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Sunn 200s Q&A

Funkster

Funkster

Supporting Member
Apr 6, 2000
3,980
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Wormtown, MA
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#1
Hi guy's. I aquired this decent Sunn 200s. It had a mixmatch/hodgepodge of tubes in it. I found out by the schematic that it uses KT88's for power a 12AX7 and 6AN8A in the pre but I couldn't make out what the Rectfier is, Is it a 5AR4?
The amp fired up and made sound so the Tranny is good. It made some cap noize and such but overall she sounded alright. I going to go through this amp and make her sing.
Is there a bias procedure for this amp or is it fixed?
Thanks all for your info in advance
Link Removed
 
#2
The bias adjustment is the pot in between the two power tubes. You need a flathead screwdriver to turn it. The rectumfrier is 5ar4 or GZ34. Either will work.

Congrads on a kickass head, i've got one myself that i'm using as a PA amp for right now, and it's clean almost all the way up. Just need to install a grounded plug so I don't get electrocuted.
 
#4
Yep The Ox beat me to it. I have a Sorado which is a rebadged 200S and I have a GZ34 rectifier.

I am pretty sure it's fixed bias. I have the original tubes in my 68 Sorado so thats a testament to Sunn's engineering. It's only been serviced once in 2000 to my knowledge for a general run though which came out with flying colors and a pot cleaning from the info the previous owner gave me.

No idea where the test points are though...

Slap some JJ KT88s in there. The 6AN8A is the tough one to find new but there are quite a few RCA or GE NOS tubes that you can find pretty easily online, but the rest shouldn't be any trouble.

Here is a site that might be able to help you with schematics and what not: Link Removed

Welcome to the Sunn family :)
 
#5
Thanks Pro, I thought it was a fixed bias but I wasn't sure.
Believe it or not I found two bran new old stock 6AN8A's, one RCA and one GE, I got a set of matched KT88 JJ's coming to me, I have variouse new 12AX7's. The one I couldn't find was the Rectumfrier 5AR4 or the GZ34. I have to do some searching tonight when I get home and try and locate a few.
Thanks all
 
#12
I've got a 200s question myself: Why does it keep blowing fuses?

It blew two the first time I tried it out. We changed the speaker output to 8ohms from 18ohms and disconnected the polarity switch which we thought was shorted; afterwards I played it at low volume (4) through an Ampeg 810 and it was fine. But then I brought it to rehearsal (using a homemade 210 and 115, both 8ohms), and it wasn't loud enough, so I cranked it up to 7, and the fuse blew again within 3 minutes of my doing so. I'm getting really aggravated.

On a side note, my 200s does not "stay clean nearly all the way up"; there is very noticeable overdrive at volume on 7. Does anyone have any idea what might be wrong?
 
#17
Funkster said:
The one I couldn't find was the Rectumfrier 5AR4 or the GZ34. I have to do some searching tonight when I get home and try and locate a few.
Click to expand...

A 5AR4 IS a GZ34. "5AR4" is the tube's American deignation; "GZ34" is its European designation. They are the same tube and several good ones are currently produced. The best is probably made by JJ Electronics, www.eurotubes.com .

The 0x said:
The head is NOT fixed bias. At least, neither of my Sunn heads are.
Click to expand...

Actually they ARE, but not what you think. A fixed bias amplifier is the ONLY kind that you can adjust bias for! I blame Mesa for using that term in reference to not having a bias adjustment pot in their fixed bias amps and saying that it means that bias CANNOT be adjusted, which is not true.

MOST amps, especially high powered ones, are fixed bias. All that means is that the bias voltage is a set or 'fixed' value applied directly to the suppressor grid which does not change. Preamp tubes and older smaller tube amp output stages are cathode biased which applies bias current directly to the tube cathode, which will vary with signal flow, hence it is not a 'fixed' value.

ihixulu said:
But where are the test points?
Click to expand...

There are none. You use an oscilliscope to bias the amp, and the pot to adjust it.

Origami Kamikaz said:
I've got a 200s question myself: Why does it keep blowing fuses?

