Sunn A212 Questions

[biltmore]

Hello everyone, first poster here. It's great to see a forum dedicated to Sunn, and it's even greater to see Conrad himself posting on here. To me, he's a legend ... and then some. I absolutely can't thank Conrad and all the other old Sunn employees enough for making such great amplifiers. Over the years I bought quite a few Sunn amps, an early Model T, alpha 212r, alpha 112pr, and a beta lead head.

I recently added another member to the family, this wonderful Sunn A212:

Front



Back



Speaker



I'm in love with the off-white tolex, and whoever put it on did a pretty good job. I found a picture of an A212 with the original tolex ...



And as much as I adore the old black Sunn tolex, I just can't bring myself to take off the off-white tolex.

For those who don't know much about this amp, it's powered by 2 x 6550's, the preamp is powered by 3 x 12AX7's, and the reverb is powered by 1 x 12AX7. The speakers are 2 x 12" Sunn Transducers (128G-SP).

Front of the amp:

Channel One

* Hi & Lo inputs
* Volume, Bass, and Treble controls
* Hi Boost switch

Channel Two

* Hi & Lo inputs
* 3 position gain switch (Lo, Norm, Distort)
* Hi Boost switch
* Volume, Bass, Mid, and Treble controls
* Mid Freq. Select switch - 705 / 1.5K / 3K
* Reverb channel selector switch - CH1, CH2, Both
* Reverb level control
* Master Volume control

Also located on the front panel is the On/Off Standby and Power switches.

Back of the amp:

* Polarity switch
* Circuit breaker (rated at 4.5A)
* 120V 60Hz 250W 3-Prong Power Cable
* Accessory 3-Prong connection (120V, 60Hz, 400W MAX)
* Reverb Footswitch input
* 600 ohm out
* Speaker outputs, 1 is capped, 2 is open (110 watts max)

And that about does it for the details.

Questions

The amp was recently serviced here in Portland at Amphead, and the tubes were purchased at Eurotubes here in town. It's currently packing 2 new J.J. KT88's and 4 new J.J. 12AX7's. I read on here that the amp is based on Sunn's traditional 2x6550 60 watt amp, but I've also read that the 2 6550 tubes are run in a "push-pull" mode for a total of 100 watts RMS. I'm curious as to what watts this thing is really pushing, as it feels more than 60 watts when I open it up. Is this 100 watts RMS thing true? It just sounds odd to me as I know that the Model T uses 4 6550's and puts out 150 watts RMS. Also, is it okay to have KT88's in there, instead of having 6550's?

I'm curious about the polarity switch on the back of the amp.  At the moment, it's currently facing down and the amp is operating well. When I went over to a friends house yesterday we moved it up, and the amp started giving off a weird feedback, so I immediately turned it off and flipped the switch back downwards. Forgive my newbie-ness and please tell me what the polarity switch is used for, and if I have it in the correct setting.

The guy that I bought the amp from told me that when he was having it serviced at Amphead, the guy (Jim's his name I think) said that the standby switch was broken, but apparently it's not a real standby switch as it doesn't idle the current. It's nothing more than a mute switch, which I found to be odd. Is this common on any other Sunn's? And while working on the amp, Jim added an internal bias pot, which I thought was pretty cool. Where some people would remove it to bring the amp back to its vintage specs, I'm not planning on doing such a thing because I believe it's practical thing to have.

All in all, I'm very pleased with this amp. I know it's Sunn's answer to the Fender Twin Reverb, and in my opinion, it knocks it out of the water. It has a more pleasing mid to low tone, and the Mid Frequency Select options are just amazing. You can dial in all sorts of tones, all the way from twangy to stoner/doom rock. It's a versatile amp, and I'm lucky to own one. I don't see these often, and I wonder how long they were manufactured. It seems that most people on here say that they're from 1973/1974, but sadly there's no official documentation that I've seen that states what amp this year was made in. The guy that I bought it from said that it was a 1974, and was told that by the guy that he had bought amp. I'm curious if Conrad or anyone else here can dig up the information on this amp.

