Expanding Biasing Range in Stereo 35

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Expanding Biasing Range in Stereo 35

Postby petercapo » Thu Feb 26, 2015 11:24 pm

I have the kit to individually bias each power tube in my Dynakitparts Stereo 35, which I believe is the same individual biasing scheme used in the diytube ST-35.

Can you swap the 10K pot for a 20K pot to expand the range of adjustment?

Thanks.
Peter
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Re: Expanding Biasing Range in Stereo 35

Postby Shannon Parks » Sat Feb 28, 2015 11:45 am

petercapo wrote:I have the kit to individually bias each power tube in my Dynakitparts Stereo 35, which I believe is the same individual biasing scheme used in the diytube ST-35.

Can you swap the 10K pot for a 20K pot to expand the range of adjustment?

Thanks.
Peter


Peter are you modding a kit that mods the original ST35? Can you post a schematic? Or something else? Thanks!

Otherwise, no: in my DIY ST35 circuit changing from 10K to 20K won't change the biasing range.

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Re: Expanding Biasing Range in Stereo 35

Postby petercapo » Sat Feb 28, 2015 12:18 pm

http://www.dynakitparts.com/dynakit-pro ... pgrade-Kit

The schematic & parts values in its accompanying documentation appear to be the same as the bias control setup in the schematic for the diytube ST-35.

The plates in my EL84s were over-spec for power dissipation (>13W), and I could not get the bias current down low enough to compensate. Without changing any of the parts on the bias PCB, I have been able to shift the range of biasing downward by adjusting the resistor in the B+ supply. So, I changed the 50-ohm resistor, which dropped the B+ a bit while also enabling me to lower the bias current a bit, too.

Problem is, I like to roll different sets of EL84s, and changing the B+ resistor in this way then made it a very close call to get the bias up high enough for a different set of EL84s. While doing it this way enabled me to reduce the plate dissipation by a combination of lowering the B+ and also the bias current, the bias current is now running at less than 35mA (IIRC).

Is the bias current supposed to be running at 35mA for best performance, regardless of the level of the B+?

I was wondering if there might be a way to broaden the range of the bias adjustment overall, sufficiently upward or downward in order to set 35mA, even after lowering the B+.
Attachments
Dynakitparts Schematic Bias Upgrade for Stereo 35.JPG
Dynakitparts Schematic Bias Upgrade for Stereo 35.JPG (566.53 KiB) Viewed 10338 times
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Re: Expanding Biasing Range in Stereo 35

Postby EWBrown » Sun Mar 01, 2015 8:30 pm

Looks like they "lifted" the design from Shannon's ST35 boards... p[

The 10K trimpot, combined with the 1K series resistor, makes the adjustment range variable between 1K and 11K, connected in parallel
with the 470 ohm resistor. Add in the 10 ohms "sense" resistor for the total cathode resistance to ground

Change R1-R4 from 470 ohms to 510 or even 560 ohms, that will lower the EL84s' cathode current adjustment range.
Changing the 10K trimpots to 20K would not significantly widen the adjustment range.

Typically, the best adjustment range would be evenly centered around 400 ohms to ground.

HTH
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Re: Expanding Biasing Range in Stereo 35

Postby petercapo » Mon Mar 02, 2015 2:43 pm

It has been my understanding that the use of the circuit is by prior arrangement with/consent of Shannon.
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Re: Expanding Biasing Range in Stereo 35

Postby paart » Mon Mar 02, 2015 6:46 pm

35 ma. of cathode current is an approximation in Dyna ST-35 amplifiers, but is subject to variations.
Plate and screen current combined, is equal to the cathode current in the 6BQ5 output tube. An example from the original Dyna manual-- Plate voltage: +370VDC Cathode voltage: +13.5VDC. Subtract the cathode voltage from the plate voltage to arrive at the working plate voltage. 370-13.5= 356.5. Multiply that by the cathode current (.0355A) to get a combined plate and cathode dissipation of 12.66 Watts. Subtract an estimated 1.5 watts of screen dissipation to arrive at 11.16 Watts plate dissipation.

