Changing voicing of an amp by valve change?

RobRoyMcCoy · Post by **RobRoyMcCoy** » Fri Sep 22, 2006 8:56 am

The JJ 6V6 is rated to 500V DC in pentode mode so can cope with the 470V HT in the Mesa comfortably. The lifespan of the tube is protected by running the tubes below 70% of maximum plate dissipation at idle as this prevents the tube from exceeding its maximum power limit under any conditions. Tube life may be extended somewhat by biasing towards 50% but you will lose some of that overdrive we guitarist love!

Rob

Post by **GrantB** » Fri Sep 22, 2006 9:21 am

sopachrga wrote:Just a note, as I noticed that the Laney LC50 has been mentioned a couple of times, I'm actually asking about a Laney VC50 Which has 2 more preamp valves, and it's a 2X12 combo....

I'm not sure if you were just using the LC as a reference or anything, I thought I should mention this.

grolschie: this is actually aimed to RobRoy... not you.

I concur the JJ83s will serve you well. But also try Tung Sol 12AX7's if you can find any. They also have a slight edge gain wise vs. standard's. I have some cryo treated ones in my AC15 and they perform well.

So, what power valves does the VC50 have? EL84? What soprt of tones are you getting from this? I keep hearing these good things about VC50's (I have a VC30 but they're a totally different beast - read modern iteration of British 30 watt combo). You can get "some" differences" by chaning power valves. For example, I have some NOS 6V6's and most of 'em sound very similar, but I have a set of JAN GE's with absolutely cook - the Deluxe breaks up earlier, sounds sweeter and is more fun in general. Trouble is all headroom goes out the window. Again, I find it hard to beat the JJ's in this amp. They are smooth sounding, and will accept my tinkering with voltages without any arguments.

While I remember, I bought some 5751's (Philips and GE) NOS. These are direct slot-ins for 12AX7's, but are supposed to have a lower gain rating. Well, to me they seem to have the same level of gain but boy do they sound better than the average 12AX7! More musical. I replaced my double deluxe pre's with these and got a tangible improvement. I also bought some NOS Sylvania 12AX7's but have been trying to find which amp these will work best in.

sopachrga · Post by **sopachrga** » Fri Sep 22, 2006 9:31 am

The power amp valves are currently some old unknown brand 5881's, my amp has a switchable bias to allow use of either EL34's or 5881's

As far as tones from it go, I'm not too sure yet, I have not had it all that long, and the preamp valves have been knackered since day 1. So once I call RobRoy, and get some JJ's, I'll be able to have a bit of a play with some power amp valves... I have some new MesaBoogie EL34's

sitting at home, so I might stick them in to compare them to my unknown 5881's.

B45-12 · Post by **B45-12** » Fri Sep 22, 2006 12:08 pm

RobRoyMcCoy wrote:The JJ 6V6 is rated to 500V DC in pentode mode so can cope with the 470V HT in the Mesa comfortably. The lifespan of the tube is protected by running the tubes below 70% of maximum plate dissipation at idle as this prevents the tube from exceeding its maximum power limit under any conditions. Tube life may be extended somewhat by biasing towards 50% but you will lose some of that overdrive we guitarist love!

Rob

Hi thanks for that - in other words there will not be sudden valve failure but it will have a shorter life as it's being operated nearer the top end of it's operating curve (which the conventional books used to say is not as linear but that's the distortion, as you say, we like).

Next question is in your experience does this 'shorter life' actually happen? and is it significant (say over 20%)? the books have allways said yes and I can understand that at times when it is operating it is at full/exceeding it's disspation limits and thus gets some heat damage BUT valves are pretty forgiving of overload in a lot of ways (like transformers) unlike say transistors which conk out at the drop of a hat sometimes.

I've never experimented myself but I've wondered if it was manufacturer self-protection that said mid curve level for operation - or possibly the hi fi niuts, who have different agendas (ultra linear broadband) than us.

RobRoyMcCoy · Post by **RobRoyMcCoy** » Fri Sep 22, 2006 3:43 pm

Hi There, no they are right tubes do have a finite life. It is based on the ability of the cathode to emit electrons and over time the metals involved lose their ability to meet the current needs of the circuit.

Where the lifespan of the tube is fundementally determinde is in the quality of design, materials used and the construction techniques. This is where NOS tube, in their day, had an advantage over modern tubes; there were so many made and the material costs were relatively lower than today (especially copper a the moment) that investments were made in quality and quality control that todays relatively modest industry cannot afford. However, in some cases the modern tubes have a more advanced design, like the JJ 6V6S. To my knowledge, no NOS 6V6GT tube was rated to 500V on the plate in pentode mode.

Rob

RobRoyMcCoy · Post by **RobRoyMcCoy** » Fri Sep 22, 2006 3:44 pm

PS I love that phrase "ultra linear broadband"

B45-12 · Post by **B45-12** » Fri Sep 22, 2006 10:00 pm

Yeah - I was thinking the hifi nuts wanted ultralinear operatrion over broad band of frequencies - we don't.

But I realise there is a finite life to tubes - I just wanted to know if in your experience that life is significantly reduced by operating in the upper part of the curve or not.

As for NOS, I see it more as a function of the fact most new ones were orignally made in China/Russia/3rd world as poor quality knockoffs of the western stuff since they depended on valve gear a lot later. If the copies are crappy then the original is going to sound good. They've improved over the years but they've still got a way top go yet.

RobRoyMcCoy · Post by **RobRoyMcCoy** » Fri Sep 22, 2006 10:37 pm

Yes by operating above 70% in class AB1 the tube will spend some of its time operating above its maximum dissipation spec and this will shorten the lifespan depending on the frequency and duration of the excursions beyond their spec.

Rob

B45-12 · Post by **B45-12** » Sat Sep 23, 2006 12:04 pm

Soi how often are you seeing the 'shorter lifespan tube' amps come back compared to normal. Say if the normal amps tubes last a year does this mod usually mean 6 months???

I appreciate it's an average and varying factors apply - just wanted to get a rough idea.

RobRoyMcCoy · Post by **RobRoyMcCoy** » Sat Sep 23, 2006 1:10 pm

Good question. I haven't seen a tube wear out in less than 18 months. It also varies by the level of usage. I reckon even frequently running your tubes really hard you should get at least a year out of them. Running at a conservative operating point and with moderate use then 10 years is do-able on a good tube.

Rob

B45-12 · Post by **B45-12** » Sun Sep 24, 2006 11:40 am

Cheers and thanks for that info - I'd expect to change my tubes about every 18months/2 years with medium/high use so in effect it would not affect what I'd see as 'normal' tube life.

Just did not want a mod that burnt them out like lightbulbs.

sopachrga · Post by **sopachrga** » Mon Sep 25, 2006 1:24 pm

Well, after a discussion with RobRoy on the phone before, I have ordered a new set of High gain graded ECC83S' for the over drive channel, and a new valve for the "Phase inverter" position.

So I'll let you all know how they go once they turn up, and I get a chance to try them out.

Thanks everyone.