5E3 goes quiet after turning up

[pbiagi]

This is my first amp, built about a year ago, sounds great. It's loud. In my small apartment I can't really turn it up, usually running at 2, and not even full on the guitar..
Recently I took it out, and have also played with a wah.
When the amp is turned up to ~3, guitar side up too, I'll be playing and when hitting a medium or low string, the amp will cough (or fart) once or maybe twice, and then it goes very quiet. Only remedy is to turn it off, and later back on. Then full volume is restored until the same sequence is repeated.
At low volume it never does this, except sometimes the wah can trigger the same.
Seems something is overloaded or overheating.. or it is oscillating.?
I tried to search on this, but probably am not using the right search terms.
Could the power cathode resistor need replacing? (why so early?)

Many months ago I had measured voltages, etc., and they were inline with expectations.
What should I be looking for?

The amp is probably typical - compressed, but good clean head room at low volumes. But I've never tried to get it to the brown distortion yet (like I remember years ago..), simply because I'd blow my ears out in my apt., plus need to mind the neighbors.

[MWJB]

Sounds like a parasitic oscillation, do you have any pics of the chassis, tube socket wiring, pot wiring?

If you can make it do it again, perhaps when the neighbours are out, before you turn it off check the dc voltages...if it's oscillating these will be pulled down compared to idle.

Shorten &/or shield your grid wires, even those to the power tubes.

Possibly an intermittent short on the OT? What plate voltage/current were you running?

[pbiagi]

Hi JB,

The grid wires on 12AY7 and 12AX7 (to tone pots) are shielded already. (You'll see elec. tape holding the shield back - I didn't have any shrink wrap at the time. They are grounded on just one side.)
The grid wires on the power tubes are longer than needed, and could be shortened. Replacing with shielded would not be easy since I'd need to get under the board.. (?)
All input/output jacks are shielded (using Hoffman washers). For the speaker jack ground, I'm taking it back to the phase inverter ground point on the board (see picture). (I think that is alright but not entirely sure - wanted to mention in case that's a clue.)
Tube socket wiring - well this is my first amp, and soldering is something I do better now, than then...
I couldn't get many good picks but here are a couple. Can try to get better if you need something specific. I think I need to clean the board of the flux.. Will deoxit? and a tooth brush? do that?

Since my original post, I found a cold solder joint on the ground for one input on the Normal channel. And the other normal input ground wire became disconnected.!!
Result is loud buzz when I try to use the normal channel. I'll get this fixed soon. Don't think this is related to the oscillation issue though, or could it?
I normally jumper the channels but have the normal channel at very low volume.

The plates current are 36.4 & 40.0 ma.

Thanks! Paul

[MWJB]

"The grid wires on the power tubes are longer than needed, and could be shortened." Good, then do so, they should be just long enough, but be able to cope with any flex/warp in the board material. If you eventually have to lift the board, then that will just be what you have to do.

Why have you insulated the jacks? The input hjack ground should be your preamp ground point for 12AX#7 cathodes, preamp filter cap & pot buss wire. Speaker jack grounds via its mechanical connection to the chassis. Speaker should absolutely NOT be grounded to the PI/preamp filter.

Power tube plate wires should be low to the chassis floor & sweep around the tube socket.

[pbiagi]

The jacks were insulated (last summer?) because I had hum/grounding issues.
If I touched the chassis, hum became louder. Since I rebuilt the input jacks and did many things I'm not sure insulating them helped, but overall there is much much less hum, almost inaudible.
The input jacks, pots, etc. are grounded near the input jacks on the side of the chassis via a bolt. See attached picture.

I read somewhere about using insulated jack for the speaker and grounding per PI. (I could probably find the reference with some searching.) I have the PI grounded back to a bolt on the power transformer, at the same point as the transformer CT and power filters ground. See picture.
Since the hole for the speaker jack was expanded, grounding to the chassis at the jack is no longer an option, unless .. maybe washers.? Or a new hole.

The electrical ground is a separate bolt on the side where the power transformer is.

