ST70 Board Experiment

knowledge base for the classic Dynaco ST70

ST70 Board Experiment

Postby Thermion » Sat Apr 30, 2005 2:23 pm

A friend has convinced me that most tube amps based on classic designs (like the modified Mullard circuit) have too much overall gain. Modern source components have a standard 2 volt max output. This is much more then tape heads and tuners 40-50 years ago. This 2 volts is more than enough to send a 30 db Eiclone into clipping. And most of us have a linestage with 20db of gain also. It just doesn't make sense to attenuate the signal from a CD player just to amplifiy it in the linestage and send a 600mv input to the 30db poweramp.

A passive linestage is one solution, but I have been less then satisfied with the musical dynamics when I implemented this approach. Another way is to reduce the gain of the poweramp. I simulated some circuits for the Ikes where the 6SL7/12AX7 voltage gain tube was replaced with a 6SN7. But, before going whole hog and modifying the Ike circuit, I thought this new ST70 board would be a great way to experiment and see what happens.

I decided to replace the 12AX7 in the ST70 circuit with a 12AU7. I changed R11 and R12 to 18.2K, R7 and R8 to 1.5K, R5 and R6 to 150 ohms. I also changed the plate resitors in the LTP to 27.4K and made R17 and R18 10 ohms to get more current flowing through the CCS. In the power supply, I changed the 6.8K resistor to 2K, and the 22K resistor to 5.1K. This gives me about 365 volts on the LTP and 308 volts on the GC.

Shannon showed up just when I finished installing the board, and we brought it out of the dungeon and hooked it up. Overall, it sounded pretty good. The next day I took it down to Shannon's and we swept it on his bench. Overall gain was about 14 db, and we were only applying about 6 db of negative feedback so the distortion numbers sucked. I have since added a 47uF bypass cap across R7 and R8 to increase the gain of the first stage. I am now listening to Fleetwood Mac Live and enjoying the amp very much. Poor distortion and all.

I hope to have a chance over the next week or two to measure the effect of the bypass cap and tweak (increase) the negative feedback so that it tests a little better. Will let you know the results. In the meantime, I think I will keep it connected to the Apogees and just sit back and enjoy the music. Yellow_Light_Colorz_PDT_06

JT
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try a 12AT7 instead

Postby tubes4hifi » Tue May 03, 2005 9:45 pm

try it with a 12AT7 instead, double the gain of the 12AU7 but half that of the 12AX7, less changes from the original.
You'll probably like it better. Just a guess.
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Re: try a 12AT7 instead

Postby Thermion » Sat May 07, 2005 7:29 am

tubes4hifi wrote:try it with a 12AT7 instead......
You'll probably like it better. Just a guess.


So far I have been quite pleased with the results of this experiment. I think the 14 db of overall gain is just about right. The bypass cap will allow a few db more of negative feedback. I considered the 12AT7, but the purpose of doing this was to investigate a similar approach using 6SN7s in a pair of Ikes.

JT
Thermion
 
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Joined: Mon Jul 07, 2003 6:04 pm
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