I have a Sunn 300 Coliseum (circa mid 80's). I replaced the blown 10A/32V fuse  but after power up with the VARAC it would draw excessive current. I disconnected the secondary and connected a DVM. Using the VARAC - I can turn up the mains voltage (fan runs) no excessive mains current but the secondary only reads 30mV across the secondary leads.

DC resistance of secondary is .2 ohms. I'm assuming the secondary is damaged  Also assuming this PT has a 100 V secondary (+ or - 50V rails). Anyone know where to find a replacement?

Mmmm let's see what I let go in the 60's and 70's:63' Gibson ES175, 63' Fender Strat, 66' Super Reverb w JBLs, 1959 Martin 0018, etc. I had a guitar teacher who had an in at Fender and loaned me demo gear that came back from the "artists" of the time. To busy trying to build my chops to think about buying or collecting.

you want to see vintage Acoustic kit in action get a copy of Wattstax - good music too. 

Quote(That is, unless the latest POD or Guitar Rig includes their digital simulation).

hey JM is that a new product? Acoustic 150 simulation - I bet guitar center would love that

This is the Fender/Sunn solid state version not the tube design. Anyone work on one and is this a design with a switching PS? Also could use a schematic if someone has one.

QuoteThe power supply is probably fine if the amp powers up.

I have worked on several Sunn Betas that power up but don't work because the =-15 V supplies aren't functioning. Just one example I can think of. My reply was a general one since there isn't much to work with.

Schematic would be helpful

Check output of the power supply

Isolate the problem - is the input signal getting to the preamp, is the preamp signal getting to the power amp stage, etc etc

Ouch - you mean these amps are supposed to perform like this?

The first picture/waveform is from an Agilent portable scope. The scaling for the verticle and horizontal are a bit confusing since you are looking at the time domain signal (blue) and the frequency domain transfer function (green). Crude but effective way of looking at realtive distortion/modulation of a test signal.

The other pictures are from an older HP scope and yes that vector CRT display is nice to look at.

Since I can't seem to attach files.... here are some links to waveforms:

http://i284.photobucket.com/albums/ll33/bonosurf/DSCF01031.jpg

This is the signal from the pre-amp output jack on the rear panel - 150mVRMS into the amp (at around 1kHz) maybe 1-2 on the volume control - the signal into and through Q101 is clean right up to the gate of Q102. Then it looks like what you see above. I have the distortion and boost disabled.

http://i284.photobucket.com/albums/ll33/bonosurf/DSCF01011.jpg

This is the output waveform from the output jack on the rear panel - 3.2 ohm dummy load same conditions for input as before but 1-2 on the volume control - as you can see this is low level output with notch in the crossover region (duh). I didn't see this before because the scope puts limit lines on the display and they hid the notch.

http://i284.photobucket.com/albums/ll33/bonosurf/DSCF01041.jpg

Moderate to high level signal measure as above maybe 4-5 on the volume control - this amp will almost "soft clip" like a tube circuit.

So I have multiple problems here. I'll check the bias on the output transistor. I tried to adjust the bias trimmer but it doesn't seem to improve the crossover distortion issue - though it will adjust the idle power (measured at the AC mains).

Can you not attach/post gif.s?

I have three Sunn Concert series amps that are showing various degrees of second order harmonic distortion in the preamp section. For example, I have a Concert Bass which doesn't shown any 2nd order harmonics at Q101 but it shows up at Q102, Q103, etc. I have the distortion feature turned off but I'm wondering if there is a ground path that I don't see

If R112 or R126 were shorts then the distortion would be "on"? I'm assuming the distortion feature works by adding the RC feedback networks into the output of Q102 and the input of Q104 via the ganged 25K pot ?

http://sunn.ampage.org/site/schematics/concertbass.gif

I spent some time testing the preamp sections on this amplifier - Sunn used JFETS in the input stages of their design (Concert series). The original part is a "glob top" (preceded TO-92) package - 2N4304. I tried using Fairchild J201's and though they had the same PIN OUT and similar IDSS specifications they produced more 2nd order spurs (without change the circuit) - the PN4303 is still available and seems to give about the same results as the orginal part - FYI

I had to rebuild much of the signal path of the second channel including sourcing a new reverb pan.

Now to dial in the bias for the best distortion/power dissipation at idle.

Thanks for the help.

Hey it works and it puts out +21Vrms into a 3.2 ohm load. The only issue now is the preamp is generating some even order harmonics. Thanks

I have new On Semi 2N3055's that I matched on a curve tracer at 1A/20V (use pulsed supply mode and work FAST). They are going in as soon as I can get around to this project.

I have repaired about 6 of these amps and have never noticed that the bias resistors get as hot as these do. (My original issue was that Q3 was getting very hot in about 45-60 sec.) We'll see what happens - thanks for the advise and I'll post the results.

Okay maybe it's not clear but though the above bias readings appear "balanced" I'm still seeing significant (>100F in 45 sec) heating of T303/304/307 only. This is of course without any active devices installed.

Something is wrong but what is it?

1) DC Voltage across
R303  = 24V
R304  = 24V
R301  = 24V
R307  = 24V
R302  =.58V
R305  =.58V
R322  =.58V
R306  =.58V

TP 16/17 =24.5V
TP 12/21/22 = -24.6v
With amp on, no signal, no speaker, no transistors.