OT and PT orientation

[dude]

I'm trouble shooting this Bass kit for a friend, he can't get working and I was wondering if the orientation of the PT and OT are good to go before I dig in.
I can't change the OT but the PT has square chassis holes so I could rotate it 45*, would this be beneficial or needed since they are so far apart?

[HotBluePlates]

... would this be beneficial ... since they are so far apart?

Magnetic Coupling decreases with the square of distance, so it's unlikely the power transformer is coupling into the output transformer.

You could test that theory by pulling the output tubes.  If the hum continues, the hum is being directly coupled into the output transformer & passed to speaker.  If the hum stops, it enters at the output tubes (or earlier) and is being amplified & passed into the output transformer.

[dude]

Thanks HPB, since the amp at this point is inoperable, l’ll just rotate the PT, just in case.

[labb]

Good time to use the old head phone trick

[sluckey]

since the amp at this point is inoperable...

What does that mean?

[dude]

since the amp at this point is inoperable...

What does that mean?

Means a guy bought a kit, no experience,  started to follow the layout, never finished, wired stuff wrong, ask me if l could finished it. Never played, l could see bias supply was wrong among many other issues. I have the layout and schematic and will finish it. Inoperable, hope that makes sense.Looks like a copy of a Sun bass amp, has two mini bias pots for each tube, one never hookup and missing, said  he burnt it up trying to solder,   .

[sluckey]

Weber 6S100

[PRR]

> he can't get working and I was wondering if the orientation of the PT and OT are good

PT and OT can point ANY which way and still operate.

That's not broken. Find all the stuff that IS broken, fix it, and it will operate.

[dude]

Weber 6S100

Yes, and l quite don’t understand the stereo bias test Jack..? I probably need your help here, never encountered this type of biasing. Looking at the schematic two 220k grids go to the wiper of two bias pods, this case mini pots. The stereo bias test Jack each go to the K (pin8) on the KT88’s,  1 ohm goes from pin 8 to ground. And the sleeve of the bias test Jack goes to ground. At least as I see it.
What plugs into the stereo test jack..? A stereo jack with just the two sides open and one just takes the mA reading on each side and to chassis to read each tubes mA’s and match then to specs. Then pull the jack out? Bias around -70 volts..? Or am l talking out my butt..! I'm totally lost here, should I just convert this bias system to the usual 1 Ohm R of pin 8 of the KT88's and read mA's across each 1 ohm R...? I know the bias voltage should be around -70..?
I can post the schematic from Weber’s site.

[sluckey]

The stereo bias test jack simply allows you to monitor the voltage across the one ohm resistors without having to pull the chassis from the cab. You understand the bias test jack perfectly. I would have preferred three pin jacks instead of the stereo jack. Look at my dual bias mod on my Sunn Sceptre.

     http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf
     http://sluckeyamps.com/sunn/mod_2.jpg

Having two bias pots is a good thing because it allows you to set the bias individually for each tube. You can perfectly match the tube bias even if the tubes are not perfectly matched. However, Weber's bias pots are connected directly to ground. This means you can set the bias voltage to zero.    If you accidentally set the bias pots to zero volts those KT88s will quickly melt! That's just stupid! You need to put a resistor under each bias pot that will prevent you from turning the bias to zero. Experiment to choose a resistor value that will provide about -35V to -40V when the bias pot is turned all the way down. I would start with 47K and increase/decrease until the bias voltage at pin 5 of each KT88 is -35 or -40 with the bias pot at minimum. Be sure you still have about -70V (or more) when the bias pot is turned all the way up. Look at my Sunn schematic for reference. You don't need any B+ voltage while doing this simple mod so just unplug the rectifier until you are done.

[dude]

OK, Thanks. I'll keep the two bias pots but eliminate the "Stereo Bias Jack", in it's place I'll use a washer and put a black pin biasing grounded jack in the big hole. To either side of that black pin jack, I'll drill two 1/4" holes and use two red pin jacks to bias the amp with a meter. I'll follow your schematic. I guess I could add the protection as you mentioned and keep the stereo biasing jack but the owner of the amp most likely isn't knowledgeable to bias the amp with that set up, much easier to just use a meter and the pin jacks to set the voltage on each tube separately.
I'll use your "bias components values" (so zero bias can't happen) in your schematic, do I seem to have explained this correctly?   Thanks again.

[sluckey]

I'll use your "bias components values"

NO! My component values work well with a 10K pot. They won't work well with your 50K pot.

Start with a 47K resistor like I suggested. Adjust up/down to get the desired lower limit of bias voltage. Experiment with only one pot. When satisfied with the resistor value, do the same to the other bias pot.

[pdf64]

… I'll … eliminate the "Stereo Bias Jack"….

