to do with pin 8 cathode....?


al

[sluckey]

Pull the 6AV6. Still work?

[dude]

Pull the 6AV6. Still work?

Yes, works with the 2K2 bypassed and the 6AV6 pulled.  Also lost that ratty distortion the amp once had. Now the volume starts on 3, before when I first got the amp volume pot only played on 8, 9 or 10. Someone has messed with this amp before me.


What next?

[sluckey]

Unbypass that 2.2K. Still work?

[dude]

Yes, now the amp plays with the 2K2 unbypassed and bypassed. 


I must have not let the amp warm up when I got a drop of 89v on the 68K plate R.


Now I get about a 60v drop with or without the cathode 2K2 bypassed. There's 301 volts on 7591 side of the 68K and 245 volts on the plate, pin 6 of the 12AX.


I'm completely lost here, working then not...?   Isn't 245 volts on pin 6 too much? once I figure out what to do with the cathode R on pin 8, maybe I should raise the 68K to 100K? Both old plate resistors were old and looked original to the amp before I replace them.


Completely lost, I have no clue if I should leave that 2K2 in or another valve. I play out a lot and use small amps, mic'd but I'm totally afraid of this amp, what's happening here?


al


EDIT: All the above is with the 6AV6 out, with it back in amp plays but volume has to be on 10 and the ratty distortion is back. This is with the 2K2 cathode R not by-passed. So, if I take the 6AV6 out, the amp plays well whether or not the 2K2 is by-passed. And leaving the 6AV6 in, as long as the cathode is bypassed, plays well. Seems like if I leave the 2K2 in or bypassed  it's doesn't make a difference as long as the 6AV6 is out.


So, maybe the problem is in the tremolo circuit as that shielded cable you see from the cathode goes to pin 2 of the 6AV6. I'll check if something is shorting out as the caps and resistors are pretty close to each other...?


Also, is 245v too high on the plate of the 12AX7

[sluckey]

LEAVE THE 2.2K ON PIN 8! The problem is in the 6AV6 trem circuit. That 3 volts that was on pin 8 was coming from the trem circuit and was keeping the 12AX7 tube from conducting. When you pulled the 6AV6 tube that 3 volts disappeared so now any voltage that's on pin 8 is just normal bias voltage that results from the tube conducting normally.

Just leave the trem circuit alone for now. There's an issue on the pin 1,2,3 side of the 12AX7 that needs some attention. Someone has put a couple 68K resistors on the input jacks, clipped out a 100K resistor and replaced a capacitor with a red wire connected to pin 2. Probably an attempt to convert the preamp to a more Fenderish circuit? Did you do that? Anyway, that's a good idea, but the job is only partially done and the bias for the preamp is messed up. Here's what you need to do to complete that conversion...

Replace the 2.2M between pin 2 and ground with a 1M. Use that new ground lug to connect the 1M resistor.

Remove the wire from pin 3 to ground and install a parallel 1.5K resistor and 22µF cap between pin 3 and a ground lug.

Now the preamp will behave like a Fender preamp and will sound better than to original grid leak biased preamp.

After making these changes, reevaluate the amp. How does it sound? Does your tone control mod work as expected?

Once you are satisfied that the amp is all it can be we'll tackle the tremolo circuit.

[dude]

OK, I'll make the changes.


Yes, I did take out the .022 cap from input to the grid, pin2.  I also clipped the 100K out and just replace two old 68K's with new, they drifted a lot.


I also added a tone pot (like a 5F2A) worked good when the played but took it out when I was troubleshooting.


So, my mistake is I didn't change the grid bias too. That grid bias 2.2M, is on pin 2 to ground and pin 3. Your saying put a 1M on pin 2 to ground. And just a 1K5 and 22uf (I like 10uf or less, don't like the bass a 22uf gives) from that cathode to ground.

[sluckey]

That .022 cap that you clipped out was a very important part of the grid leak bias circuit. That grid cap and 2.2M resistor and grounded cathode defines the grid leak bias circuit.

But, we're gonna convert to the better Fender circuit, so none of that matters now.

Your saying put a 1M on pin 2 to ground. And just a 1K5 and 22uf (I like 10uf or less, don't like the bass a 22uf gives) from that cathode to ground.

Exactly. Give us a report when done and you have played the amp a bit.

I almost have a schematic drawn from those last good pics you posted. I'll probably need one or two more for the trem circuit though.

[dude]

Here a crummy pic of the trem circuit, hope it helps. The good pics were from a friends camera, I have to get a better camera and phone soon.


