Grateful Dead Music Forum

A place to talk about the music of the Grateful Dead 

 #103858  by gpilcher2001
 Tue Sep 27, 2011 7:45 pm
Hi, guys fairly new around here but I like to play around with tubes also. I am directing this at you guys because you seem to have a good grasp on the techy side of these things. It has dawned on me that the preamp taps ,as they are, are fairly high impedance. What do you think that this did for obtaining the Garcia tone. It would seem that it would roll the highs back going into the power amp. Do you think or know of any buffer stage that was used to drop the impedance or did they just look past it? Or were they just compensating for the loss with the treble all the way up? Does the SMS utilize a low impedance output and if so did you find you needed to compensate for it? Food for thought. Greg.....
 #103863  by SarnoMusicSolutions
 Wed Sep 28, 2011 6:50 am

The way Jerry tapped his Fender preamp is, like you said, a fairly high output impedance. No buffering or impedance matching was used. The Alembic F2B is the same situation. However, Jerry's McIntosh input impedance is quite high at 250kOhms, so the match there is ideal.

Not sure exactly why you'd suggest that a high Z output would have a loss of highs. Maybe because it could be more vulnerable to cable capacitance, but with reasonably short runs of under 10' of cable, preferable 3' or so, there doesn't seem to be any effect on the audible band (up to 20kHz or so).

The one thing I alter in the SMS Classic is to take that output tube and utilize both halves of it by running the two triode sections in parallel. This effectively cuts the output impedance in half giving DOUBLE the drive capability and without changing the tone at all. Some people would suggest using a very low-Z cathode follower at the output to really have drive capability into any kind of load. But in practice, the parallel triode seems to be fine into any amp input load I've found. And really, even the single triode driving the output like Jerry's output from a Fender really seems just fine into the many load types people seem to try.

 #103869  by gpilcher2001
 Wed Sep 28, 2011 10:01 am
I get it. It's a lot like your guitar signal and the need to buffer for long cable runs. Most manufacturers probably buffer the out on there preamps for studio , effects, ect... I'm using a dual 75 power amp from groove tubes. They state there input is from 0-1.5 volts. Is there a way to convert that into impedance? I'm also looking into using the #1 channel on my weber super reverb head to install a totally separate power amp. Thus no need to haul around a rack for the power amp. Thanks for the input. Greg.....