I have both a ST 70 and an SCA 35. They both use the hard to find 7199 tube however the SCA 35 pushes the 7199 to it's limits to get the necessary gain to overcome the tone control losses. Does anyone know how they got this extra gain? I think I read somewhere it had something to do with a strange feedback loop but would really like to know how they got this extra gain. I have both schematics I can figure out any info you give me.
2 posters
Driver stage of ST 70 an SCA 35 comparison
sailor- Posts : 269
Join date : 2011-04-04
Bob Latino- Admin
- Posts : 3277
Join date : 2008-11-26
Location : Massachusetts
Hi Sailor,
The two amps (SCA-35 and ST-70) use very different amounts of feedback. The more feedback the lower the gain, the less feedback the higher the gain. The amount of feedback is determined by a resistor which feeds off the secondaries of the output transformer. The ST-70 uses a small (1000 ohm) feedback resistor which gives a lot of feedback and lowered gain. The SCA-35 uses an 82,000 ohm resistor for less feedback and more gain.
Bob
The two amps (SCA-35 and ST-70) use very different amounts of feedback. The more feedback the lower the gain, the less feedback the higher the gain. The amount of feedback is determined by a resistor which feeds off the secondaries of the output transformer. The ST-70 uses a small (1000 ohm) feedback resistor which gives a lot of feedback and lowered gain. The SCA-35 uses an 82,000 ohm resistor for less feedback and more gain.
Bob
sailor- Posts : 269
Join date : 2011-04-04
I did see that, but whatever article I read made it sound like Dynaco had done some weird voodoo to get the extra gain. I thought I was missing something. But not so, just very little negative feedback.
As always, Thanks for the straight foward info Bob. You make this the best audio board on the internet.
Here is why I asked the question. Last night I doubled up [alligator clipped] the feedback resistor [82K + 50K= 31K] and was surprised at the huge increase in detail. I will now have to play with values to find the best sound.
The drop in output did make it hard to play the TV but fortunately my CD player has plenty of reserve output. It helps that my speakers are 99db/1 watt.
As always, Thanks for the straight foward info Bob. You make this the best audio board on the internet.
Here is why I asked the question. Last night I doubled up [alligator clipped] the feedback resistor [82K + 50K= 31K] and was surprised at the huge increase in detail. I will now have to play with values to find the best sound.
The drop in output did make it hard to play the TV but fortunately my CD player has plenty of reserve output. It helps that my speakers are 99db/1 watt.