Tubes up #102 build

[andresound]

Thanks Martin, Will disconnect one at a time

[andresound]

BTW, I am getting +-800vdc after rectification. Even though the secondary is 325-0-325!! Is this a problem for the Caps?. Obviously PT is unloaded

[norburybrook]

BTW, I am getting +-800vdc after rectification. Even though the secondary is 325-0-325!! Is this a problem for the Caps?. Obviously PT is unloaded

800vDC!! how are you rectifying? Doesn't sound right to me. If you have 450- 500vDC caps in series they'll be fine.

Mc



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[sluckey]

BTW, I am getting +-800vdc after rectification. Even though the secondary is 325-0-325!! Is this a problem for the Caps?. Obviously PT is unloaded

325-0-325 is the same as 650 C/T. That 1972 BASSMAN 100 PT is meant to be used with a conventional rectifier. But the PT 026554 specified in the Robben Ford #0102 layout is rated 330-0 and is meant to be used with a full wave bridge rectifier. If you use that Bassman PT with a FWB rectifier you will get about 650 x 1.414 = 919VDC unloaded! You'll have to modify your layout for a conventional rectifier to use that Bassman PT. Should be fairly easy to do.

[martin manning]

I thought about that too, Steve, but in the photos above the rectifier board has a full-wave look about it.

[sluckey]

The red/yellow center tap is tied off with heat shrink tubing.

[norburybrook]

Sorry if I'm being stupid but Dumbles never use full wave bridge rectification with those transformers. You use the CT and full wave through the 6 diodes .

What an I missing here?

M

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[sluckey]

This is the drawing I referred. Clearly uses a FWB.

https://tubeamparchive.com/download/fil ... &mode=view

And one of the last pics posted clearly shows the center tap tied off and not being used. The B+ voltage being about 800V implies to me that this amp has a FWB used with a PT meant for a conventional rectifier.

[andresound]

Thanks guys for all the replies. Yes the Centre tap is not being used. I am quiet ignorant with regards power rectification, but still learning. I will research all the advice given, cross reference with what I have done , and revert. I will draw what I have done.

I am embarrassed and I hope I am not labeled as a “troll” due to my inexperience. I am here to learn and appologize if I am wasting the members time. I am TRULY grateful for the non judgemental attitude from the forum members and all the help.

Re the first issue I had on powering up. Martin..., you were “on the money”, it WAS a failing lytic (brand new F&T 22uf 500v). Probably due to the high HT. I replaced it and problem disappeared.

Will sort PS rectification and revert. Can’t wait to fire this this up!

[martin manning]

Should be connected like this:

[andresound]

Thanks Martin. This is how it is connected at present, with no center tap.

[martin manning]

As long as the left end of the diode strings are not joined together and grounded, then all you need to do is ground the center tap. You might as well move the two red wires to the left end, though, and use all three diodes on each side.

[andresound]

Will do sir . Easy fix. I also notice that I have no negative bias. Could this also be a bad lytic? I have bias tap - 3k3 - diode - 100uf with + to ground.

[martin manning]

Yes that cap might have failed with the high voltage. Disconnect and check the AC voltage on the bias tap after you rewire the HT rectifier and go from there.

[talbany]

Sergio
You have a Variac on your bench.I recommend you use it (even with the limiter) when bringing your transformer online while keeping an eye on both the B+ HV as well as the - bias tap and make sure you don't have something wired wrong exceed your cap voltages and blow yourself up!

Tony