very high negative bias

[jayallen]

I have a line 6 dt50 212 that blew the power transformer. So I bought the mercury magnetic bog-dt50-p.  So I hooked it up had every sounded fine than I  fried a 100uf/160v eletlctrolic cap for the bias. Now I have very little sound in class a mode and none in class a/b mode.  I replaced the cap now it start to get very hot unless I take out the power tubes to diagnose the problems. So far I get -185 to -90 on pin 5 on the el34 tube. My bias wire is 66 volt a/c goes to a diode then that bounces up to -256 d/c goes to the electrolytic cap that gets hot to a 15k resistor that turn volts into -183 dc to a trimpot then to a 220k resistor to a 2.2k resistor  to pin 5. There is also a 24k resistor off the trimpot that goes to ground  in between that voltage is -90dc. Class a mode get that cap hot fast. I can get the bias lower in a/b mod by changing the 24k resistor than I get sound but that cap get hot and sizzle.

[sluckey]

Schematic?

[jayallen]

Schematic?

line 6 won't hand one out.

[shooter]

you doing this without power tubes in?

[shooter]

this is a snippit from a Bogner, should be able to use to "create" your version

[jayallen]

you doing this without power tubes in?

With power tubes in I get a -180 to -90dc in class a/b mode on pin 5 no hot cap. If I change the 24k resistor I can get it to -40dc with sound then but the electrolyte starts to get super hot can't  touch. In class a mode I get 26.8 mv between 1 ohm resistor  on the cathode. No volt on pin 5 but once again the cap get to hot to touch. In standby mode I get -52 after the 1n4007 when I flip the on switch that when it jump to -246 dc. Another thing I don't get it when I remove the bias tap wire from the circuit I get very little voltage to anything like pin 3, diode etc.

[sluckey]

Impossible to follow what you are saying without a schematic.

[PRR]

If the cap gets hot it is either not rated for voltage or wired backward.

[jayallen]

this is a snippit from a Bogner, should be able to use to "create" your version

yes it look like that just a couple different values.

[shooter]

cool
so with NO tubes in;

does the cap get HOT
what is the "end-to-end" voltage vdc at pin 5

[jayallen]

cool
so with NO tubes in;

does the cap get HOT
what is the "end-to-end" voltage vdc at pin 5

-185 to -90 vdctube in or not in same vdc.

[shooter]

I would recommend rebuilding your circuit, so diode and caps minimum.  VERIFY all the R's
NOTE:
cap +++++ goes to ground
I'm only allowed to use crayons tonight though so DO NOT install tubes
your goal is guess -35 to -55 at pin 5, all pin 5s

[jayallen]

I would recommend rebuilding your circuit, so diode and caps minimum.  VERIFY all the R's
NOTE:
cap +++++ goes to ground
I'm only allowed to use crayons tonight though so DO NOT install tubes
your goal is guess -35 to -55 at pin 5, all pin 5s

I did all that twice. Still same problem

[shooter]

then I'm at a loss 

some thoughts;
your meter is lying
your PT is wired wrong
the ground is messed up
the parts are not spec'd for the voltage you're using

[jayallen]

then I'm at a loss 

some thoughts;
your meter is lying
your PT is wired wrong
the ground is messed up
the parts are not spec'd for the voltage you're using

when I disconnect the bias wire. Am I suppose to lose all my high voltage to everything? This morning I put a 220k resistor in front of the 1n4007 diode and tapped off the secondary wire I got -54 bias. But only had 46 dc on pin 3.

[shooter]

the transformer layout you posted has a 60VAC tap - PINK  I suspect that is your bias tap.  Is that where you wired to the diode?

[jayallen]

the transformer layout you posted has a 60VAC tap - PINK  I suspect that is your bias tap.  Is that where you wired to the diode?

yes but with it disconnected am I suppose  those all my high voltage to everything?

[shooter]

yes but with it disconnected

disconnected from what??

to answer under normal conditions NO, the bias tap is it's own thing.  there should be 2 other wires from the windings that feed HV AC to your rectifier, they don't care if there's a bias circuit or not.
UNLESS it's wired wrong, but then it's NOT normal conditions

[sluckey]

Let's assume the transformer is connected correctly. If so, the only way to get those high bias voltages is for the center tap(D/Green-Yellow) to become disconnected from ground either because of a blown fuse, standby switch, etc.

So, check for zero ohm reading between the D/Green-Yellow center tap and ground. I'm looking at the MM transformer winding. Notice there are two green/yellow center taps. I'm not talking about the one for the 6.3V filament winding (B/Green-Yellow).

If you happened to get the two green-yellow wires mixed up then the PT is not connected properly and could be the problem. Need to verify proper connection.

[jayallen]

Let's assume the transformer is connected correctly. If so, the only way to get those high bias voltages is for the center tap(D/Green-Yellow) to become disconnected from ground either because of a blown fuse, standby switch, etc.

So, check for zero ohm reading between the D/Green-Yellow center tap and ground. I'm looking at the MM transformer winding. Notice there are two green/yellow center taps. I'm not talking about the one for the 6.3V filament winding (B/Green-Yellow).

