Comments for page: PS Mods for ST-35

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Posted by Bill May 21, 2014 - 11:15 am
Duh, I didn't take into account that second output tube. makes sense now.

I built a ST-35 from a kit with Magnequest OPT's, which have the desired 16R pot. I used decent parts and incorporated Dave's EFB. It works great. I use the basic set-up, and with carefully matched tubes, both channels bias measurements are always within 0.5%-2% of each other. In my next build, I will probably go for the individual channel bias - not the individual tube bias. Which brings me to a question: If I were to go to mono amps (I'm kinda thinking about it), is there a suitable PT on the market that would handle the "reduced" requirements? Or could I modify the original somehow? Also, if I stay with the original stereo design, is there a PT out there that runs cooler? I have heard that there is an equivalent Edcor that runs cooler, but wonder if there is an "over designed" one available. Maybe more questions to follow. By the way, I like the use of diodes on the input tubes idea.

Thanks Art

Posted by Dave May 20, 2014 - 10:19 pm
Hi Bill --
The .27 vdc figure mentioned produces a current flow of 27 ma per tube.

The cathodes of the output tubes are connected together in each channel, with both tubes then drawing current through the 5 ohm cathode resistor for that channel.

Since Ohm's Law dictates that I = E/R, substituting the .27 vdc and 5 ohm resistance into the equation means that 54 ma of current is flowing through these resistors when the bias is set for a voltage drop of .27 vdc across them.

Since the 54 ma current flow represents the current for both output tubes in a given channel, dividing that figure by two then means that each tube individually is drawing 27 ma, if they are well matched tubes.

Thanks for the interest, and I hope that helps!


Posted by Bill May 20, 2014 - 06:13 pm
In the simple EFB method, the recommended bias setting is .27vdc. It mentions in the article that this is equivalent to a current of 27mA measured through a 5R resistor. How does ohm's law work here?


Posted by Art Grannell March 01, 2014 - 08:05 pm
Hi Steve-
An ideal power supply would provide pure DC to the amplifying circuits, and that “purity” includes all frequencies from 0Hz to high RF frequencies. While electrolytic caps do well filtering 60 or 120 Hz, they don’t do nearly as well at high frequencies, because of their internal construction. It is beneficial therefore, to add smaller capacitors in parallel with the filter capacitors to eliminate high frequency interference which may appear before or in the power supply or in the following wiring. Many recommend these bypass caps should be about 1% of the value of each electrolytic, but a 4.7mfd 600 volt cap, isn’t practical to fit in this chassis, so I used smaller values from .47 to .1 mfd. Polypropylene caps have electrical characteristics that make them well suited for this application. While not always possible, it’s also good practice to have the physical location of the last bypass cap in the supply located as close as possible to the active device or tube to negate the effects of “stray” inductance in wiring. While meeting the exact requirements isn’t always practical, adding any bypass caps is beneficial. If you look at the photo on page 5, two of the bypass caps can be easily seen. The first (brown) is wired across the first electrolytic filter cap on the lower right, and another (orange) is visible across the second electrolytic at the lower center.

The same theoretically perfect power supply would provide infinite current at 0 ohms output impedance. Since this is impossible, there are always undesirable interactions that occur between the channels of a multi-channel, i.e., “stereo” amplifier. A common solution is to utilize two completely separate amplifiers (mono-blocks), each with its own power supply. Short of doing that, there are ways to reduce these interactions in single power supply “stereo” amplifier. Dave’s EFB Bias circuit excels at doing this with the output stage, as he described in his article. Large power supply filter caps with bypasses also are helpful. A third effective method is to isolate the two channels from each other with diodes between the power supply and the circuits. This at least partially “fools” each channel of the amplifier into acting as if it has its own separate power supply. While I cannot post a schematic in this Q &A column, a very good example of the technique is available in the “net. If you search for Eli Duttman’s “El Cheapo” amplifier schematic (available on numerous sites), and locate the power supply diagram, you will see that he uses a diode to isolate each of the two channels of the amp at the power supply. While he isolated each complete VA and power output section, I chose to use this technique only on the VA/phase splitter, again mostly because of chassis space issues.
Referring to the Dynaco ST-35 schematic, a 1N4007 diode would be placed between the junction of C8c and R11 and eyelet 7 of each printed circuit board, with the cathode or stripe facing eyelet 7, which is the junction of R5 and R6. On an original ST-35, it may be easiest to attach the diodes at the quad section can, possibly using a terminal strip, then attach the PC board feed wires to the end of the diodes. To further improve the isolation, I added a small electrolytic cap and bypass cap between eyelet 7 and ground on each channel. The value isn’t critical as the current draw of the 7247 tube is low. Any value between 10 and 50 mfd rated for at least 350VDC would be fine. In the same photo, the black electrolytic and bypass caps are seen on each side of the chassis near the chassis supports. The diodes are more difficult to see, mounted one above the other on the opposite side of the terminal strip from the R11 power resistor.
I hope this detail helps clarify my statements.

