Fender Ultralinear SR 6v6 mod

[Gobind Khorana]

I just kicked 6v6's in my SR ultralinear. Sounds really good .... question is what will happen if I continue playing it like this? Would it burn down something ... ? Using 8 ohm load right now.

[Chuck H]

The voltages are likely too high for practical safety margins. That doesn't mean it won't continue working. Modern 6v6's are often of unknown specs. But there may be an issue with stresses. Especially screen voltages.

[Justin Thomas]

I'd use JJs and hope for the best. Bias reasonably!

Justin

[Richard]

If the tubes are run too hard what is the likely failure mode? What damage could it do the amp itself?

[Chuck H]

If the tubes are run too hard what is the likely failure mode? What damage could it do the amp itself?

Well... The most likely failure would probably be a power tube short. In that amp, then, it could take out some peripheral circuits and/or components. Which is no biggie. Worst case scenario would be the OT. A lot of people believe the OT will be protected by the fuse in the event of a power tube short. It often is, but not always. So it might be a good idea to add a fuse to the B+ line and see that the bias is conservative as Justin noted. Also, That amp is old. See that ALL the electrolytic caps are operating as they should. In fact I would suggest straight up replacing the bias circuit electrolytics as these are often overlooked in recaps anyway and, if they fail, can cause the most collateral damage.

[Gobind Khorana]

My Ultralinear has solid state recifyer ... I don't play it in insane volumes ... But sound is somehow so much better with 6v6 ... it really smoothes out of some harshness of a thing ...Im not too worried about transformer though, I already decided if it will blow .. Im going to modify it to 6v6 es anyway ... Also it takes FX much better right now. Smoothes out highs, High mids nicely .... Ofcourse payng 100$ after blowing a OT will not be fun @ all, but temptation is very big to continue playing like this right now and I have no reasonable tech in reach who would measure thing all over. I have never blown OT so far and have been playing 30 years .... Also no blown speaker is on my account. I even have Marshall 2203 MK2 MASTER LEAD (100W) ... that I play sometimes with 25W speaker (Jensen p10r!) or 1 solitary greenback and never blown em either, just being carful. Some person would never do this ... I have been playing this Marshall with 2 output tubes for a while ... NO Problems ... So would I let u know after 2 weeks if im still alive and havent been exploding with my practice room ? ... Ofcourse would be good if I would be able to do secure the situation in my abilities .... I can solder. To put 1 extra fuse would be something I probably would manage myself. I could also probably go to tech, but they cahrge a hell and they would like to do proper job in mods, not just make something u can get away with (maybe) ... well, if I will not blow it in few weeks , then I have to take a good look for proper MOD. Im also little bit curious just for a sake of experiment, that could it be run with 6v6 es without doing anything to it

[Chuck H]

The bottom line is that amp has nearly 500V on the plates and about the same on the screens of the power tubes. +6v6=bad. The bias isn't adjustable, but rather only balance-able and almost certainly inappropriate for the 6v6 tubes. And the amp likely has 40yo electrolytic capacitors. Simply plugging power tubes into an amp with no measuring or adjustments is a bad idea. Doing it with differently designated and under rated tubes is worse. Doing this with an old amp that is likely in need of service (even if it doesn't show signs) is about as bad as it gets. The truth is that there's stink all over this. A proper tech could probably pull it off by idealizing some particulars and testing operation. Sans that, I'd recommend against this.

Just because you haven't suffered a failure due to gear abuse should not be taken as an endorsement to continue. Just as some people have actually fallen out of airplanes and lived doesn't mean you should expect to survive if that happens to you (don't try this at home).

[Gobind Khorana]

Ok. I found the reasonable guy and he made full service to my amp measuring all the plates. We brought down plate to 480V and it's running on JJ 6v6 now ... it was 530V actually. All together took around hour. So this mod can be easily done. Now I don't have answer, what would have happened if I just continued playing the amp without checking ,.... but I can play it few days without any paranoid feelings and see if I like it "New" way better, or it's just a temporary mood.

[g1]

Now I don't have answer, what would have happened if I just continued playing the amp without checking

With the lower current of the 6V6's, the 530V would have climbed even higher with them in. I think any 6V6's would have been very short lived when subjected to that kind of voltage.
The cost of that would have soon equaled what you paid to do it right, and you may have saved some caps as well (depending on what their voltage rating was).

[Chuck H]

True that. I figured the voltages as the amp was would have been higher. but not having the facts I just defaulted to the schematic figures.

What I'd like to know... How did the guy knock 50V off the plate supply? I don't think most techs keep components on hand to do that sort of thing, so I'd be interested.

Just the same, 480V is still very high. Higher than any sane tech would endorse I think. But way better than 530V. I don't know if there are any problems doing this, but the addition of screen resistors, rather than having the screens straight on the UL taps, would probably improve the still questionable circumstances a little more.

