Setting Bias on a Sunn Concert Bass Head?

[AMPREPAIR]

Can anyone provide guidance on setting the bias on a Sunn Concert Bass Head?

[gbono]

Which version of the amp do you have?
post schematic if possible

[AMPREPAIR]

Hmmm, you ask what version I have; gosh, I don't know. The faded (hand written) model/serial stick-on label on the bottom is unreadable. If it helps, this amp has a trim pot on the PCB.

[nashvillebill]

How many rectifier bridges are there coming out of the power supply transformer? The "old" version has 3. The "new" version has 1 with a conventional split-rail power supply (two 6800 mfd power supply caps).

[AMPREPAIR]

This amp has three bridge rectifiers. One large bridge mounted on the chassis and two smaller bridges mounted on the PCB.

[AMPREPAIR]

Whoops, I forgot to mention this amp also has two 3400 uF filter caps.

[nashvillebill]

Sounds like the old version then. Here's a link to the schematic http://www.webphix.com/schematic%20h...oncertbass.pdf

The 100k trim pot on the poweramp board is labelled as "bias adjustment" but I will have to defer to someone more qualified than I to advise on the best way to adjust it.

[AMPREPAIR]

Yes! The schematic link you provided is the same as the Sunn Concert Bass (older version) on my bench. Thanks!

[gbono]

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. 1972 version of the power amp board didn't have a bias adjustment.

[J M Fahey]

Excuse the curiosity, but, why do you want to adjust bias in that amp?
What are the symptoms or what are you trying to correct?
Because itīs an old transformer driven design, and what we usually call "bias" (idle current of the output transistors, to avoid crossover distortion) is *not* adjustable there (unfortunately).
"Bias" there in fact refers to a "symmetry" adjustment, a very different thing.

Thatīs a very unconventional amp: transformer driven ; bridged output (which would mean a balanced output) ; one output side grounded so you artificially get only one "hot" output (but now your supply rails swing with the music, á la QSC, although QSC started doing that many years later) ; itīs a "split supply" design but uses a single filter cap, etc.
Thatīs one reason to have separate floating power supplies.
Oh well.

[AMPREPAIR]

Where do you see "6mV/2kHz" and "all controls including volume are at max."? All I see is: "ALL WAVE FORMS AND VOLTAGES TAKEN WITH 8 OHM LOAD AT 1KHZ WITH MIL. VOLTS IN." (the 8 looks like a 6). I wonder if you have a different schematic than I have. I did check the wave form at the base of Q301 and decided not to touch the "bias" adjustment because the wave form was nicely symmetrical and balanced. I agree, this setup is quite confusing and it is nice to consult others. The reason I want to know about biasing this amp is mostly curiosity!

[gbono]

Thatīs a very unconventional amp: transformer driven ; bridged output (which would mean a balanced output) ; one output side grounded so you artificially get only one "hot" output (but now your supply rails swing with the music, á la QSC, although QSC started doing that many years later) ; itīs a "split supply" design but uses a single filter cap, etc.
Thatīs one reason to have separate floating power supplies.
Oh well.

Acoustic used a simialr design on the 140/150/ ect. Was it cheaper (1970's $) to use a transformer driven output section?

[J M Fahey]

Vox and Standel too ! Even some odd Peavey and Danelectro !
Fact is, there were no cheap or easily available complementary driver transistors.
As a side note, a quad secondary driver transformer simplified high power amp design, classic examples are the Vox series connected output transistors stages and Sunnīs bridge ones.

[AMPREPAIR]

I find it interesting how many manufacturers used a cheap design. However, it seems we are getting "off topic". How can we keep this discussion on Setting Bias on a Sun Concert Bass Head?

[J M Fahey]

Dear amprepair, the question has already been answered:
1) the "real" bias canīt be adjusted, *because* this is an obsolete and cheap power amp design.
Thatīs why the discussion later followed that path.
2) the so called "bias" pot is not such a thing, but a "symmetry" adjustment preset.
Since you already checked itīs working properly, donīt mess with it.
Good luck, and enjoy your amp.