67 Super Reverb ugly distortion

[vintagekiki]

Does this look ok?

[ATTACH=CONFIG]55508[/ATTACH]

I have the shield connected to ground...

Excellent
I do the same.

[Helmholtz]

Does this look ok?

[ATTACH=CONFIG]55508[/ATTACH]

I have the shield connected to ground...

If that's a conductive black sleeve around the center conductor, you should cut it back/peel it off to avoid accidental shorts.

[earache]

vintagekiki

I'm going to do the tests you've described and will post results. I have a question about one of your earlier posts...I said that I have a variac that I can use for the transformer test. You replied:

"You can use variac only if it is galvanically separated from the mains."

Does that mean the variac would have to be plugged into an isolation transformer? Please explain....thanks in advance

Would a standard lab type bench power supply be more suitable for this test?

[earache]

vintagekiki - here’s some results.

I got a sine wave signal from that website on the IPod. I choose a 1000hz sine wave.

I took some baseline output measurements from the iPod before I hooked it up to the amp. The iPod was at about 70% volume.



I then injected the signal into the amp without changing the iPod settings - here’s the results



Lastly I turned the output of the iPod up full



The sine waves look pretty clean, right?

The load measures 2.8 ohms

I then resolderred the .001 capacitor in place, I put the V1 tube back in and plugged the guitar into the normal input, and also put the speaker back into the speaker jack.

The signal sounds like a slightly amplified guitar, pretty quiet, but no distortion.

[vintagekiki]

The signal sounds like a slightly amplified guitar, pretty quiet, but no distortion.

That you can hear the guitar pretty quiet, it can be pre-listening normal ch through vibrato ch through common cathode connection (point A) if you set tubes also for vibrato ch.

Your photo does not show that there is a 2 x 220k resistor network connection with normal ch (red point x)

- Check all DC voltage on tube for normal ch
- Disconnect .001uF from mix network 2 x 220k and temporarily connect to test wire with anode (pin 1 normal ch ).
Turn on the guitar in normal ch and try the sound.
If the sound is loud and clear, test wire temporarily connect to g1 (pin 7) and try with a guitar.
If the sound is loud and clear and tone and volume controls works, test wire temporarily connect to anode (pin 6) and try with a guitar.
If the sound is loud and clear and tone and volume controls work, check .047 (pin 6), 220k (network resistor) and .001uF




EDIT 191012

[vintagekiki]

Does that mean the variac would have to be plugged into an isolation transformer? Please explain....thanks in advance

Would a standard lab type bench power supply be more suitable for this test?

The standard lab type bench power supply provides DC voltage and cannot be used to test transformers.
Only the AC voltage source can be used to check the transformer.
If you use variac, variac must be plugged into an isolation transformer.
As AC voltage source you can use any kind of mains transformer that supplies a voltage of 6.3VAC

[Helmholtz]

The last picture with full iPod output shows 0.7Vrms across the 2.8 Ohm load. This corresponds to an output power of 0.175W = 175mW.
From your description of the problem it is unlikely that the distortion would show at such low power.

To test for full output (ca. 10Vrms) you would need a PI input signal of over 2Vrms.

[earache]

Helmholtz- you’re saying I need a stronger test signal - I will work on that

[earache]

The standard lab type bench power supply provides DC voltage and cannot be used to test transformers.
Only the AC voltage source can be used to check the transformer.
If you use variac, variac must be plugged into an isolation transformer.
As AC voltage source you can use any kind of mains transformer that supplies a voltage of 6.3VAC

So, would a “wall wart” type of transformer work as a power supply?

[Helmholtz]

Helmholtz- you’re saying I need a stronger test signal - I will work on that

Please see my edited numbers. From the first 2 pictures, I calculate a power stage voltage gain of around 4.2. So 10Vrms at the output requires 2.4Vrms at the PI input.

[earache]

Vintagekiki - first the DC voltages for V1
P1. 298
P3. 2.42
P6. 254
P8. 1.9

Additionally - AC voltage measured across P4/P5 - P9. 6.39VAC


Next I lifted the .001 cap and used a clip lead to connect it to the wire at V1 P1 (which I lifted from the PCB)



I got no clear guitar sound when playing guitar thru channel 1, just hum.

Should I have left the wire connected at the PCB and just clipped the lead right to the pin on the tube?

Additional note - I see where the wire has been moved from the “X” to the .02 cap connected to P6 of V2 ( I think that’s “Z”).



I must have followed some mod directions to allow reverb and vibrato on both channels when I serviced the amp back in 2011. It’s my sons amp so I know no one else has serviced it since and it has worked fine.

Further notes.... I reattached the lead to the PCB AND connected the clip directly to Pin 1 and now I have amplified guitar sound! There is no volume control BUT...if I hit it real hard there’s a *hint* of that distortion from before.

I will take a video of the sine wave input to channel 1 and post it next

[earache]

I ran the 1000hz signal from the iPod thru input 1 channel 1, and measured with the scope and the multimeter across the speaker output, with the speakers connected. This is with the clip lead still connected to V1 P1 and the .001 capacitor.



I turned up the signal generator a little more until I saw clipping of the wave form



Here’s a closer view of the scope



Comments welcome, as I need help interpreting what I see.....thanks in advance!

[vintagekiki]

So, would a “wall wart” type of transformer work as a power supply?

I doubt. Wall wart transformers give mostly DC voltage.
You talked about the standard lab type bench power supply. “Wall wart” is not a lab type bench power supply.

What does it say on “wall wart”. For what voltage (value in V) and what voltage (AC or DC) it is.
Any transformer that is galvanically separated from the mains supplying an AC voltage of 6 - 10 VAC can be used to test transformers.

Do you have any amplifier on the workbench with mains transformer, which among other voltages has a voltage of 6.3VAC ?

For testing uses only AC voltage.

[Helmholtz]

I doubt. Wall wart transformers give mostly DC voltage.

Just use the nice scope to find out.

If all OT wires are disconnected one can use the heater voltage from the same amp for testing.

I don't really suspect the OT- based on the existing measurements.

[Helmholtz]

with the speakers connected.

Why not use the dummy load? Not good for the speakers and your ears.

Otherwise the signals look good. Looks like you could get around 10V clean volts at the output. 10Vrms clean sine at the output means 50W into 2 Ohm or 36W into 2.8 Ohm. No signs of OT damage.

I 'd say power stage is good.