Posted by Steve February 28, 2014 - 03:34 pm
Hi Art,

I have a few questions about the SCA 35 article: "High Performance Power Supply Modifications to an ST-35 Including Individual EFB Bias".

The questions in (bold) were based on page 6 excerpts below. From page 6:

"Additional changes that I made to the power supply include small polypropylene bypass capacitors, at each
filter capacitor section," (what size ?)

"and the electrical isolation of each 12DW7/7247 from the other, with a 1N4007 diode," (what does this mean - can I get a schematic drawing of how to accomplish this ?)

"and adding additional electrolytic and bypass capacitors for each channel mounted as shown in at right." (does this mean you simply added an additional RC section the the end of the power supply?)

Posted by Art Grannell February 26, 2013 - 12:45 pm
The 6BQ5 is a true pentode, meaning that the internal construction of the tube features a true suppressor grid, rather than electron beam directing structures, that are common in most other audio output tube types. The stated purpose of this suppressor grid is to keep electrons that may be dislodged from the plate, from being attracted to the positively charged screen grid, raising the screen current and possibly exceeding the screen grid’s current capabilities. The suppressor grid is internally connected to the cathode in the vast majority of 6BQ5/EL84/7189/7189A tubes, with the Russian 6P15P series of tubes being the only exception of which I’m aware.

The Hazen mod that you mention, places a small film capacitor between the cathode and suppressor grid, and claims sonic improvement in the specific example shown on their website. That example is a triode strapped SE EL34. I’ve been able to find little information on other configurations, such as 6P15P tubes connected in push-pull ultra linear or pentode operation.

If you decide that you want to try adding this capacitor, keep in mind that the only tubes that will allow this are the 6P15P. Others types will internally bypass the capacitor.
If I were doing this, I’d bench test the amp, under a wide variety of operating conditions, carefully checking for problems such as oscillations in either the audio range, or higher. I’d then listen to the amplifier both ways, with and without these capacitors, to see if both myself and others can actually hear a difference between the two configurations. Please report your results if you do this: it would be interesting.

It should be pointed out that a serious wiring incompatibility exists between certain specific types of these tubes. The 6BQ5, EL84 and 7189 use pin 3 for both the cathode and suppressor grid connection. The screen grid is connected to pin 9, while pins 1 and 6 are unused. The 6P15P uses pins 1 and 6, internally connected together for the separate suppressor grid connection. The 7189A tube (includes Seimens/Telefunken EL84 and E84L tubes) uses pin 1 as an extra control grid connection, and pin 6 as an extra screen grid connection. If the socket is wired for a 6P15P with pin 6 connected to pin 3, connecting the suppressor grid to the cathode, and a 7189A tube is inserted into the socket and powered up, disaster awaits! High screen grid voltage will be applied to the cathode, destroying the tube, and/or causing other damage. Be careful when “rolling tubes”!

As far as your regulator to adjust the suppressor grid voltage: it’s just too difficult to understand what you are proposing without seeing a diagram of the circuit. I’d suggest that you create a diagram of your idea and post on one of the audio forums. There, others can look at your diagram and comment on it.

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