These parameters were set to give good performance with good tube life.

Independently increasing either the cathode current or B+ voltage will raise the dissipation, and also result in a slight increase in the operation of the output stage under Class A conditions. This might (but not necessarily) result in a slight improvement in sound. In practice it will also slightly decrease tube life, but that’s subject to a lot of other variables, particularly the quality of the output tubes. Some 7189 tubes (mostly no longer available) were rated as high as 15.4 Watts of combined plate and screen dissipation.
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Re: Expanding Biasing Range in Stereo 35

Postby paart » Mon Mar 02, 2015 9:17 pm

"Multiply that by the cathode current (.0355A) to get a combined plate and cathode dissipation of 12.66 Watts."
That's of course, a "typo". It should read:
Multiply that by the cathode current (.0355A) to get the combined plate and screen grid dissipation of 12.66 Watts.
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Re: Expanding Biasing Range in Stereo 35

Postby petercapo » Tue Mar 03, 2015 9:22 am

I'll have to rework some of my calculations.

Input from this thread gives me a good direction to move in.

Thanks, guys.
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Re: Expanding Biasing Range in Stereo 35

Postby Stunch » Tue Mar 03, 2015 1:00 pm

Following up on PAART's post, using the PA-774 (330-0-330, 180 mA) PT, what would be the current draw on the original ST-35 at idle? Let's assume that the EFB is employed at 27mA.
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Re: Expanding Biasing Range in Stereo 35

Postby EWBrown » Thu Mar 05, 2015 5:08 pm

The original ST35 draws 35 mA per 6BQ5 and perhaps a total of 10 mA for the two 7247s, approx. 150 mA continuous at idle.
Yes, the PA774 Power Trannie gets very hot! Combine that with the very close tube and transformer spacing, and the perforated top cage,
it' runs hot. Nearly too hot (666) to touch, without raising a blister :/ :'(

The SCA-35 added two more 12AX7s for the phono stage, two 50 ohm "hum balance pots", and a 150 mA pilot light, loading the filament windings to the maximum.
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Re: Expanding Biasing Range in Stereo 35

Postby paart » Thu Mar 05, 2015 7:41 pm

4 EL84 output tubes drawing 27ma. each =108ma.
2 12AX7s drawing .63 ma. = 1.3 ma
2 12AU7s drawing 3.9 ma. =7.8 ma
Voltage divider for EFB Bias= 1ma.
Add it up and it equals 118.1 ma. at idle.

But that’s just the actual circuit itself, if that's what you need. There will be additional power consumed by the power supply-particularly the transformer. Some will be used by the power supply series resistors or chokes. A small amount (should be very small) will be used up by the filter caps. Some small amount will be used in the output transformer, even at idle. Additional bleeder resistors will use a small amount.
Probably the best way to determine the idle current drawn by any particular amplifier would be to measure it.
I'm am a little curious why you have asked for this on two occasions.
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Re: Expanding Biasing Range in Stereo 35

Postby Stunch » Thu Mar 05, 2015 8:43 pm

The reason I asked here was because I never got a response from my original post. I've always been curious about the current draw and wondered about the 180 mA rating of this PT. You would think it would be enough, but maybe most of the heat is generated by the filament draw.
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Re: Expanding Biasing Range in Stereo 35

Postby EWBrown » Fri Mar 06, 2015 9:41 am

The EFB circuit does run the EL84s more gently than the original Dyna configuration (4 6BQ5 / EL84 sharing a common 95 ohm, 5W resistor), the PA774 should run at a lower temperature. Also, not having the confined air space under the perforated top "cage" helps to keep the heat down to a more manageable level. [:)
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Re: Expanding Biasing Range in Stereo 35

Postby paart » Fri Mar 06, 2015 7:44 pm

The tube heaters draw about 25% of the total power consumed by the amplifier. My current (and next) amplifier uses an external SMPS heater power supply, which helps keeps the transformer primary current lower and therefore cooler. This should in theory, help B+ regulation, although my testing doesn't show much improvement. (???)
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