Ampeg Rocket Reverb 2X12

[noah_swanson]

Hey all,
I've got an Ampeg Rocket Reverb 2X12 combo amp. I've had it since August of 2005, and i've loved it since. However, 2 weeks ago while practicing with the band i noticed i lost some attack and gain. I replaced the stock 12ax7R's with new 12 ax7R's (x3) and the stock 6L6R's with Fender 6L6 (sovtek 5881WXT - x2). The attack is back but i still have a lot less gain. What i used to get with the gain at 6 i barely get with it at 10. I checked the bias on the tubes and that was okay after the switch, but still cannot match the gain. Today I checked things again and the bias was way off...about 1/8th of a turn on the trim pot. What else could be wrong? I've had this amp for a little over a year so i dont want to believe its already messed up.

Any help and suggestions are much appreciated..Thanks!

[noah_swanson]

Just to add a thing or two...the amp sounds like it did before...everything is great, but when i seek higher volumes the power seems to fluxuate..it seems to stray down then up again. Mostly at high volumes. and when this happens i lose gain and such as well.

[stokes]

What kind of 6L6's did you have before?6L6GC,6L6G,6L6WGB,you said 6L6R but there is no "R" designation.A 5881 has somewhat lower gain than a 6L6GC,tends to break up sooner and wont have as much headroom in a given circuit.Could also be the choice of preamp tubes,if the other ones were only a year old,they shouldnt need replacing yet,try putting them back in with the new 5881's and see if that helps at all.If you had 6L6GC's before,try lowering the bias on the 5881's about 5 to 7ma's,below where the 6L6's idled,they wont break up so fast.

[Amp Kat]

Try patching a chord from the send jack to the return jack and see if it comes back up. Your jacks may be corroded.

[noah_swanson]

Stock 6L6's were Groove tube Russian 6L6-R is what they said.

The more i've played now it seems i start to loose power after a period of play or if i turn to a higher volume. the power seems to be fluxuating a bit. Could this be something overheating?

[stokes]

It could be heat related,could be any number of things,the easiest to check would be the tubes.I would put the old preamp tubes back in and see if that changes things,like I said before your old preamp tubes shouldnt need replacing after only a year.Your first post you said the amp seemed to be weaker,that was why you changed tubes.You didnt mention it fluctuating.If that is the case put the old tubes back in and see if it stops fluctuating,if it does try the new power tubes,if it starts fluctuating,you will know it is the new power tubes causing the problem,if it doesnt fluctuate with the new power tubes and the old preamp tubes you will know there is a problem with one or more of the new preamp tubes.If the problem cant be narrowed down to the tubes then it is time to look inside,but eliminate the tubes being the problem first.

[noah_swanson]

Sorry Stokes for all the confusion.

First problem i had with the amp was that i lost attack and an ammount of gain. It did not fluctuate at this point. I recently put all new tubes in it...preamp and output. Now the attack and gain were back but it began fluctuating.

I did as you said...first put the old Pre-Amp tubes in and no resolution. Then put the old power tubes back in...I have a little less gain (what i had on 7 originally i get at 8 - no big problem) and the fluctuation almost gone. I noticed it dipped momentarily then came back and remained for an hour or so. I then let the amp on standby for about 15 minutes. I flipped it back on and it momentarily fluctuated a few minutes after i started then came back.

From that experiment those power tubes i bought have something wrong with them correct? and the momentary fluctuation now is possibly from the power tubes starting to show wear? Thanks for the help so far and sorry for any confusion.

[stokes]

No problem,Noah,no real confusion.I know its tough to relate in text audio related problems.If you notice a much bigger amount of fluctuating volume with the new power tubes then I would say it could be the tubes,its tough to say definitively from where I sit.I assume the amp was new when you got it,so a tube issue is more likely than other components like filter caps.5881's should be biased a little lower than a 6L6GC.For instance at 350 plate volts a 6L6 at 70% should be 60ma's while the 5881's should be about 52ma.That shouldnt cause the tubes to cut out tho.Try switching between the 2 sets of power tubes a few times to be certain that it is the new power tubes and not some other problem in the amp.I would try to return them if they are faulty.When you get a new set I would stay away from the 5881's,they will soften up the tone and break up sooner.Stick with the 6L6GC's,they will give you more headroom,and I'm not sure what plate volts you have in that amp,but if the volts are over 360 the 5881's wont last as long or deliver as much power as the 6L6GC.

