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Old 26th December 2011, 03:03 PM   #61
jgf is offline jgf  United States
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mr2racer:

I have a 32.1k feedback resistor in there now, on the 16 ohm tap. Same size cap across it.
I may lower it just a little more (maybe I'll try 29 or 30k) I'm getting just a hint
of upper midrange harshness on horns and lead guitar sometimes, which was not
there with the original feedback resistor value.

Power supply filtering has been increased also.

Last edited by jgf; 26th December 2011 at 03:08 PM.
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Old 27th December 2011, 02:16 AM   #62
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Hey jgf,
I'm using ST35 output boards. And they originally had 27k resistors in the feedback loop.
That might explain where the extra gain comes from. The SCA35 used 82k.
Kevin
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Old 27th December 2011, 12:11 PM   #63
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Racer -- Since the NFB level is reasonably similar between the ST and SCA designs, you can't just look at the value of the FB resistor and determine from that if one design has more operating gain than another -- or conversely, more feedback than another. Rather, it is all dependent on the amount of open loop gain (OLG) of the stages that the feedback is placed around, and the level of FB voltage returned to the input as determined by the speaker tap used and the ratio of the resistors used in the feedback network.

In the case at hand, the extra gain largely comes from using a pentode input stage in the SCA (which easily provides about 5X the gain of the triode input stage of the ST), and from only a slight reduction in the effective FB level in the SCA design.

The FB network of the SCA design only applies about 13.5% of the FB voltage around the power amplifier section that the ST design does. But with the added gain of the pentode input stage within the FB loop, the effective FB level of the SCA design is still about 70% of that used in the ST design. Since it is not 100%, that slight reduction in FB level, coupled with the 5X gain increase provided by the pentode input stage reduces the 1.0 vac sensitivity level of the ST design, down to about .15 vac for that of the SCA amplifier board. Of course, in the SCA design, the gain advantage is put to good use by allowing the installation of losser type tone controls ahead of the amplifier proper, so that the effective operating sensitivity of the two amplifiers (with the SCA tone controls set flat) is basically the same.

I hope this helps with your project!

Dave
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Old 27th December 2011, 03:59 PM   #64
jgf is offline jgf  United States
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Kevin, Dave,

That's a much better explanation than I could give

John
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Old 29th December 2011, 02:03 AM   #65
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Thanks for the help guys. I was counting on the standard 2 volt output common with a CD player to make up for the loss in gain. The question is will that doubling of input overdrive the 12AX7 half of the 12DW7? And as mentioned earlier in the thread. Might it be better to use a 12AX7 instead of the 12DW7? What changes would be necessary to the ST boards to make that work? Other than cathode resistors?
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Old 29th December 2011, 03:50 AM   #66
rmyauck is offline rmyauck  Canada
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How about just dropping a 12AT7 in as it has less gain?

Last edited by rmyauck; 29th December 2011 at 04:04 AM.
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Old 29th December 2011, 05:23 AM   #67
rmyauck is offline rmyauck  Canada
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Maybe this would help.

PS-4 & PS-5 Tube Power Supplies


Anyone ever made the EL84 P-P circuit referenced on the Tubecad Journal site?
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Old 29th December 2011, 01:33 PM   #68
jgf is offline jgf  United States
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No problem with overdrive with just disc player -> volume control before the 7247/12dw7. That's exactly what I've been running for a while. I'm not a huge
fan of the 12au7 either, but it sounds ok in this circuit. 12ax7 in the splitter position is very iffy, you can't run much current through it. That 6n1p circuit and Morgan Jones' 6dj8 circuit are the only ones I've seen fully worked out that might sub given easy power supply mods if you really wanted to get rid of the 7247.
I suppose a 12at7 could be made to work there, but just dropping it in to the existing circuit could well be worse..
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Old 29th December 2011, 06:15 PM   #69
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Have you removed the positive feedback?

Thanks,
Chris
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Old 29th December 2011, 08:42 PM   #70
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Right, until you mentioned it I assumed they used the 12AU7 as the phase splitter and the 12AX7 as the gain stage. In the Dynaco catalog there is an EL-84 amp that uses a single 12AX7 as a combination gain/phase splitter. I can only assume that the 12AX7 has sufficient amperage output to drive the EL-84's. And the position of the tubes has more to do with matching impedance's than anything else.
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