High negative voltages on Plexi build

[Al_T]

I am at the stage of this build when the initial voltages are checked. Good news is the amp fires up without blowing any fuses, but the only sound coming out of the speaker are clicks coming through the speaker when certain voltages are checked. On V5 the readings are as followed Pin 3 (492) Pin 4 (492) Pin 5&6 (-106) 2&7 (6.1acv) and zero from 1&8. I double checked all of the connections and everything "seems" to hooked up properly. I could use a little guidance on the trouble shooting of what could have been missed. I do not have access to a camera at this time to take pictures, but was hoping that someone has encountered this problem before. I am using ClassicTone transformers; 18023-18025-18058 choke. Thanks in advance.

[Willabe]

Hi Al and welcome.

The grid dcv of -106 would put the output tubes into shut off. Depending on what your using for power tubes it should be in the range of somewhere from -30 to -50.


                Brad     

[sluckey]

Pin 5&6 (-106)

Yes, that's way too high. Should be in the neighborhood of -30 to -50.

I'm assuming pin 5 on both power tubes is the same -106???

Check your bias supply. What is the voltage across each bias cap? Will the bias pot adjust that voltage down to the correct range?

[Al_T]

I'm assuming pin 5 on both power tubes is the same -106???

Check your bias supply. What is the voltage across each bias cap? Will the bias pot adjust that voltage down to the correct range?

Yes, I am having the same readings on both tubes. The voltage across the caps is around 130 and the bias control has zero effect.

[sluckey]

The voltage across the caps is around 130 and the bias control has zero effect.

The most likely suspect is you forgot to ground the bottom side of the bias pot. See pic...

[Al_T]

I grounded the 100 ohm heater resistor to the main PT ground along w the 1st filter and the two taps from the PT. I had bought this board pre made and the under board wire was hooked up. I would assume that this would be the grounding point. I have kept everything stock and have inexpensive Chinese el34's in the unit now. I do have a matched set of tungsols when the time comes.

[Willabe]

I grounded the 100 ohm heater resistor to the main PT ground along w the 1st filter and the two taps from the PT.

The high voltage (B+) should have a CT. The heaters (6.3) most times has a CT if it does not then you use 2 @100ohm R's for a faux CT. You do not use both a CT and 2@100ohm R's.

The grounding you described is good. And all the grounds in the -bias supply could be brought to that same ground point. You can tie/string them together and then connect 1 wire to the string and bring that over to your main PT ground point.

You have to go through all the component's in the -bias supply and make sure their all wired up correctly. Use the schemo and the layout drawing to do this.

The most likely suspect is you forgot to ground the bottom side of the bias pot.

If you don't have that pot grounded it wont work.

Is there a pot in your -bias to adjust it? If so with both power tubes out clip the positive probe to pin on the tube socket the other probe to ground so your hands are free. Set meter for dcv, take a reading with the bias pot turned full and full down, 1st on 1 tube socket then the other. Please post what you get?

have inexpensive Chinese el34's in the unit now. I do have a matched set of tungsols when the time comes.

That's a very good thing to do.



                Brad      

[Willabe]

Sluckey, he's got a Marshal clone PT, 18023. No bias tap. So he should have a coupling cap coming off 1 leg of the B+ secondary.

Here's the link.


http://www.classictone.net/40-18023.pdf


                  Brad        

[sluckey]

You only use a cap when you have a bridge recto. A plexi has exactly the circuit I posted above. The AC input comes from one of the secondary HT leads.

I'm still betting the pot is not grounded.

[Willabe]

You only use a cap when you have a bridge recto. A plexi has exactly the circuit I posted above. The AC input comes from one of the secondary HT leads.

Yes your right, my mistake. I just went and looked at some Marshall schemos.


              Brad     

[Al_T]

I ran the two red leads from the PT to the bottom of the standby switch (same 4 leg switch as in layout) and then ran the bias tap off of one of the 2 bottom legs. I grounded out both the brown and black wires to the PT central ground.

[Willabe]

Ok, but you need to test measure the -bias with your meter to see if the pot is working and post the results.


