Newbie problems with startup!

[sst4270]

First of all I agree with RJ. Great photos. It's always a big help when a builder takes/posts hi-resolution photos of their build.

Since you've only posted photos of the power supply section so far... I would suggest de-soldering your mains from the power switch and twisting them up to help reduce hum. Mind you... they'll shorten as you twist them so may not have enough length afterwards. You'll have to be the judge. They don't have to be twisted tightly though. Even just 5 or 6 twists can go a long way to help reduce the induced hum from those mains wires.

Good luck and keep plugging away at it. The end result will be so worth it.

Regards,

Steve

[RJ Guitars]

Great report Matt. Sounds like you have all the major pieces in the right place. The Express is famous for challenging your tweaking skills but as Steve mentions, the satisfying results are equally famous.

You should hear some big differences when you try out the new tubes. Ken Fischer had the reputation of swapping tubes repeatedly until he got the sound he wanted.

It turns out that 35 watts can be pretty loud... in fact depending on who the guitar player is, 5 watts can be more than loud enough (which isn't really a matter of volume).

Do you have the ability to make some audio clips? Those along with your photos would really complete the documentation.

rj

[M Fowler]

Glad you got the amp going great job.

[Big Jim]

Congrats on getting your Express up and running. Good glass made a huge difference in my build. Enjoy your new amp!

[KellyBass]

Congratulations on your build! The adventure begins...

Of the two expresses I've built, my first I barely got time to tweak it before the player picked it up. It's rude and unstable, slightly michrophonic and a absolute blast to play/hear. On my next one I had plenty of time to tame her down. Now she's as tame as a kitten and boring as hell. I guess the trick is to just get them stable enough...

[Matt J]

Here's the latest update:
I have eliminated most of the noise problems. I tidied up the wiring in the power section and shortened the shielded input wire leading to V1. I also made sure the wire was not touching anything else, and added a 33k grid stop resistor. I know the input jack is supposed to shield itself when attached to the chassis, but everyone knows how these things can loosen over time. Because of this I ran a grounding wire from the input jack to the nearest ground connection on the board. Any noise you hear now is probably from the p90 pickups.

The biggest issue that remains is that when I'm playing and really bite into a note or hit a chord, the amp begins to distort then becomes frazzled sounding, as if something is overloading. The sound is just like what you hear when you have a guitar cable that is shorting out or has a poor connection (I did check my guitar cable). I tested the amp with and without my Dr. Z airbrake attenuator, the same thing happened both times, the amp starts to distort then you get a farty/overloaded sound. The Scumback speaker is rated at 65 watts too. I am still using the EH tubes, the other ones I have ordered have not arrived yet. I checked all the voltages and they are the same as before. I am really starting to wonder if it is because of the low plate voltages. What does everyone think of this? How would you recommend I bring up the plate voltages?

Here are some more photos of the amp after I had worked on it:

[Matt J]

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[Matt J]

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[Matt J]

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[Structo]

My 2 cents are, you should have your power cord green ground wire go to a grounding lug by itself. It should not share that point with any other wire.

Ideally, the wire should be longer than the black hot wire so that in the event that the cord is yanked out of the strain relief, the ground wire will remain attached even if the hot wire is pulled free.

This is one of the reasons I like to use the IEC connector instead of a hard wired power cord.
If somebody trips over the cord, it simply pulls free of the amp (in a perfect world that is).

[M Fowler]

I have seen and started doing this myself grounding the IEC with the CT of the PT and noise free.

Sounds like blocking distortion, bad solder joint, or a bad cap I believe can cause the problem your having.

[Matt J]

Here is the latest update:

I went and placed the mains ground on it's own tab. The noise did go down further so that it is no longer a problem. I agree that IECs are the way to go, I had trouble getting the power chord holder to fit properly and had to do some filing. Any future builds (I'm considering a Rocket after I take care of getting a new guitar) will use an IEC.

The staticy/overloaded sound still occurs when you really did into the amp or hit a hard chord. I double checked the voltages, see the attached chart. The preamp voltages went up to what you would expect yet the power amp voltages are still low. I do not know exactly how to raise them. One thing that caught me off was the -80.7 bias voltages on pins 5 of V4 and V5. Should I be worried about this? There's no red plating, and I tested the bias trimmer, the lowest setting it will go to is around -69.0. I checked the solder connections and resoldered some of them to be sure. I also measured the voltages coming from the 6 large filter caps themselves. Here is what I got:
C15 - 370.7
C16 - 370.1
C17 - 371.0
C18 - 307.9
C19 - 290.3
C20 - 275.1

Anybody see anything that really stands out to them?

If it is blocking distortion, how could I tell exactly and how would I diagnose and fix this?

Thanks again for your advice and help everyone!
-Matt J.

[M Fowler]

Start by checking each tube to see if that eliminates the noise your hearing. Once you have eliminated the tubes then we start on other problems areas.

Mark

[jjman]

That change in the bias voltage range is a problem. Check the 15k and 47k and the pot. I would suspect that the 47k has moved up out of spec.

Can you check the idle current of the output tubes? You could use the transformer shunt method or the voltage drop method, if you don't have idle sensing resistors installed.

What is the heater voltage now?

[Matt J]

I checked the 15k, 47k, and bias pot. All were fine except the 47k, it had drifted up to 55K. I replaced it. Note: on the bias trimmer both the wiper and the outer tab connect to the ground via the same wire, the inner tab connects to the 47k resistor. Should I remove the ground connection to the outer tab so that only the wiper attaches to it?

I check the idle current by the transformer shunt method. I connected the negative black probe to pin3 of V4 and connected the positive red probe to the center tap where it connects to the standby switch.
The meter read .044, 44Ma. Pins 5 of V4 and V5 now read -28.8.

I started removing tubes to see if they were causing trouble. Removing V1, V2 and V3 result in the signal just going out, no sound. However, whenever I would pull either V4 or V5 the resulting tube operates fine. The sound has a much higher gain and very low headroom, but there is no staticy breakup or overloaded sound.

The heater voltages are 6.4 VAC.

-Matt J.