Weber 6M45P build. No bias voltage. Help please.....

[Marcus]

So I just finished my first build. I don't have a variac so I know I'm going to get slammed for not bringing the voltages up slowly. Everything seemed good until I got ready to bias the tubes. The heaters are good. All tubes are lighting fine. Where should I start looking? Thanks 

[eleventeen]

1: Electrolytic caps in backwards? (Their POSITIVE terminals go to ground, this is different from EVERY OTHER e-cap in your tube-life)
2: Circuit not completed from bias supply output (a NEGATIVE voltage) all the way thru a pot (if present, for bias adjust) and "range setup" or "offset" series resistors to ground.
3: Connection not made from bias supply out (more likely the wiper of the bias adjust pot) to the junction of 2 qty symmetrical resistors whose other ends go to the G1's on the output tubes.

With power all the way drained, hook your ohmmeter to G1 (pin 5 on an EL34, do this for both tubes) and trace connection all the way back to the anode (the non-bar end) of the rectifier diode that is the bias rectifier.

Post a schematic and or pix. Has to be simple.

[Willabe]

I don't have a variac so I know I'm going to get slammed for not bringing the voltages up slowly.

Most guys around here use a light bulb current limiter for 1st start up and when replacing power supply parts, like filter cap jobs, PT's.

Everything seemed good until I got ready to bias the tubes.

So your saying you have no -bias voltage? More information will be helpful.

On amps that use negative grid bias it's best to start the amp without any tubes (if you have a bias tap, if you don't have a bias tap and are taking the -bias from the HT B+ and have a rectifier tube then you need to have that in) and 1st make absolutely sure you have -bias on ALL the power tube socket grid pins and if adjustable make sure that the -bias voltage changes in the range needed, say ~ -30 to -55? Depends on the power tube type but that's ~ pretty typical voltages. After you confirm that then you can put in the power tubes and bias them up. Always start with the -bias full up (largest negative dc voltage) so the power tubes run cold at 1st then bring them up to where you want/need them to idol.


            Brad      

[stratele52]

Marcus , first , don't plug any Output Power tubes before you fix bias circuit . If you think you find the problem , redd the negative voltage at the grid on each Output Power tube 's sockets before you plug tubes . you must have around -30 volts .

I write this as an average for many amps . I don't know this amp .

To power ON safely your amp you can use the well know current limiter ; a 100 watts bulb lamp in serie on AC input line .

If amp work good ( with low power because the 100 watts bulb ) you can plug the amp direct in wall outlet.

[Willabe]

Here's another drawing of the limiter and it has some instructions with it.

http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf


            Brad   

[Marcus]

1: Electrolytic caps in backwards? (Their POSITIVE terminals go to ground, this is different from EVERY OTHER e-cap in your tube-life)
2: Circuit not completed from bias supply output (a NEGATIVE voltage) all the way thru a pot (if present, for bias adjust) and "range setup" or "offset" series resistors to ground.
3: Connection not made from bias supply out (more likely the wiper of the bias adjust pot) to the junction of 2 qty symmetrical resistors whose other ends go to the G1's on the output tubes.

With power all the way drained, hook your ohmmeter to G1 (pin 5 on an EL34, do this for both tubes) and trace connection all the way back to the anode (the non-bar end) of the rectifier diode that is the bias rectifier.

Post a schematic and or pix. Has to be simple.

Really? My layout shows the grounding on the negative side. Going by the layout I cant find anything wrong. I'll try tracing it later.

[alerich]

Really? My layout shows the grounding on the negative side. Going by the layout I cant find anything wrong. I'll try tracing it later.

Both the layout and the schematic of the 6M45P on the Weber web site show the + side of the cap going to ground.

https://taweber.powweb.com/store/6m45p_schem.jpg

https://taweber.powweb.com/store/6m45p_layout.jpg

[Marcus]

I'm sorry. You guys are talking about the cap in the bias circuit specifically. Yes I have that one in correct with the positive end to ground. Really stumped here.

[eleventeen]

Simple. Power on with the amp in standby. The operation of the stby switch will remove all B+ (except for C4, the .047) and should have no effect on the presence of bias on the output tube grids. Trace out where you are losing your bias. I'd start right on the wiper of the bias pot. My guess the most likely culprit is the bias pot itself, unless you've just omitted a wire. Most importantly in the cut-to-the chase department, if you have (guess) -65 volts on one end of the bias pot and -25 on the other yet nothing on the wiper, you have a bad pot = the wiper is not making contact.

I realize the way the schematic is drawn that even with an open wiper you *should* be getting ~~neg 40-50 volts bias, I would not make the assumption that the schematic absolutely matches the connections. There are a couple of ways to misdraw a rheostat (eg; a pot with one end connected to the wiper) simple as it may be. Maybe the short little jumper connection from the wiper to one end of the pot is missing, the pot is bad, and the bias circuit wire is taken from the wiper. That could do it.

Otherwise, R29 could be open. Maybe the bias winding itself is open. Maybe the ground at the bottom of R30 isn't there.

But any and all of these are very non-subtle faults which you should be able to trace down in less than a minute each.

The other question I have is, just in case I am hallucinating: Your main rectifier is a full wave NOT-bridge, like a Twin Reverb. All the diodes face the same way. The loose ends of the operative windings are both red/whites. They go nowhere. The schematic also shows a center tap, designated red/yellow. To the right of the center tap is the designation "not used"

It's quite unclear to me whether the red/whites above and below the writing "not used" are the things that are "not used" or is the center tap (which there shouldn't be in the first place if there are split windings with both ends available) is "not used"....even though it is drawn as going to ground.

