johnheath

Hi all...

I faced with a concern. I have ordered some high priced transformer and one of them is the mains transformer for a tube HiFi-amp.

The order was placed in august last year and due to working abroad I haven't had the time to take a proper look at them but when I did I saw that the 50VAC tap was missing. I told the producer (who is, by the way, a very good guy) who told me there has been a misunderstanding in the communication between the two of us.

That said I still miss the 50VAC tap for the Bias circuit and as I see it there are two solutions but I don't which one is the best suited?

First I can tap from the HT+ and lower the voltage by using  a resistor... I must confess that I am not sure about current draw... the amp is planned to use two pairs of 6550 power tubes. The resistor might be huge and/ or very warm? I have calculated to draw some 140mA - 150mA per pair.

Second is to get a small transformer with suitable secondaries to get the desired 50VAC. Problem with the transformer option is size which is limited in this build... it needs to be less that 70mm i diameter.

I have an idea if it works?... to get a small transformer with dual 24VAC secondaries and connect them in series... would that give 48VAC?... still a bit concerned about current draw to determine the VA of this small transformer? And still I have the issue with placing this small transformer inside the chassi... on the outside is not an option due to lack of space.


Thoughts?


Best regards


/John
Nothing is impossible - It just takes some more time


Rob Flinn

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #1 on: February 04, 2019, 02:06:24 PM »
If I understand you correctly, assuming that the h.t is positive, it's not easy to work out how you would tap a negative bias voltage from it.
regards Rob

johnheath

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #2 on: February 04, 2019, 02:32:34 PM »
Well, the "normal" 50VAC tap in a transformer is also positive... using a diode in reverse, in series with a resistor,  will provide the negative voltage... or am I completely wrong here?

The difference would be, if the HT+ is 345VAC, to use a higher value resistor than if the tap were 50VAC I suppose?

I have seen schematics of this in some guitar amps... both where they tap from one of the HT+ secondaries but also where they tap from both the HT+ secondaries (higher value resistor needed in the latter case).

Best regards

/John
Nothing is impossible - It just takes some more time

CJ

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #3 on: February 04, 2019, 03:31:08 PM »
season to taste
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #4 on: February 04, 2019, 06:37:41 PM »
You dont need to worry about current draw much  ,your only providing a voltage for the grids of the output stage , the same circuit exists in most guitar 50-100w amps in one variation or another ,The Marshall one provides a pot which varies the voltage on both tubes  , some of the Fender ones employ a balance control instead . Hiwatt uses a seperate winding in the transformer , full wave rectifier and two pots one + one - phase .While current draw at the grid is normally small, if the amp is driven into distortion it could draw a bit more ,I think the full wave supply and seperate winding  in the Hiwatt contributes a lot to the big clean tone. Seperate transformer for bias would have the advantage that no matter whats going on  in the amp ,your bias voltage would remain nice and stable .
Any small 48V transformer would probably do ,
You didnt say what you were using the amp for ,if its guitar very much down to what kind of sound you want ,if you want maximum clean ,go for the full wave or bridge setup with either seperate TX or seperate winding ,which you dont have.
The half wave setup tapped from the main HT winding does add a little sponginess but only noticeable if your really driving the amp .If its for hifi ,your probably never going to be intentionally driving it into distortion anyway ,if its studio monitor amp maybe the extra headroom of a completely seperate bias supply might be beneficial.
 

squarewave

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #5 on: February 04, 2019, 07:27:30 PM »
I think you might need to worry about current draw. It's not grids that you need to worry about. It's the bias resistor from ground. It depends on the circuit but it could be 10K or 5mA from HT which is potentially an issue. Again, it depends on the circuit. Post a schematic. Also, many of the guitar circuits referenced are center tap to ground and use half of the AC to get negative. So again, post a schematic.

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #6 on: February 05, 2019, 03:20:24 AM »
It's easy to generate a negative bias supply with a capacitor-coupled rectifier don't worry about it. Assuming the current is roughly constant you can drop most of the voltage across the capacitors with no heat dissipation.
« Last Edit: February 05, 2019, 04:32:21 AM by merlin »

johnheath

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #7 on: February 05, 2019, 06:07:13 AM »
season to taste

Thank you Sir

That is also what I have been thinking of. I just wonder if there are any drawbacks from tapping of just one side of the HT?... imbalance?


Best regards

/John
Nothing is impossible - It just takes some more time

johnheath

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #8 on: February 05, 2019, 06:26:33 AM »

Any small 48V transformer would probably do ,


Thank you sir

I was asking about a small transformer. Could you tell me if I could use a dual 24VAC and couple these secondaries in series to get 48VAC? BTW it is a HiFi amp and as you imply it won't get driven into distortion... but also I would like to avoid any "sponginess".


Best regards

/John
Nothing is impossible - It just takes some more time

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #9 on: February 05, 2019, 09:29:33 AM »
That is also what I have been thinking of. I just wonder if there are any drawbacks from tapping of just one side of the HT?... imbalance?
You can tap from boths sides and use a bridge rectifier, if you want.


johnheath

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #10 on: February 05, 2019, 09:38:14 AM »
You can tap from boths sides and use a bridge rectifier, if you want.

Thank you Sir

I have that as a plan if that will be the final solution.


Best regards

/John
Nothing is impossible - It just takes some more time

clintrubber

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #11 on: February 05, 2019, 12:08:13 PM »
For grins, would it be a nasty idea to generate the negative bias from the heater-winding ? (with a diode & cap ladder)

Possibly it's a better idea to read this article  8)

http://sound.whsites.net/valves/design2.html#s9

johnheath

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #12 on: February 05, 2019, 12:27:15 PM »
For grins, would it be a nasty idea to generate the negative bias from the heater-winding ? (with a diode & cap ladder)

Possibly it's a better idea to read this article  8)

http://sound.whsites.net/valves/design2.html#s9

Thank you sir...

Fact is that I have been talking to the manufacturer of the transformer and they will provide me with a single transformer with a 50VAC secondary (free of charge) and I will try that solution before fiddling with tapping of the HT+


Best regards

/John
Nothing is impossible - It just takes some more time

clintrubber

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #13 on: February 05, 2019, 12:32:08 PM »
Nice, sounds good!

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #14 on: February 12, 2019, 12:48:34 PM »
Look at the circuit on any Ampeg V4 or VT40 etc...the bias is just derived straight from the b+ (which is pretty high on these amps) with a big capacitor and resistor diode network as above...that capacitor is the first thing I check when Ampegs lose their bias.

abbey road d enfer

Re: Tapping from HT or get a small transformer to get -50VAC for Bias
« Reply #15 on: February 13, 2019, 03:17:56 PM »
Fact is that I have been talking to the manufacturer of the transformer and they will provide me with a single transformer with a 50VAC secondary (free of charge) and I will try that solution before fiddling with tapping of the HT+
That is a much better solution IMO. Although there are many amps that use the capacitive divider to generate bias, it has some drawbacks. One is that the bias voltage takes a long time to establish, generally short enough to be more or less stable when the heaters get hot. Another is that it takes some time to sort out the proper capacitor value. As mentioned earlier, failure of the cap results in loss of bias, which is a serious threat to the output tubes and transformer.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.


 

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