If you happened to get the two green-yellow wires mixed up then the PT is not connected properly and could be the problem. Need to verify proper connection.

Everything right. Here an article I went by https://mytubeaudio.com/2016/11/11/all-the-way-to-mercury-and-back-for-a-line-6-dt-50/. 

[sluckey]

So, check for zero ohm reading between the D/Green-Yellow center tap and ground.

Did you do this? What is the meter reading?

[jayallen]

So, check for zero ohm reading between the D/Green-Yellow center tap and ground.

Did you do this? What is the meter reading?

yes Ik did it read nothing unless I did it wrong. I hooked the g/y wire with the label b+ center tap written on it to the red probe then touched the black probe to the chassis. Multimeter is on 200 ohms.

[shooter]

Multimeter is on 200 ohms.

thinking it should be closer to 2 ohms, but it possible there's a ground lift 
with connectors connected properly;
what OHM reading do you get from chassis to the bias caps + lead?

[sluckey]

it read nothing

Nothing as if both probes are held in the air without touching anything? Or, nothing as if the two probes are touching each other and the meter reads zero ohms? There's a big difference! To say it read nothing is ambiguous. I've never seen a meter that had a reading of "NOTHING". Lot's of meters that read 0 or OL.

[jayallen]

it read nothing

Nothing as if both probes are held in the air without touching anything? Or, nothing as if the two probes are touching each other and the meter reads zero ohms? There's a big difference! To say it read nothing is ambiguous. I've never seen a meter that had a reading of "NOTHING". Lot's of meters that read 0 or OL.

  50 ohm is g/y disconnected.
.4 ohm is with g/y back in circuit.

[jayallen]

Multimeter is on 200 ohms.

thinking it should be closer to 2 ohms, but it possible there's a ground lift 
with connectors connected properly;
what OHM reading do you get from chassis to the bias caps + lead?

.3 ohms from bias cap.

[sluckey]

.4 ohm is with g/y back in circuit.

That's what I was looking for. Unfortunately that's not the problem.

[shooter]

my call; we're at the limit of what's possible, with what's available.

As you can see, the board is very complex, it might have fault detection circuits, it could be roasted under, lots of possibilities that can't be guessed without a schematic.

Have you looked up Tech's locally?  make sure they know what you have cuz if you  called me, Id pass, not because of complexity, but lack of schematic.

[jayallen]

my call; we're at the limit of what's possible, with what's available.

As you can see, the board is very complex, it might have fault detection circuits, it could be roasted under, lots of possibilities that can't be guessed without a schematic.

Have you looked up Tech's locally?  make sure they know what you have cuz if you  called me, Id pass, not because of complexity, but lack of schematic.

I took it to a tech last week and he couldn't figure it out. I am gonna gut the chassis and build something with el34 power tubes. Leading toward a 50 watt hiwatt clone. Thanks for every one help.

[sluckey]

I took it to a tech last week and he couldn't figure it out.

Well SHIT! That would have been a nice piece of info to put in your first post!

[shooter]

some days the toast falls butter side up, sometimes it doesn't 
good luck.

fwiw, here's Doug's schematic choices for hiwatt;
https://el34world.com/charts/Schematics/files/Hiwatt/Hiwatt_Schematics.htm

[sluckey]

And some days it's just a pile of shit no matter which way is up!

[shooter]

 

YUP!

[jayallen]

Shit happens. Doesn't bother me Just order all the stuff for a hiwatt dr504.

[Ritchie200]

Jayallen
Did you happen to unplug all your PT connectors and check voltages at the connectors?  As in the “all the way to mercury” article, did the new PT wires match the old one?  Did you verify like in the article or did you just assume?  If all is well check the cheap connectors, evidence of other components overheating, burnt traces, arcing, etc. Thats a lot of jack for that tranny. I wouldn’t give up quite yet.


Jim

[Ritchie200]

Also you mentioned that after you changed out the transformer everything was fine then you fried the bias cap. Fine in both AB and A?Did you check bias before this happened?  What was it?  Was the cap failure immediate, hour, days? Have you swapped out tubes?  Like the blank schematic shooter posted, this is not a complex circuit in its own right.


Jim

[jayallen]

Also you mentioned that after you changed out the transformer everything was fine then you fried the bias cap. Fine in both AB and A?Did you check bias before this happened?  What was it?  Was the cap failure immediate, hour, days? Have you swapped out tubes?  Like the blank schematic shooter posted, this is not a complex circuit in its own right.


Jim

I just used everything I had to build this. More of a really tube amp. No digital stuff. It sound great especially  with the helix lt. Thanks for trying  to help. Just got to find decal to print off.

[Williamblake]

Are you still working on this? It was suggested you had the centertaps from heaters and high voltage mixed up, if this amp has heater elevation it maybe would explain the bias issue.

[shooter]

Now that's a great example why IC chips aren't needed 

Nice work

[Williamblake]

Ah... Nicely done!