Ummmmm.....errrr? This is goofy. This type of rectifier REQUIRES a (grounded) center tap. The dwg shows a grounded center tap all right but there cannot be the two red/white "inside" winding-ends AND a center tap. Am I making sense?

There ISN'T a center tap on this kind of dual winding. If there were, the two side-windings would have, well, a center tap and thus they would be (presumably, internally) connected. Follow me? You would in effect have three wires coming out of the tranny, all of them going to the same place, eg; the center tap of the HV winding, but that surely isn't likely.

There would not be a red white, either side.

So...neither red/white is connected to anything.... and the center tap is "not used"....?

This cannot work. There is no ground connection to the power supply rectifier and thus there is no ground connection to the bias tap.

None of us can resolve at a distance whether this is a drawing error or what the heck it is, but it should not be hard to resolve.

But a full-wave NOT-bridge can't work as this is drawn.

[sluckey]

It's quite unclear to me whether the red/whites above and below the writing "not used" are the things that are "not used"

Well, they are not used. Pretend they don't exist. Now you have a conventional HT winding with a center tap.

That PT has a special HT winding with a choice of two voltages. The red wires give you 720V with center tap and the red/white wires give you 660V with center tap. Use ONLY one or the other.  The center tap (and bias tap) is used regardless. See pic...

[Marcus]

Update.... I traced the circuit and I do have bias voltage all the way to pin 5. Weird thing is when I hook up my bias rite I don't get a reading. I checked the sockets on the bias rite and they are fine. When I plug the tubes in the bias pot seems to have no affect on the tubes. Obviously I'm also not getting any signal to my speaker cab. So confused and frustrated at this point.

[alerich]

Just for kicks - with the power tubes removed (or all the tubes, for that matter) power up the amp and insert your voltmeter probe into the sockets at pin 5 in the same way the tube pin would contact it and see if you have negative bias voltage. Check all power tube sockets in this manner. If the negative voltage is present in this check then I might begin to suspect your bias rite tool as the culprit.

[stratele52]

Just for kicks - with the power tubes removed (or all the tubes, for that matter) power up the amp and insert your voltmeter probe into the sockets at pin 5 in the same way the tube pin would contact it and see if you have negative bias voltage. Check all power tube sockets in this manner. If the negative voltage is present in this check then I might begin to suspect your bias rite tool as the culprit.

What I wrote  Feb 01 ;

Re: Weber 6M45P build. No bias voltage. Help please.....
« Reply #3 on: February 01, 2014, 04:23:29 pm »
 

Marcus , first , don't plug any Output Power tubes before you fix bias circuit . If you think you find the problem , redd the negative voltage at the grid on each Output Power tube 's sockets before you plug tubes . you must have around -30 volts .

[Willabe]

When I plug the tubes in the bias pot seems to have no affect on the tubes.

Everything seemed good until I got ready to bias the tubes.

So your saying you have no -bias voltage? More information will be helpful.

On amps that use negative grid bias it's best to start the amp without any tubes (if you have a bias tap, if you don't have a bias tap and are taking the -bias from the HT B+ and have a rectifier tube then you need to have that in) and 1st make absolutely sure you have -bias on ALL the power tube socket grid pins and if adjustable make sure that the -bias voltage changes in the range needed, say ~ -30 to -55? Depends on the power tube type but that's ~ pretty typical voltages. After you confirm that then you can put in the power tubes and bias them up. Always start with the -bias full up (largest negative dc voltage) so the power tubes run cold at 1st then bring them up to where you want/need them to idol.

[stratele52]

When I plug the tubes in the bias pot seems to have no affect on the tubes.

Everything seemed good until I got ready to bias the tubes.

So your saying you have no -bias voltage? More information will be helpful.

On amps that use negative grid bias it's best to start the amp without any tubes (if you have a bias tap, if you don't have a bias tap and are taking the -bias from the HT B+ and have a rectifier tube then you need to have that in) and 1st make absolutely sure you have -bias on ALL the power tube socket grid pins and if adjustable make sure that the -bias voltage changes in the range needed, say ~ -30 to -55? Depends on the power tube type but that's ~ pretty typical voltages. After you confirm that then you can put in the power tubes and bias them up. Always start with the -bias full up (largest negative dc voltage) so the power tubes run cold at 1st then bring them up to where you want/need them to idol.

+1

[alerich]

Just for kicks - with the power tubes removed (or all the tubes, for that matter) power up the amp and insert your voltmeter probe into the sockets at pin 5 in the same way the tube pin would contact it and see if you have negative bias voltage. Check all power tube sockets in this manner. If the negative voltage is present in this check then I might begin to suspect your bias rite tool as the culprit.

What I wrote  Feb 01 ;

Re: Weber 6M45P build. No bias voltage. Help please.....
« Reply #3 on: February 01, 2014, 04:23:29 pm »

Marcus , first , don't plug any Output Power tubes before you fix bias circuit . If you think you find the problem , redd the negative voltage at the grid on each Output Power tube 's sockets before you plug tubes . you must have around -30 volts .

Which is not necessarily what I suggested. He says he has bias voltage at pin 5. Many people read this from inside the amp on the wiring side of the socket. Many people do that by clipping their voltmeter lead onto the wire that is soldered to pin 5. If there is a poor solder joint or a defective pin in the socket that could be a problem that is overlooked. I asked him to read these voltages at the pin side of the socket where the tubes are inserted. If the voltage is present there and then he gets no reading with his bias rite in place I might then suspect a faulty bias rite tool. It must be a tremendous burden being right all the time. 

If there were ever a thread that is begging for a few high def pics of the power socket wiring this is it.