Anyway, the resistors from left (power supply) to right are: 68K, 4K7, 1M and 220K. The tremolo on/off pot has a 220K to ground and is regulates speed and intensity. The tremolo caps are .01, .02, .02.


I've learn a lot from this conversion about grid bias, what I now read on the subject makes better sense, thanks,


al

[sluckey]

What's the value of the caps in the can?

What's the value of the 3 resistors connected to the cap can? I'm guessing the big one is 100Ω, the mid-size is 4.7K, and the little one is 68K.

[dude]

What's the value of the caps in the can?

What's the value of the 3 resistors connected to the cap can? I'm guessing the big one is 100Ω, the mid-size is 4.7K, and the little one is 68K.

That's exactly right.


I did the change on the V1, the amp sounds pretty good. Probably better if I had left the tone pot hooked up but I can do that last.


Since the amp sounded good without the tremolo tube in, I figured I'd replace all the components that I didn't before. I had the tremolo working pretty good a few days ago so I figured the wiring was correct.  I made a big mistake replacing the caps and cleaning up the joints. I put the end of the 1M on the leg where the 68K and 4K7 were instead of on the .02 cap. I tried the tremolo after I replace all the components but that 1M on the wrong tab I didn't know...?


After playing the amp for 4 or 5 minutes, smoke was coming out of the PT. I have a 2 amp fuse but it didn't blow. I shut down the Amp, it was so hot I couldn't touch the PT.  After it cooled, I looked at the last thing I did in the trem circuit and saw the 1m on the wrong tab. Changed it to where it was and checked all the other R's, there good. After the PT cooled down, I was praying I didn't blow it. I turned the amp on, sounded good, no tremolo tube. But the PT started to smoke again, even with the 1 meg in the right place.


Is it possible I ruined the PT. The amp plays but the  PT gets hot pretty fast. This amp is my son-in-laws, I have a Princeton PT I might have to give him. All this started with that 1meg to ground on the power to the tremolo

[sluckey]

You could miswire a 1M resistor anywhere in that amp and never cause the PT to smoke. I suspect you have accidentally done something else. Look around the area you were working in. Look around the diodes and cap can. Something is touching something that it shouldn't. Or maybe the PT has given up. Disconnect the red wires. Still smoke? Disconnect the green wires. Still smoke?

Invite your friend with the good camera over for a beer.

[dude]

You could miswire a 1M resistor anywhere in that amp and never cause the PT to smoke. I suspect you have accidentally done something else. Look around the area you were working in. Look around the diodes and cap can. Something is touching something that it shouldn't. Or maybe the PT has given up. Disconnect the red wires. Still smoke? Disconnect the green wires. Still smoke?

Invite your friend with the good camera over for a beer.

I didn't answer on the cap can, 32/32


Well, I just had a gin and tonic and I'm pissed at myself. A 1M R, isn't going to burn anything up sounds logical. Blobs of solder were falling through the amp cleaning up the all the old tabs (old solder is dirty and needs heat). But I thought of that and didn't see anything shorts to the chassis. All the wiring is as before. Last thing I did before it smoked was after the tremolo mistake with the 1M, hot to ground. But that's debunked. Hey, maybe with all the stress I put on the PT when it was not playing and being left on for too long, it just went. I doubt it. I did something with that last try that's the problem. I need a rest or maybe another gin & tonic, ha. Tomorrow's another day. al

[dude]

You could miswire a 1M resistor anywhere in that amp and never cause the PT to smoke. I suspect you have accidentally done something else. Look around the area you were working in. Look around the diodes and cap can. Something is touching something that it shouldn't. Or maybe the PT has given up. Disconnect the red wires. Still smoke? Disconnect the green wires. Still smoke?

Invite your friend with the good camera over for a beer.

I looked everywhere for a possible short, none outside the PT.  The only thing I did right before it smoked was change some the tremolo components, and miswired a 1meg between 170v input voltage to tremolo and ground. I always use a 220K bleeding resistor on the power supply nodes with no problems, so like you say 1 meg would do nothing.


I think the PT just gave up, I tried it a few times, I get voltage and the amp plays but the PT gets hot fast, then smokes. At this stage I'm not giving up, I have another workable PT but it's a laydown. Thinking about rigging it up, attached to the outside of the chassis like the original in the pic.  But I'd rather get an inexpensive 290-0-290 PT. like the original mounts. Any suggestions where I can find one, ebay has nothing now in vintage electronics.