[noah_swanson]

Thanks again Stokes. They run at 390 for plate voltage. Here is a link with the schematic in case your curious:

http://www.schematicheaven.com/ampeg...brocket_ri.pdf

I'll spend the day tomorrow playing and swapping and so forth. I play for a but more distortion so headroom isnt that big of an issue for me but good to know for the future! I was thinking...if i have to...getting groovetubes with a lower "distortion rating" so they'll break up easier..or so they say.

Before that...just to be sure i'm setting the bias correctly. Im doing the output transformer shunt method as described here: http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#howtobias putting my multimeter on dc current from the center tap to the other leads on the primary of the output transformer correct. according to that site i should set it to 30-35 ma with the 6L6 tubes.

like i said i'm new to tube amp stuff but i really would like to learn.. Thanks for the help!!!

[stokes]

To get the "correct" bias you have to figure your plate dissipation,lets take the 6L6GC.It has a max plate dissip.of 30 watts.Class AB1 which is most push-pull amps,such as yours,calls for 70% dissipation.For a 6L6GC that would be 21 watts plate dissipation at idle.So you multiply your 390 plate volts times 35ma's,which is 390X .035 and you get 13.5 watts.So you can see for the 6L6GC this would be cold,but is very safe for that tube.On the other hand the 5881 has a max plate dissipation of 19watts,so the 35ma would be the max for this tube in AB1.As you get higher than the 70% you getinto Class A territory,which sounds great but at those plate volts tube life would greatly diminish.Once you have the tubes sitting at a safe idle,you can adjust the bias up or down to suit your taste,to what sounds best to you.As long as your plates dont start to glow red you will be okay.This is a link to a site that has tube data so you can see the differences between all the various 6L6 types.Contrary to what is assumed by many people they are different. tdsl.duncanamps.com/tubesearch.php

[noah_swanson]

having all the originals in, the amp sounds fantastic again. I checked the bias again (output shunt method) and measured only 22ma. Is this going to hurt the amp going that low? Thats only 10.78 watts on a 490Volt plate. I'd like to keep these settings the way they are because it sounds soo nice! Thanks again for the help

[stokes]

No,that bias setting is fine,in fact if those tubes are not 6L6GC's,since we cant be sure what GT is calling that "R" designation,if they are one of the others, like a 6L6GB,that 22ma at 490 volts is just about 70%.Like I said before,as long as your plates dont glow red you are okay.One point I forgot to mention is that as you increse the current draw the plate volts will decrease,and vice versa,so if you decide to adjust the bias keep that in mind as you do the calculations.It is possible that your old tubes werent bad at all,over time the bias will shift and it seems they just needed to be readjusted.I try to check all my amps at least once a year and always seem to tweak them a little,especially the ones that are used alot.Also,I wouldnt sweat the preamp tubes at all,unless you start getting symptoms related to these tubes such as microphonics.I have 2 old Ampeg Gemini's that have the same preamp tubes from the 1970's in them and they are still fine.It is a good idea to change the phase inverter tube when you change power tubes,as this tube is actually part of the power section and not really part of the preamp,it gets hit with a fairly large signal and tends to wear out along with the power tubes.

[noah_swanson]

Thanks so much Stokes for helping me out! You saved me a lot of money and stress! also thanks for the new information!

[stokes]

No problem,Noah,glad I could help.

[Enzo]

GT added the R as a part identifier.

6L6-R would mean Russian

just like - and I am making these up -
6L6-C might mean China
6L6-Y might mean Yugoslavia

Magic parts does that, their JJ tubes have CZ on the end of the number to represent Czech Republic.