I've been thinking of replacing the power transformer (Thordarson Messner) and output transformer (Sensor Corp? NSC041318) with Heyboer's. Don't really know, but I think these existing transformers are on the cheap end..??
Given some of the cruddy soldering, maybe redo or get a new board and redo wiring, connections. Maybe even replace the tube sockets. Are ceramic better?
So this is turning into a project... that I'll need to find some time for..
You recommend using shielded cables on the power tube grids. Which side, and how would you attach the shield to ground? (Wouldn't want high voltages arching to the ground on the shield.)

Thanks again for your advice!

[MWJB]

"I have the PI grounded back to a bolt on the power transformer, at the same point as the transformer CT and power filters ground. See picture." the PI & preamp filter cap shoulf be grounded AWAY from the PT bolt, at the same point as the 12AX#7 cathode grounds & input jack ground.

"Since the hole for the speaker jack was expanded, grounding to the chassis at the jack is no longer an option, unless .. maybe washers.? Or a new hole." try it to the PT bolt ground, or, yes, try a new hole with a solder tag next to the exiasting jacks.

If your PT is deficient (too little current rating) then I'd change it, but if current rating & voltages are OK, why change it? Why change the OT, or for that matter the sockets, or any other parts until you have identified what is actually causing the problem symptoms. Face the fact that it is probably something that you have done that is causing the problem, rather than the wrong brand of resistor, or cap.

"You recommend using shielded cables on the power tube grids. Which side, and how would you attach the shield to ground?" Ground the shielded ends at the junction of the 220K grid resistors, heatshrink (don't tape) the free ends that feed pin 5 (via pin 6 & 1500ohm resistors). But first I would revise the plate wire layout feeding the 6V6s and shorten the grid wires as best you can.

[pbiagi]

Why change the OT, or for that matter the sockets, or any other parts until you have identified what is actually causing the problem symptoms. Face the fact that it is probably something that you have done...

Oh definitely agree. The amp sounds great but I was thinking of changing out the transformers (anyway) in search of even better tone..
Now looking at this mess ... I know I could do better.. That was my thinking.. do it all at the same time.
But you're right, I'll fix this first... It may be this weekend before I get a chance tho.

[pbiagi]

Cleaned up the wiring and in the process found a cold solder joint at V2 grid going to the 1M resistor. (The wire actually was loose.)
Also redid the normal channel input ground wiring.

Like usual overall the amp is sounding great. Haven't got a chance to turn it up.. so I really don't know yet for certain the cough, low power issue is resolved, but I suspect it is per the cold solder joint above.

B+ is same voltage as before, 415V, after 5K resistor, 352V, after 22K, 243V. Other voltages are a bit higher than last time read (months ago).
Across power cathode resistor is 21V.
Output tubes current is 37.7/41.7 ma.
V1-1: 127V
V1-6: 125V
V2-1: 151V
V2-6: 190V
V2-7: 3V
V2-8: 51V

I'm still learning... Nothing is hopping out at me.. The current appears a little on the high side. Should I think of bringing it down? (Amp sure sounds good so far.) If so, how - add resistance? where?
Or is this just great - leave it alone?

Thanks, Paul

[MWJB]

Best to keep under 40mA. Do you have a NOS 5Y3 in there? Russian 5Y3 put out too much voltage.

Either increase the 6V6 cathode resistor to 330ohms, or add a reverse polarity zener diode (or diodes) between the PT centre tap & ground (do a search). Don't add resistors in the B+ rail.

With the stock 250ohm cathode resistor you want to keep 6V6 plate voltage <385vdc.

[pbiagi]

All tubes are NOS.

I live in Europe and use a step-down transformer. Recently I tried a balanced isolation step-down transformer, but it's putting out 129V. That's what I had used for the previous measurements.

Back to the 750W step-down transformer which puts out 117V and I get:
B+ 370V, output tubes current, 36.2/33.2ma.

Could the 50 Hz cycles cause heat or grounding issues?
The amp is quiet ... but when connected with the Fulltone tube tape echo, the echo unit introduces hum which gets louder when turning up the echo. (Don't have this issue w/ another amp.) Breaking the elec. ground on TTE helps some. Both use the same 117 V power line.

It's possible to convert the TTE to use 230V. I'm thinking of replacing the 5E3 power transformer with a 230V model. At least that would make it more convenient to get out with the amp.

I'm diverging from the original thread - which is solved as far as I can tell, with all my thanks to MWJB!