Ampeg V-6B SS amp repair

[LHPcope]

Posts: 9

Hey all,
I just got done with an Ampeg V-6B repair that I thought I'd pass on.

The amp came in blowing fuses/not working.  I replaced all the output transistors as 2 were shorted, 4 were leaky, and all were unobtainable.  The driver transistors were also leaky so they got replaced.  After that the amp had bias problems which are difficult to run down in direct coupled power amps.  In the end it appears that the three transistors on the input which are used in a differential amp stage may not have been specified for high enough Vce by Ampeg.  They are rated at 60 volts with about 50 to 52 volts being applied to them by the circuit.  After the original transistors and the replacements all tested either shorted, open or leaky I substituted a set of 2SA970s with a Vce rating of 120 volts which worked nicely.  These amps seem to date from the 1970s judging from the construction and produce a good 135 watts at clipping in this case.

I bring this up because after I got into the repair I began to hear stories from different people about how this particular Ampeg model was impossible to keep out of the repair shop and blew speakers, all of which makes sense considering the input transistors. 

[smackoj]

hi; that is great info to pass along. thanks. what process do you use to test the pwr transistors for 'shorts'? (do you remove them from the circuit)? also, what method did you use to find a replacement tranny without having to buy different ones and try them in the amp?

thanks, smacko jack   

[LHPcope]

My experience is that for the most part all transistors need to come out of the circuit to get an accurate read on their condition; a good variable heat iron really comes in handy for this task.  Yes, you can catch the shorted and the open transistors in circuit but the leaky junctions really need to be isolated to test.  I use a B&K 520B, a Sencore TF151A, and the fellow in figure 2.7 of this page: http://www.tpub.com/neets/book21/87d.htm I use a couple of 1K resistors in the circuit instead of the listed resistors.  The procedure is to check the base/emitter junction, check the base/collector junction and then check emitter/collector (which should be open.)  My signal generator can put out about 12volts p-p so you get a graphical view of what the junctions are doing from -6 to +6 volts.  I used a very similar circuit to check transistors in RF equipment years ago when I was doing bench work full time.  This will catch problem transistors that my other two testers will miss. 

As far as replacement transistors go generally you need to get a datasheet on the old transistor and line up the specifications of a prospective replacement to be as good or better than what's coming out.  In the case of power transistors things like type (PNP or NPN) case (T03, T066 etc), power dissipation, max voltage base to emitter, collector to emitter, and so fourth.  You may be able to find replacement recommendations on line or in a cross reference.  Bipolar transistors for the most part really have fewer variables than vacuum tubes.