Also, I feel like the Hi Boost switch doesn't do much, could it be that it's broken?

And for my last inquiry, it says SUNN Model A on the back. Was there any other Model A series amps, or was this it? I've read on here that the preamp is similar to that in the Model T, and I'm curious to see what else this amp borrows from other Sunns.

I really wish that there was a larger and more extensive catalogue for finding out information on old Sunn amps, but it's great to see that you guys have the Sunn Museum up and are adding information to it. I can only hope that we get more models and information added to it so that we can preserve Sunn's history as much as we can. I know that we're not getting any younger, and it pains me to think of all the valuable information that's out there in the minds of former Sunn employees.

If you made it all the way down to here, thanks for reading.

[loudthud]

There is a schematic of the Model A in Aspen Pittman's book and you can find it online here:

http://www.el34world.com/charts/Schematics/Sunn_MODEL_A.pdf

The schematic is dated 9-14-74 so that's around the time it came out. It's not in the 74 catalog. The preamp is kinda like the 2nd gen Model T with James type tone controls and the inductor midrange on CH-2 but there is an unusual biasing scheme for the preamp tubes and a JFET on the input to CH-2. The High Boost on CH-2 will only give about a 3dB boost, different from CH-1.

The schematic shows a three wire line cord. Back in the days before 3 wire outlets were standard power wiring in the US, the polarity switch was used to reduce hum from guitars and give a path to ground for leakage currents. It really doesn't do much if the 3 wire feature is working properly.

It's not easy to get 100W out of a pair of 6550's. The Model A uses an Ultra-linear connection for the screen grids so it's probably about 75W. KT88's should work fine if the bias is adjusted for them. I hope your tech had a schematic and didn't just install a bias pot willy-nilly. There is a 22V zener diode that regulates the voltage that powers the reverb and biases the preamp that needs to be taken into account.

The standby switch is just a mute. The Enforcer used the same type standby. Your switch probably just needs a good shot of contact cleaner. There is a replacement switch that will fit but it's not an exact replacement. Search the forum for posts by me for a Model T power switch.

It would be nice to have a better picture of the chassis where the tubes are. Could you remove the back board and shine some light in there?

No telling what the weird noise was. Next time it happends, turn down the master volume. Could just be a bad preamp tube. It has nothing to do with the polarity switch.
 

[biltmore]

Thanks for all the info loudthud!

Looks like 1974 was the year that these guys first came out, according to the schematic. Sweet. I wonder how long they stayed in production and how many were made. Of the 5 A212s that I've seen, everyone of them had a serial number starting with 50 (50XXX).

I mainly play in CH2, so maybe that's the reason why I'm not hearing a huge difference with the Hi Boost switched on.

I guess the polarity switch is fine where it is then, since the amp's working just fine with it switched down.

I'm pretty sure that Amphead has a schematic of the Model A as they work on a lot of Sunn amps, so I'm willing to bet that he did a good job with it. Still, I'm going to call him today and make sure that he took the 22V zener diode into account. You never can be too cautious. I'm also going to ask him if the standby switch is dead, because if it only needed a cleansing I would think that he would've taken care of that. I *might* get it replaced, but since it's nothing more than a mute, I can just turn the volume knobs down. Thanks for the heads up on the replacement switch.

I'll get a nice shot of the chassis later on today for you.

[MisterTV]

Wow, great information! I'm also a first-time poster and am REALLY glad to have found this forum.

I just picked up a Model A myself.... my first Sunn.    As you can see, it doesn't have a baffle board. I'm looking to get a replacement board made and am curious about these little plastic nub pieces that must have locked the board on like a couple of Lego pieces. I've never seen this before. Are they standard on Sunns??

