Expression trouble, need help

[ivan]

Hi,
this is my first Trainwreck build. Already built few Fenders and Marshalls and repair many. Tube only.
I used Ceriatone layout just as a path to get to some point where I could continue on my own or with the help of the net.
Metal chassis. Tubes' voltages are OK.
The trouble is in oscillations and hum.
Oscillations starts with the tone and measuring voltage on output/power tube's anode.
As soon as I dime volume pot voltage goes from +395 to +408.
It's only at the one side, the other is constant with the same voltage on every volume pot position.
I will gratefully take any advice!

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[pompeiisneaks]

Express and the like are prone to being on the edge of going wild. If you read the trainwreck documentation in the threads there it should help. You often will need to adjust some of the component values and/or add some grid stoppers etc, to the amp to get it 'tamed'.

See the page on the Express:
https://ampgarage.com/forum/viewtopic.php?f=3&t=5691

Look at the schematic, and see if you think there are components there that may help. For example there's no input grid stopper, that's going to make the first stage a bit hot. Some of the others that have built that specific amp are likely to have a better idea what may help but adding one there may assist. You may also consider putting in a grid stopper between the output of the bright switch and V1a. Basically these amps do need a bit of tuning quite often.

Another thing to test, though without changing components is to take a solid wood chopstick and probe around inside the amp, testing for any noises on touching, tapping or pressing on components and leads. This could indicate a bad solder connection, or a faulty component.

~Phil

[Winder]

I'd consider "tidying up" the bright switch wiring. That area can cause issues and you have a lot of wire flying about there. I like the concept of rotating the volume pot for the shortest possible distances ... works well for me. Not sure who came up with this method (Martin Manning?). There are some other ideas on the site as welll:

https://ampgarage.com/forum/download/fi ... &mode=view

Looks like you went with some level of star grounding. Nice. That said, not sure that's the best path chosen for the NFB connection. You might consider the more "traditional" path on the other side of the caps, etc. Seems like it might invite hum on it's current path.

Also, can't see if there's more than one ground point for the preamp section. If it's a bus configuration separate from the power supply "star", isolate the end close to the PI from the chassis and ground the bus close to the input jack.

[jjman]

Is the chassis paint removed below all the ground-wire connections?

[RockinRocket]

Add a grid stopper on V1. Start with 10k. Almost always needed and if only to suppress radio pick up (why did Ken not always do this!)

Hum could be tubes.

[ivan]

Thanks so far!
I put all new tubes and they are not/should not be the reason for the hum and oscillations.
I cleaned up the paint off the chassis where I attached the ground.
I eliminated bright switch to lower possibility of oscillations since I am aware of its sensitivity.
Pots are being isolated of the chassis and ground buss is connected to the C20 and C19 caps.
The sound is still harsh and I doubt in oscillations.
Anode on the first el34 still "jumps" to the volume pot starts from zero.
Second el34's voltage is constant.
Anyone here had the same situation?
I put 2k2 on V1 and b820 on V2 and I'll try starting with 10k.
When I put tone caps from the board directly to the pots hum is getting lower but still there is oscillations.

[ivan]

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[Mark]

What material is your circuit card made of?

I built a Rocket (twice) and the problem was a conductive phenolic board. I was lucky as Jelle Welagen put me on to this, who would think the board would be conductive?

I didn't have oscillations but I did have noise. Basically, I lifted the components and wiring off a turret (and rewired them in free air). I then measured the turret for any small DC voltages. The voltages were as low as millivolts and as high as 4VDC. Naturally the board was replaced with a Garolite G10 board and the amp worked fine.

The anode of the output valve changes when the volume control is turned up and no signal is applied?

[ivan]

Thank you all.When i add grid stopper on first stage oscillacion become less.Stil persists on one side output stage,with or without plaget cable.Circuit material is textolit-izotex.

[Mark]

http://www.exmega.si/en/izotext-textolit.html

http://merrenaudio.com/perf_boards_and_end_blocks

I'm not convinced the board material is okay, I would check for spurious DC voltages around the board where they shouldn't exist. The grids of the PI stage is usually a good place to start, look for millivolts of DC.

[ivan]

Little better

[ivan]

Hi again!
It's coming to sense, slowly but for sure.
How up shold I go with the resistors?
Now I have 220k at the input and 100k at volume pot

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[pompeiisneaks]

That seems a bit high for the first grid, usually it's as low as possible 10k, 22k, standard amps use 33k to 64k as the most really.

~Phil

[ivan]

I know,but det walue doesnt work

[pompeiisneaks]

Did you check the board material for conductivity as was mentioned by Mark? Usually a very minor resistor or cap change is about all that should be needed to tame it, if it's still not tamed then I'd look for other areas as potential problems, like Mark's hint.

~Phil