ST70 PT Bias Current

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ST70 PT Bias Current

Postby Geezer » Sun Jul 15, 2012 5:31 pm

I have an OEM ST70 PCB I'm using in a "why not" build using an Antek toroid PT and additional small trannies for the rectifier and bias...the toroid is 360V@ 550mA x2, 6.3V@4A x2 (more than enough). I have a 5V/4A Hammond that should serve the 5AR4. I can't find info on bias current required in the ST70, and the stock PA-60 55V tap appears to be high amp. I have a 36V/140mA Hammond, is this adequate for the bias supply of the VTA70? If not, what transformer rating do i need?
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Re: ST70 PT Bias Current

Postby 20to20 » Sun Jul 15, 2012 6:41 pm

Geezer wrote:I have an OEM ST70 PCB I'm using in a "why not" build using an Antek toroid PT and additional small trannies for the rectifier and bias...the toroid is 360V@ 550mA x2, 6.3V@4A x2 (more than enough). I have a 5V/4A Hammond that should serve the 5AR4. I can't find info on bias current required in the ST70, and the stock PA-60 55V tap appears to be high amp. I have a 36V/140mA Hammond, is this adequate for the bias supply of the VTA70? If not, what transformer rating do i need?


I wouldn't call the VTA board an OEM board. That would be the Dynaco board.

The original bias supply was 55v and assuming the VTA board uses the original supply voltage it would be suprising if your 36v was adequate. I don't know if the adjustment range of the VTA board is wide enough to still get the bias set correctly. You should contact tubes4hifi since they supply the boards for that VTA mod.

HTH

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Postby Geezer » Sun Jul 15, 2012 7:03 pm

You're correct, I shouldn't correctly call the VTA board an OEM...I believe the voltage is adequate, the VTA bias supply has a diode followed by a 10K dropping resistor to deliver 35VDC at the bias trimmers. My question is amperage...what does the bias supply require and does anyone know the amperage of the PA-60 55V bias tap?
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Postby 20to20 » Sun Jul 15, 2012 8:04 pm

Geezer wrote:You're correct, I shouldn't correctly call the VTA board an OEM...I believe the voltage is adequate, the VTA bias supply has a diode followed by a 10K dropping resistor to deliver 35VDC at the bias trimmers. My question is amperage...what does the bias supply require and does anyone know the amperage of the PA-60 55V bias tap?


The VTA board will have its own requirements based on a 55v supply. It may divide and load your 36v down well below the @32v for biasing without modding the 10K resistor.

The original St-70 had 55v loaded by @30K for about 2ma. (If I'm in the ballpark) I don't know what the VTA board draws on the bias circuit but It's probably well short of 140ma.
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Postby Geezer » Sun Jul 15, 2012 9:12 pm

Thanks much, I'm just a paint-by-the-numbers diy'er and I thought the question might have general interest not specific to the VTA circuit for folks interested in using alternate PT's in any ST-70. Looking now, the VTA does use a different bias voltage on the EL34's than the stock Dynaco.

I picked up a pair of the Antek toroids on Ebay for $38 ea., as I recall, when I saw that the B+ was identical to the PA-060. There are many posts discussing different ST70 PT's but none mention alternate transformers for the bias and rectifier circuits...the 5V/4A tap in the PA-060 could light a pair of GZ34/5AR4's if needed, it's truly overbuilt and I didn't know about the bias tap.

I personally see no liability and possibly some (theoretical) benefit in going with the high-current toroid for B+ and 6.3V plus a pair of small inexpensive transformers for the 5V and bias supplies, I guess I'll see. I did email tubes4hifi and am waiting on an answer, I just wanted to see if others had input (I'm always bugging Roy with dumbass questions and he's not shy about letting me know).
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Postby 20to20 » Sun Jul 15, 2012 9:42 pm

Geezer,

Don't forget for the 5ar4 you'll see that the B+ is present on the 5v winding so the tranny winding needs to be rated for the full B+ DC too not just the 5v 4A.
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Postby Geezer » Mon Jul 16, 2012 6:33 pm

20to20 wrote:Geezer,

Don't forget for the 5ar4 you'll see that the B+ is present on the 5v winding so the tranny winding needs to be rated for the full B+ DC too not just the 5v 4A.


