Oscillation problem with Express Clone

[Matthews Guitars]

Excessive bass can itself be a symptom of oscillation, in rare instances.

Do you have an oscilloscope? Use it to watch for oscillation.

I have to say, I don't like those orange drop caps in there very much. I favor MKT1813 series Vishays. They're about as close as you can get to a modern Philips Mustard cap.

As for that bass issue, what happens when you bypass the bass pot?

The Express, particularly if built close to Ken's original designs, is very sensitive to tube choice. I run a lower gain V1 preamp tube in it than I would in my Snorkler clone or even a Superlead. If I run a hotter tube it'll oscillate at settings that I believe should be playable.

Definitely shielding the bottom is a great idea. I planned to do that, I just haven't done it yet. Shame on me. That's a huge entry point for unwanted interfering signals.

My Express clone absolutely sounds its best with the volume control set to 3 o'clock, almost all the way up. It'll put a grin on your face every time.

[wizardof_oz]

then something seems wrong. recheck the wiring of the tonestack

Will do,thank. I'll keep you posted after the check.

[wizardof_oz]

Excessive bass can itself be a symptom of oscillation, in rare instances.

Do you have an oscilloscope? Use it to watch for oscillation.

I have to say, I don't like those orange drop caps in there very much. I favor MKT1813 series Vishays. They're about as close as you can get to a modern Philips Mustard cap.

As for that bass issue, what happens when you bypass the bass pot?

The Express, particularly if built close to Ken's original designs, is very sensitive to tube choice. I run a lower gain V1 preamp tube in it than I would in my Snorkler clone or even a Superlead. If I run a hotter tube it'll oscillate at settings that I believe should be playable.

Definitely shielding the bottom is a great idea. I planned to do that, I just haven't done it yet. Shame on me. That's a huge entry point for unwanted interfering signals.

My Express clone absolutely sounds its best with the volume control set to 3 o'clock, almost all the way up. It'll put a grin on your face every time.

Nope, don't have access to an oscilloscope, unfortunately.

Can you give me some ideas why you don't like orange drop caps in an Express? KF used them too...

What Do you mean with "bypass"? Connect C3 and C4 (same as connecting pins 1 and 2 of the bass pot)?

I currently use a 5751 in V1 (JJ, selected), still trying to find selected low microphonic tubes and try them. Several members of the TAG swear on Amperex for V1, I still struggle to spend the money...What lower gain V1 preamp tube do you use?

[Colossal]

I'm not convinced it's the root cause of the problem (and you definitely have an issue if the bass is boomy), but have a look at the cathode lead (yellow) on V1b, pin 3. I see a tiny puncture in the insulation where the wire was bent to follow the drop off the turretboard. Solid core wire can be cracked surprisingly easily with handling and you might not know it because the crack is hidden by the insulation. Some of your leads have an irregular, almost angular flow to them, rather than a gentle, smooth, and soft curve, making me think they may have been handled a little roughly.

Some other things:

Definitely recheck the blue and yellow leads with a chopstick that run under the control pots, and the positioning of the bright caps, as Martin said. That region is quite sensitive in these amps.

The yellow cathode lead for V2_pin 3, the solder connection of the wire around the turret looks a little iffy.

It may not be an issue, as it's hard to tell from the photo, but on V1_pin 6, the anode connection, it looks like the heatshrink is pushed through the solder pin, leaving a suspect connection of the wire to the pin. I would remove that, make sure the wire is well exposed and tinned, then resolder that connection.

I would bend the 10k input grid resistor on V1_pin 7 slightly up and away from the blue anode lead directly below it.

Finally, where is C2, your treble cap? I see two empty turrets on the board, but no C2. It looks like your Treble pot is wired to C3 (?).

[wizardof_oz]

Any ideas at all where the bass heavy behavior of my amp could come from? The low end is booming and really annoying

then something seems wrong. recheck the wiring of the tonestack

I re-checked all pots and connections:
Tone stack pot values:
- Vol (1MA): Alpha A1M (measured value 1,03M) but now shows weird reading on the VM
- Tre (1MA): Alpha A1M (measured value 1,04M) reading in the chassis OK
- Mid (25kB): Alpha B25k (measured value 25,3k) reading in the chassis OK
- Bass (1MA): Alpha A1M (measured value 0,94M) reading in the chassis OK

pot pin connections:
Vol 1 --> GND
Vol 2 --> C5 (100pF), C6 (500pF) and V1 pin2
Vol 3 --> Tre 2 and bright switch mid position
Tre 1 --> Bass 2
Tre 2 --> Vol 3
Tre 3 --> C2 (500pF)
Mid 1 --> GND
Mid 2 --> Bass 1
Mid 3 --> NC
Bass 1 --> Mid 2 and C4 (22nF)
Bass 2 --> Tre 1 and C3 (22nF)
Bass 3 --> NC

I checked with the Kelly '90 schematic and the Francesca photos, seems like the connections should match. I will replace the vol pot just to be on the safe side.

[wizardof_oz]

Colossal, thanks for you time to have a close look at the photos.

Good catch with the cathode lead to V1b p3, it is punctured indeed, I will replace it.

I will check and re-do the anode connection on V1_pin 6.

10k input resistor: done.

C2 is hidden from C3 (huge orange drop). I was told that micas may be microphonic, too. So I decided to replace the mica with a 470p MKT1813. Value is almost the same, so this should not be noticable from the signal.
The Tre pot is NOT connected to C3, but C2 - as it should be.

[Colossal]

Colossal, thanks for you time to have a close look at the photos.

Good catch with the cathode lead to V1b p3, it is punctured indeed, I will replace it.

I will check and re-do the anode connection on V1_pin 6.

10k input resistor: done.

C2 is hidden from C3 (huge orange drop). I was told that micas may be microphonic, too. So I decided to replace the mica with a 470p MKT1813. Value is almost the same, so this should not be noticable from the signal.
The Tre pot is NOT connected to C3, but C2 - as it should be.

Ok, I wasn't sure if C2 was hiding in the shadow of C3. I just couldn't make it out in the photo. Then the tonestack there is wired correctly.

Good work!

[Matthews Guitars]

About Orange Drops...there's something about their sound I just don't like. I prefer the MKT1813s. I've also seen a surprising number of Orange Drops that became leaky/shorted. They have a reputation for reliability but I've seen a lot that weren't.

[Colossal]

About Orange Drops...there's something about their sound I just don't like. I prefer the MKT1813s. I've also seen a surprising number of Orange Drops that became leaky/shorted. They have a reputation for reliability but I've seen a lot that weren't.

When you are referring to Orange Drops' sound and reliability are you referencing the SBE6PS/225p/416p polyester film series or the 715/716p polypropylene film foil? Or both?

[wizardof_oz]

Good question. FWIW I used only SBE6PS and 416P in my build.