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Old 29th December 2011, 09:35 PM   #71
jgf is offline jgf  United States
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Chris --
I tried removing the positive feedback, but liked it better with. I guess they optimized everything around that.

I've only seen schemos of amps with both halves of the 12ax7 making up the splitter driving el84s, but not a concertina. Also, if I read the theory right,
gain would be lower using a pair type splitter, and it seems just about right, if not
just a bit low as is (without a preamp).
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Old 30th December 2011, 02:26 AM   #72
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I think the positive feedback is there to balance the concertina.
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Old 30th December 2011, 02:31 AM   #73
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Racer -- the positive FB loop has nothing to do with the balance of the concertina, but everything to do with making the effective gain of the first stage as high as possible. Remember, the higher we can make it, the lower the input driving requirement will be for a given FB level. In trying to shoot for a 20 db FB factor, and an input sensitivity of 1.0 vac, the positive FB loop helped to create enough gain within the loop to achieve those figures.

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Old 5th January 2012, 01:21 AM   #74
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Hey All,

I found some 6P15P tubes online from a Russian retailer and bought 17 of them. I was under the impression that they were identical to an EL84 but now find that to be untrue.

I plan on using them in a Dynaco SCA-35 I'm building. I'm substituting ST-35 boards to swap the 7199 for the 12DW7 preamp/phase splitter. And removing everything but the volume control from the circuit.

Obviously, on the EL84 the suppressor (?) grid is internally connected to the cathode on pin 3. The 6P15P does not have this connection. If my tired old eyes aren't failing me the suppressor grid on 6P15P is connected to pins 1&6. The ST-35 manual voltage chart lists 13.5 volts on pin 3 and 0 volts on pins 1&6. This is important because of the 150 volt limit on the suppressor grid for the 6P15P.

I also found the "Hazen" mod for Decware amps where the EL-84 cathode and suppressor grid are connected by a non-polarized film cap. Since there is an internal connection for these I'm assuming the cap is a bypass cap?

So these are my questions. Is the 150 volt limit on the grid for a pentode amp where the grid is driven? This not the case in the ST-35 as it is an ultra-linear amp. Second, should I connect the cathode to the suppressor grid? Third, Does anyone know the value of that cap? And lastly, could I damage my new amp by experimenting with this connection either by cap or by a wire?

Since the ST-35 boards use a chassis mount socket for the EL-84 rewiring the output tube sockets is easy.

As always, thanks for any help you could provide!

P.S. I also posted this on another thread, the 6P15P thread. I hope I haven't committed a faux pas here?
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Old 7th January 2012, 05:30 AM   #75
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I want to eliminate all but the low level phono input on this SCA. And to be honest these old selector switches confuse me completely. It looks like the signal hot is connected to lug #6 on the phono board (left channel). And there is a 56k resistor from this hot input to ground. #1 is signal ground to the chassis via lug #4. #12 is for B+ via both connections "x." #11 is, I think, output to the power amp. I think then I need to jumper from lug #7 to #8 (to bypass the switch). And leave lugs 2 and 3 unconnected. OR should I just connect the input to #8 and leave 2, 3, 6, and 7 unconnected?

Dave, when I was building a pair of Mark III's someone told me the positive feedback to the cathode side of the phase splitter was to balance the section. It seems Dynaco made this connection on all their amps.

Thanks if anyone can help me out here.

Last edited by mr2racer; 7th January 2012 at 05:31 AM. Reason: grammar
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Old 7th January 2012, 06:13 AM   #76
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Originally Posted by mr2racer View Post
I found some 6P15P tubes online from a Russian retailer and bought 17 of them. I was under the impression that they were identical to an EL84 but now find that to be untrue.
6P15P is an SV83, A 6P14P is an EL84 (EL84M). SV83 is a lighter duty tube.

IIRC SV83 doesn't take to UL well.

That said, Chris swapped in 6P15P to replace the 6P14P in our Class A triode PP so that he could try the hazen mod (which he likes, i have no opinion)

community sites,, ........ commercial site planet10-HiFi
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Old 7th January 2012, 06:21 AM   #77
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Hi guys

Here some pix of my refurbished SCA-35 together with my Rex Baldock Tricolumn parabolic horns
Btw thanks Dave for the EFB-regulator.
I tryed original bias, regular bias with balance pot, and finallt the EFB-regulator, wich sounds great.
I built the EFB-reg with the four separate bias adjustment pots and ended up with a Ia of 34mA sounding best to my ears

Click the image to open in full size.

Built-tread in swedish: - Pix - Dynaco SCA-35. En gentleman i ny kostym

Click the image to open in full size.

