potentially reversed OT primaries? Now Express trouble shooting

[aceofbones]

Hey everyone,
I've started testing my Express build using a light bulb limiter. Using a 25 watt bulb everything seems fine, without tubes and then with tubes. However, when I go up to a 40w or 60w bulb I end up with a high pitch sound as soon as I take the amp out of standby. I can only assume it's because of reversed OT primaries, but thought it was strange that it doesn't do it on the 25w bulb. I just wanted to check if this was a common issue using a LBL or if I should be watching out for other issues before I continue?
Thanks.

[martin manning]

25W is pretty limiting. Most likely there just isn’t enough gain for it to break into oscillation. I’d swap the primaries and try again.

[aceofbones]

25W is pretty limiting. Most likely there just isn’t enough gain for it to break into oscillation. I’d swap the primaries and try again.

Awesome, Thanks Martin. I will make the change and report back.
Cheers.

[Stevem]

Make life easy on yourself!
Unsolder one end of the feedback resistor ( 47K) and add in series a 100k resistor, if the amp clams down a lot then , yes swap the primary's on the outputs.

[aceofbones]

Alright, swapped primaries and no change. I'll start back at the beginning to try and figure this out.
The noise almost sounds like an 80s video game sound. When I take the amp out of standby the sound sort of powers up and when I put it back in standby or power off the amp the sound powers down.
I'll redo all my checks and take some notes so I can report back here with some useful information.

[Phil_S]

You may or may not have more than one problem. It is important to do this methodically. If I had your amp here, I might give a different answer, but here is what I think I'd do.

Finish the power up testing. You've come this far. Finish getting it to full voltage. Dial all your controls down to zero in an effort to eliminate the noise. Work your way through the bulbs until you get to the 100W bulb. If the bulbs stay dim all the way through, then it's fairly certain there are no shorts causing excessive current draw. Obviously, if you get a bright light, shut it down and figure out what's causing the short. Assuming all is OK so far, eliminate the dim bulb tester. Power up. Get a good set of voltage readings at each tube (plate, grid, cathode, and screen on power tubes. Post a voltage chart.

Is there a bottom plate on the amp? Is this just an open chassis not yet mounted in a cabinet? If so, spread glue on cardboard and attach a piece of aluminum foil. Cut to fit over the chassis. Does a cover change the noise? This makeshift bottom will block RF interference, which may be what you are hearing.

While you are at it, turn all the knobs to 5 and see if the noise is present. Turn it up more if that's needed to get the amp to make noise. See if you can record the noise and post a file here. Most phones will let you make a video with sound. Tell us what you did to get the amp to make noise.

Try to determine where in the amp the noise originates. Start by pulling all the tubes. Run only with power tubes. Then add the PI tube. Work backwards to the first preamp tube. Power off/on each time you add a tube. At what point in this test do you hear the noise?

Of course, any time you hear or smell something you don't like, don't hesitate. Shut the amp off right away. Really, don't wait. Shut it off in a hurry.

[Stevem]

Remove one preamp tube at a time starting with the first one after the input jack.
If doing that kills the problem then at least you have narrowed it down to the the build or parts related to that tube or that tube itself.

If you place a short with a clip lead or whatever across a plate load resistor you will kill off the gain from that A or B side of the preamp tube and narrow the issue down even more.

[aceofbones]

Thanks Phil_S and Stevem, I'll start doing all of that. I only had time today to run through a couple voltage checks. No tubes installed, on a 40w bulb, then 60w and everything appears fine. Bulb only lights up very briefly at start-up then barely glows as I perform voltage checks. My plate values all seem high but I believe that's because of no tubes being installed. I'll post voltages later. I believe the sound only occurs when tubes are installed but I'm going back through right from the beginning and keeping track of everything so I can narrow this down.
Also, when I'm checking over everything I have all controls turned fully down so the sound was occuring with no volume or anything turned up.

I'll take some good photos of everything and try the above suggestions and report back. I'll try and record the sound too.

[Stevem]

If it has this issue with all the controls off, then the problem has to due with V1B and on down stream.

If the problem was originating in any thing dealing with V1A then the volume pot would turn it off.

[Stevem]

If it has this issue with all the controls off, then the problem has to due with V1B and on down stream.

If the problem was originating in any thing dealing with V1A then the volume pot would turn it off.

If your wires feeding the outputs from the PI coupling caps are too long, then this can make for oscillating problems.
Sometimes depending on your layout 1/2” too much of added wire length will do it.

The rule of Thumb is that the wires from the plate of any preamp tube to the. Coupling cap can be long, but once on the out bound side of the coupling cap your now on the high impedance side of the next tube and high impedance acts like a antenna!

If you need to make a long wire run to get from the plate of a preamp tube to let’s say a pot, then that wire should be shielded and that shield should be grounded at only one end, like to the case of the pot it’s going to.

You might try installing a 10k resistor right on the tube socket spins on the grid inputs one at a time to see if this helps out.
You can try this same thing with the output tubes , but install the resistor on each grid, not just one at a time.

[aceofbones]

Here's some pictures of the build. I've narrowed down the sound to V3. If I remove it, the sound goes away. If I have the power valves in while on the bulb limiter, the pilot light doesn't always light up, but I only have up to a 60w bulb, so it doesn't seem to be getting full voltage. I need to pick up a 100w. I've been too nervous to give it a whirl without the bulb limiter until I figure out some issues. Having the bottom plate on made no difference to the sound.
Here's my voltages with no tubes in and a 60w bulb in the limiter.
Mains 115vac with the power tubes in this shows as 99.5
Secondaries 570vac
B+1 385v
B+2 385v
B+3 380v
B+4 382v
B+5 382v
bias min -27.9v
bias max -19.5v
heaters 6.4vac
I haven't had time to look into much else.
Oh, fyi, the board at the back is VVR and it seems to be functioning as it should.

[aceofbones]

Here is the schematic and layout I followed.

[Stevem]

You have your bias terms reversed !
It looks like a trainwreck with EL34s wants like -33 volts to show up on pin 5 of each output tube to keep them out of burn up city!

I would want to see you have a bias voltage range of -23 to -36 volts.
I would use the alternate 150k as the schematic calls for in the bass circuit to replace the 220k to produce more negative voltage.

I would not put the output tubes back in until yo get that -33 volts on pin 5!

Also power supply filter grounds should not be daisy chained in series, at least not the filter ground for the first preamp tube.
This filter should get its ground where the input jack gets its ground at .

If you want to refine the amps noise floor later by playing around where the screen filter gets its ground then that’s fine once the amp is up and running normal.

[bepone]

for the first look, heater wiring looks sloppy. remove all heater wires from the preamp and redo again, twist the wires properly..
second where is grounded first capacitor for powering the first stage? must be close to input jack, yours looks like at the power stage?

[Stevem]

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