Express build done- almost there

[ubxf]

Thanks to RJ for the transformers, chassis and boards and the wealth of info that is shared in this Forum I was able to build the Express; but it needs tuning. When all the dials are at 12noon the amp is really loud but clean and bright sounding with a nice sustain. If i go to 1 o'clock on the volume or presence i start to get a feedback that stays even if i lower the volume on the guitar. All my DC voltages are proportionally lower than the ideal by 7%.
I'm doing more more checks and i still need to do the output tube biasing.

[ubxf]

here are some pictures of the work in progress

[John_P_WI]

I think I would start by looking at the long feedback wire to the presence that is intertwined with the power supply filter cap wires. Also some general wire management / lead dress would help. The bright caps also can be problematic.

[ubxf]

Thanks, should i use a shielded cable for the feedback wire.Do you have any pointers for lead dress. I tried to follow the layout the best i could but i'm sure i made some rookie mistakes.

[hans-jörg]

Hello,
what you have to do is to twist the blue and yellow wires comeing from the 2x 220k (bias grid leaks) to pin 5 via 5k6 at powertubes. Here they go a long way parallel and this produces oscilation and unwanted effects. Some builders use even shielded cables, but twist is enough.
Oh, yes, and the wire from presence to output is very lousy. This one has to run at least in the corner of the chassis. Here its like antenna. The problems you have comes 90% from there and the wires from above. Here is leaddress very important as already written from John.
I didn´t tried shielding this one but read that its not good. Just put it around the corner.
Sorry, an other question: are you shure the resitor (10k) comeing from PI cathode to coupling cap has a good contact? From the picture I see not. Like it leans only to the junktion of the 3 resistors.
ok, double checked - looks fine

Best

Hans-Jörg

[John_P_WI]

Hans has good suggestions as well. Regarding the feedback wire, simply disconnect it from the presence end and see how the amp reacts. Of course don't let the free end touch anything....

[ubxf]

Thank you guys, i disconnected the feedback and the amp was working well
but pretty gritty. I redid the feedback line shorter and away from the other lines as well as pushing the 0.1uf cap against the chassis and all seem well now.

[ubxf]

I also noticed that after adding the wooden faceplate it seemed to eliminate some microphonics . Now i can start changing things one at a time and listen to the changes in sound. There is still a fair amount of hiss (white noise).
There used to be some hum but that went away with a tube swap.

[Randy Magee]

The hiss is not unusual for an Express... it's a high gain amp. That being said, my Express clone doesn't hiss as much as my 1969 Marshall Super Lead when both are cranked...

[ubxf]

Yes it's not too bad but i need to try to lower it a little. I'll try looking for better power tubes after i improve my wiring.

[ampgeek]

Sounds like you are making excellent progress. Well done!
Start off by focusing on V1 regarding hiss. My builds always benefitted from some tube rolling there to identify quieter tubes.
Rock on,
Dave O.

[John_P_WI]

Any preamp tube stage is subject to thermal noise / hiss. As a general rule, using a larger size (power rating) resistor will be quieter than the same resistance value in a smaller package. Additionally, using high quality Metal Films for the first preamp stage 1M grid leak, grid stop, plate load and cathode will net the most results.

[ubxf]

So i've swapped tubes and it helped but didn't really fix the problem. So i decided to try to clean up the wiring. I added a 68k metal film grid stop resistor. The hiss got better but now i have a hum and some oscillation when i pass the half way mark on the gain or presence. Oscillation as well when i engage either bright switch settings. My beginners luck ran out and i'm realizing that i should i've picked an easier first project : )
I appreciate all the help so far, many thanks .

[ubxf]

I've been trying to move the wires one a time and i can hear some differences but nothing drastic . Still the same problem of oscillation after
12 on the volume, another concern is that the sound is pretty much clean (very little distortion ) at the max setting (before oscillation) with the guitar wide open. Something is definitely not right. My voltages all look within range. I tried swapping the 2 output transformer wires and i got instant oscillation. I see that i didn't twist the transformer 6.3V wires or the Output transformer wires could that be a problem ?

[ubxf]

I re-read all your advice as well as a lot of threads and rewired the amp.
Now it is hum free and i only get a little oscillation if it is wide open and the presence is past 1 o clock. I can use the 2 bright settings. I'm definitely hooked and will try to find the last few problems. Many thanks to everyone
on this forum.