What’s the benefit?
I use stereo mini jacks for that same purpose, seems pretty simple and convenient to me.

[dude]

Ok, pdf64 that would save me from drilling holes in chassis. Thanks.
Sluckey, l assume starting with a 47k on one bias pot, l’d put that resistor between the leg to ground where as in the Weber schematic there is no resistor..? So turning the pot down would never go to zero, use a value in the end that gives me the correct range for KT88s.
To bias: Insert a stereo jack, black meter probe to Ground, the chassis, Red to tip for one pot and the tube it biases, the other shorter sleeve (not ground side) to the other tube and set mA’s to specs.
The 47k across the biasing pots keeps each pot from going to zero bias?
Also, use the values on the Weber amp. Only change l make would be the 47k (or whatever valve gives me the range l want) on each bias pot, I think l got it. Forgive my ignorance.
But Weber’s way on the schematic seems crazy, if one accidentally turns that pot, like a newbie, he blows up his expensive KT88s!

[PRR]

> if one accidentally turns that pot, like a newbie, he blows up his expensive KT88s!

Weber will sell you more KT88s.

[sluckey]

l assume starting with a 47k on one bias pot, l’d put that resistor between the leg to ground where as in the Weber schematic there is no resistor..? So turning the pot down would never go to zero, use a value in the end that gives me the correct range for KT88s.

yes
 

The 47k across the biasing pots keeps each pot from going to zero bias?

Arrrgh!  The resistor is not across the pot!

Also, use the values on the Weber amp. Only change l make would be the 47k (or whatever valve gives me the range l want) on each bias pot, I think l got it. Forgive my ignorance.

yes

But Weber’s way on the schematic seems crazy, if one accidentally turns that pot, like a newbie, he blows up his expensive KT88s!

That's the reason we're even having this discussion. If I didn't like you, I would have said nothing!   

[Willabe]

Arrrgh!  The resistor is not across the pot!

The R is used as a tail R for the pot. It's inserted between the pots ground end and chassis ground. The pot 'stands' on that R.

It's in series with the pot, not in ||, parallel, across the pot.

Like a bass pot stands on the mid range set R, 6K8, in a Fender TB tone stack.   

[dude]

I screwed up in the word “across”, meant to say to ground, like I did in the sentence before that… that was affirmed.
I can get redundant at times, like you guys probably already know .    I’m just an old anal crazy “dude”. But without you guys l’d be playing through a Solid State Peavey   
And thank you all again!

[sluckey]

l’d be playing through a Solid State Peavey   

You mean like Lynyrd Skynyrd? You wish...   

[dude]

Well, maybe another bad choice in word use.   Yeah, lf l could play leads like Lynyrd Skynyrd, l’d be stoked. I’ve covered some of their songs but nothing like the real deal.
I’d buy you a beer if l could 

[sluckey]

I'd gladly drink it with you.

[Willabe]

Naw, Skynyrd played tube Peavey's.

I read an interview with them where they said when they had 3 guitarists, during sound check, 1 of them would go stand in the back and listen. They said the Marshall's sounded thin, too bright. So they switched to Peavey's and during sound check, they said they sounded much fuller.     

[sluckey]

Live and learn

[Fresh_Start]

The stereo bias test jack simply allows you to monitor the voltage across the one ohm resistors without having to pull the chassis from the cab. You understand the bias test jack perfectly. I would have preferred three pin jacks instead of the stereo jack. Look at my dual bias mod on my Sunn Sceptre.

     http://sluckeyamps.com/sunn/sunn_sceptre_1971.pdf
     http://sluckeyamps.com/sunn/mod_2.jpg

Having two bias pots is a good thing because it allows you to set the bias individually for each tube. You can perfectly match the tube bias even if the tubes are not perfectly matched. However, Weber's bias pots are connected directly to ground. This means you can set the bias voltage to zero.    If you accidentally set the bias pots to zero volts those KT88s will quickly melt! That's just stupid! You need to put a resistor under each bias pot that will prevent you from turning the bias to zero. Experiment to choose a resistor value that will provide about -35V to -40V when the bias pot is turned all the way down. I would start with 47K and increase/decrease until the bias voltage at pin 5 of each KT88 is -35 or -40 with the bias pot at minimum. Be sure you still have about -70V (or more) when the bias pot is turned all the way up. Look at my Sunn schematic for reference. You don't need any B+ voltage while doing this simple mod so just unplug the rectifier until you are done.

Question:  are the dual bias pots noticeably interactive?  I’m sure it’s totally manageable if they are, just wondering.

Off topic, but that is a really cool circuit Steve. I mean the whole amp.

[sluckey]

Question:  are the dual bias pots noticeably interactive?

The two bias pots are completely independent. No interaction.