metalockpick
Well-known member
See that big ground symbol connected to the Center tap on the schematic? Connect directly to the big filter cap negative for best performance. Not sure how you were getting any HV without it. Or bias!
EL-34's won't bias.
- Thread starter Amled87
- Start date
Help Support GroupDIY Audio Forum:
At pin 5 no tubes installed, with red probe in pin 5 and black to ground it's 100-200 millivolts.
If using a full wave rectifier, yes. I'm using a bridge rectifier with the negative DC going to ground. As I said, I can rewire the transformer with a center tap, and I will right now.
And after doing so, where do i connect the 1N4007 to the bridge rectifier for my bias supply?
metalockpick
Well-known member
Bridge... that's tricky.
Might be best to add a small xfrmr for bias tap, as I'm not sure you can phase shift the AC with a series cap and another rectifier
In all 100w marshalls you should see the negative volts at the grid before standby is switched to on ,
some of the earlier 50W models tap the bias supply after the standby ,
If you have no bias volts on the powertubes , yes they will red plate within 30 seconds ,then procede to melt down .
some of the earlier 50W models tap the bias supply after the standby ,
If you have no bias volts on the powertubes , yes they will red plate within 30 seconds ,then procede to melt down .
There is no bias voltage (except a few millivolts no matter where standby switch is located. I know the tubes will redplate and then melt down without bias voltage hince why I'm trying to get bias voltage (it's done ate one set of tubes lol)
Rewired my power transformer with a center tap (I'm using a bridge rectifier instead of (4) 1N4007 to convert my AC to DC and accidentally connected my center tap to DC negative side and blew the mains fuse. Transformer checks out ok, so replacinf the fuse and moving the center tap to the nut on the first filter cap.
At this point, it is just going to a tech. I've exhausted all possible fixes I can.
CJ
Well-known member
Check the polarity of your caps in the bias circuit. They need to have the negative side connected to the bias supply and positive to ground.
Get a highlighter pen and trace out your bias supply on a paper schematic. I am sure you will find a missing ground or some other gremlin.
Can you post a picture of
1) your el 34 tube sockets
2) your bias supply circuit
.
Get a highlighter pen and trace out your bias supply on a paper schematic. I am sure you will find a missing ground or some other gremlin.
Can you post a picture of
1) your el 34 tube sockets
2) your bias supply circuit
.
That's the thing though and why I'm scratching my head with this one. I've retraced my bias supply 100x already, but one more won't hurt. I've checked all grounds, made sure the caps had the + lead grounded and that the - was connected to the bias circuit. When the diode is connected or disconnected from the caps I have 230-240vac where voltage is being sent to the cathode, but the anode (and I've changed the diode several times now) always has just a few millivolts, no reading of anything higher. I'll post photos of both in just a moment, thanks for looking CJ!
Voltage comes in from power transformer, goes to a bridge rectifier, 1 leg of rectifier connects to 220k resistor which feeds the bias supply. Power transformer center tap is connected to ground now. I was using a bridge rectifier previously, now using center tapped full wave rectifier. I have continuity between caps and ground, they are facing the correcr way. I have 1k 5w resistors on pins 4&6, they read within spec, and show 470v when powered on. The 5k6 screen resistors measure within spec and show now sign of issue except no negative voltage.
I have continuity between the 15k resistor which connects to the 47k resistor that is on the input of the bias pot. Bias pot measures from 0-25k when adjusted. No matter, there is no negative voltage present at at the bias resistors.
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metalockpick
Well-known member
Ideally, the center tap is grounded, the outside lugs each go through diodes, with the anode inside together, positive high voltage. One side has the reverse biased diode and resistor for the bias supply. With this circuit, you'll get positive voltage from the full wave, and you'll get negative bias from the single diode and capacitor. Just get that working. I believe in you! You can do this.
You should analyze the circuit. The voltage taken from the transformer varies between 0 and B+, so never negative.
2 possibilities:
2 possibilities:
- revert to half-bridge rectifier
- use a separate winding.
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The toroidal transformer isn't suitable for a half bridge rectifier, the data sheet says it will saturate the transformer and isn't recommended.
I've got the center tap ground, the outside lugs (325v tap 1 and 325v tap 4 of the power transformer) then goes to the AC in of the bridge rectifier. Tap 1 of the bridge rectifier then jumps from the ac input of the bridge rectifier to my bias circuit. Would this be correct? I'm out of 1N4007's now, so hoping to make this work with the bridge rectifier providing my DC high voltage, with one side of AC connected to the bias supply.
Sometimes this easiest solution really is the easiest. Any reason why I can't skip the dropper resistor and single diode and jump directly off the negative side of my bridge rectifier for my negative bias voltage? I have filter caps rated high enough to take the punishment. Look where the arrow is pointing in the included picture . At that point it would be finding suitable dropper resistors so i end up with -50 to -65v at my bias supply.
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MagnetoSound
Well-known member
You CAN’T use a bridge rectifier, and still derive negative bias volts from the same winding.
With a bridge rectifier, the negative side must be grounded to complete the B+ circuit.
This means that in any duty cycle there is NO negative voltage available for bias.
It’s fine until you hit the STANDBY switch, but when that switch closes your negative half-cycles no longer exist.
With a grounded CT and full-wave rectifier you can do it, and that’s the schematic you have.
What’s all this about the transformer saturating?
With a bridge rectifier, the negative side must be grounded to complete the B+ circuit.
This means that in any duty cycle there is NO negative voltage available for bias.
It’s fine until you hit the STANDBY switch, but when that switch closes your negative half-cycles no longer exist.
With a grounded CT and full-wave rectifier you can do it, and that’s the schematic you have.
What’s all this about the transformer saturating?
I understand that, which is why when I was using (4) 1N4007 for the power supply I had the center tap. When I switched to the bridge rectifier I was only using one pair of the 325v tap, the second was going to my bias supply. Either way, there is no negative voltage.
However, in this instance, using the 325v taps with a center tap could I not use a bridge rectifier and then use my negative DC side for my negative bias voltage?
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MagnetoSound
Well-known member
No. Consider the output current. The negative side of the bridge must be grounded to complete the B+ circuit.
If you are only using one of the secondaries for the B+, you can use the other for bias, but your voltage will need a lot of dropping. Again, ground the other end of the winding to complete the circuit.
If you are only using one of the secondaries for the B+, you can use the other for bias, but your voltage will need a lot of dropping. Again, ground the other end of the winding to complete the circuit.
Of course. You would need a different xfmr.
Ok, lets make sure I understand. If using one set of secondaries going to the bridge rectifier (requires removing the center tap) to produce my DC high voltage I ground the negative side of the bridge to complete the circuit.
Using the other set of secondaries I connect one tap to the bias supply, and then ground the other?
No. Read again. It's not suitable for single-wave rectification.
