Advantages of power supply w/ dual choke

knowledge base for the classic Dynaco ST70

Advantages of power supply w/ dual choke

Postby Brinkman » Sun Oct 12, 2008 9:34 pm

Hello everyone,

I have seen threads where DIYers have split the ST-70's power supply into two channels requiring two chokes (and/or current inrush limiters) and/or builds which utilize lower-DCR chokes (than stock) or Miller chokes to improve the power supply. I have even heard from tube loyalists who recommend I remove the choke altogether ("too high DCR")! I am wondering what methods are the best for improving the fidelity of the ST-70 in terms of it's power supply.

I am currently using the SDS cap board in conjunction with the VTA driver board (low-gain 12BH7 model).

I understand there is a split between builders who try and stay true with a 5AR4 rectifier and others who wish to use solid state rectification and I am open to opinions from both camps. I would just like to know more about different ways to improve the ST-70's power supply, even if that money could be better used towards a better amp. I alternate between the 5AR4 and the Weber CopperCap rectifier, though I seem to prefer the Weber.

I may end up asking this question on other forums, but I felt here was the best place to start. FYI, I am using the dynakitparts.com transformers.

Best,
Brinkman
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Re: Advantages of power supply w/ dual choke

Postby kheper » Sun Oct 12, 2008 11:20 pm

Brinkman wrote:Hello everyone,

I have seen threads where DIYers have split the ST-70's power supply into two channels requiring two chokes (and/or current inrush limiters) and/or builds which utilize lower-DCR chokes (than stock) or Miller chokes to improve the power supply. I have even heard from tube loyalists who recommend I remove the choke altogether ("too high DCR")! I am wondering what methods are the best for improving the fidelity of the ST-70 in terms of it's power supply.


A split choke supply may not be the best option due to the design of the ST70. The multi-cap or cap-board feeds the driverboard with two leads for both channels (not four) with the voltages to the driver stage.

I suppose people use two chokes to keep the heat down or provide a little better regulation for the B+ to the A470s.

For a point-to-point wired ST70, a choke for the voltages feeding each channel's requirements may be advantageous.

I am currently using the SDS cap board in conjunction with the VTA driver board (low-gain 12BH7 model).

I understand there is a split between builders who try and stay true with a 5AR4 rectifier and others who wish to use solid state rectification and I am open to opinions from both camps. I would just like to know more about different ways to improve the ST-70's power supply, even if that money could be better used towards a better amp. I alternate between the 5AR4 and the Weber CopperCap rectifier, though I seem to prefer the Weber.


With the upgraded PA-60 power transformer, a 5V3 or a 5AU4 can be substituted for the 5AR4.

They will drop more voltage than a 5AR4, but they have a current capacity of around 350ma. The result is a tighter bass response and a wider range in dynamics than with the 5AR4.

I may end up asking this question on other forums, but I felt here was the best place to start. FYI, I am using the dynakitparts.com transformers.

Best,
Brinkman
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Re: Advantages of power supply w/ dual choke

Postby Brinkman » Mon Oct 13, 2008 5:28 pm

Thanks for the response. I know my question was worded a bit strangely, so I need a little clarification:

kheper wrote: A split choke supply may not be the best option due to the design of the ST70. The multi-cap or cap-board feeds the driverboard with two leads for both channels (not four) with the voltages to the driver stage...

For a point-to-point wired ST70, a choke for the voltages feeding each channel's requirements may be advantageous.


So if I had multiple boards (one for each side), a split choke could be advantageous?

Here's a series of quotes regarding the ST70's power supply AA:
Eli Duttman wrote: IMO, the frequent comments made about not using SS rectification in a ST70 are utter drivel. Yes, the B+ rail voltage comes in high. You turn that liability into an asset by switching to combination bias. Install a shared RC network under each pair of O/P tubes. Now, the fact of a single bias adj. pot. in each channel is no longer a problem, as the RC network compensates for minor differences between the EL34s.

I suggest the use of what I call a "cockeyed" bridge, for the rectification scheme. Connect each end of the rectifier winding to the "free" cathode of a snubbed series wired pair of UF4007s. Ground the "free" anodes. Connect the CT of the rectifier winding to the anode of a 1200 PIV Cree SiC Schottky diode. Take the "raw" B+ from the Schottky's cathode. Install a CL150 inrush current limiter between the Schottky diode and the PSU filter, to slightly soften B+ rise. The instant on bias supply electrostatically protects the EL34s against cathode stripping. A 15 μF. 'lytic is plenty in the 1st position. Follow with a decent value inductor and clobber the remaining ripple in the 2nd position of the CLC setup. A Hammond 159T looks like a suitable choke. Save the salvaged C354 for an "El Cheapo" build.

There's little doubt in my mind that the choke is important. If you increase the doubler stack capacitance over the OEM value, the choke's importance grows. Small conduction angles, with their sharply triangular shaped ripple waveforms, imply LOTS of ripple overtone energy. Space permitting, consider inserting a LC section made from a J.W. Miller RF choke and a mica or NPO ceramic cap. between the doubler stack and the 159T to kill "hash" that might be passed on by 159T's capacitance. The background should be quite black.


The original thread is here.

kheper wrote: With the upgraded PA-60 power transformer, a 5V3 or a 5AU4 can be substituted for the 5AR4.

They will drop more voltage than a 5AR4, but they have a current capacity of around 350ma. The result is a tighter bass response and a wider range in dynamics than with the 5AR4.


