Why are Expresses so unstable?

EtherealWidow · Post by **EtherealWidow** » Sun Nov 10, 2013 4:11 am

Sorry, I tried looking this up elsewhere. I just don't get how the Express is such a deceptively simple circuit, but so incredibly sensitive to layout. Would someone please take the time to enlighten me? Thanks!

martin manning · Post by **martin manning** » Sun Nov 10, 2013 12:34 pm

Simple circuit, but it is almost unrestrained. Note there are no grid stoppers on the pre-amp stages , and there is no attenuation of the signal other than the volume control and the tone stack between stage one and stage two. It's running wide-open, so to speak.

passfan · Post by **passfan** » Sun Nov 10, 2013 1:24 pm

Build it right , no problem. Deviate at your own risk.

EtherealWidow · Post by **EtherealWidow** » Sun Nov 10, 2013 3:26 pm

Ah. Thank you Martin. So it's all the potential ultrasonic frequencies, depending on the layout? What about doing a point to point Express to cut out slightly more parasitics than turret boards? Would that just be asking for frustration?

martin manning · Post by **martin manning** » Sun Nov 10, 2013 4:38 pm

Here's my alternate layout, inspired by rj's previous pcb version. This amp is very stable (without the Presonance, but that is it's own subject...). It has small grid stoppers on V1, like many people have used.
https://tubeamparchive.com/viewtopic.ph ... highlight=

EtherealWidow · Post by **EtherealWidow** » Sun Nov 10, 2013 5:11 pm

Awesome. Lots of good info to sort over. Is the wide open frequency response also what makes the Express so magically dynamic?

M Fowler · Post by **M Fowler** » Sun Nov 10, 2013 7:20 pm

I built my last Express using Martin's excellent layout and info.

Some one posted in thread about his PTP express see photo.

Reeltarded · Post by **Reeltarded** » Sun Nov 10, 2013 7:36 pm

That is freaky good.

EtherealWidow · Post by **EtherealWidow** » Mon Nov 11, 2013 5:56 am

Yeah. That's a great idea for builds. All the benefits of p2p without having to have a million little screws.

ampmike · Post by **ampmike** » Mon Nov 11, 2013 12:09 pm

That is impressive!!!really nice Mark!

M Fowler · Post by **M Fowler** » Mon Nov 11, 2013 6:19 pm

ampmike wrote:That is impressive!!!really nice Mark!

Not my build Mike I forgot to label the photo with the builder's name my bad, I should just find the thread this photo came from to give the builder credit for a nice PTP Express.

ampmike · Post by **ampmike** » Mon Nov 11, 2013 6:24 pm

Your just being modest Mark!!!You built that!!

ampmike · Post by **ampmike** » Mon Nov 11, 2013 6:30 pm

I wander how much that PTP helps with the Express?Mine is OK but the express is not the build for the faint of heart.I think that is why Ken was such a incredible amp builder.He knew how to bring each one in and get them so stable.The master of voicing his creations.I still have never played one, and Im sure mine isnt even close to a real one.But it was sure fun trying

That PTP one looks real nice!!!!

EtherealWidow · Post by **EtherealWidow** » Tue Nov 12, 2013 3:49 am

I plugged in the values of the PI and found out that it's totally unbalanced. The non inverted side is like 3.5V higher than the inverted side. This amp design is so weird. Ken was a genius. How does someone get an amp to sound amazing with an inefficient tone stack, no grid stoppers, and a poorly balanced PI?

jelle · Post by **jelle** » Tue Nov 12, 2013 4:15 am

Distortion. Controlled, gradual distortion, till all hell breaks loose. Like flying an F-16.

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Why are Expresses so unstable?

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