Help! I tried to recap my amp!

[docz]

Ok, I did the measures. Here are the values

Filter caps: 288V

6X4
1: -8v
6: -8V
7: 294V

EL84
1: 0v touching this pin made more hum, it was a higher pitch hum - the kind you get when you short the guitar cable with your finger.

2: 8V
7: 275V
9: 280V

12Ax7
Everything 0V
And my ears are ringing, two of the pins made a really loud pop in the speaker. After the pop the static went away, only to return again.

Volume pot controls how much of the hum and static enter the speaker. But hum is present even with the volume all the way down.

DocZ

[martin manning]

Okay, so there is no high voltage getting to the 12AX7. Referring to the schematic, check the path back to the power supply from the plates (pins 1 and 6).

The effects are working, so there is high voltage at the right side of the 5K resistor that is between the two 10uF filter capacitors (10uF and 33uF as you have it in your amp), but there is an open connection downstream of that point (to the left), before the two plate load resistors (100K and 250K)...

Maybe you missed something when you replaced the 10uF filter cap on the left? Check the connections at the terminal lug with the positive end of the 10uF, the 5K resistor, and the red wire that goes over to the 12AX7. Looks like you're almost there!

MPM

[docz]

Just a quick question..

Those resistors hooked to the filter caps. Is there a way I can calculate their watt requirement? Since they sit between the caps, and I know the power supply is 260V 60mA, does that mean that these resistors need to be able to handle 15W? Or is there something that lowers the current on the way to those resistors?

DocZ

[martin manning]

Just a quick question..

Those resistors hooked to the filter caps. Is there a way I can calculate their watt requirement? Since they sit between the caps, and I know the power supply is 260V 60mA, does that mean that these resistors need to be able to handle 15W? Or is there something that lowers the current on the way to those resistors?

DocZ

It's only the current passing through that counts. Taking the 1.5K between the last two filters, it supplies current to the two triodes in the first 12AX7. As a rough guess, they will idle at about 1.5 mA each, and being Class A stages, that will be the maximum current. Power is I^2*R, so 0.003^2*1500=0.014W. A half-Watt resistor would therefore be ~35x what is required in operation, but it is not uncommon to oversize components liberally for reliability, and perhaps for mechanical properties too. If you replace any of them, just use the same wattage rating.

MPM

Edit: corrected calculation

[Phil_S]

Ok, I did the measures. Here are the values
Filter caps: 288V

Looks good.

6X4
1: -8v
6: -8V
7: 294V

1 and 6 are AC voltage from the transformer. No DC should be there.

EL84
1: 0v touching this pin made more hum, it was a higher pitch hum - the kind you get when you short the guitar cable with your finger.

2: 8V
7: 275V
9: 280V

Are you sure? Pin 1 is not connected to the tube. Is there something tied to pin 1? What is it?

Pin 2 is the grid. This is where input is provided to the tube. 8V is possible, but at idle it is often closer to 0V.

Pin 7 is the plate and pin 9 is the screen grid. These voltages seem backwards. Maybe when we get a signal going through, it will change. I suggest this goes on the list of things to look at later.

12Ax7
Everything 0V
And my ears are ringing, two of the pins made a really loud pop in the speaker. After the pop the static went away, only to return again.

Volume pot controls how much of the hum and static enter the speaker. But hum is present even with the volume all the way down.

I'm not sure how you got a loud pop all the way from the 12AX7 to the speaker with no plate voltage. Maybe you are not measuring this correctly? This just seems odd to me.

See if you can trace the voltage. There is a filter cap that feeds a plate load resistor. On the power (B+) rail find the 2nd 10u cap. What is DC voltage at the + side of the cap?

Then follow to the 100K plate load reistor (upper right on the schematic). What is voltage on each side of that 100K resistor?

Then, the other half of the 12AX7 has a 250K plate load resistor. What is voltage on each side of the 250K resistor?

[martin manning]

I'm not sure how you got a loud pop all the way from the 12AX7 to the speaker with no plate voltage. Maybe you are not measuring this correctly? This just seems odd to me.

Touching the plate of the second 12AX7 stage hits the output tube's grid through the last coupling cap. My guess is that the red wire going from the last power supply node (junction of 1.5K and 10uF) is not connected, but the B+ ends of the plate load resistors are still tied together. So, touching the plate of the first 12AX7 stage would hit the output tube's grid through the plate load resistors and the last coupling cap.

