MEDIC ! My Carvin just crapped out on me

JohnW63

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Yes. I see that. Now that we have determined that I don't have that, I need to discover why.

Nuuska,

How "durable" is the output transformer. For a few tests, I have the speaker unplugged because it got so annoying sounding. It's not bad, after I soldered the tube pins. How would I verify I didn't damage it in testing ? I'll look up amp transformer testing.

I checked out a video. The output transformer checks out with ohm tests. So, it's not wide open or shorted. Not grounded to case.
 
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JohnW63

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Since V4 and V5 are EL84s, right, then I don't know if the measurements above fit. Default mentioned that the heaters were AC and the rest DC. That would be the pins I mentioned, if it was a 12AX7, but EL84s list filements or heaters only on 4 and 5. 9 is the screen grid, so should 9 be AC or DC ? It's 369 vdc.
 

JohnW63

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Figured it would be. So, I have 390vdc on V5 and 377 vdc on V4 at pin 9 ,
 
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Prince of Darkness

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Don't know if anybody has already answered this, but triode mode does not turn any valves off. It changes the way the output valves are connected. With pentode, or beam tetrode, valves you have five elements to the valve, a cathode heated by a filament, a control grid, a screen grid, a suppressor grid (beam forming plates in a beam tetrode), and an anode. In most cases the suppressor grid is internally connected to the cathode (EL34 is an exception). In pentode mode the screen grid is connected to a fixed voltage, which is less than the anode voltage. For triode mode it is connected to the anode by a resistor. This results in the valve becoming much more linear, but also significantly reducing the output. There is another way of connecting the screen grid called "ultra linear", where it is connected to a tap on the output transformer. In theory at least, this gives the power of pentode with distortion somewhere between that and triode.
 

JohnW63

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Prince of Darkness

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Here is a link to my power amp tubes data sheet:

Here is a link to my preamp tubes:

V1 is actually a Groove Tubes gt12ax7r, which I have not yet found the datasheet. I would assume the specs would be very much the same as a stock 12AX7 tube.
Yes, as far as the specs go, the GT12AX7R is just another 12AX7 or ECC83. Groove Tubes have their valves made by a number of factories in Russia, China and Slovakia. They do their own testing and quality control on the valves. As a major distributor (owned by Fender) they are able to have valves produced to their specifications, but still very much within the normal specifications for the valve type :)
 

JohnW63

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Testing voltage on the 12AX7 tubes. Schematic says 230V. I'm not getting that.

V3:
Pin 1 = 103 vdc
Pin 6 = 203 vdc

V2:
Pin 1 = 156 vdc
Pin 6 = 209 vdc

V1:
Pin 1 = 170 vdc
Pin 6 = 149 vdc

I also verified the switch on the back ,to change to triode mode, works. The center pin connects to the upper or lower pole on both sides, when I slide the switch back and forth.
 
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JohnW63

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I thought I might only get half the 230vdc on each plate of each 12AX7, but the schematic shows 230 on each part. Either way, I don't have 115 or 230 volts, so something is amiss.
 

DThomasC

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Testing voltage on the 12AX7 tubes. Schematic says 230V. I'm not getting that.

V3:
Pin 1 = 103 vdc
Pin 6 = 203 vdc

V2:
Pin 1 = 156 vdc
Pin 6 = 209 vdc

V1:
Pin 1 = 170 vdc
Pin 6 = 149 vdc

I also verified the switch on the back ,to change to triode mode, works. The center pin connects to the upper or lower pole on both sides, when I slide the switch back and forth.

The schematic that you provided on page 1 of this thread shows that the plate supply voltage for all of the 12AX7 is 325V. But then there should be a voltage drop across each plate resistor when the tube is conducting, right?
 

Nuuska

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V3 gets it's supply from "B" @ PSU - and it is 335V there.

It goes thru R27 & R28 to V3 anodes, and it is supposed to be around 230VDC there - absolute voltage is not critical - but it should be same at both V3 anodes.

Next I would check all passive components between R24 - the feedback resistor - and output tubes.

Between here means signal flow - there are 5 caps and 10 resistors.

What about DC voltages at pin 2 in output tubes? both same? should be.
 

JohnW63

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"What about DC voltages at pin 2 in output tubes? both same? should be. "

They start out higher and drop down until they stablize at the values below.

V5:
pin 2 = 1.04vdc

V4:
pin 2 = 0.63 vdc

Will give other checks a bit later.
 

JohnW63

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Well, by data sheet the grid voltage should be -7.3 vdc, but , in this amp, voltage should be the bias voltage at -17vdc, I think.
 

DThomasC

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Well, by data sheet the grid voltage should be -7.3 vdc, but , in this amp, voltage should be the bias voltage at -17vdc, I think.

Right. The control grid of most tubes needs to be negative relative to the cathode. The Brits call tubes valves because that's what they are they are. They're valves that control the flow of electrical current. The valve needs to be set part way open at idle so that it can be opened and closed by the signal. That's called the bias voltage, and according to the schematic, the bias voltage in your amp it should be something like -17VDC. It doesn't need to be exactly -17V, but we should expect something in that neighborhood.

You're seeing something like a volt, not -17V. So look around the bias supply to see if you can find where things are going wrong. In the schematic, the bias supply is this part of the circuit. It should be making -17VDC or so, but apparently it's not. See if you can find where the problem is.

BiasSupply.PNG
 

JohnW63

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Wouldn't the voltage on pin 2 be related to the amount of voltage applied to the bias circuit ? If the voltage is correct and pin 2 is out of spec, then one of the components is bad. But if the voltage is out of spec, then properly functioning components won't be able to correct that. I'm trying to follow the voltage path and determine where it comes from to get to pin 2.

Voltage from the power transformer hits C25 at almost 300vac in my case and passes to D3 and R33. D3 half rectifies that voltage and allows it on to R32, P7, and R31. Out of P7 it moves up to the middle of R27 and R28 which allows it through to P2 on each tube, V4 and V5. C22 and C23 keep any DC coming from V3, so it would seem that the bias voltage only comes from the power transformer directly.

I suspect I am missing something along the way.
 

DThomasC

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You're on the right track. There is one capacitor that you can't see in the clip, but you can see it in the full schematic. That cap smooths the half wave rectified voltage from D3. If you could look at the junction of D3 and R32 on an oscilloscope it would just be a bunch of bumps. The one capacitor can't do anything about that. But by the time you get to the junction of R32 and P7, things should be fairly smooth. Almost DC. I've marked on the diagram below approximately what DC voltages I would expect to see at those points in the circuit. Even if everything is working perfectly, the voltages won't be exactly what I guessed at, but they should be close.

BiasSupplyWV.png
 

JohnW63

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All measurements are to chassis ground.

P7 side of r34 = +5.5 VDC
P7 side of r32 = +3.6 vdc
other side of r32 nearest D3 = -16 vdc
P7 center is +5.1 vdc

P7 measures 16K. 9K on the R32 side and 8.3K on the R31 side.

I get 80 vac on the D3 and R33 intersection. Only 140 vac on the far side of C25. Which is the same at D4/D5 point. I would expect a lot more , unless the full wave rectifier is altering this, even though these spots don't go though it.
 

DThomasC

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That's very weird. All of those voltages should be negative, not positive. You say P7 center is +5VDC. Do you get the same at the grids (pin 2)?
 
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