Thoughts on dropping B+ in Ampeg V4 (another blasted V4!)

[g1]

And I thought you guys were talking about the increased heater current.
As to the plate current, it will be more at idle as you say (if biased at higher mA), but when delivering the same amount of power while playing, I don't think they will run hotter.
6L6GC and new production 7027's are rated as 30W tubes.

[Chuck H]

Chuck, I don't understand what you mean. JJ 6L6GC are 30W tubes. I thought old 7027s are listed as 35W, but the JJ 7027 is just a 6L6GC re-labeled, so shouldn't it also be 30W?

Well, you never said that's what was in there (the current project). You mentioned that the 7027's looked bad and that you were going to use used 6L6's. You did say you've been using 6L6's in YOUR amp. But YOUR amp has a lower plate voltage so I thought we were trying to find a solution for the current project.

I can't say about JJ tube specs other than what the data says and that I have had unfortunate circumstances happen well within margins. I always miss that "GC" thing and just consider typical big bottle guitar amp tubes (KT66,77 EL34, 5881 and plain ol 6l6) at 25W. So my bad on that (I still don't trust that max spec though )

Also if a 6550 is either 35W or 42W plate dissipation wouldn't a 6550 always draw more current than 6L6GC or current 7027, BECAUSE you need to adjust bias. You could bias a 6L6GC to 60% and throw in some 6550s and probably leave it idling around 30-40% and call it the same current draw but no one would do that because the tube is way underbiased. To be at the same operating point as far as idle dissipation you would have to increase the current draw, and since the tube is higher plate dissipation won't the current draw always be higher, since change in plate voltage when biasing is minimal.

Agreed. That sure seems to make sense. It does to me too, but I took a bit of a beating for it here in an argument about "power scaling" circuits. It was explained to me that a tubes max dis is physical/electrical limitation and absolutely not an operating parameter. If I were better with trace curve charts I wouldn't have gotten into that mess, but what it came down to was that I don't know enough finite details about tube operation. Ever notice that the higher output tubes always have a higher max Vp? To achieve the max watt spec you pretty much need to hit that voltage, if you dare. Conversely, if you run a pair of 6L6CG tubes at only 200Vp you won't bias them at 100mA of current ea. keeping to the 70% rule and even if you did you're never going to get 60 watts from that amp.

Not saying your're wrong obviously. I am just confused by your post.

I'm wrong often enough. I could be now too and I'm certain to get disagreed with, but I've had my experiences with JJ tubes seemingly failing because they don't meet spec. when the Ruby's do. So I was just offering my first hand caution on the matter. A proclamation such as "Well these tubes have a max plate voltage of X and a max dis of Y" is nice and tidy but it may or may not be reality. Just like getting run over in a crosswalk shouldn't have happened "Because they have to stop." For my experience, JJ is that bad driver.

[nsubulysses]

Oh yeah it's hard to see eye to eye when we're all talking about different stuff LOL

[Justin Thomas]

The JJ 7027 data sheet lists plateV/screenV/plate dissipation as 500/450/30W respectively. Actually, it lists the plate dissipation as 30V and shows that pins 1& 4 are both connected to the screen, but pin 1's legend says g3, so whonreally knows?

All these numbers lead me to conclusively believe they are a rebased 6L6GC and NOT a genuine 7027, although it's been pointed out that the original 6L6GC & 7027's plate curves were IDENTICAL! So, whatever.

I found all this out researching alternatives for unobtainium SED 6L6GCs. I let my usual tube vendor know of the hinkiness with the datasheet, as they may have more pull with JJ. JJ has also apparently issued a 5881, too... hope all this helps with some of the confusion!

I'll keep buying their tubes that I've had good experiences with, as anyone can make some typos. But I'll wait to try the 7027s or 5881s...

Justin

[g1]

The JJ 7027 data sheet lists plateV/screenV/plate dissipation as 500/450/30W respectively. Actually, it lists the plate dissipation as 30V and shows that pins 1& 4 are both connected to the screen, but pin 1's legend says g3, so whonreally knows?

The g3 is a typo, pins 1&4 are g2.
The JJ7027 is their JJ6L6GC with the extra pins wired. That's why they had to rewrite the max voltage & max wattage ratings to the 6L6 values. Notice the extra capacitance at Ca/g1 from having the extra wire hanging off g1.

[nsubulysses]

Alright got this retubed with some Tung Sol 6L6GC-STR from Jim McShane. I"m still on the fence of trying to drop B+. Now I know of a few methods but still wondering if it's just negligible to drop from it from ~560V to what seems more regular to me, 520-540V. Set now to 29mA at 555Vp, line voltage is 121VAC. I know G1 said that's not far off for these amps; I've just never seen one this high V myself.

[Randall]

Glad to see Jim McShane's name coming up. He has helped me in the past. One of the real good guys.

[nsubulysses]

Awesomely, when I emailed him saying I need tubes for a V4 with extra high B+ the first thing he said in his reply was to change 470 ohm screen grid resistors to 1K

[trobbins]

Were you able to confirm a 'shorted' power tube? The other common fault would likely be pin 2 to 3 flashover.

If a 10 ohm 5W anode resistor exploded, then that would indicate a current in excess of 1.5A. Was the associated tube ok? That would have to be a short to cathode, or to heater (did you replace the humdinger pot?).

Even with a lower B+, it sounds like some extra over-current protection would be a good idea. If the heater was involved then some cleaning of the base and maybe some heatshrink over the anode terminal.

[nsubulysses]

Yeah one was definitely shorted. I was too lazy to pull out the tube tester so I just put the amp on a light bulb limiter and insterted one power tube at a time until it lit the light.

Hum balance pot was destroyed and was replaced by two 100 ohm resistors to ground.

[bobshbob1]

For what it's worth, since it sounds like you've already re-tubed it, I went through two sets of Tung-Sol 6L6GC-STRs in about two months in an amp with higher-than-normal plate voltage (Fender 140, B+ ~525V). That amp has an ultralinear OT, though, so maybe not directly comparable to the V4 circuit.

In the end I was advised against using Tung-Sol tubes (even the STRs) in amps with B+ over 500V, and told that JJ and SED are more tolerant of high plate volts. I went with JJs and have had a set in it for over two years now with no problems. JM2CYMMV

[nsubulysses]

It's so hard to say because I have used primarily JJ 6L6GC in my V4 and in other ones I have serviced. So far no problems. In this case however the JJ 7027 bit the bullet. Jim McShane says he will not stock JJ becasue they are unreliable, and he and Tedmich say Tung Sols have a better chance of withstanding this high voltage compared to JJs......

Maybe there is no real answer......except 6550 or EL34!

[Jazz P Bass]

Sovtek 6L6 WXT tubes can take it.

[trobbins]

Taming the aspects of 'high voltage' that cause tube failures can be a way to proceed. There has so far been no definite confirmation of what the failure mode was.

Dumbing down B+ was the first option raised. Lowering idle power dissipation was an option, although that raises idle B+. Raising screen stopper was an option, as that goes to peak power dissipation, and screen protection. Another option is to suppress the highest peak voltage that plate experiences, which can be simply done with some RC loading, or better with some MOVs across the plate-to-CT windings of the OT.

[nsubulysses]

I must laugh. I didn't really know what to do because it's still unclear to me whether lowering B+ from ~560V to 520-540V range is a negligible drop anyway. I decided to test my luck and just crank the amp full volume for a few hours into a load, playing music from my phone, to see if it would last. It was fine for four hours so I felt ok releasing this as is.

Today I receive a V2 and give it a quick look because I'm curious. It idles around 17mA at 585Va !! It has JJ 7027 just like the other one.