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Quick fixes for 7591A amps
1. Quick change from 7591 to 6L6-GC or Russian 5881 (Scott and Fisher amplifiers)
On these amps the cathodes of the 7591A's are usually tied together. Find the wire connecting between the two cathodes and remove it. Some amplifiers have a balance control between the cathodes, this must be disconnected.
NOTE: If a bias rectifier is used and has not been changed we recommend doing so as the original part is selenium and will deteriorate and eventually fail. Also, the replacement silicon rectifier will give slightly more negative voltage. If the amp has tubes (usually 12AX7's) with filament voltage sourced from this rectifier, a series resistor (usually between .5 to 2 ohms) should be installed in series with the filaments to produce a DC voltage across the filament about 10% or so less than the normal filament voltage. (e.g. about 5.2 to 5.7 volts for a 6.3 volt filament or 10.5 to 11.5V for 12.6 volt filament).
Assuming that amp uses fixed bias (e.g. negative voltage on grids, not a big cathode resistor), do following:
A. Find the base diagrams for 6L6 and 7591 (remember they are the BOTTOM view of the socket.) Rewire socket as follows:
|Control grid (g1)||Pin 6||Pin 5|
|Cathode (k)||Pin 5||Pin 8|
Other connections are the same.
7591A has two screen leads pin 4 and pin 8. If there connections on pin 8, move them to pin 4.
B. Between the cathode of each tube and ground, install a 390 ohm 5 watt resistor. Parallel to that a 220 uF 50 volt capacitor. If the tube is drawing the correct amount of cathode current the voltage across the resistor will be between 19 and 20 volts.
If the voltage is too high, make the resistor higher in value, or vice versa. A small value pot could be used for this adjustment. The voltage drop should be equal to 50 ma no-signal current.
Optionally you can install parallel to the resistor a 1 watt zener diode (in place of the capacitor) equivalent to the correct voltage drop or a little higher, this gives 100% fixed bias operation. If amp is cathode biased, changing to 6L6 tubes is a simple matter of changing the connections as noted above, then increasing the size of the cathode resistor by a factor of about 2.1 or 2.2 times the original. For example, if the original was 170 ohms, increase to 390 ohms. The replacement should be twice the wattage of the original, and the bypass capacitor parallel to it should have twice the voltage rating.
C:\>The other, more difficult way to change bias is to change the bias supply either to (A) (if bias supply is 26VAC, check it first!)to a voltage-doubler (if you need explanation, probably better to skip this idea), this requires upping voltage rating of the bias-filter caps, most of these amps derive DC for 12AX7's from bias supply, and will require rewiring filaments of 12AX7 from series-parallel to straight series connection, possibly with a series resistor to bring filament voltage back to correct level (between 10.5 to 11.5VDC across each tube). Then rechange the bias resistors ie voltage divider to produce approx -37 volts. Alternately, you could install pots, and copy bias circuit (except for rectifier arrangement) out of ST-70 Dynaco, installing 10 ohm 1% 1/4w resistors for metering cathode current (set at 50ma per tube)
Some amps have a 50 or 52VAC bias winding, this simplifies getting the higher bias voltage need and only requires making changes in the voltage divider as noted above.
You may wish to fiddle with the feedback resistor. Since the gain of the amp is cut more or less by half, try cutting the value by half.
Sometimes the preamp section will not put out enough voltage to cover this change in sensitivity, most of them will. You will notice you must increase the volume control a bit more for the same output.
However, not adjusting feedback levels with the tetrode-connected output arrangements usually employed will often result in pretty flaccid bass response.
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