The Official Dual Rail Thread

[SoulFetish]

In principle a separate screen PT would allow for a stiffer screen voltage - if that's the purpose.
Some tube Hifi amps even use active screen voltage regulation for max. dynamic response (= least screen compression).

Yes, this is true! You wouldn't have the drop in voltage under the full plate load to contend with. I wonder if the difference between a separate winding and separate PT would be much more noticeable from each other to make it worth the extra space needed.

[hylaphone]

Fusing idea: Install a 1N4007 such that the plate supply draws current from the screen supply (and through the screen supply fuse) when the plate supply fuse blows. The plate supply can never go lower than a diode drop below the screen supply. I see this all the time in test systems with multiple power supplys primarily for analog circuitry.

Is there any reason this trick wouldn't work if the fuses were on the secondaries rather than after the rectifiers?

[nickb]

I wonder if/how they took care of Vhk limit of the series regulator tube.

It uses a separate winding on the power transformer for the heater.

[mhuss]

Here's an example of the stacked supplies idea: https://hiwatt.org/Schematics/DR_200wPS.gif

[hylaphone]

Is there any reason this trick wouldn't work if the fuses were on the secondaries rather than after the rectifiers?

Can anyone advise on this?

[nevetslab]

That's what Audio Research has in their aged D70 Stereo Tube Power Amp that I just received from MEF member sgelectric. Amp has two pairs of 6550's with a third 6550 as part of the regulated screen supply. At present, the screen supply is blowing fuses, so I'll have to dig into circuits to find the cause and solution. I'll post schematics when I get a chance.

In reading thru this 'The Official Dual Rail Thread' for ideas on building a MosFET Screen Supply for my KT88 Power Tube Test Fixture, I stumbled upon my post from 3/24/21, having a client's Audio Research D70 Tube Power Amp, which uses a KT88 in a Screen Supply for the power tubes. Turned out the amp was NOT blowing fuses, or at least not while I had it on the bench and running. I did trim up the supplies and ran it for some time. I found I didn't have the schematics for the D70, but only the D70 Mk II, which I forgot to post.

D70 Mk II Schematics.pdf

I was able to obtain the Owner's Manual for the D70, which had the adjustment procedure for the Supply Regulator per channel. That's posted here:

D70_Manual.pdf

[nevetslab]

i guess the issue is really the dynamic effects on the power supply. if you are running the plates at high voltages, you will need to have the screens running lower.

using separate supplies for the plate and the screens avoids all these problems. you just need a second rectifier. one winding is rectified to feed to the plates, the other is rectified and fed to the grids
the things to consider are

- ensure that the way you fuse the power supply is done in a way that the grid supply will be shut off if the plate supply is. if the grid supply is on when the plate supply is off you will blow your output tubes very quickly. not cheap when you have 3 pairs of 6550's or similar in a monster amp.

I've been reading thru this thread, posted by Wilder Amplification "The Official Dual Rail Thread" on 4/07/2010 regarding using separate supplies for the HT Plate Supply and Screen Supply for tubes like the KT88, 6550's. I also found later in this post two circuits....one very similar to that used in the Hiwatt DR201 200W Amp, where there are two separate HT secondary windings, each with it's own bridge rectifier, and wired them in series, with a common HT fuse at the bottom of the lower Screen Supply rectifier. A second solution, that Wilder Amplification showed was that using a HT secondary winding with a C/T, and had the C/T connected at the center of the stacked filter caps to yield the HT Plate and lower tap at the C/T for the Screen Supply. I hadn't thought of that in my efforts to use a separate transformer for the screen supply. The HT Plate Supply winding in this Blackstar amp I have has a CT, so this FWB Doubler circuit could be used. This was from their post # 43 on 4/11/2011.


The only thing though is that I can't really see much of a difference between the floating and the FWB doubler arrangements. Either way the screens are pulling from one cap while the plates are pulling from both while the caps are getting charged to 1/2 the total plate supply voltage. The only difference I see is that the FWB doubler resembles a grounded center tap rectifier whereas the floated arrangement FWB rectifies both windings.



The other thing too is that the only way I can see to fuse both supplies with 1 fuse would be to fuse the ground feed from the negative side of the supply. I know that's not "standard practice" but it's the only way I can see to have a fuse that's common to both supplies.

Later on in this post, R.G. got involved in the discussion to suggest creating a MosFET Screen Supply derived from the HT supply. That's what I was looking for.....a solution to make use of the large heat sink in this Blackstar PSU from their S200 Bass Amp that I inherited and am building up as a KT88 Power tube test fixture for screening/sorting used Power Tube pulls, accumulated over the past 13 years, while the cost of power tubes has been impacted by Putin's war on Ukraine and the US Sanctions on Russia has made life 'interesting' for folks keeping backline gear running.

In Wilder Amplifications later post after much discussion with R.G, Steve Conner and others in this very interesting & well-thought out thread, this circuit evloved:


09-10-2010, 09:20 PM



OK...here's the scheme I've come up with for the Dual Rail PSU utilizing the transformers I originally had wound for this thing. Included pretty much every idea from this thread in it. Anything else that we feel needs to be added feel free to contribute.

A bit overengineered?



Jon Wilder
Wilder Amplification

So, I have a lot of information now and several directions to go, besides just settling with the cobbled two-power-transformer solution to achieve the separate HT and Screen Supply, off of which is the Driver and Preamp stages of the Hiwatt DR201 amp circuit that I'm in the process of building into this discarded Stage Prop Blackstar S200 amp.

I just love what can be found and re-discovered here on this forum!!