Sunn 1200S - Head and Cabinet Impedances

[Starliner]

Hi everyone,

I have a 1970's era Sunn 1200S head. No cabinet. And, I need to overhaul the amp head, and buy (or build) a cabinet.

I got an old Sunn catalog with some info and pics, and I see the original cabinet had six 12" speakers.

What I would like to know is:
A. What is the impedance of the standard six speaker cabinet, and
B. What is the output impedance of the amp head?

Any help is appreciated. Thank you.

Starliner
 

[EdBass]

Not sure what the impedance of the 6X12 was, I'm thinking it's 6 ohms nominal. Your 1200S has three taps on the OT; 4, 8, and 16 ohms. Whichever taps are connected to the output jack assembly will default to parallel when both are used.
So if you connect the 8 ohm and 4 ohm taps,  one output soloed will be 4 ohms, and the other will be 8, both used together will default in parallel to the 4 ohm tap.
If you use the 16 ohm and 8 ohm taps,  one output soloed will be 8 ohms, and the other will be 16, both used together will default in parallel to the 8 ohm tap.

If I'm correct about the 6X12 being 6 ohms nominal, you would want it connected to the 8 ohm tap on the Output Transformer.

[Starliner]

Thanks for the info Ed!


It looks like I might just get an 8-ohm 4 x 12 Marshall cabinet, unless I find a deal on building or buying a 6 x 12 cabinet. I fear even if I find an original, I will just be paying for wood, as the speakers likely have seen better days.

I actually got mine apart yesterday (a tough job, due to a frozen chassis screw).  Now, I  have more questions...

Was surprised by the fact I have nine tubes in mine: (1) 12AX7A,  (1) 12AU7A,  (1) 6AN8A,  (4) 6550, (2) 5AR4 / GZ34.

I have a schematic with waveforms, but it does not show the two GZ-34's. So, I am unsure if this close to the correct schematic?

This amp overheated in the studio one night, smoked a little (drummer saw it), and at least one of the 6550 plates got really red (assume a bias problem). I quickly shut it down, and it's never been re-powered for years.

Oddly, I noticed no serious signs of overheating on the chassis, but did see two slightly bubbled 6.8K carbon resistors on the filter can, and a 1,000 mfd. cap (C101) on a PC board with a lead broken loose from the PC board. 

Curious as to whether a number stamped into the chassis between two of the 6550's is the serial number, or whether a yellow sticker on the chassis rear is the serial number? Any ideas here?

Another sticker on the side of the chassis (visible only when it is pulled out) shows a set of initials (DB) and a date of either 7-16-67 or 7-16-69 and "E-test".  Guess a final Q.A. stamp?

Starliner

[loudthud]

Check these two threads:

https://forum.sunnstillshines.online/index.php?topic=3833.msg13448#msg13448

https://forum.sunnstillshines.online/index.php?topic=4844.msg18880#msg18880

[Soundmasterg]

Since the amp had a failure I'd suggest that you replace the filter caps before trying to use it again regularly. Also sounds like those two resistors you mentioned might have cooked. It is also a good idea to check all the carbon comp resistors for value drift, especially the plate load resistors, and replace as necessary.  There are many threads here about cap replacement, but the main thing to remember is that the original can cap is often over voltage these days with higher wall voltages compared to when these amps were made, and using series connected caps with parallel resistors is a good idea to get sufficient voltage headroom for the caps in the power supply. Leave the can in place for cosmetic reasons, but electrically disconnect it and connect instead to the new caps that you've installed under the chassis. You can change value as you like as long as you remember a couple things. First, going larger than 60uF in the first stage would be a bad idea if you intend to use the GZ34/5AR4 tube rectifier, as those have a limit of 60uF for the first filter cap after the rectifier, and with today's tubes probably 50uF is a better choice. Second, changing filter cap values in the preamp will change the tone and bass response as those stages are RC coupled. Changing to larger values in the power amp or screen supply will change the feel in that the bass will be quicker and tighter, but the actual tone doesn't change if you change the value of those caps. YMMV.

Greg

[EdBass]

Thanks for the info Ed!


It looks like I might just get an 8-ohm 4 x 12 Marshall cabinet, unless I find a deal on building or buying a 6 x 12 cabinet. I fear even if I find an original, I will just be paying for wood, as the speakers likely have seen better days.

I actually got mine apart yesterday (a tough job, due to a frozen chassis screw).  Now, I  have more questions...

Was surprised by the fact I have nine tubes in mine: (1) 12AX7A,  (1) 12AU7A,  (1) 6AN8A,  (4) 6550, (2) 5AR4 / GZ34.

I have a schematic with waveforms, but it does not show the two GZ-34's. So, I am unsure if this close to the correct schematic?

This amp overheated in the studio one night, smoked a little (drummer saw it), and at least one of the 6550 plates got really red (assume a bias problem). I quickly shut it down, and it's never been re-powered for years.

Oddly, I noticed no serious signs of overheating on the chassis, but did see two slightly bubbled 6.8K carbon resistors on the filter can, and a 1,000 mfd. cap (C101) on a PC board with a lead broken loose from the PC board. 

Curious as to whether a number stamped into the chassis between two of the 6550's is the serial number, or whether a yellow sticker on the chassis rear is the serial number? Any ideas here?

Another sticker on the side of the chassis (visible only when it is pulled out) shows a set of initials (DB) and a date of either 7-16-67 or 7-16-69 and "E-test".  Guess a final Q.A. stamp?

Starliner

The good news is that old Sunns are pretty straightforward and easy to work on. As Greg mentions the power supplies are the weakest link. Even with the 110 VAC wall currents of the era the peak voltages to the filter caps was marginal as far as the filter caps were concerned. My regular gigging amps have been fortified for reliability, however I do have some original 60's circuits and components amps that still function well 50 years later, a little hum but still usable.
Another suggestion, particularly if you plan to gig the amp would be to get grounded AC rather than the the old two plug. Just common sense; no point in using your body as a conductor...

If you post a pic of the amp, we can probably solve that 7-16-67 or 7-16-69 question pretty fast. The sticker would lean towards '69, most of my pre '68 amps have the inspection written directly on the chassis in grease pencil, but there is no hard fast rule.

[Starliner]

Thanks for the replies, everyone!

I had a major computer crash and lost lots of logins, including this one, or I would have responded sooner.

I did secure a Marshall cab for testing the unit, but have not started restoring the amp yet.
Lots of good points above... again, thanks!

I will update as I progress with this.

Starliner