Also, since you've been pretty tolerant with me, could you look at the tremolo wiring and give me your opinion? It's 98% as the original was, just new components. I sure don't want to blow a new PT, I say 98% because nothing's 100%.

[EKDENTON]

Looks like you may also have some grounding issues around the pots and jacks that probably need looked into.

[dude]

Looks like you may also have some grounding issues around the pots and jacks that probably need looked into.

Yeah, I know. Once I get everything up and running, I'll fix the grounding issues.

[sluckey]

I have another workable PT but it's a laydown.

Go to Lowe's and buy a package of 3/4 x 3/4 x 1/2 angle brackets. They're called corner braces. Cost under $2. Mount these to the laydown PT and then you can stand it up.

http://www.lowes.com/pd/Stanley-National-Hardware-4-Pack-0-75-in-Zinc-Corner-Brace/3169855

or get these...
http://www.lowes.com/pd/The-Hillman-Group-4-Pack-1-2-in-Zinc-Plated-Corner-Braces/3478185

could you look at the tremolo wiring and give me your opinion? It's 98% as the original was, just new components.

I think the tremolo will work now that you have fixed that 1M resistor error. That wiring error was causing that original 3V on 12AX7 pin 8 that was preventing a voltage across the 68K plate resistor. The only thing I see in the trem circuit that looks odd is that 4.7K + 68K that's connected between the 6V6 pin 1 and the filter cap can. That's an easy fix though if the trem doesn't work.

Your schematic is attached...

[dude]

That 68K and 4K7 in series feeding the trem was odd to me too, I assumed the trem circuit need a 72K7 R, which is not made. I thought why not use a 68K or 82K, it's within 20% specs...? But I left it as it was, maybe that tremolo circuit needed to that precise...?


The tremolo did work at one point wired the way it is now, that's good news.


I never had a PT go right after I made changes (except human error on those changes) but just could not see any shorted wires externally. This amp is pretty simple and the problem would most likely be around the power supply. No solder blobs, broken wires.


This amp was most likely sitting for years in storage before my son-in-law bought it for $15 at a flea market. It's an old paper PT, two hole stand-up, probably at least 50 years old. Maybe stressing it a bit with the 1Meg resistor and wiring error, at times I had no sound and was on for a while.


I'll take a longer look at the  PT I have and either get some small brackets like you posted or maybe even cut the chassis if I can get everything to fit. 


Once I get the PT and the grounding issues fixed, I'll post the results.


Also, thank you for the schematic,  al

[sluckey]

I would suspect that cap can more than the PT. Hook up a power cord to the PT with it out of the circuit and measure the secondary voltages.

[dude]

I would suspect that cap can more than the PT. Hook up a power cord to the PT with it out of the circuit and measure the secondary voltages.

Man, you are good, 306vac on each high voltage line (red to CT). Just like it did when when in the amp but no cap can hooked up. Ha, only difference is without the can the PT is cool as cucumber.

I can't win with this amp, who would think a brand new Antique Electronics Cap Can would go bad after an hour or less of use? To bad for me, I just cut the chassis for the new laydown PT, fits nice. I took a pic of the new PT in the amp and was going to send a picture before I wired it up. Glad I thought of that and saw you last post.       I hear you laughing, it is kind of funny. I probably should have left my tinkering skills to fixing guitars and making cabinets (I'm a cabinetmaker self-employed for 30+ years). But I'm a tone freak guitar player, I just can't can stop...?


Oh well, on I go.  THe only thing I can think of to make that can short is a solder blob or something metal stuck on one of the 32uf lugs. And that's not possible, I look at those two legs for a long time, even used small metal brush. Nothing was shorting it out.


How often goes a  new can go bad....?  For $7.50 it's not worth it to send back,  I Would have ordered from Doug but I needed a speaker too, they had a speaker sale. Sh*t.   


I need to test this can, how can I do that, my cap meter only goes to 20uf, maybe a continuity test on the common to each leg?

al

[sluckey]

I didn't realize you had a new can in it. The can is probably good. That old PT has good voltage on it just sitting on the bench but it may fail when under load.

Check resistance from common to each cap terminal on that new can. If no shorts or low resistance readings then hook it up in the amp and check voltages on each cap. If the voltages look good and the hum level is OK AND the cap does not get warm, I'd say it's good.

[dude]

I didn't realize you had a new can in it. The can is probably good. That old PT has good voltage on it just sitting on the bench but it may fail when under load.

Check resistance from common to each cap terminal on that new can. If no shorts or low resistance readings then hook it up in the amp and check voltages on each cap. If the voltages look good and the hum level is OK AND the cap does not get warm, I'd say it's good.

