Please help with a 100s

[mrelliott]

Hi everyone,

I'm new to Sunn and this forum and have a few questions for you guys.

I am attempting to repair a 1967 Sunn 100s (no reverb). I dated it and all the like, but I am confused as to why it may be acting up.

To diagnose: The amp is still two prong, and when I plug it in and turn it on, there is a buzz/hum. I'm actually not sure if I would describe it as a buzz or hum, but more like something in between. I understand how amps work (I built a D*mble clone  ) but I am lost on why its buzzing while the amp is in standby. It also is known to shock, and I'm pretty sure nothing has been replaced or updated.

I attemped to look up schematics for it, but the values of everything do not match.

If you gurus have any ideas, feel free to chime in.

[HRobert]

The amp probably needs a good going through by a qualified technition.  First I'd have the power cord switched over to a 3 prong grounded and have the polarity disabled.  If it's a 67 it is probably need to have the capacitors replaced.  I have a 67 Sentura II and that, plus some new tubes and a re-bias worked wonders.

[mrelliott]

Thanks a lot for the help, I did a little bit of fiddling last night and feel like I could fix it. If not, I'll just do the most I can before giving up and handing the reigns to someone else.

Could the shocking and buzz/hum be from the filter caps, I know they usually need to be replaced, but I can't help but think that something could be really wrong if its buzzing while in standby. That almost seems like PT problem to me.

Any advice?

[mckinnon audio]

   Hi there,is this "buzzing" coming from the spkrs. when the amp is in stnby ?
   If it isn't,I would check my transformers to see if they are mounted loosely,or if any of the individual trans.plates/leaves are loose.Good luck and be careful,Mel.

[mrelliott]

Mel,

Yes its coming from the speakers. The tranny is not buzzing itself, but I really dont want to disconnect the secondaries to test the tranny. I just dont feel comfortable with that, I'll take it to a tech and get that done.

Could it be possible the filter caps are inducing that much buzz?

[biltmore]

Could the shocking and buzz/hum be from the filter caps, I know they usually need to be replaced, but I can't help but think that something could be really wrong if its buzzing while in standby. That almost seems like PT problem to me.

You're probably getting shocked because you have the original 2 prong cord. Do what HRobert suggested, get the power cord switched over to a 3 prong and have the polarity disabled.

[loudthud]

A normally operating amplifier should not make any sound through the speaker when the Standby switch is in the down position. The only possible situation I can think of that would cause this is an arched tube socket with a short between pins 2 and 3. Take the amp to a tech, operating will only make things worse.

If the amp gives you a mild shock, try moving the Polarity switch to the other position. Get a grounded power cord ASAP!

[mrelliott]

Loudthud,

I will be switching to a grounded cord this week. Could you elaborate on the arched socket? Would it be in the preamp section or power section? I can't imagine the preamp effecting the signal while still in standby. Also, the buzz stays on when I engage standby as well, so it has to be running from the amp.

I didnt know about the cap can that housed the filter caps, so that took some research to find, but I am ordering a new one as well. I'm new to the Dynaco circuits, so this is all Japanese to me.

Thanks!

[loudthud]

Power tube sockets tend to breakdown between pins 2 and 3. Pin 2 is the heater connection that gets about 3.15VAC any time the amp is on. Thats one side of the 6.3VAC that has a grounded center tap. Pin 3 is the plate connection that goes straight to the output transformer. If the amp is played really hard, the output transformer can generate inductive spikes that can be in the thousands of volts. This breaksdown the plastic in the tube socket. Most of the time you can see a little charred spot on the terminal side of the socket. Ceramic tube sockets generally can withstand higher voltages and higher temperatures without breakdown. In your amp the output transformer is probably damaged.

[mrelliott]

Loudthud,

Just looked at the output tubes terminal side, they are definately charred, but the plastic is fine. It looks as though it was bad solder joint on pin 3 to begin with, but that could also be from the voltage spike that apparently happened.

I will be putting in new sockets this week, I might as well do the preamp and rectifiers also.

With any luck, the OT will be fine, is there a way to test it before buying one?

Also, I want to give everyone a big thanks for helping me out, its because of genuine people that music and pieces of history like this amp are still around.

-Matt

[loudthud]

This is the short form OT test. If you need more detailed instructions, go over to Ampage and ask over there.

With amp off, unplugged and caps discharged... Connect a small AC output wall wort to the speaker output. It's important to know what tap you are connected to. Search for red yellow orange and you should find a thread where I detailed the OT wire colors. Measure the voltage at one of the taps, then measure the voltage between the output tube pin 3s. (You'll need good tube sockets for this or just disconnect the wires.) Expect about 20 to 30 times the first voltage you measured so it could be several hundred volts AC. Now divide the two numbers to get the turns ratio.

The impedance ratio is the turns ratio squared. So if you get 23 for a voltage ratio from the 8 ohm tap the primary impedance would be 23*23*8=4232. This is about the right number for a 60W OT. If the OT is bad, the turns ratio will measure very low, like 5 or you may not be able to get a reading at all.