Help...Rogue r88 stopped working...tube newbie

usp1 · « **on:** 22 Nov 2009, 12:45 am »

I recently bought a used Rogue R88 amp from Agon. I have played it on and off for a few weeks without incident. Today the amp just stopped working in the middle of a song. When I open and look inside, I can see the KT88 and 12au7 tubes are lit but the 12ax7 tubes (both of them) are unlit. Since this is the first time I am looking inside I am not sure if that is how it is supposed to be.

Appreciate any help or pointers on what I should do or look for.

Niteshade · « **Reply #1 on:** 22 Nov 2009, 01:17 am »

It sounds like a loose connection to me. Did you try wiggling the tubes in their sockets? If that isn't it, maybe there is a cold solder joint on the PC board in the region of those sockets. The tube filaments might be in series too.

usp1 · « **Reply #2 on:** 22 Nov 2009, 02:02 pm »

I tried moving the tubes around but that did not help. Since the two tubes are in the two different channels I am wondering why both should stop working at once.

Niteshade · « **Reply #3 on:** 22 Nov 2009, 02:41 pm »

If the filaments are in series (like Christmas tree lights sometimes are) a bad filament or a break in the series connection between the filaments could be bad. Do you have a spare tube to try? If they're not in series, than the circuit that provides current the the filaments is open. Sometimes there are independent filament voltage supplies to different areas of an amp, usually the ones that are hum sensitive, like preamp or phase splitter circuits.

Steidl Guitars · « **Reply #4 on:** 22 Nov 2009, 03:51 pm »

Call Rogue, I'm sure they'll help you get it sorted. Very nice folks.

usp1 · « **Reply #5 on:** 22 Nov 2009, 04:07 pm »

Blair - I don't have a spare so I will need to get one before I can try swapping.

Steidl - Thats my next move. I am hoping it is something minor.

usp1 · « **Reply #6 on:** 24 Nov 2009, 12:59 am »

Called Rogue Tech support. They think it is a failed bridge rectifier. Hopefully that is all it is.

Thanks for all the help.

Niteshade · « **Reply #7 on:** 24 Nov 2009, 01:05 am »

It's a cheap fix if that's it.

Quote from: usp1 on 24 Nov 2009, 12:59 am

Called Rogue Tech support. They think it is a failed bridge rectifier. Hopefully that is all it is.

Thanks for all the help.

usp1 · « **Reply #8 on:** 3 Dec 2009, 03:15 pm »

I finally got a new bridge rectifier put in but it did not change anything. So that was not the problem it seems.

Niteshade · « **Reply #9 on:** 3 Dec 2009, 03:45 pm »

Sounds like there's a break in the circuit. Did you trace it yet?

richidoo · « **Reply #10 on:** 3 Dec 2009, 03:50 pm »

I would replace the 12ax7s first. Get new tubes from local music store.

Check inside the amp that all fuses are fine. There are different fuses for the main powerinput and the B+ circuit, and maybe also the heater circuit. Test them with a meter.

Look for something wrong, burnt cap, resitor, bad loose connection.

JakeJ · « **Reply #11 on:** 3 Dec 2009, 04:09 pm »

And I'll add - USE GREAT CAUTION!!

I hope you are aware of the voltages you are exposed to inside a tube amp. Clip your negative test lead to the chassis/ground plane. Use your right hand to probe with the positive test lead. Keep your left hand in your back pocket.

Good luck and work safe.

JakeJ

richidoo · « **Reply #12 on:** 3 Dec 2009, 04:30 pm »

Excellent advice Jake!

You can check the voltage across the PS caps to make sure they are dead before you do anything. If they are still hot, wait until they drain.

usp1 · « **Reply #13 on:** 3 Dec 2009, 04:42 pm »

Thanks for the advice guys. I was aware of the voltages and so I dont touch anything when the amp is on or for a couple of hours after.

All the tubes are glowing. I am not sure about the fuses. Just seems like such a hassle. I may just end up sending it to rogue for fixing...another 200 it seems for what I was hoping would be a cheap way to get into tubes.

richidoo · « **Reply #14 on:** 3 Dec 2009, 04:54 pm »

If all the tubes are glowing then it is just a B+ fuse, and like Uptown says below, a bad tube to blow the fuse.