Already did that with the cap can, no continuity, no ohm reading, zero resistance on the two 32's to common. I always put in new e-caps on old stuff like this, that's a first with me.


I need to learn from this. So, a PT hook-up in an amp (and no other issues in the amp) and it smokes but "out" it doesn't....,   Doesn't necessarily mean it's good as no load is on it. A bad PT can read ok without a load.

So, in my case it's either the PT or the can, I also have another 22uf e-cap. Could be either of them. Both caps read no continuity and zero ohms. so there is most likely no short inside. 


What if I use my laydown PT, it's brand new and the other is mostly bad...?


Then I put another 32uf cap in parallel and check each leg for 16uf, my meter will read under 20uf. Or even better I just wire the two 32uf tabs in parallel and measure the capacitance, looking for 16uf? Since no shorts and values read good, cap can and single 22uf is probably good and the PT is the culprit..?  Your take. I know nothing is 100%.


I'm just paranoid at this point. I've fix much more complicated stuff than this simple little amp. Live and learn. I appreciate you help,


al

[sluckey]

Or even better I just wire the two 32uf tabs in parallel and measure the capacitance, looking for 16uf?

The value of caps in parallel ADDs, so two 32µF in parallel would be 64µF.

If the cap holds a good B+, does not hum, and does not get warm, I'd call it good. The actual µF value is not that critical. If it is marked as a 32µF, then I believe it.

Hook it all up and see how the amp behaves.

[dude]

OK, will do.


I need a break from this amp, I'm done for today. I'm thinking the opposite of correctly wiring series and parallel cap values, this is  correct: Series halfs the values, parallel doubles the value but keeps the same voltage rating. Resisters just the opposite. Also, with series caps use a 220K balancing R between each series wire cap. Caps are usually not wired in parallel, waste of a cap. Series is used to double the voltage rating. 


Tomorrow is another day, sure hope this SE 7591A, (Princeton with tremolo topology) sings,
al

[dude]

Amp is all wired with new PT, just one question.


On the 7591, pin 6 you have a 100K to ground. The original amp had a 180K to ground, I would think it probably doesn't make much difference.


Should  I just make it 100K?   


 


al

[sluckey]

It's obviously a 180K. I made a mistake on the schematic. I would leave the 180K in the amp.

Here's the corrected schematic...

[dude]

No luck with this amp, put the new PT in, checked everything, no volume, nothing but voltages. The voltages are higher due to the PT. 


B+ of 410, drops to 400 after the 100 ohm, the 400v goes to the OT and feeds the tremolo circuit through a 68K, 4K7 drops the 400v to around 350v to the grids of the 7591A, then two plate resistor, 100K and 68K, so the 12AX7 gets hit by 225 on the 100K side and 250 on the 68K side.


Want to know if these voltage are too high to the 7591 and 12AX7? I was thinking I could use two 15 watt Zeners in series on the CT to lower about 30v on the B+, up the last dropping resistor to the 7591 and 12AX7 to a 10K. I"m thinking around 150v and 180v to the plates of the 12AX would be good, what do you think? 


I plan to disconnect the tremolo circuit by pulling the 6AV6 and snipping the 68K that feeds voltage. Then just get the amp to play. When I first got the amp, it was set up with grid bias and played but had a raspy distortion and only produced sound at 9 or higher on the volume.


I'll fool around with it some more before I post some clear pic's of what I did, see if you see any mistakes. Just wanted to know if the voltages are too high. When I turned the amp on for a minute or so warming up I thought I saw a small flash around the base of the 7591...? PT doesn't seem to get hot but it's inside a metal cover. I took the cap can out and hooked up a single 32uf to each leg (in series)and got a reading on both sides of the 32uf cap of 18uf (reads oK, my meter only goes to 20uf) , no shorts with meter on lugs of cap when out. I replace the 22uf first filter, still nothing.


Other than what I mentioned, can you think of anything? So, I'll keep checking my work but have a feeling it may have something to do with changing the bias of the 12AX7...? I know you just drew the schematic from what I had, but look at it and let me know if anything was missed. This is the first Amp I can't get working and also one of the simplest SE circuits...? i just can't seem to see any wiring mistakes except maybe the input jacks. I also plan on cleaning up solder blobs and rewiring the tone pot, it's sloppy but just want to get things working first. 


I think I noticed when checking the voltage on the 12AX7, pin 1, I didn't get a single noise.... maybe I need to check my input wiring. I didn't use shorting jacks...?


al