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Hello,

I just finished assembling a stereo version of the poseidon boards. I powered up a single side two nights ago with no problem, and then I added th eother side last night and the side from the night before was oscillating after the tubes warmed up. Any thoughts on where to begin to check this?

Thanks for any help,

Blair

Hi Blair,

Are you using a common power supply? What happens if you disconnect the negative feedback leads from the output transformers?

When I first (mis)read your post, I thought you were having a problem with the second channel and it was just a matter of phasing. This is more interesting. After disconnecting the NFB, If you're still having problems, unplug the driver tube in the non-oscillating channel and see what happens.

Thanks Eric!

That fixed the oscillation, but I have a driver tube not lighting. The right channel 12AU7. I know th etube is good, because I ran it in my st35 a few minutes ago. Any ideas why just the 12AU7 woudn't light?

Blair

You didn't say what fixed the problem, but it sounds more like getting rid of a symptom--the problem's still there.

As far as the dark tube goes, check pins 4, 5 and 9 on the socket to make sure solder has flowed completely around them and that they're shiny. Best bet is just to resolder the entire socket with a tube installed in it--any 9 pin tube will do if you don't want to use your good stash.

If that solves the heater problem, I'd say your PC board is a good candidate for a thorough inspection with a magnifying glass. There could be other gremlins on the board that are causing, or contributing to, your oscillation. A second set of eyes also works wonders. They don't have to be trained--just connected to someone smart enough to say "Should this connection look like this?"

Keep us posted.

Okay,

I resoldered the tube, and yes one of the leads was broken. Thank you for the advice. I'm making music now, but on one of the channels, when the volume is at moderate levels, it pops kinda like a zappy sound. Other than that, I just need to fix shome sheilding issues, and it should be good to go. I have resoldered all the shunt to ground resistors, and tried changing C1, but the amp still gives me this poppy sound. Any ideas on this?

Thanks again for all your help Eric.

Blair

Boy, Blair, you don't give me much to sink my teeth into.

Are you sure the problem is in one, and not both channels? If it's coming from both, the problem might be in the power supply--but that's not set in stone. One channel could be intermittently taxing the PS to the point of affecting the other channel.

Listen to the amp with one channel's input shorted, then with the other input shorted to determine which channel is the culprit, if any. If you get noise in both channels, unplug one of the driver tubes--left or right channel, doesn't matter--and see if the noise goes away. If not, replace the tube and remove the other channel's driver and see what happens.

If you're at the flailing-hands stage, PM or email me your phone number and I'll give you a call (I have WATS out) so we can nail it down with dispatch.

Good hunting.

Hi Blair,

Is this a DIY design or a MKIII upgrade? If a DIY amp, what is the voltage at pins 3 & 8 on the 12AU7s?

Did you sort out your phases? Do you have your negative feedback reattached?

Hello Shannon,

You probably do not remember my emails from about a year ago, but I was planning to build a stereo version of the poseidons. I have exact circuitry on both boards, and the only thing different is the PS. The PS is upgraded on th ecap bank to 100uf + 100uf + 20uf + 20uf with the original configuration all in order. I did up the choke to a 15mh Hammond choke, and I am using a 550ma Antek tranny. As Eric stated, I am kind of at the flailing hands stage, but I couldn't manage to throw this beast if I tried . I will test those pins you suggested and get back to you. Thank you and Eric for all your advice. Oh, and yes the negative feedback is running straight from the 16 ohm tap on the OPT because I do not have all the binding posts yet.

Thanks,

Blair

Okay Eric,

Here are some real voltages for you to see:

Left channel: Pin 1 225, Pin 2 0, Pin 3 10.9, Pin 4 1.5, Pin 5 1.5, Pin 6 2.3, Pin 7 10, Pin 8 0, Pin 9 220.

Right Channel: Pin 1 220, Pin Pin 2 0, Pin 3 10.56, Pin 4 .6, Pin 5 .5, Pin 6 225, Pin 7 0, Pin 8 10, Pin 9 9.1.

I hope this is informative enough. I will send you my number as well.

Thanks again!

Blair

deicide67 wrote:Left channel: Pin 1 225, Pin 2 0, Pin 3 10.9, Pin 4 1.5, Pin 5 1.5, Pin 6 2.3, Pin 7 10, Pin 8 0, Pin 9 220.

Right Channel: Pin 1 220, Pin Pin 2 0, Pin 3 10.56, Pin 4 .6, Pin 5 .5, Pin 6 225, Pin 7 0, Pin 8 10, Pin 9 9.1

Are you sure the left channel voltages you listed aren't jumbled around? Those look way off. Also, switch your DMM to VAC and measure from pins 4 or 5 to pin 9.

On the 'poppy' channel, the LM234Z current source could have been damaged during oscillation, but if pins 3 & 8 are biased around 11V, then I think it should be OK.