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 Post subject: Valve tolerances.
PostPosted: Mon Jan 11, 2016 12:14 pm 
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Mark III

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Hello.
I'm thinking of trying some EL34's in my Mark V. However I'm balking at the idea of £100 on valves as an experiment. I may not like the sound of EL34's in this amp. Which is a little outlandish for me as I tend to favour them. I've a 2203KK that I switched from KT88's to EL34's because I didn't like the excessive low coupled with the hollow sort of low mid sound that kinkless tetrode valves tend to give.
I fear I may be hearing the same hollowness and excessive low (as compensation) in 6l6's.
Further more, I do like the 6l6 sound on clean sounds, which I don't use much.

I'm wondering, I've two 2203's. The KK and a JMP. Both have fairly new valves in them. The 6l6's in my Mark V are rated as green. I don't really understand Mesa's grading system, or what it means in comparison to normal valve ratings.
What I'm considering is measuring the plate voltage of the Mark V with the valves out and switched to EL34 mode. If that is comparable to the plate voltage of my Marshall's (476v and 482v). Bung em in, give em a listen. If I like em, go buy some green rated Mesa EL34's.

Does anyone see a danger with this, anything I'm missing? I'm OK at modding amps, working in chassis. I'm not an electronic guru, I may have missed something.

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 Post subject: Re: Valve tolerances.
PostPosted: Mon Jan 11, 2016 1:07 pm 
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The Mesa rating system is not rated for hot vs. cold. They keep it deliberately mysterious. Even if it were a tone thing, I wouldn't compare a rating for EL34 to a rating for 6L6.

First, I would not put tubes from other amps into a Mesa without checking bias. They may be fine, or they may not. Mesas need a pretty tight tolerance and tubes vary a LOT.

Second, you should be able to get some lightly used Mesa (or other brand that were sold as compatible with Mesa) EL34 for pretty cheap.

I'm probably over-conservative, but you wouldn't want to red-plate the tubes. Tough on the amp.

You could probably find someone in your area with a Mesa that would loan you a set of tubes to try.

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 Post subject: Re: Valve tolerances.
PostPosted: Mon Jan 11, 2016 1:33 pm 
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Mark III

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Elvis
sadly not mate. I live in the back end of nowhere England.
So you, with experience advise not to do it. Mesa needing tightly matched valves, is that right?
Guess I'll just bite the bullet. There's a shop 30 miles away that stocks Mesa valves and tests valves. Luckily it's one of 4 valve specialist places in the UK. So I could probably take a couple of my 6l6's in for them to get a reading from.

Thank you for saving me from blowing resistors.
:P

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 Post subject: Re: Valve tolerances.
PostPosted: Mon Jan 11, 2016 2:55 pm 
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I don't think they need a super tight match as much as the fixed-bias means the bias-point has to be spot-on.

You might also try one of the reputable non-Mesa dealers who can sell tubes spec'd for Mesa amps. Might save some pounds there.

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 Post subject: Re: Valve tolerances.
PostPosted: Mon Jan 11, 2016 6:48 pm 
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Mark III

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Good point, thank you sir. It's always worth asking isn't it.

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 Post subject: Re: Valve tolerances.
PostPosted: Mon Jan 11, 2016 8:16 pm 
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Mark IV
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EL34's in the MK5 - I'd look for the cooler running tubes, and don't forget to run the amp on the Tweed/Variac setting. Thanks!

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 Post subject: Re: Valve tolerances.
PostPosted: Mon Jan 11, 2016 8:49 pm 
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Mark III

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Thank you very much Authorised Boogie.

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 Post subject: Re: Valve tolerances.
PostPosted: Fri Feb 12, 2016 5:27 am 
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Mark III

Joined: Sun Sep 27, 2015 2:50 pm
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Hi authorised boogie, sorry for my ignorance but i run el34's in full power mode in my mark v. Am i risking damage or working my amp hard at all if not running in variac mode?

Thanks for any advice.

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 Post subject: Re: Valve tolerances.
PostPosted: Fri Feb 12, 2016 2:03 pm 
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I believe it's worse for the tubes than the amp to run full plate voltage.

