How to set bias on the Sunn Concert Bass

[AMPREPAIR]

Does anyone know how to set the bias on a Sunn Concert Bass?

Thanks in advance!
AMPREPAIR

[loudthud]

There is a trim pot on the Concert series power amp PCB, but I can't find instructions anywhere on how to adjust it. It adjusts the bias on the driver stage, not on the output transistors. Most solid state amps have a bias adjustment for crossover distortion, but in the Concert, it won't do that. It probably adjusts the clipping so that one side of the waveform does not clip before the other does. You would need a scope, generator and dummy load to adjust it.

[AMPREPAIR]

First off, I like your sign-on name!

It sounds like you have a good handle on this.  For what it's worth, this Sunn Concert Bass has a trim pot on the PCB and I have all the test equipment you mentioned.

Question:  Do you attach the 'scope to the wiper of the trim pot to make adjustments?  Also, what level is used for an input signal (I assume the level should be just enough to make the scope readings you mentioned)?

Thanks for the help!

[loudthud]

The info I have is for the scope waveforms you see on the schematic. It specifies 6mV at 2Khz with all the tone controls all the way up, distortion and boost off for most waveforms. It also specifies a 6 ohm dummy load. That's right, six ohms. You could just apply a signal to the Power Amp Input on the back panel and monitor the output to the speaker with a scope. Increase the signal until you observe clipping.

[george]

It looks like Sunn set the bias on Q301 with 6mV/2kHz on the input of Q101. all controls including volume are at max. Output of Q1 - collector lead - is where the signal is shown on the schematic (figure 7). Very confusing setup.

[AMPREPAIR]

Figure 7?  I have a single page schematic with no reference to "Figure 7".  Do you have a Service Manual with more pages of information or a better document?  If so, can I talk you into a copy?

[george]

Sorry figure 7 is given on the schematic listed here http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/newamps/sunn_concertbass.pdf

[AMPREPAIR]

Thanks for the link to the schematic; it looks the same schematic I currently have.  This will sound really dumb, but I just can't find any reference to "Figure 7".  Can you provide any guidance?  This is not important, but I am curious.

Thanks

[loudthud]

I think what was refered to as figure 7 is really one of the oscilloscope waveform diagrams. There is a number in the lower left hand corner of each one. They may be hard to read.

[AMPREPAIR]

Hmmmmm, I'm still trying to understand.  I see the wave form blocks on the schematic diagram, however, I only see a blurry figure in the lower right (not left) corner.  Most of these figures are unreadable.  What appears to be "Figure 7" points to the collector of Q1 and shows "2 VOLTS 1 CM" (I guess this means 2 volts per centimeter?)  On a separate note, I am still trying to understand were "6mV/2kHz" is shown on the schematic (per previous post)?  To me, 6mV seems very low - regardless where it is measured.

Thanks everyone!

[loudthud]

Sorry, I ment lower right. I've got a page from the Sunn Service manual titled Concert Series Test Specifications And Trouble Shooting Information. Not much to it really. Just a list of steps with waveform numbers, scope settings, input signals and control settings. Waveform 7 is the one from the collector of Q1, the driver transistor.

Edit: Look at the schematic near the bottom of page 1 in this thread: http://sunnforum.ampage.org/sdp/index.php/topic,4515.msg17048.html#msg17048

Is it any clearer? I am having trouble downloading it and sent a message to Admin

[AMPREPAIR]

Thanks, Loudthud!  Now we are getting somewhere!

I tried to download the .gif files you mentioned.  Like you, they were not working when I tried this morning.  For what it's worth, the information I have gathered from this "thread" makes me believe we similar documentation and my understanding of setting symmetry is understood.  It would have been nice if this information was revealed in the currently available documentation.

This has been a good "thread" and I have learned a lot.  You are an excellent mentor and I hope you stay around for a long time.

[loudthud]

I contacted tboy about the issue and his advise was try another browser. Screw that! I was able to download the file by using the "save target as" option. Right click on the file link or thumbnail and select "save target as".