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another question on tweaking my 18 build

2/20/2004 4:59 AM
another question on tweaking my 18 build
I know i've been relentless in asking questions here, but i am getting closer. Today i found a big part of my problem was the old marshall SS combo cab i used to build it. I found it's made of what appears to be compressed paper ! No, not particle board. So i tried a plywood cab and it made a big difference. (using the same speaker)  
So I'm at the point now where my only remaining major concern is the amp's feel. It lacks sag or sponginess in the feel and isn't very smooth. Is there anything in perticular i should be focusing on? It does use a rectifier tube, but you wouldn't know it by the feel.  
the other thing i wanted to ask is if there are any schematics you could point me to that have pre-tone stack set EQ sections that EQ the pre-distortion tone of the signal. I know some amps have this, and i feel if i could boost some mids pre-distortion it would smooth out the sound. Any tips or links would be appriciated. thank......  
Oh, and the faster you can help me get this thing sounding right, the sooner i'll stop pestering y'all ! *LOL*
2/20/2004 3:13 PM

Hey there. OK, without a schematic I can't help much, however due to the fact that you're using a tube rectifier and getting no noticeable sag, I am assuming that you're using a 5AR4/GZ34 rectifier tube. On these they might as well be a solid state rec tube because they have an indirectly heated cathode, meaning that instead of using the heater itself as the cathode, the cathode is actually a small metal cylinder wrapped around the heater and the heater is used just to heat the cathode. Tube recs with indirectly heated cathodes are much more efficient than directly heated cathodes(where the filament itself is used as the cathode), however the lack of efficiency is what you need to produce more sag.  
Try using either a 5U4 or a 5V4 rec tube and see what happens. They are interchangeable since most if not all rec tubes are all pinned the same.
2/20/2004 3:37 PM
Actually it's a EZ81 9 pin tube. Looks like an EL84. But i've had mostly SSrectifier amps and none of them were this stiff sounding.
2/20/2004 4:13 PM

check this out, paste the following under search in the design and construction forum:  
Tube Rectifiers vs Sag Resistors  
pretty good thread on this  
2/20/2004 7:09 PM
thanks, i'll check it out [nt]
2/20/2004 5:00 PM
Chuck Re: another question on tweaking my 18 build
So what you want is an 18 watt with tight bass and a spongy attack. That doesn't seem like too much to ask :D. Just giving you a hard time. You can try reducing the PI voltages a little. To help keep the bass with an "implied" tightness you can adjust the voltage dropping resistors in the power supply so that you have the same voltage on your preamp. Raise the value of the resistor feeding the PI plate resistors and lower the value of the resistor feeding the preamp plate resistors so that you end up with the same sum resistance you have now between those two resistors. You can also try heating up the bias a bit with a smaller value cathode resistor for the EL84s if you still have room for more current. The bias adjustment is one of the best things you can do to fine tune the "feel" of your amp.  
2/20/2004 7:10 PM

Tried most of that already, but i'll mess with the bias resistor a bit more. Thanks.
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