[mckinnon audio]

  Hi there,yeah they're orig.You see them on the 70's amps,not too much on the late 60's ones.You just banged the grill board with your fist and it snapped right in.Nice amp by the way,good luck Mel.

[MisterTV]

Good to know, thanks. Is it possible to find these fasterners anwhere? Or would I have more luck tracking down a Bigfoot?  I'm not keen on the notion of drilling holes to screw in a replacement baffle, so even an idea for something similar that I might be able to pick up at Home Depot would be greatly appreciated! 

[biltmore]

Looks like you've got yourself one nice A212 there MisterTV! Sweet! What a nice introduction to Sunn.

I totally need to get that chassis shot up on here. I forgot all about that.

As far as the fasteners go, I have no idea where you'd find them. I don't even know what they're called. Hopefully someone here can fill you in on this. Sucks that the board is missing. I wonder what the story is with that.

[cromag]

i have several sunn cabs with these types of fasteners.  and i just removed them and stapled some regular velcro in the same spot.  works just fine.

[MisterTV]

As a new guy around these parts, are there some DIY forumites I could contact about having a replacement grill cloth frame made for this? (With grill cloth attached, of course!)

[biltmore]

It would be nice to have a better picture of the chassis where the tubes are. Could you remove the back board and shine some light in there?

I finally got around to opening up the back board and taking some pictures. Hopefully these will satisfy! BTW, these pictures are all taken from left to right ... and sorry for some of the blurriness, my camera is a piece of crap.

[biltmore]

i have several sunn cabs with these types of fasteners.  and i just removed them and stapled some regular velcro in the same spot.  works just fine.

That's a wonderful idea. Some of my fasteners are pretty old and brittle, and I just might end up going the velcro route in the near future.

As a new guy around these parts, are there some DIY forumites I could contact about having a replacement grill cloth frame made for this? (With grill cloth attached, of course!)

Just to finish this up, MisterTV contacted me via email and found someone to build the grill frame for him online. I provided the measurements for him, so if anybody needs them in the near future ... just ask.

[mckinnon audio]

  Hi there,the chassis looks like it's in really good shape,you should try to get some tube shields for your preamp tubes,where they hang upside down they'll keep them tighter in the sockets.The output trans.doesn't look right to me,different cutout in the chassis and it looks a little small for a 60 watt output trans.Anyone else got any thoughts about this ?
 Still a nice amp though,good luck Mel.

[biltmore]

  Hi there,the chassis looks like it's in really good shape,you should try to get some tube shields for your preamp tubes,where they hang upside down they'll keep them tighter in the sockets.The output trans.doesn't look right to me,different cutout in the chassis and it looks a little small for a 60 watt output trans.Anyone else got any thoughts about this ?
 Still a nice amp though,good luck Mel.

I've got the original tube shields, they weren't on when I bought the amp. Good thinking tho, I'll definitely throw them back on there.

About the Output Transformer, I thought the same exact thing Mel. It has to be a replacement, since there's that space around it and there's 4 holes. I'll get some information off of the sticker on the Transformer, and see if we can find out what's up with it.

[biltmore]

Hey Mel, this is all the information that I could get off of the Output Transformer ... it's off of the sticker that you can see in the photos.

TF-127
T540918
10520209

----

I did a google search for "TF-127 Output Transformer", and I found a couple of links:

http://www.magicparts.com/new/transformers.htm

According to MagicParts, here's the scoop on what the TF-127 is:

TF127     Fender Pro Reverb, 2 X 6L6GC - Output Transformer Bandmaster 4ohm output Mounting Centers: 3 9¼16"

-----

On Vibroworld, it says the same thing essentially .. with a little more info:

TF127 - Fender Pro Reverb, Bandmaster, Tremolux - 2 x 6L6GC - Speaker output Transformer 4-ohm, Hole mounts 3.50-inches center-center. Primary resistance: 20,000-ohms; secondary: 4-ohms; Frequency response: 20Hz to 20kHz.