Sorry but I don't see where you're going...I'll be supplying 720VAC to the 5AR4 via the Ankek toroid; the toroid lacks a 5V filament winding so I propose to supply that with a separate 3A transformer. Is this a problem or am I missing something? How is the B+ present on the 5V winding?
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Postby 20to20 » Mon Jul 16, 2012 6:52 pm

Geezer wrote:
20to20 wrote:Geezer,

Don't forget for the 5ar4 you'll see that the B+ is present on the 5v winding so the tranny winding needs to be rated for the full B+ DC too not just the 5v 4A.


Sorry but I don't see where you're going...I'll be supplying 720VAC to the 5AR4 via the Ankek toroid; the toroid lacks a 5V filament winding so I propose to supply that with a separate 3A transformer. Is this a problem or am I missing something? How is the B+ present on the 5V winding?


The 5AR4 heater pin 8 is common to the B+ output from the cathode so the B+ DC is present and the 5v AC rides on top. The Dynaco tranny winding insulation can handle the B+ on the 5v winding. I was just pointing this out in case you had a 5v transformer that wasn't meant to be used for a tube rectifier circuit. If the specs on it say it HiPOT test was 1000-2000v then it'll be good to go.

HTH

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Re: ST70 PT Bias Current

Postby burnedfingers » Wed Jul 18, 2012 5:11 am

Geezer wrote:I have an OEM ST70 PCB I'm using in a "why not" build using an Antek toroid PT and additional small trannies for the rectifier and bias...the toroid is 360V@ 550mA x2, 6.3V@4A x2 (more than enough). I have a 5V/4A Hammond that should serve the 5AR4. I can't find info on bias current required in the ST70, and the stock PA-60 55V tap appears to be high amp. I have a 36V/140mA Hammond, is this adequate for the bias supply of the VTA70? If not, what transformer rating do i need?


The Dynaco bias tap is 55VAC and after it is rectified it is -75 roughly.
It doesn't take much current wise for the bias supply. Your transformer in my opinion will work better if you make a negative voltage doubler circuit in order to get the supply higher in voltage. This will mean that you will have to play with the resistor values a little to get a nice compromise. My 70 has a switch that I installed so I can run KT88's and bias them correctly. I built a bias supply for a pair of Wurlitzer 6420's that I own. I used a voltage doubler in that circuit and it works great as I have individual tube bias and can run a number of different output tubes as a result.

A while back I built a Dynaco with an HP transformer that didn't have a bias tap. As a result I mounted a small transformer under the chassis and made my own doubler circuit.

I think there is a schematic on my site of the bias circuit I made.
Last edited by burnedfingers on Wed Jul 18, 2012 5:15 am, edited 1 time in total.
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Postby burnedfingers » Wed Jul 18, 2012 5:14 am

20to20 wrote:
Geezer wrote:
20to20 wrote:Geezer,

Don't forget for the 5ar4 you'll see that the B+ is present on the 5v winding so the tranny winding needs to be rated for the full B+ DC too not just the 5v 4A.


Sorry but I don't see where you're going...I'll be supplying 720VAC to the 5AR4 via the Ankek toroid; the toroid lacks a 5V filament winding so I propose to supply that with a separate 3A transformer. Is this a problem or am I missing something? How is the B+ present on the 5V winding?


The 5AR4 heater pin 8 is common to the B+ output from the cathode so the B+ DC is present and the 5v AC rides on top. The Dynaco tranny winding insulation can handle the B+ on the 5v winding. I was just pointing this out in case you had a 5v transformer that wasn't meant to be used for a tube rectifier circuit. If the specs on it say it HiPOT test was 1000-2000v then it'll be good to go.

HTH

20


If you have purchased a 5v filament transformer for your amp you are good to go.
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Postby Geezer » Sun Jul 22, 2012 4:01 pm

Your transformer in my opinion will work better if you make a negative voltage doubler circuit in order to get the supply higher in voltage.


Burnedfingers, thanks for the info, I believe the voltage doubler is the way to go. Roy Mottram, the disigner of the original circuit, tells me the same.

You mentioned a schematic on your website...how do I find this?

Best Regards,
DC
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Postby burnedfingers » Mon Jul 23, 2012 4:26 am

I hope no one minds

http://dynaco2forum.forumsmotion.com/t15-wurlitzer-6420

There is a primitive hand drawn schematic that I drew with the Wurlitzer schematic. There are pictures in my thread showing a transformer and circuit operating showing a -70 volt supply. It will get you going with your 70 and if not you will find support there also.

There is actually 4 pages in that thread and if you go thru the whole thing it might be helpful. I made a small turret board and mounted the components and it looks and works ok.
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