                Brad     

[Willabe]

(same 4 leg switch as in layout)

What layout? Could you post the layout and schemo drawings your using?


                 Brad     

[Al_T]

When I check the voltage at the 220k (where the bias tap is hooked up) resistor going into the bias it fluctuates quite a lot. There is 66 volts coming out of the resistor.  The layout is the standard Plexi found on the Hoffman web site.

[Willabe]

When I check the voltage at the 220k (where the bias tap is hooked up) resistor going into the bias it fluctuates quite a lot. There is 66 volts coming out of the resistor.  The layout is the standard Plexi found on the Hoffman web site.

OK.

But now you need to try this test below.

Is there a pot in your -bias to adjust it? If so with both power tubes out, hook up your meter, clip the positive probe to pin 5 on the tube socket the other probe to ground so your hands are free. Set meter for dcv, take a reading with the bias pot turned on full up and full down, 1st on 1 tube socket then the other. Please post what you get?

        Brad     

[Willabe]

From looking at Doug's layout drawing, your -bias pot is standing on a 47K R. That R is the -bias pots path to ground.

There's a mistake in Doug's drawing for the heaters. He shows 6.3 heater wind with a CT and with 100ohm faux CT R's. Loss the 100R's keep the heater CT grounded.

I see the under board wire you mentioned now, that's for the -bias feed going to the power tube grid leak/grid return R's junction.  

                  Brad    
  

[Al_T]

I get -106 dcv all the way to the cc and only drops to -104.5 when turned all the way clockwise. I did try a 150k and 270k range resistors and they made a little difference, but not more than a couple of points either way.

[sluckey]

The layout is the standard Plexi found on the Hoffman web site.

I've never seen a standard plexi layout on this website.  Hoffman has a layout but it ain't what I'd call 'standard'. Please post a link to the exact layout you are using. Then we can quit dancing and get to fixin'.

[Willabe]

Ok Al, that's your problem.

Sluckey's right. That pots not working and must not have a ground connection. Changing the range R wont fix it as you found out. You can change that if you need to change the over all range after you get the -bias pot working .

Maybe bad solder joint? Maybe a missing connection?  You have to find it.


                   Brad    

[Willabe]

The layout is the standard Plexi found on the Hoffman web site.

Is the drawing I posted it exactly?


             Brad      

[Willabe]

With the amp off, take your meter and put the + lead on the 47K R and -bias pot junction, put the - meter lead to the chassis PT ground. You should get around 47K, if not that 47K R is not going to ground.

2nd test to find out if the bias pot is good. Only 2 of the 3 pot lugs are hooked up. Put your meter on resistance, put the probs, 1 on each of the 2 lugs that are hooked up, take a reading of the pots resistance full up and full off and post what you get.


                 Brad      

[Al_T]

Is the drawing I posted it exactly?

Yes, that is the layout I used.
 
When testing the 47k R the meter reads 0.4

The bias pot goes from 41 cc to 0.0 fully clockwise.

[Willabe]

That 47K is not connected to ground. You need to find why it's not going to ground.

Doug's layout drawing shows the -bias circuit having the 2 bias caps, that 47K tail R, the 2@1ohm cathode (K) r's for measuring bias current and the 2@100R faux CT heater R's that need to be taking out are all tied together in a ground buss and then there should be a wire connected some where to that buss string going to ground some where. Trace it down.

On the pot, 41 what? 41ohms? 4K1(4.1K), 41K? Is it supposed to be a 50K pot? If it's a 50K pot and it reads 41K then that's within 20% tolerance.  


             Brad    

[Willabe]

Here's the layout drawing with the ground buss string I'm talking about high lighted in a red box for clarity.


             Brad     

[Al_T]

Thanks Brad- I should have some time later in the week to rebuild that string.

[HotBluePlates]

Thanks Brad- I should have some time later in the week to rebuild that string.

You might not have to rebuild anything as long as you solder a wire to the point in that box with the upside-down triangle, and attach the other side of that wire to ground.