Welcome to tubes. It happens. It takes more maintenance to run a tube amp and this makes it inconvenient and annoying in the beginning. But eventually you get better at it and understand what it takes. Keeping spare tubes and fuses on hand is essential to avoid frustration. Usually you can fix anything yourself unless you operate the amp incorrectly and break it, like wrong tube in the right socket, or overheat it, etc.

The B+ is inside the case.

You can do it!!!

You will be so proud of yourself when you get it working again.

Uptown Audio · « **Reply #15 on:** 3 Dec 2009, 04:55 pm »

Those are the only options I would suggest. 1) The output tubes need to be tested and replaced if bad. 2) The fuses need to be tested and replaced with the exact same types and values if bad. 3) If you can't do those things or if they doesn't work, send it to Rogue for repair.

Never poke around inside of a tube amplifier. You can't always reliably see if filaments are glowing. You can't always reliably tell when and where voltages are present. It's a very nice amp and it deserves service by the best techs, which of course would be Rogue in this case. It likely has a bad output tube in addition to a blown fuses. That is the #1 cause for a troubled amplifier. Expect to have to buy a matched set of those also.
-Bill

usp1 · « **Reply #16 on:** 3 Dec 2009, 06:49 pm »

By output tubes do you mean the KT88 tubes? Or is it the 12ax7 or the 4 12au7? I have some spare 12au7s.

How do I find out what the fuse values are? Would the local radio shack carry them?

hmen · « **Reply #17 on:** 3 Dec 2009, 06:53 pm »

Quote from: usp1 on 3 Dec 2009, 06:49 pm

By output tubes do you mean the KT88 tubes? Or is it the 12ax7 or the 4 12au7? I have some spare 12au7s.

How do I find out what the fuse values are? Would the local radio shack carry them?

If you don't have a manual call Rogue and they can tell you the fuse values. Radio Shack or any hardware store will have replacements.

usp1 · « **Reply #18 on:** 3 Dec 2009, 07:26 pm »

The manual does not give the fuse values. I will call them and check.

richidoo · « **Reply #19 on:** 3 Dec 2009, 08:59 pm »

Open up the amp and look on the fuse for the value. You need to test it anyway, with ohmmeter after removing it from the amp.

The output tubes are the KT88s, the big tubes that make up the output stage of the amplifier. They provide the current to drive speaker drivers at the increased voltage made by the smaller signal tubes. When the output tubes (aka power tubes) fail they will frequently take out the B+ fuse also. The B+ fuse is usually the sum of the bias currnt times 4 tubes plus headroom for peaks during musical peaks. .5-1 amp is typical for B+ fuse. But you have to know whether sloblo or fast blo is required. It is labelled on the fuse.

After you get the fuse replaced, you can determine which tube is failed if you know how to adjust the cathode bias. There is no instruction on how to do this in the manual. If you already kow how to adjust the bias yourself, then you can turn down the bias all the way on all tubes. Turn on the amp with new fuse. With bias meter connected, add bias current slowly to one tube or the pair if one bias screw controlls two tubes, all while watching the meter. If the reading starts to climb by itself without you increasing the adjustment (called "run away") then you have found the bad tube. Reduce the bias to zero before the fuse blows, then replace or swap out that tube with a different one and start again. All tubes should hold specified bias steadily. The bad one will want to rise by itself when set to recommended bias. It will not hold a steady bias setting. If two tubes are controled by one bias pot, you can narrow down the bad tube by process of elimination from swapping. A bad tube will not blow B+ fuse if there is no bias current applied, so leave all 4 tubes in the amp while you are testing. Only the tubes with bias current added can cause fuse to blow again and you are watching them with the meter. This allows you to use the amp to find the bad tube. Always have 4 tubes in the amp when it is on so the circuit is complete and the procedure is safe. Of course there may be visible damage inside the tube that would make it easy to identify. Your other options are to replace all 4 tubes available from local guitar store, or get yourself a tube tester. Sovtek and Electroharmonix are fairly reliable. SED 6550C is a very reliable tube that sounds very good and will work in your amp. KT88 is preferred.

If you can't adjust the bias, then just get 4 new tubes for now so you get the music back asap. It's good to have a back up set anyway for this kind of thing. Send the old ones to someone who can test them for you. Ask Rogue to send you written bias adjustment procedure.
Rich