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 Post subject: Re: Valve tolerances.
PostPosted: Sat Feb 13, 2016 5:13 am 
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Mark III

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elvis wrote:
I believe it's worse for the tubes than the amp to run full plate voltage.


Sorry to pester, but please elaborate.

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 Post subject: Re: Valve tolerances.
PostPosted: Sat Feb 13, 2016 8:37 am 
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Wayno wrote:
elvis wrote:
I believe it's worse for the tubes than the amp to run full plate voltage.


Sorry to pester, but please elaborate.

It is bad practice to put "unknown rating" output tubes (valves) into an amp without someway to measure their idle plate dissipation (Plate Current). This is asking for trouble.

Also, B+ (plate voltage) should always be measured under load (tubes in). The reading will be abnormally higher without a load on it.

You must never exceed the tube's maximum plate dissipation (rated in Watts) or damage to the tube and or amp will occur. The general rule of thumb is to remain under 70% at idle.

For a 25W tube, that 70% is 17.5W. If the B+ is 470V, you should never exceed a Cathode Current of 37mA (470V * 0.037A = 17.4W) or you risk overheating (red-plating) and destroying the tube. When a tube fails like this it will usually also take out components in the amp (plate resistors etc).

And above all, be safe. Your amp will store that 470 B+ voltage in it's power capacitors, even with the amp off and unplugged. Unknowingly poking around inside a tube amp can release that voltage, which is enough to stop your heart if the voltage travels through you to ground. A wise old tech once told me to always keep my other hand in my back pocket so as not to be (unaware of) holding onto a source of ground, creating a pathway for voltage to travel across your chest if you get shocked.

HTH,
Dom

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 Post subject: Re: Valve tolerances.
PostPosted: Sat Feb 13, 2016 3:27 pm 
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Mark III

Joined: Sun Sep 27, 2015 2:50 pm
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Location: Good ole England
My el34's are ( supposedly according to the shop i bought them from anyhoo ) spec'd for the mark V, im just a little concerned about what authorised boogie posted in reply to nicklotsaguitars about being sure to run in variac/tweed when using el34's???

Does it apply to all el34's spec'd or not?

Does running in variac/tweed power mode increase tube life, and does it run the tubes a bit cooler, ie, less chance of red plating?

Or did they just mean it as a precautionary measure when running possibly out of spec tubes?

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 Post subject: Re: Valve tolerances.
PostPosted: Sat Feb 13, 2016 4:25 pm 
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Tweed mode lowers all voltages in the amp, reducing stress (lowering cathode current) on the output tubes in the process.

It's really hard to comment on if it is always needed without actually measuring the cathode current on the class A sockets.

I will say this, I had some JJ E34L's that were fine (Mesa spec'ed) in my Roadster, but way too hot in the class A sockets of my MKV in full power.

Without proper bias measurements it is just guesswork.

Dom

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 Post subject: Re: Valve tolerances.
PostPosted: Sun Feb 14, 2016 6:14 am 
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Mark III

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domct203 wrote:
Tweed mode lowers all voltages in the amp, reducing stress (lowering cathode current) on the output tubes in the process.

It's really hard to comment on if it is always needed without actually measuring the cathode current on the class A sockets.

I will say this, I had some JJ E34L's that were fine (Mesa spec'ed) in my Roadster, but way too hot in the class A sockets of my MKV in full power.

Without proper bias measurements it is just guesswork.

Dom


Thank you, think i might try adjusting my settings to work with variac if its easier on the tubes, also being in the arse end of england decent tubes can get pretty pricey out here so anything to extend the life of them is worth considering. I know the Mark V is biased a little hotter then usual for mesa. Is the roadster biased typically cold like most mesa's? Would explain why yours didnt sit right in the Mark V if they where spec 'd for the roadster specifically???

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 Post subject: Re: Valve tolerances.
PostPosted: Mon Feb 15, 2016 5:31 am 
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I asked the vendor to supply tubes to be the same as a Mesa "GRN" or Groove Tubes 4-5 set. Should have been fine in any Mesa, but as you already know the MKV class A sockets are biased hot, much hotter than anything up to that point in their offerings.

Dom

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