From: email@example.com (Ralph & Diane Barone)
Subject: Re: Rebiasing a solid state amp
Date: Fri, 13 Dec 2002 14:35:05 GMT
NNTP-Posting-Date: Fri, 13 Dec 2002 07:35:05 MST
Organization: Shaw Residential Internet
In article <firstname.lastname@example.org>,
email@example.com (Russellsit) wrote:
I would be immediately wary based on the "Class A+" designation. I suspect
that the amp was never Class A to begin with. Given the power supply
voltages, you could expect the amp to be capable of supplying about 250 W
into 8 ohms and you couldn't fit 250 W of Class A amp into 1.5 rack spaces.
I am, however, a bit worried about the power supply voltages exceeding the
DC rating of the capacitors, so something may well need repair or
>I have acquired an solid state amplifier. On the front panel it said:
>Monogram Professional Audio
>handcrafted in England
>It is the usual rack-mount size width and would occupy one and a half
>rack spaces. The interior shows the power supply in the middle, and
>two amp modules at the sides. There are only 2 TO-3 transistors each
>side (2n3773). There is a sticker beneath the left circuit board with
>a date of 17th Sept, 1978, so this is a rather old amp. From the size
>of the heatsinks and the power supply (2 capacitors of 10000uF, 63V
>and a big EI transformer), to make it running in class A is not
>difficult when it was manufactured. I have also done some crude
>measurements on the amp. The power supply is +/- 65v. When there is no
>signal in, the voltage across the 20ohm resistors is not recordable (?
>0 volt) (? class B operation).
>I have noticed that the TO-3 transistors do not feel hot. In fact it
>is only a little bit warm to touch, may be about the same as body
>temperature. The heatsinks are even cooler. In contrast, the drivers
>and the heatsinks are very hot. I found this very puzzling as the amp
>claimed to be class A+ but yet feel much cooler than my NAD 302. There
>is no infomation on the amp about the power output, but I guess it is
>at most 20w from the 2 transistors.
>I tend to believe that it was originally made to run in class A, but
>was somehow modified later to give a higher power output (class B
>now?). So I am looking into whether I can re-convert it back to class
>A (or at least class AB).
>On a closer look to the circuit boards, there are 4 labelled trimpots
>each side. They are
>All of the resistors on the boards are low wattage type, except 2 next
>to V4. On the resistors I saw R20 5% CB5. I guess they are 20ohm
>resistors, may be 5 watts rated.
>I guess V4 is the control of the idle current. However, if the higher
>wattage resistors is for gauging the idle current, the maximum current
>through it will be limited to a small value by the 20ohm resistor. In
>addition, there are 2 of them each side, and I do not know on which
>one I should base my measurement.
>I would be most grateful if you could tell me how to do the rebiasing,
>or if there is any catch inside or something I need to be careful
>Thank you for your time in reading this long post.