Bias tap from power transformer

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johnheath

johnheath

Well-known member
Joined
Jul 31, 2014
Messages
890
Location
Sweden
Hi all...

I am about to build a HiFi amp for a friend as a wedding gift and I have the schematic ready and all but it seems pretty hard to find a suitable power transformer for the task.

I would like to have a bias tap (secondary) of around 50V or so and I also need two secondary windings for 6,3VAC @ 4A each ( a lot of heater current with two EL34's on each channel). Looking at Edcor it seems that I can chose between the two options but not together.

So... my question is if it is just to tap from the HT secondary to get a 50VAC tap? I have seen this in some guitar amp schematics and I thought I could do the same. Pros and cons of this?

Any input is valuable I guess :)

Best regards

/John
 
Are you closed to the idea of using more than power transformer for a one off build?  2x 6.3VAC @4A is getting a little bit power hungry like you said.

Maybe someone or some other people would be willing the split the cost of custom design from edcor for the xfrmr specs you're after. They ship in about 3 months usually..
 
Yes, it is mainly the heater arrangements that eats all that current... the rest is not so much. I have e-mailed Edcor and asked the about their options and if they don't have one that suits me, if they can build a custom made one. I guess it will be more expensive for sure but waiting three months is not an option since the wedding is in the late July.

I do not have a complete schematic at the moment but simply put there will be a 6GH8 and two EL34 in each channel. The El34 draws 1,5A each and the 6GH8 draws around 450mA each so roughly 3,5A for each channel... heating that is :)

Hmm...what to do?


Best regards

/John
 
for bias, small 120 VAC : 12.6 VAC transformer,
backwards off a 6.3 VAC winding.
inexpensive, low hassle factor, low magnetic field.
 
gridcurrent said:
for bias, small 120 VAC : 12.6 VAC transformer,
backwards off a 6.3 VAC winding.
inexpensive, low hassle factor, low magnetic field.
Click to expand...

Thank you sir...

I have not tried such a solution before but I guess it could work?

Best regards

/John
 
A high wattage resistor can be used to  shift the ht  low side of coil away from 0v and rectify a negative bias voltAge...  Might be inefficient and require some redesign of circuit-- found this idea in the Garnet book
 
shabtek said:
A high wattage resistor can be used to  shift the ht  low side of coil away from 0v and rectify a negative bias voltAge...  Might be inefficient and require some redesign of circuit-- found this idea in the Garnet book
Click to expand...

Thank you sir

You do not have a schematic of your suggestion... perhaps? :)

Best regards

/John
 
Why not use a guitar amp transformer? 4xEL34 is a common configuration, so it should have  enough heater current,  plus a bias tap is pretty standard as well.

I would think there would also be this configuration from folks who make HiFi transformers,  Hammond is a common manufacturer.
 
johnheath said:
Thank you sir

You do not have a schematic of your suggestion... perhaps? :)

Best regards

/John
Click to expand...
You mau want to look at the schemo of Ampeg VT40/22; bias is derived from the HT secondary via a cap and a resistor in series. That reduces heat dissipation. Some tweaking needed, but works well.
 
sounds like you need a Marshall 100 watt  pwr xfmr?  plenty to choose from as far as price,

then use either a resistor network or cap for your bias tap. 

1960's diodes were not rated for the hi voltage needed for tube work, so they used a center tapped pwr xfmr. even so, Fender needed  6 diodes to get the job done, 3 in each series string. so using 12 for a bridge circuit would have been a hassle.  and then you need 3 more for your bias circuit.

don't know where Marshall got those A10D10 diodes for their bridge circuit, have not seen a data sheet either.

so after the 1N4007 types became cheap, everybody dumped the CT transformer because they could do a bridge circuit with four diodes. this means you can make the power transformer smaller as the VA rating drops with the bridge circuit as opposed to the CT circuit.  also, you get rid of 1 tap (CT) so they were chasing pennies any way they could to compete, thus, the bridge circuit. CT transformers are still used as you need them for the 5U4 or 5Y3.

it seems the bias tap went away with the CT xfmr, so now  everybody uses a resistor  voltage divider or a capacitor. Fender also uses a voltage doubler off their solid state +/- wind.  See the Marshall 1959 model for the resistor circuit or the Ampeg as mentioned above for the cap circuit.  use a cap with a stable temp coef so you do not drift to far. 

you might want to add a surge resistor of beefing up the microfarads, usually audio amps use more filtering than a guitar amp.

 
john12ax7 said:
Why not use a guitar amp transformer? 4xEL34 is a common configuration, so it should have  enough heater current,  plus a bias tap is pretty standard as well.

