1200s 5ar4 tube not on???

[paco)))]

Hello, well I just got this amp & I have a dumb question. When it's on all the tubes are fired up except the 5ar4 (rectifier?) tube on the far left. It doesn't seem to be on, I mean there's no glow in it. I can't find a manual on it but it sounds GREAT anyway. The amp is in better condition than I expected & for $275 I think it was a pretty good deal. Other than the tube issue the reverb doesn't work but if I mess with the box I can hear the springs through the speakers & there's a difference in the sound when I turn up the reverb level. Also my mid boost switch only works sometimes but I know that's an easy fix. Does anyone have any info on my issues I stated? I 'll probably end up taking it to a tech to get it a check up in the near future & I want to be able to address the problems. Oh yeah, I ended up getting the new 4x12 at a pawn shop a block from my house for $200 so I think I did good overall.

Thanks in advance 

~Chris

[mckinnon audio]

   Hi there,does the 5AR4 get hot to the touch after the amp has been on awhile ?
   You should be able to see some heater glow from inside the tube.Have you had the amp apart ? A long shot but maybe someone wired up diodes,disconnected the tube and just placed it in the socket for looks ?
   The fact that you can hear the rev. springs in the spkrs. means the tank and the output of the rev. circuit are working.Your prob. will be on the input side,or rev. drive.
   The mid boost sw. intermittancy is probably just a cleaning or a loose "body" around the sw. Hope this is of some help,good luck Mel.

[paco)))]

Thanks Mel, I'll try & check it today but I don't think the tube gets hot. We're going out of town for the weekend so I probably won't get to mess with it until I get back, I'll try though 

[loudthud]

Swap the two 5AR4's to see if the problem is the tube or the socket.

[djc]

wow, I'd buy it for that price in a virtually wrecked state!  $475 total for the cab and head?  I think you did very well.  nice rig.  hasten thyself to a good tech for tender loving care.  spare no expence.  the price you bought it for gives you some room to refurbish it.

[paco)))]

An update on the 1200s. That tube on the far left is just a dummy in an unwired socket. I finally opened her up & took a look inside. I was able to fix the reverb which was just an unhooked spring. I love that old twangy reverb, sounds cool at around 4-6 with the ric. However, the mid boost is still funky, I tried cleaning in a bit (not much) just blew it out with high pressure air. It will just kick on while playing awhile & is kind of startling really Is that more than likely a prob with the switch or is there a tube that runs that too?

Thanks,
Paco)))

[Ryan Phelps]

Paco,
So the tube on the left is a dummy in an unwired socket? If so, the amp is running on a single 5AR4 rectifier (unless someone added diodes inside the chassis). The 1200S was designed with two 5AR4's to handle the current drawn by by the circuit, primarily the quartet of 6550's. So, if the amp is really running on a single 5AR4, it will likely burn itself out eventually trying to deliver more current to the amp than it is designed to deliver. Conversely, if the dummy is actually a solid-state rectifier replacement that is wired to the circuit, then the amp should run OK.

Ask your amp tech to check how the rectifier portion of the circuit is currently wired. For reliability, you want adequate current capability; either two 5AR4's or a combination of solid-state diodes and a single 5AR4 or all solid-state diodes.

Regarding the mid-boost switch......it doesn't add another tube to the circuit to create the mid-boost. It's simply a switch in the the tone stack (tone controls) that allows more mid-frequncies to pass to the next amplification stage. If compressed air doesn't clean the switch, try some electronic contact cleaner (or ask you amp tech to do it). If that doesn't correct the problem, check for loose or funky wiring. If that's not the problem, then just replace the switch.

And be extremely careful poking about inside the chassis. The filter capacitors store lethal amounts of voltage! So, if you're not sure what you are looking at, or don't know how to bleed the charge off the capacitors, then take it to your amp tech.

Good luck!

Ryan

[paco)))]

Thanks man! I wasn't sure about that mid boost. I'll probably try some contact cleaner or maybe just replace it, looks pretty easy to do. I do need to get it up to the tech guy but it's hard to get it up there to him. I did read somewhere that there was some sort of replacement for that rectifier tube that was done by Sunn. The socket looks like it's never been wired/soldered so I dunno. I've worked on arcade games before so I've learned my lesson on filter caps I'll just get it up to the pro & see what the deal is.

~Paco)))