EL-34's won't bias.

[Tubetec]

Could that little blob of solder in the middle of the photo be causing a short ?
Could one of the tubes have a shorted grid ?
Sometimes arcing across the tube socket can cause a conductive path in the phenolic material .

 

[MagnetoSound]

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?

Yes, you seem to have got it but your terminology is odd.

You have two secondary HT windings. Each of those is a coil with two ends. So one pair of WIRES for the HT and the other pair of WIRES for the bias.

You don’t want 300-odd volts for the bias, so tinker the resistors in the bias circuit until they’re in the ballpark.

 

[Amled87]

This is what I drew up guys. Would this be suitable and work?

 

[MagnetoSound]

This is what I drew up guys. Would this be suitable and work?

 

[abbey road d enfer]

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 think there is a big misunderstannding as to how you have wired the xfmr and rectifiers.
Your early description tended to indicate you used a full bridge, as in the South-West picture of the datasheet, but this last description tells different.
Would you please draw a good schemo of the actual arrangement that doesn't bias properly?

 

[abbey road d enfer]

This is what I drew up guys. Would this be suitable and work?

No.
Check attachment. This is what you should do.

 

[abbey road d enfer]

No, sorry. This is bridge rectification with negative grounded.
For correct operation, it's the CT that needs to be grounded.

 

[Amled87]

I think there is a big misunderstannding as to how you have wired the xfmr and rectifiers.
Your early description tended to indicate you used a full bridge, as in the South-West picture of the datasheet, but this last description tells different.
Would you please draw a good schemo of the actual arrangement that doesn't bias properly?

I had the xfmr and rectifier wired exactly as in the schematic and exactly as how you posted. However, I was still getting no negative voltage at the bias resistors.

I've since changed the topology, and I'm using a bridge rectifier using only 1 set of secondaries for the HV and the other for bias voltage with the mock up schematic I drew. I'm out of 1N4007's, so I am now having to use the bridge. So, the question is will this work?

 

[Amled87]

No, sorry. This is bridge rectification with negative grounded.
For correct operation, it's the CT that needs to be grounded.

In order for me to have a center tap, it requires both HV coils to be connected together with the inner two (#2 and #3 grounded). As I'm out of 1N4007's now it's either wait for more or proceed to use a bridge off one secondary and my bias voltage off the other. Why will this not work?

 

[johnR]

Why will this not work?

As others have pointed out, ditch the bridge rectifier and use separate diodes plus grounded CT as on the Marshall schematic. If you've run out of diodes you need to stop and get some more, as it's the only way it'll work.

 

[Matador]

Abbey's shem is how a gazillion tube PSU's are wired. If that isn't working then that is what needs investigating.

Do you have a bleeder resistor on your B+ voltage. To discharge the B+ rail when powered off?

 

[Amled87]

As others have pointed out, ditch the bridge rectifier and use separate diodes plus grounded CT as on the Marshall schematic. If you've run out of diodes you need to stop and get some more, as it's the only way it'll work.

Not sure why everyone says it won't work, because it does? Amp is biasing correctly now. I have however ordered diodes and I'll switch topology to follow the schematic. However, I assume the starting issue will be there and I'll have no negative voltage at the bias resistors again.

 

[Amled87]

Abbey's shem is how a gazillion tube PSU's are wired. If that isn't working then that is what needs investigating.

Do you have a bleeder resistor on your B+ voltage. To discharge the B+ rail when powered off?

I do not have bleeders installed, however I'm comfortable discharging filter caps and always take every safety precaution! I am going to revert back to the schematic, and follow abbeys directions. But, for now it is working as I've posted.

 

[Amled87]

As others have pointed out, ditch the bridge rectifier and use separate diodes plus grounded CT as on the Marshall schematic. If you've run out of diodes you need to stop and get some more, as it's the only way it'll work.

Older Marshalls have bridge rectifiers in them (not diodes) so not sure why (it's the only way it'll work)

 

[johnR]

Not sure why everyone says it won't work, because it does? Amp is biasing correctly now. I have however ordered diodes and I'll switch topology to follow the schematic. However, I assume the starting issue will be there and I'll have no negative voltage at the bias resistors again.

From your sketch it looks like you're using one secondary for HT and the other one (separately) for bias. Is that correct? If so you'll burn out the HT secondary as it only has half the required current capacity. You might have bias and HT voltages at the moment, but the first time you overdrive the amp you can probably say goodbye to the transformer.

 

[Amled87]

From your sketch it looks like you're using one secondary for HT and the other one (separately) for bias. Is that correct? If so you'll burn out the HT secondary as it only has half the required current capacity. You might have bias and HT voltages at the moment, but the first time you overdrive the amp you can probably say goodbye to the transformer.

Even though each secondary is rated separately for more current then the amp can pull? Each secondary is rated for 400ma? Care to explain why the transformer would go if it's underloaded?

Also, please don't take me as rude. I'm just asking questions and trying to get this correct. I've order UF4007's and will be converting back to the schematic using a center tap and etc. But, as is the way I have it currently seems to work.


The actual Drake transformer was rated at 690v and 150mA (213mA max)

Mine is rated at 325v (455-470v after rectification) at .4A (400mA) so even at half (200mA) why would this not be suitable if both secondaries are rated at 400mA?

 

[moamps]

Yes, you are right. If you use that transformer of that power, one winding is enough. And you can use the second winding for - bias. According to the schematic from your post 43. It would be good if you implement a standby switch so that you can first turn on the heating and set a sufficiently negative bias before turning on the anode voltage.

 

[sortedaudio]

Are you sure the o/p valve cathodes go to ground. and it's the same ground as the bias pot is connected to?

 

[CJ]

You can try the Ampeg bias circuit, they use a cap as an AC resistor to feed the single diode. Look up the circuit on say a V4 or VT 22 etc.

 

[Amled87]

Yes, you are right. If you use that transformer of that power, one winding is enough. And you can use the second winding for - bias. According to the schematic from your post 43. It would be good if you implement a standby switch so that you can first turn on the heating and set a sufficiently negative bias before turning on the anode voltage.

That was just a rough drawing, I do have a standby switch implemented. The schematic I posted was just to get a working bias supply. I can't wrap my head around why the transformer when even at half current capacity (200mA or half of max rated 400mA) would go though. Because even at half current, it is in spec for the amp (it's actually closer to the spec called for as it only calls for a 150mA (213mA max) HV secondary power transformer.