Strange squealing in amp when powered up.

[Matt J]

Hey everyone. I just finished building a #102 ODS. This is my first ODS build so I'm progressing delicately with it. Today I was going through some start up procedures and noticed something strange. I plugged the amp in to a current limiter, first with a 25 watt then a 60 watt bulb for the first power up. The bulb dimmed as it should. Voltages are of course high as I did not install any tubes. I wanted to let the amp burn in a bit and let the capacitors charge up. However when the amp is powered up I can hear a noticeable squealing sound coming from inside. It's hard to describe really. It's not a sizzle like something burning and I cannot smell anything that might be a burning part, but it is just a squeal sound that exists and goes erratically in and out for a bit then slowly goes away. From the best I can tell it sounds to be coming from the large capacitors on the power supply filters board. Should I be very concerned about this? Or is this something some of you may have noticed when first powering up your builds and is just the capacitors settling in? Any feedback on this would be much appreciated! I don't want to do anything else on it until I have this resolved.
Thanks,

- Matt J.

[Matt J]

Also, the power transformer I'm using is a Mojotone #763. Looking at the schematic there are both a red/yellow wire and a green/yellow wire listed at 0v. Which of these should I connect to the ground? Both or a specific one? I've attached the schematic.

[dcribbs1412]

Hi Matt
I'm not the most experienced here by a long shot, but I don't remember any builds having a noticeable squeal from the filter caps. You have it plugged in to speakers or a load? Voltages are good with out limiter? Assume you have 2 220uf 350V caps. Have you loaded it with tubes?
Also both r/y and g/y go to ground one is HV CT(R/Y)one is heater CT(G/Y)

Darin

[Matt J]

Darin,
I've got it plugged into a speaker. The two caps are 250uf 350v in a totem pole formation. Would I still need to ground the G/Y wire for the heater if I have an artificial CT consisting of two 120 ohm resistors running from V4 pin 2 and V5 pin 7 to ground? I just tried it again, no squealing this time. I'm going to try it without the limiter, then with tubes in a bit.

Thanks!
- Matt J.

[dcribbs1412]

Darin,
I've got it plugged into a speaker. The two caps are 250uf 350v in a totem pole formation. Would I still need to ground the G/Y wire for the heater if I have an artificial CT consisting of two 120 ohm resistors running from V4 pin 2 and V5 pin 7 to ground? I just tried it again, no squealing this time. I'm going to try it without the limiter, then with tubes in a bit.

Thanks!
- Matt J.

Matt
no need to ground the G/Y, but I would make sure heater voltage is good before tubing. Double check voltages and I say tube it and let the amp
sing...loudly if possible. pics if any...everyone deserves accolades after building one of these...

Darin

[Matt J]

I ended up removing the 120 ohm resistors and just grounding both the R/Y and G/Y wires. I didn't want to trim and cover the G/Y wire just yet because I can always just resolder some 120 ohm resistors. Here's the latest: After grounding the wires I tried the amp with no tubes installed. Now the limiter does not dim, none of the heaters are active, and the pilot light does not light up. I don't want to just throw the limiter off to avoid blowing another fuse (or something else). Here are some pictures of the inside. Since this is my first Dumble build the wiring's not that great. Also note that the grid resistors on the power tubes are held up by terminal strips, I wanted that extra stability when wiring the four of them up. If anyone can spot anything amiss or thinks of anything they want a closer picture of just let me know.

Thanks everybody for the help!
- Matt J.

[Structo]

Amps can make almost mechanical noises in transformers and capacitors.
You don't normally hear them because you are playing guitar through it.

My 100w amp has a noisy power transformer that buzzes a bit when first powered up on standby.

On the other hand, your amp could be oscillating, not sure if that is possible without tubes but I would be ready to shut it down quickly if it starts to scream when you power it up with tubes.

If it does squeal like a pig, then it is most likely positive feedback on the presence circuit.
That means the primary wires to the OT are backwards.
You have to swap positions with them on the power tubes.
That's why on new builds it is a good idea many time to leave the primary wires full length until you are sure the feedback is negative as it should be.

[Matt J]

Here's some more:

[martin manning]

After grounding the wires I tried the amp with no tubes installed. Now the limiter does not dim, none of the heaters are active, and the pilot light does not light up. I don't want to just throw the limiter off to avoid blowing another fuse...

