Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dinkyguitar on June 05, 2020, 07:12:41 pm

Title: Trying to figure out why my ac bias in voltage is high?
Post by: dinkyguitar on June 05, 2020, 07:12:41 pm
Hi All,

So I'm building a Soldano SLO 50 watt clone and I'm following this guide and I'm at this point measuring voltages.

http://c3amps.com/images/SLOHeadBuild/Wiring2%20Power%20Section/Wiring2Pages/116Test%20Voltages.html (http://c3amps.com/images/SLOHeadBuild/Wiring2%20Power%20Section/Wiring2Pages/116Test%20Voltages.html)

I'm using a variac and while slowly bringing up the voltages I noticed my AC Bias In is around 100 vac and it should be around 60.

I have the O'Netics PT & OT and the wire color I'm using is red/yellow.

(https://photouploads.com/images/E4qI.th.jpg) (https://photouploads.com/image/E4qI)

Here is my powerboard;

(https://photouploads.com/images/E4qD.th.jpg) (https://photouploads.com/image/E4qD)

Here are the schematics.

(https://photouploads.com/images/E4qk.th.jpg) (https://photouploads.com/image/E4qk)

Can someone help?

dinky
Title: Re: Trying to figure out why my ac bias in voltage is high?
Post by: sluckey on June 05, 2020, 07:46:22 pm
Do you have 60VAC ***BETWEEN*** the red/yellow and red/white wires.
Title: Re: Trying to figure out why my ac bias in voltage is high?
Post by: dinkyguitar on June 05, 2020, 08:43:21 pm
So I have a star ground setup and one of the wires in that star ground is the red/white wire.

This is where I clip my negative when taking voltages.

So when I put my neg. clip on the star ground and pos. on the red/yellow wire which is soldered on the turret of the powerboard I still get 100vac.

I'm doing same thing as in the 6th pic down on this site.
http://c3amps.com/images/SLOHeadBuild/Wiring2%20Power%20Section/Wiring2Pages/116Test%20Voltages.html
Title: Re: Trying to figure out why my ac bias in voltage is high?
Post by: sluckey on June 05, 2020, 08:54:55 pm
Are all the other voltages reading as expected?
Title: Re: Trying to figure out why my ac bias in voltage is high?
Post by: dinkyguitar on June 05, 2020, 09:19:03 pm
So as far as voltages:
My line voltage was 117vac same as the C3 link.

From the PT schematic in my first post: (BTW wires 5 & 6 should be 360v each, 720v is combined)

Wires 5 & 6 I get 346vac - In the C3 link I posted he got 358vac
DC voltage after rectifier I got 474dc - he got 520
Power on lamp I got 3vac - he got 3.18vac (each side)

Title: Re: Trying to figure out why my ac bias in voltage is high?
Post by: sluckey on June 05, 2020, 09:38:55 pm
OK, so the bias winding is putting out more voltage than advertised. It's very easy to compensate for that. Just use a larger resistor for R67 on your schematic.
Title: Re: Trying to figure out why my ac bias in voltage is high?
Post by: dinkyguitar on June 05, 2020, 09:51:23 pm
Here is what the note for that says in the schematic:

(https://photouploads.com/images/E4q4.th.jpg) (https://photouploads.com/image/E4q4)

This is the board I'm using...problem is that there is no R67.

http://c3amps.com/images/SLOHeadBuild/PCB/BoardPages/03PowerboardStockTop.html

Not sure if it's clear, but the diode on the left next to the bias in turret is D5, and the resistor next to it is R52.

My R52 is 10k, the schematic lists 15k.

The green resistor up top is R56 10k.

Could R52 be increase to lower the ac bias in? The notes don't say anything about it...but then again they used 15k.

Not sure if 15k is enough though.
Title: Re: Trying to figure out why my ac bias in voltage is high?
Post by: tubeswell on June 05, 2020, 10:18:47 pm
Quote from: dinkyguitar on June 05, 2020, 09:51:23 pm
Here is what the note for that says in the schematic:

(https://photouploads.com/images/E4q4.th.jpg) (https://photouploads.com/image/E4q4)


This is the board I'm using...problem is that there is no R67.

http://c3amps.com/images/SLOHeadBuild/PCB/BoardPages/03PowerboardStockTop.html (http://c3amps.com/images/SLOHeadBuild/PCB/BoardPages/03PowerboardStockTop.html)

Not sure if it's clear, but the diode on the left next to the bias in turret is D5, and the resistor next to it is R52.

My R52 is 10k, the schematic lists 15k.

The green resistor up top is R56 10k.


So that says "R67 varies", so whoever drew that schematic knew you might have to change R67- depending on what VAC you get from the bias supply winding/tap. Try a few different resistances 27k, 47k, 100k see where you land
Title: Re: Trying to figure out why my ac bias in voltage is high?
Post by: dinkyguitar on June 05, 2020, 10:46:16 pm
So this is interesting....The datasheet that came with my PT is different than what I just found in many documents.

(https://photouploads.com/images/E4qQ.th.jpg) (https://photouploads.com/image/E4qQ)

You'll notice is has 700v combined between wires 5-8 I measure 346v each...so pretty close.

And between wires 6-7 - 100vac.....which is what I get.

I bet my spec's sheet is wrong....seems like it, considering my voltages match up with this new datasheet I found....not sure where i got it from though...

Now the question is...does the 100vac need to be lowered?
Title: Re: Trying to figure out why my ac bias in voltage is high?
Post by: 2deaf on June 05, 2020, 10:57:11 pm
If your capacitors are really rated for 100V, the 100Vac will lead to an over-voltage for your capacitors.  Increasing R52 could get your required bias voltage, but it would still leave C25 with an over-voltage problem.

You need R67 somehow and 10K would be a good first try with 15K for R52.
Title: Re: Trying to figure out why my ac bias in voltage is high?
Post by: dinkyguitar on June 05, 2020, 11:12:05 pm
And they are rated at 100v....33uf...I just checked.