SCA-35 Dynaco - Page 6

dcgillespie · 21st December 2011, 05:06 PM

Kevin -- The main difference between the SCA-35 board and the ST-35 board is that the SCA board has significantly more gain to it (hence the use of the 7199) to account for the loss through the tone control networks. If you are bypassing those networks and only using a level control, then use of the ST board will produce a very similar gain characteristic to that of the SCA board with the tone controls left in place. Alternately, the 6AN8 can replace the 7199 in terms of performance, but would require not only a rewiring of the socket, but also an adjustment to the screen dropping resistor, and an adjustment to the HF stability circuits as well.

Dave

mr2racer · 21st December 2011, 08:11 PM

Dave,
I can't find a mu spec for the 7199 but is it really higher than the 12ax7 half of the 12DW7?
And it appears the 6AN8A pentode and triode are reversed from the 7199?
Kevin

SY · 21st December 2011, 08:15 PM

Mu of a pentode is really, really high- it's rarely spec'ed. If it were me, I'd skip the 7247 and use 12AX7 and 12AT7. You can build it on a perf board.

Chris Hornbeck · 22nd December 2011, 12:25 AM

And if using the PC-13 (ST-35) boards, it's possible (if maybe not completely optimum) to use a 12AX7 as the driver. You'd want about 75-100K ohm split load resistors - make it 75K.
Ultralinear isn't terribly hard to drive.

Thanks,
Chris

rmyauck · 22nd December 2011, 01:00 AM

"I'd skip the 7247 and use 12AX7 and 12AT7. You can build it on a perf board."

An amp builder designer on another form suggested a 6AQ8/6CG7 as he is one of a number that doesn't like the 12AX7.

I have a used pair of Z-565 OPT's and a used 272HX (300-0-300V, 200 mA) and so interested too.

Would these tubes just plug in with socket rewiring? Both those tubes are in production so I like that too.

Thanks in advance!

Regards,

Randy

mr2racer · 22nd December 2011, 01:08 AM

Thanks Sy and Dave,
This amp won't be a powerhouse regardless. The purpose of this exercise is to build a nice little amp that will go to someone that hasn't had a tube amp before. Without completely re engineering the amp.

With that in mind I can use the 12DW7's with the ST35 values. Remove the 'silly switches' Stereo/Mono, Filter, and the Loudness Contour. Dave's EFB feedback design and let the gain fall where it will. Then make it pretty with a new faceplate and sell it to BUY MORE TUBE PARTS!

Thanks again Guys

mr2racer · 22nd December 2011, 01:10 AM

Chris, I thought about the 12AX7. But the 12DW7 seems easier. Less math!

rmyauck · 22nd December 2011, 01:22 AM

You may like it too much to sell after all that!

jgf · 22nd December 2011, 04:29 PM

mr2racer:

You've about described the SCA mod I've built, I used stereo 35 repro boards,
added EFB mod, used bigger power supply filter caps, and 3 rc sections between the power amp and driver sections. It sounds good! With the EFB mod, it can go loud cleanly even with moderate effeciency speakers. Typically I've got the volume knob at 8 to 11 o'clock with CD source, so the gain is fine.
I have lowered the feedback a little from stock, but before that, I still
wasn't higher that 12 o'clock on the volume in normal listening.

mr2racer · 26th December 2011, 04:12 AM

jgf,

How did you lower the feedback. The original used a 27k resistor from the 16 ohm output.
did you raise the resistor value or use the 8 ohm tap? If you changed the resistor do you remember the value?

Kevin

21st December 2011, 08:15 PM	#53
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	Mu of a pentode is really, really high- it's rarely spec'ed. If it were me, I'd skip the 7247 and use 12AX7 and 12AT7. You can build it on a perf board. __________________ If there's a sucker born every minute, where do the rest of them come from?

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21st December 2011, 05:06 PM	#51
dcgillespie diyAudio Member Join Date: May 2010 Location: Ball Ground, GA	Kevin -- The main difference between the SCA-35 board and the ST-35 board is that the SCA board has significantly more gain to it (hence the use of the 7199) to account for the loss through the tone control networks. If you are bypassing those networks and only using a level control, then use of the ST board will produce a very similar gain characteristic to that of the SCA board with the tone controls left in place. Alternately, the 6AN8 can replace the 7199 in terms of performance, but would require not only a rewiring of the socket, but also an adjustment to the screen dropping resistor, and an adjustment to the HF stability circuits as well. Dave

21st December 2011, 08:11 PM	#52
mr2racer diyAudio Member Join Date: Jul 2009	Dave, I can't find a mu spec for the 7199 but is it really higher than the 12ax7 half of the 12DW7? And it appears the 6AN8A pentode and triode are reversed from the 7199? Kevin

22nd December 2011, 12:25 AM	#54
Chris Hornbeck diyAudio Member Join Date: May 2011 Location: Little Rock	And if using the PC-13 (ST-35) boards, it's possible (if maybe not completely optimum) to use a 12AX7 as the driver. You'd want about 75-100K ohm split load resistors - make it 75K. Ultralinear isn't terribly hard to drive. Thanks, Chris

22nd December 2011, 01:00 AM	#55
rmyauck diyAudio Member Join Date: Apr 2009	"I'd skip the 7247 and use 12AX7 and 12AT7. You can build it on a perf board." An amp builder designer on another form suggested a 6AQ8/6CG7 as he is one of a number that doesn't like the 12AX7. I have a used pair of Z-565 OPT's and a used 272HX (300-0-300V, 200 mA) and so interested too. Would these tubes just plug in with socket rewiring? Both those tubes are in production so I like that too. Thanks in advance! Regards, Randy

22nd December 2011, 01:08 AM	#56
mr2racer diyAudio Member Join Date: Jul 2009	Thanks Sy and Dave, This amp won't be a powerhouse regardless. The purpose of this exercise is to build a nice little amp that will go to someone that hasn't had a tube amp before. Without completely re engineering the amp. With that in mind I can use the 12DW7's with the ST35 values. Remove the 'silly switches' Stereo/Mono, Filter, and the Loudness Contour. Dave's EFB feedback design and let the gain fall where it will. Then make it pretty with a new faceplate and sell it to BUY MORE TUBE PARTS! Thanks again Guys

22nd December 2011, 01:10 AM	#57
mr2racer diyAudio Member Join Date: Jul 2009	Chris, I thought about the 12AX7. But the 12DW7 seems easier. Less math!

22nd December 2011, 01:22 AM	#58
rmyauck diyAudio Member Join Date: Apr 2009	You may like it too much to sell after all that!

22nd December 2011, 04:29 PM	#59
jgf diyAudio Member Join Date: Jan 2010 Location: PA	mr2racer: You've about described the SCA mod I've built, I used stereo 35 repro boards, added EFB mod, used bigger power supply filter caps, and 3 rc sections between the power amp and driver sections. It sounds good! With the EFB mod, it can go loud cleanly even with moderate effeciency speakers. Typically I've got the volume knob at 8 to 11 o'clock with CD source, so the gain is fine. I have lowered the feedback a little from stock, but before that, I still wasn't higher that 12 o'clock on the volume in normal listening.

26th December 2011, 04:12 AM	#60
mr2racer diyAudio Member Join Date: Jul 2009	jgf, How did you lower the feedback. The original used a 27k resistor from the 16 ohm output. did you raise the resistor value or use the 8 ohm tap? If you changed the resistor do you remember the value? Kevin