First Power Up

[Gregt01]

Hi guys I was able to power up my first channel of the Poseidon, initial power up I got -50vdc from Gnd to H, all ok.
Shut down and added the 5AR4 and powered up again (from start I had a 8 Ohm speaker connected) as the tubes warmed up the cone started to vibrate, by the time it took me to start taking measurements the cone went silent. The speaker was dead, I found that the cause was coming from the 5AR4 at first.
Once I disconnected between pin 8 of the 5AR4 and #1 of the cap and also the choke from #1 I ran another test I connected pin 8 back to #1 and all seemed ok but once the choke was reconnected, there it was again.
What have I done wrong? I’m getting 420VDC between pin 8 and ground.
Can anyone help?

Greg

[TomMcNally]

Hi Greg -

If you turn on the amp to listen for hum - make sure the input RCA jack is shorted out, or you have a source connected and turn off. Otherwise, the amp is running at full gain with no input, and may pick up some AC hum out of the air.

... tom

[Gregt01]

Hi to I'm using a 7 Ohm 200W resistor on the output. I have just did as you said and still getting it (hum?)
It is hard to describe the sound is comming from the resistor and not any other component. The souond is more like a AC resonating pulse threw the resistor.
Just measured 19vac on the outputs, I only run it for a few seconds.

Greg

[Gregt01]

Tom I have traced the fault comming from beyond terminal K on the Poseidon board.

Greg

[Shannon Parks]

Greg,

It sounds like the negative feedback connection is reversed. You're getting massive oscillation due to positive feedback being applied with mismatched phases. What's the old saying? "Want an oscillator? Make an amplifier..."

First just remove the negative feedback at connection I. If this fixed it, then swap the grid lines to the power tubes or switch the transformer leads to the power tubes. This is a very common mistake on new push-pull amps.

[Gregt01]

Thanks Tom,
It took time but I got so a simular conclusion.
I think I spoon and forked up.
In my rush I forgot what Shannon had told me.
Due to using Hammond Irons I should change C3 from 120pf to 220pf and R5 from 2.7k to 2k.
Im using the boards and parts as sent from the States and not the ones that I have put together with my parts.
Tom do you think this could be the problem?

Regards
Greg

[Gregt01]

I just realised it was you Shannon sorry buddy.
Here I go again spoon and forking up. It is 12:45am and I have been with the kids all day.LOL
Thanks Shannon

Regards
Greg

[Shannon Parks]

Hi Greg,

My tweaks are just minor tweaks unrelated to this. With 19VAC on the outputs, you have an oscillator. Please check out my previous post.

[Shannon Parks]

No prob! Just drinking my morning coffee on the other side of the world. :)

Using the Hammond iron means with the Poseidon board means a 50-50 shot of hooking up the phases right. Let us know what it is when you are done. I know my buddy Thermion would just tell me to figure it out by looking at the drawings, but I prefer the non-thinking method.

[Gregt01]

Thanks for that, I will have to make the changes tomorrow I must go because I must be up at 6.
I will be making the changes and I'll get getting back to you.
Have a great day or afternoon or evening guys.

Many Regards
Greg

[Gregt01]

Have a great day then
Thank you


Greg

[TomMcNally]

Hi Greg -

I was typing my reply when Shannon came up with the same conclusion, so I didn't send it. Let us know how you make out and how it sounds.

I don't start up an unknown amp into my little test speakers anymore after having had this happen. One of my big amps burned the lead off the woofer before I sut it down. Fortunately, it could be resoldered ! I now do what you are doing, feel the heat on a set of 10 watt dummy load resistors first, then put the 50 watters on for real testing.

... tom

[Gregt01]

You know I’m supposed to be at work?
I couldn’t wait for it came home did what the God’s
told me, and now I have music :o , no hum no distortion
just music. Yellow_Light_Colorz_PDT_03
Guys you are truly good, real good and I don’t care
What the others say LOL LOL Yellow_Light_Colorz_PDT_02
That was it, if I only followed the way I drew it on your
Drawings Shannon I would have had the second channel
finished. I have been thinking about this for three days
Along with all my must do things. Yellow_Light_Colorz_PDT_13
Thanks again to both of you. Yellow_Light_Colorz_PDT_12
You guys are Yellow_Light_Colorz_PDT_11

Ok Ok let's not get a big head now Yellow_Light_Colorz_PDT_07
Shannon can you tell me the significance of the change to R5 and C2?
I have yet to change them on these boards I am using.


Greg

[Gregt01]

Ok
Now part two
getting the -55vdc is no problem
where do I get the 600mV for the KT88 or 6550, what points?

Greg

[TomMcNally]

Greg -

Pin 5 and 8 of the octal socket on the front panel to ground, those pins are wired across the 10 ohm cathode resistors.

... tom