Tweaking :

How to improve tube equipment , especially commercial amps

From a manufacturers point of view , (mine and other really big ones ) there are a few factors that determine what will be build and sold ,

the first rule of commerce is profit !!!!

Everybody in business tries to make some , the question is how ?

Super quality amp with a hefty price tag . 7500$ or more
High end amp with hefty price tag 2500 $ or more
Std so called high end amp with budget price tag . 1000 $ or more

This is for power amps but same applies to pre amps .

Where the money goes :

Actual design cheap (50 to 100 $)
Chassis and finish expensive (250 to 1500 $)
Components usually (400 to 2500 $)
Gadgets ex remote control ( 50 to 250 $)

Where is the money saved :

Where you can't see it as a customer :
Certainly not in the chassis , this has to look expensive and feel expensive

So usually most (95%) use std industrial components , which are not bad , nowadays most try to use mkp capacitors for the signal paths , and cheap potentiometers .

Most also use 12AX7 and 6SL7 and ECC88 tubes , because they are low cost and the first 2 especially because they are low current tubes , this saves a bit of money in the power supply design , little current means low hum .The ecc88 has other advantages , it's a low cost high transconductance tube , but certainly not the best in its class , it's quite microphonic , 6922 are less microphonic but not really better .

Tweaking practical :

First draw the circuit with the correct values
Most tubes are wrongly biased :
Example ECC83 needs 1mA and most of the time its used with less then 0,5mA
Example ECC82 needs 5mA and most of the time its used with less then 2mA
Example ECC81 needs 5mA and most of the time its used with less then 2mA
Example ECC88 needs 10mA and most of the time its used with less then 4mA

This is the main cause of bad sounding amps ,how to improve , change cathode and anode resistors but be sure to improve residual hum of powersupply .
Then you can reduce feedback , as the bad sound is compensated with feedback

Once this is done , you'll ask yourself why the big companies cant do that , well I also wonder.

Next step , if the bug has bitten you :

Start bypassing all electrolytics with small values mkp and smaller mpp
The signal caps eventually change with copperfoils
The potentiometer replace with Panasonic .
And only then try new / different brand of tubes .

The hum problem :
Filtering is best done with a choke , add chokes wherever necessary , also to separate left from right .You can also use a current source made with a transistor or fet to simulate a choke , its cheaper .

By know you have a killer amp , about that I am sure .

Copyright 2002
Benny Glass

info : email sales@diyparadiso.com