MPM

[docz]

Ok, I did another set of measures. I don't know if it is me who is just not good with a multimeter, but at least now I got a reading

12AX7 #1
pin 1 before 100k 278v after 165v
pin 6 before 250k 278v after big pop and 191v

12AX7 #2
pin 1 before 100k 282v after big pop and 220v
pin 6 160v can't find a resistor here.. only a .005 uF cap

The hum is definately arround "E" my guitar tuner says an Eb but the needle is on the top so it borders the "E"

The "pop" comes with a pop, followed by a short loud squeal, then it is gone, and a loud hiss when probe is removed. It is sort of like when you pull a microphone cable out of the mixer without muting first.

Any clues that might solve my problems?

DocZ

[Phil_S]

I asked to probe both sides of the resistor to see if there was some sort of wiring error or disconnect. I think not. B+ voltage will fluctuate a bit. You have it at either 278 or 282, so that's good -- call it 280v.

I think you might eventually find the schematic helpful to write on. The more you look at it, the more sense it will make and it will start to translate to the layout. Make a copy to record voltages if you haven't already done so.

Where you found 160V at the .005 cap, trace it to the tube. There is another leg that goes to a 100K plate load resistor, and then back to the 10u cap.

The squeal may be a sign of a bad tube. Try swapping the 12AX7's and see if the problem follows the tube. It could also be poor lead dress (where the wires are located). That amp is a rat's nest of wires and parts, which doesn't help. V2, in particular is a mess to sort out.

[martin manning]

Based on those voltages it looks like the first two triodes in the signal path are conducting, and they're pretty consistent with expectations for the plate loads and a reasonable bias point. Trying a swap of the 12AX7's can't hurt, though.

I've lost track here... does a signal at the input jack (guitar) get through to the speaker or not? The reverb and trem work, so is the issues just between the input jack and the first gain stage?

MPM

[MBD115]

"rats nest" is an understatement

Reading your first post again I would like to think what happened that day was V1 preamp tube went out. I base this on the fact the amp worked after being stored and quit while you were playing. I could be wrong of course.

The photo looks complicated but the schematic does not.

Man ain't turret boards nice!!!!

"loud pop". V1a has the tone/volume controls which might be keeping that stage from popping (signal attenuated). V1b does not. Any pop here when connecting the probe would be wide open to the power tube and much louder/harsher. Maybe that is whats going on here... ?

Also a good idea... Check the voltage on each side of a plate resistor, like the 100k on V1a, as you did and record the voltages. Then check the voltage just after the coupling cap to see if its leaking DC. Your schematic shows this cap is a .01uF. A leaking cap can cause excess noise.

A complete and accurate voltage chart is extremely helpful. Its helpful now and also as a future reference if something should go wrong later.

Just a thought
Good luck

[Cornelius]

A tought:
If the first preamp tube are used for the preamp stages, and the second are only used for the effects, and turning the effects up and down doesn't affect the noise; try swapping those two tubes. If now the noise varies with the effects, that tube are the guilty one....

I think that by replacing the first preamp tube with a new one would get you far in your debugging.

I'd try to get a direct replacement instead of those russian you got that needed some of the pins to be reconfigured. (In the 'debug before mod' spirit... )

[docz]

Ok guys, I've been probing around all night.

And changing the tubes around didin't make a difference.

I'm not quite sure what a voltage chart is, so I just wrote them down on the schematic. Sorry for the big image, but it has to be big to be readable

[img1167]http://farm3.static.flickr.com/2775/425 ... 83d5_o.jpg[/img]

Any clues here?

One thing that puzzled me was that there was high DC voltage going out to the output transformer. I thought that the speaker only wanted AC? Again this might be me being totally stupid...

I've ordered a new set of tubes, since I didn't want to mod the thing to use those russian tubes.

Strangely this "rats nest" is beginning to make sense to me I can now easilly spot the different sections on the schematic and inside the spagetti of components.

DocZ

[MBD115]

Here is a little voltage chart I made. It should give you an idea.

I think you need to know the voltages on the cathodes also, and I like to check the voltage after the coupling caps (you did for a couple of them).

Its getting easier isn't it

[docz]

Ok, now I understand, here is the voltage chart.

[img:774:195]http://farm5.static.flickr.com/4043/425 ... 7277_o.jpg[/img]

I measured all sides of the capacitors, the ones that have a volt reading on both sides have that reading listed on the side in the schematic with volts, those that didn't have a reading I didn't write anything.

DocZ[/list]

[Cornelius]

Pin 3 (Cathode) on both V1 and V2 should usually read around 1-2V... (like pins 8 does.) Though, i'm not sure about the cathode voltage on the tremolo tube-half?