[Gobind Khorana]

I don't know, but I will ask. He does this already 40 years (old guy), was pretty confident. He was also pretty confident that JJ's will handle that ... what I also have read from other forums that people have been running 6v6 24h under 500V without failure. As much as I can tell for now. First I was very impressed with the change. But this might be just because I was bored from the sound of amp. I will play it few more days like this, BUT it started to look for me that does not matter that I like this sound now and amp works, it is still little bit making cat out of dog thing. It was powerful ultra clean amp, which was too heavy and loud for giging. Now it's still as heavy as it was, but not half as loud. It is not by far DR either. Something in between thouse 2. As stated the sound is really OK.It would not open up like small W amps but is much smoother in this area. Also I started to think that maybe this 530V what was feeding 2 6L6'es is still a better sounding thing than 480V that feeds 6v6 ses .... It's just the fact that u will realize what the thing really is in few weeks constant playing. I have had a guitar PU's that I really liked , when I putted them on but after a month playing I realized that the old ones where more my sound ... Ok. At least I have the amp serviced right now and I can go back to 6L6's if I want. I will look pretty stupid when, I after 2 weeks go back with tha same amp and say "reverse please", ... I have to try it also with different speakers to see, how universal it sounds right now. This will be little complicated to ask the tech how he brought down voltage, because I am in Berlin and he is German who does not speak english well. And I would not understand German too as good, for detailed procedure description ... Well ... anyway the truth will come in a week, if I will keep the amp New way or reverse it back. Bottom line still is that, why to make teaspoon from excavator. But it still takes 6L6 as it is right now. SO maybe its easily reverseble fro 1 thing to another and that is the point. Because the voltage is down, just kick in 6L6 ... go back to 4 ohm and there u have it reverse. Well well ... and Im not the guy who mods the amps to play first 20sec of stairway to heaven. I compose original music ( mostly experimental) ... here my last record with band where I have been also producer/composer. And I don't post it here because I want to promote my record/music ... Just because there are lot's of people out there who would endlessly modify their gear and then make extremely stupid music OR even worse ... play solos or intros of famous songs. Other guys would do great stuff with what so ever they get theire hands on , even transistor amps & it sounds awesome. That is something not to forget about, that @ the end music is important. One time there was situation in a studio where left main monitor blew (Not in my studio). I instlanty used situation and re-amped some guitar tracks through the blown speaker and some of it made it to the final mixes . https://soundcloud.com/rainerjancis/...ter-full-album

[Chuck H]

Music is art. And every artist should be allowed their process! Be it just plugging into anything and discovering it's musical qualities or obsessing over the tone in your head. Obviously one is easier, but that's not the point.

Any sound difference in the amp may be a consequence of HOW the voltage was dropped. FWIW, newer small bottle tubes (6v6's, el84's) seem to handle high voltages better than the vintage tubes. Not so with new big bottle tubes though (6l6's, el34's). The big bottles don't seem to take the higher voltages anymore. I know they're still rated with higher max voltages on the spec sheets, but I've blown up enough of them to know better by now. If the method used to drop the voltage is primarily responsible for the change in the sound you may want to try to find another way to drop volts that won't affect the sound as much. Because the lower voltage you have now is definitely more appropriate for new 6l6's too. I haven't said that 6v6's and 6l6's won't operate at 530 plate volts, but don't ever crank them into overdrive. Just don't do it. Don't overdrive the 6v6's at 480 volts either. 6l6's will clip in overdrive just fine at 480 volts though.

JM2C

[Gobind Khorana]

NOW. I played whole day this amp and I have impression, that together with voltage dropped, the sound became more flat and with less dynamic headroom. Is it possible ? I have a feeling that it actually had warmer tone before voltage drop. Kind of I feel that higher voltage on plates was driving tubes hotter what actually ended up in wider dynamic range and more sensitivity. Maybe it's just an imagination but I can kind of hear more the flatness of a solid state-recifier it has = kills dynamics. With 6v6's and more limited clean headroom it's even more obvious. Yeah, it really seems that I liked it more when the voltage was almost killing em. Now it's kind of cold in a way. Some transistor like character is noticeable somewhere. Probably this is the thing of a Ultralinears and it just sticks more in my ear, because I pay close attention .

[The Dude]

I'm going to guess that all of this tube swapping and voltage lowering did not involve any changes to bias or bias supply? Unless modified, I don't think these have adjustable bias. You can't (or at least shouldn't) just take an amp designed for 6L6's, install 6V6's, and not expect to have to make some changes to bias or the bias circuit/supply. I suspect, as far as what you are describing tonally, that the difference you hear is not as much to do with plate supply as it is bias. I'll also echo what others have said here. It's unwise to run tubes much over their rated specs. They have specs for a reason. I suppose a light bulb would be brighter at 150V also.

[Chuck H]

What Dude said. To get it done, with a side order of gabbing, in an hour I'm guessing your tech guy looked at the bias and figured it would be safe, but didn't make an adjustment to the actual bias voltage. That and...

I'll also guess that he used a big honkin' resistor to drop the volts. That would be the "flatter" sound you hear. Somewhat compressed. More current, like you would have on pick attack, would create a larger voltage loss across a resistor. Less current, like when a note is decaying, would result in less voltage loss across the resistor. Since higher voltages generally mean more power and higher fidelity, and conversely with lower voltages, the end result would be a loss of dynamics. The amp will sound more like the attack and the decay are at equal power. Slightly more implied sustain, but with a lot less punch. This is why I asked about how the voltage was dropped.

480 volts is PLENTY for 6v6's or 6l6's to generate huge, punchy and dynamic tone. If the voltage was dropped with a resistor it might be better to change that to a method that doesn't produce the same voltage sag on dynamics. But absolutely do keep the voltage drop. It won't be a problem for the tone you're after provided the method doesn't compress the dynamics.

JM2C (actual value $.02)