Rex Baldock Tricolumn parabolic horns with Philips 9710 Alnico driver:
These speakers are a great match to the SCA-35 using no filters hence really sensitive and easy to drive.

Click the image to open in full size.

Click the image to open in full size.

Built-tread in swedish: - Pix - Rex Baldock Tricolumn, Paraboliska horn

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Old 7th January 2012, 12:48 PM   #78
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Racer -- I am finishing up moving to a new location, so all of my material is still packed up for now. I should be able to get to it in a few days and can help with the phono board connections.

Regarding the positive feedback loop, there is no doubt that when that concept is used, the phase splitter must be adjusted to account for it, since it is the source of the feedback. But its purpose is not to balance the phase splitter itself. Its sole purpose is to increase the gain of the input stage to maximize the global NFB level available in conjunction with the targeted input sensitivity level.

In the Dynaco amplifiers, the splitter's balance is determined by the value of the plate and cathode resistors used in the splitter circuit. When a positive feedback loop is NOT used (as in the case of the MK II, MK III, MK IV, MK VI and Stereo 70), you will note that the plate and cathode resistors for the phase splitter are exactly equal. When the positive feedback loop IS used (as with the ST-35 and SCA-35), the cathode resistor of the splitter is raised in value slightly over that of the plate resistor, to account for the impedance of the positive feedback loop connected there. When that impedance is accounted for with the higher cathode resistance, the overall effective resistance at the cathode is then identical to the value of the plate resistor used, so that balanced drive is achieved.

Now, all that being said, Dynaco did put out their own hype about the capacitive NEGATIVE feedback loop from the "bottom" output tube screen connection back to the cathode of the input tube, saying its purpose was to help balance the phase splitter. It is an important loop in the overall design, providing the necessary response roll off of the amplifier circuit proper (which does not include the OPT), so that the main NFB loop (which does include the OPT) will remain stable in the face of capacitive loads applied to the amplifier. However, in reality, it has no more to do with the balance of the actual phase splitter than the main NFB loop does. At the time these amplifier's were introduced, the "split load" phase inverter was under much criticism by those who (incorrectly) felt that this type of inverter was inherently unbalanced at high frequencies. It can be if the following stage loads are not identical at all frequencies as well, but the inverter itself is not inherently unbalanced. However, since the little capacitive loop from the screen connection was quite unique at the time, and the criticism of the inverter was in full play, Dynaco tried to avert criticism of its use by saying that the capacitive loop helped to balance its performance across the entire audio spectrum. It was all pure hype to be sure, and required only because of the misconception in play about the concertina's performance -- but it certainly worked, as their sales and history certainly shows.

I hope this helps clarify the action of the positive feedback loop that Dynaco used in SOME of their designs, versus that of the capacitive negative feedback loop from the screen grid that they used in ALL of their commercial designs.

I'm sorry you ended up with the wrong Russian tubes for your amp. Hopefully, if you cannot use them otherwise, you can resell them to cut your losses. The EL84M is the tube you want, and has proven to be capable, durable, and sounds good as well.

Pixworld -- Thanks for the kind words about my EFB modification. If you have the facilities available to you to measure your amplfier's performance, it would be instructive to watch the THD or IMD produced as the bias is adjusted when the EFB modification is used. During my initial development work, I used more sets of output tubes than I care to remember in determining the best current draw to specify as the target quiescent operating point. The average, and most often actual low distortion point was 27 ma per tube. As I recall one set nulled at as low as 25 ma, but in no case was more than 29 ma required for a null. The vast majority of sets (at least a dozen or so) nulled very nearly dead on at 27 ma.

There will be no damage caused by running the tubes at 34 ma when EFB is used, but I can virtually guarantee that distortion is significantly elevated at that level, and, you are not getting the benefit of the increased tube life that the lower quiescent level provides. I just wanted to make sure you were aware of these facts as you determine what setting is best for your needs.

Your SCA-35, speakers, and installation look really great, and I'm sure sound equally great as well!

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Old 7th January 2012, 01:07 PM   #79
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Someone more cynical might speculate that the real reason for that cap is that the Dyna transformers didn't have fabulously good balance at high frequencies from one half the primary to the other...
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 7th January 2012, 02:40 PM   #80
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Dave, the 6P15P's won't go to waste. I bought a Sansui AU-70 chassis with, I think, tango transformers. It was a pentode amp. So that's four of them right there. It was driven by a pair of 6AN8's. Which I have because I originally bought them for the SCA before I decided to use ST boards.

And I even found a use for some very low impedance 20 watt OPT's I salvaged from some old Magnavox consoles. They can be driven by a pair of 6550's in triode class A.

Its only a loss if you don't hang on to it long enough to find a use for it. Tubus Packratis!

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