It blew two the first time I tried it out. We changed the speaker output to 8ohms from 18ohms and disconnected the polarity switch which we thought was shorted; afterwards I played it at low volume (4) through an Ampeg 810 and it was fine. But then I brought it to rehearsal (using a homemade 210 and 115, both 8ohms), and it wasn't loud enough, so I cranked it up to 7, and the fuse blew again within 3 minutes of my doing so. I'm getting really aggravated.

On a side note, my 200s does not "stay clean nearly all the way up"; there is very noticeable overdrive at volume on 7. Does anyone have any idea what might be wrong?
Click to expand...

Yours may have a partially shorted output transformer or output tubes. At higher output impedance, flyback causes the tubes to start drawing too much current and your fuse blows. You probably damaged it with too high a load. Are you using the proper output tap now?

Origami Kamikaz said:
The load is now 8ohms, it was 16 (oops). There's a three-prong cord so I'm not worried about the shocking...I just put in 250V 4A slow-blow fuses from Radioshack, didn't think there'd be anything wrong with them.
Click to expand...

That's over 8 amps at 120 volts.

Origami Kamikaz said:
I'll see what I can do about the rec tube. Any chance I could go for a higher amp fuse or will that endanger the other components?
Click to expand...

If your rectifier tube was bad, NOTHING would work at all. If you go any higher in fuse value, you risk the amp catching on fire. Find a good tech, because you need one.
 
#18
Psycho Bass Guy said:
A 5AR4 IS a GZ34. "5AR4" is the tube's American deignation; "GZ34" is its European designation. They are the same tube and several good ones are currently produced. The best is probably made by JJ Electronics, www.eurotubes.com .



Actually they ARE, but not what you think. A fixed bias amplifier is the ONLY kind that you can adjust bias for! I blame Mesa for using that term in reference to not having a bias adjustment pot in their fixed bias amps and saying that it means that bias CANNOT be adjusted, which is not true.

MOST amps, especially high powered ones, are fixed bias. All that means is that the bias voltage is a set or 'fixed' value applied directly to the suppressor grid which does not change. Preamp tubes and older smaller tube amp output stages are cathode biased which applies bias current directly to the tube cathode, which will vary with signal flow, hence it is not a 'fixed' value.



There are none. You use an oscilliscope to bias the amp, and the pot to adjust it.



Yours may have a partially shorted output transformer or output tubes. At higher output impedance, flyback causes the tubes to start drawing too much current and your fuse blows. You probably damaged it with too high a load. Are you using the proper output tap now?



That's over 8 amps at 120 volts.



If your rectifier tube was bad, NOTHING would work at all. If you go any higher in fuse value, you risk the amp catching on fire. Find a good tech, because you need one.
Click to expand...

Thank you Psycho, I was waiting for you to stop into this thread.. The procedure you sent me for my SVT worked pisser. I was just wondering if this amp had bias adjustment and if there are any quirks I have to watch out for or do.
Thanks bro.
 
#19
Hmm. Maybe I ought to clarify what I meant: the 200s originally had 1 16ohm and 1 8ohm output, and I ignorantly ran both outputs into 8ohm cabs, blowing the fuse.

We then disconnected the polarity switch and modified the outputs to be 8ohms and 8ohms. I ran one of the outputs into an Ampeg 810 (4ohms) with the volume on 4 and it seemed to work perfectly fine.

We then ran both outputs into the same two 8ohm cabs, and the fuse still blew after running the amp on 7 for about 3 minutes.

I guess the best thing I can do is take it to a shop now...but what should I tell them? Also, does anyone know a good amp repair place in the Baltimore/Washington DC area? :hiding:
 
#20
Psycho Bass Guy said:
Actually they ARE, but not what you think. A fixed bias amplifier is the ONLY kind that you can adjust bias for! I blame Mesa for using that term in reference to not having a bias adjustment pot in their fixed bias amps and saying that it means that bias CANNOT be adjusted, which is not true.
Click to expand...

*adds one more reason to list of why I hate Mesa/Boogie.*

:spit:
 

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