I would think there would also be this configuration from folks who make HiFi transformers,  Hammond is a common manufacturer.
Click to expand...

Thank you sir

I have been thinking of this option but really not looked into it yet... shall do :)

Best regards

/John
 
abbey road d enfer said:
You mau want to look at the schemo of Ampeg VT40/22; bias is derived from the HT secondary via a cap and a resistor in series. That reduces heat dissipation. Some tweaking needed, but works well.
Click to expand...

Thank you sir

Yes, at least on the VT40 (schematic that I found) they tap from the secondary HT for the bias. On the VT22 schematic I found they seem to use a C.T from the "screen-windings" for bias.

Best regards

/John
 
CJ said:
sounds like you need a Marshall 100 watt  pwr xfmr?  plenty to choose from as far as price,

then use either a resistor network or cap for your bias tap. 

1960's diodes were not rated for the hi voltage needed for tube work, so they used a center tapped pwr xfmr. even so, Fender needed  6 diodes to get the job done, 3 in each series string. so using 12 for a bridge circuit would have been a hassle.  and then you need 3 more for your bias circuit.

don't know where Marshall got those A10D10 diodes for their bridge circuit, have not seen a data sheet either.

so after the 1N4007 types became cheap, everybody dumped the CT transformer because they could do a bridge circuit with four diodes. this means you can make the power transformer smaller as the VA rating drops with the bridge circuit as opposed to the CT circuit.  also, you get rid of 1 tap (CT) so they were chasing pennies any way they could to compete, thus, the bridge circuit. CT transformers are still used as you need them for the 5U4 or 5Y3.

it seems the bias tap went away with the CT xfmr, so now  everybody uses a resistor  voltage divider or a capacitor. Fender also uses a voltage doubler off their solid state +/- wind.  See the Marshall 1959 model for the resistor circuit or the Ampeg as mentioned above for the cap circuit.  use a cap with a stable temp coef so you do not drift to far. 

you might want to add a surge resistor of beefing up the microfarads, usually audio amps use more filtering than a guitar amp.
Click to expand...

Thank you sir

Very interesting facts... like it.

A problem that still remains is that every transformer I look at that will handle four EL34's doesn't have more current than 7,5A @ 6,3VAC. Using the four EL34 and two 6GH8 will draw near 7A... I guess that it would be on the limit of what such a transformer can handle... Am I wrong on this? It feels like I should have a bit more margin?

Best regards

/John
 
After some scouting I have found a Hammond 373FX 110-240VAC primary/ secondary 650V C.T. @ 460ma/ 5V C.T. @ 6A/ 6.3V C.T. @ 9A and a bias tap of 50VAC.

It has a fair price as well so it would work.

Could you gentlemen say anything about the difference between let's say an Edcor and a Hammond. I know that I have had some trouble with Hammonds before concerning mechanical vibrations transferring into noise in a LA2A build a couple of years ago.

Best regards

/John
 
abbey road d enfer said:
Unfortunately, that's often a problem with cheap transformers.
Click to expand...

Yes, I guess so. I am awaiting answers from Edcor to see if they are willing to make a custom made... and keep the cost to a human level? :)

Best regards

/John
 
the heater winding in a big pwr xfmr could probably carry 20 amps or more, that is how big the wire is. primary wire is also usually way overkill, so no problems on that side either. the 7.5 amps spec is used so that you get 6.3 with a load of 7.5 amps, you can go up or down on the load and that 6.3 will move up and down just a tad.
 
CJ said:
don't know where Marshall got those A10D10 diodes for their bridge circuit, have not seen a data sheet either.
Click to expand...
10D10's were made by International Rectifier back in the day. The A designates the Avalanche version. They were 1A 1000Volts.
My first job after leaving college was working for IR in their Englsh factory, we made high voltage/current diodes and thyristors. Fascinating to see the whole process from a silicon wafer being milled to the right thickness, alloyed into a junction, soldered to a base, welded to its top, then plated with tin. And tested at every step. Happy days!
 

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