Thumping from amp on startup

[bal704]

I'm in the middle of a home brew amp build. The amp is pretty much built, and I'm in the fine-tuning phase to get the sound I want. When I power the amp on, then kick on the standby switch, I get a weird thumping from the amp. It lasts for about 15 seconds, and is not heard again. Just seems to happen soon after I kick on the B+.

I'm using a bass speaker cabinet as I test this, and I have some towels on it to keep from scratching the tolex. The towels flap in the breeze during this thumping phase.

Any tips or suggestions on what's causing this?

I've attached a hand drawn schematic below:

[IMG764]http://i755.photobucket.com/albums/xx19 ... ematic.jpg[/img]

[JazzGuitarGimp]

I think I might be inclined to try the divide and conquer method. Start by removing the phase inverter valve and see if the problem goes away. If it does go away, then you know that the problem is either in the phase inverter, or before it. You could then reinstall the PI and remove the preamp valve to see if the problem is in the preamp.

[martin manning]

I'm having a hard time understanding your power supply schematic. Are there really no resistors between the filter capacirors?

[LOUDthud]

Does it thump with the volume control all the way down? Is that feedback that goes from the output transformer back to before the tone and volume controls?

[bal704]

I mis-drew the power supply. It's basically the AX84 20w PP schematic. I'll add the resistors in and re-post.

[Firestorm]

I mis-drew the power supply. It's basically the AX84 20w PP schematic. I'll add the resistors in and re-post.

I'm having a hard time seeing the standby switch, too.

[Phil_S]

I realize I am not in the same league as many here, so take this with a grain of salt. I am going out on a limb. My guess is that it is cap related. You've got a bad cap that is not charging properly or is not discharging properly. Instead of smoothing, you are getting thumping until it gets ramped up. It does away after the power supply comes up to full tilt.

I know it is a bit of a reach. I typically use generic caps in my builds, not the brand name stuff. Sometimes, on first power up, the amp seems like a dud, but after a very short time under power, the caps will charge and the amp starts to happen. One thing that happens in this situation is that the speaker will thump. That is what makes me think its a cap.

I suggest that you power it up and leave it on for a while. Do something to cause the caps to discharge, like run a signal through it, and run it hard.

My 2¢, fwiw. Good luck.

[bal704]

Here's what the power supply looks like:

c1->120 ohm -> c2 -> 470 ohm -> c3 -> 4k7 -> c4 -> 2k2 -> c5

[labb]

Take a read through this over on EL34 world

http://www.el34world.com/Forum/index.php?topic=14406.0

[bal704]

That was kind of my thinking. This just started happening during the tweaking process. I've been fiddling with values trying to get the tone I want, and the thumping just started.

Is there any way to test a cap with a multimeter to see if it's charging properly?

Also, this amp actually contains 3 of those preamps, feeding 2 weber channel switchers. The preamp has enough flexibility that you can get anything from nice cleans to really nice grit depending on where you turn the pots, so I decided to build it into 3 independent channels. The schematic just shows the preamp and poweramp side for simplification purposes. The 3 channels sound really good right now, with no buzz or hum. I'm just twiddling with the values trying to get exactly what I want.

Thanks for all the help so far.

I realize I am not in the same league as many here, so take this with a grain of salt. I am going out on a limb. My guess is that it is cap related. You've got a bad cap that is not charging properly or is not discharging properly. Instead of smoothing, you are getting thumping until it gets ramped up. It does away after the power supply comes up to full tilt.

I know it is a bit of a reach. I typically use generic caps in my builds, not the brand name stuff. Sometimes, on first power up, the amp seems like a dud, but after a very short time under power, the caps will charge and the amp starts to happen. One thing that happens in this situation is that the speaker will thump. That is what makes me think its a cap.

I suggest that you power it up and leave it on for a while. Do something to cause the caps to discharge, like run a signal through it, and run it hard.

My 2¢, fwiw. Good luck.

[Structo]

Take this with a grain of salt but it sounds like you are not decoupling the power nodes properly.

Look at how a similar amp like yours is built and take that information and use it on your amp.

Generally you want at least 1K between power nodes.

Are you using two different feedback loops?

[bal704]

If your talking about the NFB loop, I'm using one 22k resistor, but each preamp has it's own lead to that resistor.

Talking to a EE, he mentioned it sounded like a filter cap problem. He suggested I add another filter cap in parallel with each cap in my filter circuit to see if that stopped the thumping.

I'll try that later today.

Take this with a grain of salt but it sounds like you are not decoupling the power nodes properly.

Look at how a similar amp like yours is built and take that information and use it on your amp.

Generally you want at least 1K between power nodes.

Are you using two different feedback loops?

[Phil_S]

...t it sounds like you are not decoupling the power nodes properly.

Once again, I'll admit I don't know much. I've gone as low as 470R to decouple without a problem. 120R makes me nervous, although that is similar in R value to a choke. Though I don't fully comprehend the technicalities, I'm inclined to agree with Structo. Why is it that you selected a 120R? I have a feeling using a 470R wouldn't make much of a difference at all.

[gar13]

If your talking about the NFB loop, I'm using one 22k resistor, but each preamp has it's own lead to that resistor.

I see a couple of problems with the way the NFB is employed here. First, that cap looks like a dead short to ground to the NFB circuit. Look a the schematic for a tweed champ to see how to empoy NFB to cathodes of gain stages with cathode bypass caps.

Second, if you are using this in three like preamps, and all of the cathodes of all 3 preamps connecting to the 22k feedback resistor, this means that all three of these tube stages are essentially connected at the cathodes, parralleling all of their cathode resistors.

I don't think you can employee NFB in this fashion, it may be best to move the NFB to the PI.

[Tillydog]

I see a couple of problems with the way the NFB is employed here. ...

To add to the above, the volume and tone controls are inside the NFB loop as drawn - which will cause much weirdness (maybe not your thumping, though). Another reason to move it to the PI.

Also, I don't see a grid leak resistor on the top half of V2 - this could be related to your thumping. The cross-coupling of all the V1B cathodes could cause funny things to happen, especially if all the V1s are fed from the same B+ node, but overall, I tend to concur with the thought that the power supply filtering is insufficient.

Andy