I've been using the WZ68 Coppercap and have noticed these improvements, probably because it acts like two 5AR4s. I may need to install a pair of inrush limiters...
CL-90s or CL-120s?
Brinkman
 
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Location: Portland, OR, North America

Re: Advantages of power supply w/ dual choke

Postby kheper » Mon Oct 13, 2008 8:19 pm

Brinkman wrote:Thanks for the response. I know my question was worded a bit strangely, so I need a little clarification:

kheper wrote: A split choke supply may not be the best option due to the design of the ST70. The multi-cap or cap-board feeds the driverboard with two leads for both channels (not four) with the voltages to the driver stage...

For a point-to-point wired ST70, a choke for the voltages feeding each channel's requirements may be advantageous.


So if I had multiple boards (one for each side), a split choke could be advantageous?


For a dual mono st70 (two driverboards, two power supplies and two chokes), yes. But a dual mono st70 is not an st70. It is a MKIV.

Splitting the B+ and going through two chokes and two filter banks, one for the A470s and one for the driverboard may be beneficial, as well.

If you build from scratch, a choke with more Henries and more amperage capacity will be the option I would choose.

Here's a series of quotes regarding the ST70's power supply AA:
Eli Duttman wrote: IMO, the frequent comments made about not using SS rectification in a ST70 are utter drivel. Yes, the B+ rail voltage comes in high. You turn that liability into an asset by switching to combination bias. Install a shared RC network under each pair of O/P tubes. Now, the fact of a single bias adj. pot. in each channel is no longer a problem, as the RC network compensates for minor differences between the EL34s.

I suggest the use of what I call a "cockeyed" bridge, for the rectification scheme. Connect each end of the rectifier winding to the "free" cathode of a snubbed series wired pair of UF4007s. Ground the "free" anodes. Connect the CT of the rectifier winding to the anode of a 1200 PIV Cree SiC Schottky diode. Take the "raw" B+ from the Schottky's cathode. Install a CL150 inrush current limiter between the Schottky diode and the PSU filter, to slightly soften B+ rise. The instant on bias supply electrostatically protects the EL34s against cathode stripping. A 15 μF. 'lytic is plenty in the 1st position. Follow with a decent value inductor and clobber the remaining ripple in the 2nd position of the CLC setup. A Hammond 159T looks like a suitable choke. Save the salvaged C354 for an "El Cheapo" build.

There's little doubt in my mind that the choke is important. If you increase the doubler stack capacitance over the OEM value, the choke's importance grows. Small conduction angles, with their sharply triangular shaped ripple waveforms, imply LOTS of ripple overtone energy. Space permitting, consider inserting a LC section made from a J.W. Miller RF choke and a mica or NPO ceramic cap. between the doubler stack and the 159T to kill "hash" that might be passed on by 159T's capacitance. The background should be quite black.


The original thread is here.

kheper wrote: With the upgraded PA-60 power transformer, a 5V3 or a 5AU4 can be substituted for the 5AR4.

They will drop more voltage than a 5AR4, but they have a current capacity of around 350ma. The result is a tighter bass response and a wider range in dynamics than with the 5AR4.


I've been using the WZ68 Coppercap and have noticed these improvements, probably because it acts like two 5AR4s. I may need to install a pair of inrush limiters...
CL-90s or CL-120s?
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Re: Advantages of power supply w/ dual choke

Postby Brinkman » Mon Oct 13, 2008 8:57 pm

kheper wrote:For a dual mono st70 (two driverboards, two power supplies and two chokes), yes. But a dual mono st70 is not an st70. It is a MKIV.

Splitting the B+ and going through two chokes and two filter banks, one for the A470s and one for the driverboard may be beneficial, as well.

If you build from scratch, a choke with more Henries and more amperage capacity will be the option I would choose.


Sorry. I meant two cap boards; I'm using the triode electronics cap board and have an extra.

I boldfaced what you said that really caught my attention. What I'm wondering about is how the choke specs would have to be changed if one decided go this route. In other words, could splitting the power supply duty into two chokes present an opportunity to also use lower-DCR (or otherwise higher quality) chokes, in addition to the benefits of separate filter for both the A470s and driver board?

Any chokes you would suggest off the top of your head?

I have sincerely appreciated your help.

-Brinkman
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Posts: 32
Joined: Sat Jan 06, 2007 10:00 am
Location: Portland, OR, North America

Re: Advantages of power supply w/ dual choke

Postby kheper » Mon Oct 13, 2008 9:20 pm

Brinkman wrote:
kheper wrote:For a dual mono st70 (two driverboards, two power supplies and two chokes), yes. But a dual mono st70 is not an st70. It is a MKIV.

Splitting the B+ and going through two chokes and two filter banks, one for the A470s and one for the driverboard may be beneficial, as well.

If you build from scratch, a choke with more Henries and more amperage capacity will be the option I would choose.


Sorry. I meant two cap boards; I'm using the triode electronics cap board and have an extra.

I boldfaced what you said that really caught my attention. What I'm wondering about is how the choke specs would have to be changed if one decided go this route. In other words, could splitting the power supply duty into two chokes present an opportunity to also use lower-DCR (or otherwise higher quality) chokes, in addition to the benefits of separate filter for both the A470s and driver board?

Any chokes you would suggest off the top of your head?

I have sincerely appreciated your help.

-Brinkman


With the st70's chassis size, there are few options. You may be able to fit another c354 under the chassis, but that is about it. 2 chokes with the same specs would run cooler and be more efficient, than one. A c354 for the driverboard voltages may not produce a noticable improvement. Separate chokes on each channel, smoothing out the ripple voltages for the output transformer and the driver circuitry is incorporated into a number of schematics. But, I would opt for one big choke instead.
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