Do you mean the bulb lights brightly and stays that way, and you don't have heater voltage at the socket pins? Sounds very much like there is a wiring error, with the heater circuit shorted somewhere.

[Matt J]

Exactly. The bulb lights and does not dim. No voltage on the heaters either. By the way, here's how I wired the power transformer wires, I know in a lot of layouts and schematics I see there is a white and a black primary wire. The Mojo 763 has two black primary wires, so I don't know if there is a difference between them:

Red - Bias Board
Red/Blue - Bias Board 3.3k resistor for bias supply
Red/Yellow - Ground
Red - Bias Board

Green - Heaters
Green/Yellow - Ground
Green - Heaters.

I'm still looking over for any shorts. I haven't seen any yet but it's probably something that's right in front of me.

[dcribbs1412]

After grounding the wires I tried the amp with no tubes installed. Now the limiter does not dim, none of the heaters are active, and the pilot light does not light up. I don't want to just throw the limiter off to avoid blowing another fuse...

Do you mean the bulb lights brightly and stays that way, and you don't have heater voltage at the socket pins? Sounds very much like there is a wiring error, with the heater circuit shorted somewhere.

If the only thing you changed was the heater CT then...
I agree with Martin...check for shorts after removing resistors...I know from experience a heater short is an easy miss. It only takes a small piece of wire. I use a flashlight and look at all heater connections closely
Nice looking build btw...you are almost there.

Darin

[Structo]

Look at V7 the green heater wires go to pins 2 and 7.
On pin 7 it almost looks like the wire is touching pin 8.

Look for that type of thing.

If the heater circuit is shorted to ground in the slightest way, it will be as you describe.

[Matt J]

Here's an update: I looked all over the heater wires again. The V7 heater connection to pin 7 was a little close to pin 8 so moved that back. Still nothing when I turned it on. I started looking at the ground wires from the Power Transformer. I disconnected the Red/Yellow wire and left the Green/Yellow wire still grounded. I powered it up with no tubes, this time the limiter dimmed and the pilot light came on. The heater voltages where screwy though, they read 26 volts on my multimeter! When I turned the standby switch to operate there was the sound of electrical activity then a quick 'zzzzzt" noise with a small spark around the large filter capacitors and the amp shut off. Blew my last fuse so I'll have to make a stop by the store tomorrow after work and pick some more up.

I'll just have to remain patient and keep plugging away at it!

- Matt J.

[Matt J]

Here's the latest update, I went through and tested a lot of things:

- I disconnected the power transformer secondaries and left the primaries connected. With the power on all the secondary voltages were ok so the power transformer checks out.
- I double checked the heater connections with the continuity tester on my multimeter, all came out ok with no shorts.
- Wiring things back up once again the limiter would not dim when both the Red/Yellow and Green/Yellow wires were grounded. ONLY when the Green/Yellow wire is grounded will the limiter bulb dim.
- I changed the dropping string resistors from 2 watts to 3 watts.

With this all set and the limiter turned ON, turning the mains on will cause the limiter to dim, switching from stand by to operate causes the limiter to light back up again.With the limiter OFF the amp comes on when the mains is switched on, however switching from stand by to operate causes the amp to short out and the fuse to blow.

I started looking over the power supply filter capacitors. All of them read out ok on the multimeter except for the 20uf/500v capacitor that connects to B+ 2 on the schematic. I could not get a reading on it with it attached to the board. After desoldering it and testing it on its own there was still no read. So, I think I'm dealing with either a faulty or blown capacitor. I should have caught this earlier when building the board in the first place! (smacks hand on forehead)

So, as it stands I'm probably going to build a new power supply filter board from scratch. I've ordered some new parts and hopefully they will arrive in the next few days. Instead of just using the large radial capacitors with the screw terminals I'm considering just using axial ones with a layout similar to the "OTS Ford Mustang 100" from Ceriatone.

Any thoughts on this?.

- Matt J.

[markr14850]

I hope this isn't it, but...

I see that your circuit boards are black. I cringe every time I see that, as I once had a batch of black circuit board from McMaster which turned out to be conductive. I assume that some anti-static board got mixed up with the normal board.

To test this, you could apply power to just the PS board and see if you can measure a voltage between a powered eyelet and the edge of the board. (For some reason, the faces weren't conductive, and some voltage threshold had to be reached for the effect to happen - so an ohm meter didn't show the effect.)

https://tubeamparchive.com/viewtopic.php?t=7756

Good luck,
--Mark