Who is running EL34s in their V:90s?

[bandit2013]

I really liked the winged =c= EL34s I had in my first mark V 15ish years ago. Just running 6L6s in the one I just got at the moment though. Sounding really good so I'm not in a rush to experiment.

Out of curiosity... how serious is the manual update about ONLY using variac mode with EL34s? I had my old one before they updated the manual to say this. I never had any issues using regular power mode pretty heavily for the 2 or so years I had the amp. Anecdotally never heard of any issues from anyone else either (and =C= EL34s were a pretty popular pairing with the Mark V at the time as I recall)

This is from the online manual,

NOTE: When using the BIAS SELECT switch on the Rear Panel to accommodate the use of EL34 Power Tubes, ALWAYS switch to VARIAC POWER (switch down) on the main Front Panel POWER switch and operate the amplifier in this power mode until swapping back to 6L6 Power Tubes. This will ensure uninterrupted performance and avoid running the EL34s over their rated safe operating voltage.

 

[spawnofthesith]

This is from the online manual,

NOTE: When using the BIAS SELECT switch on the Rear Panel to accommodate the use of EL34 Power Tubes, ALWAYS switch to VARIAC POWER (switch down) on the main Front Panel POWER switch and operate the amplifier in this power mode until swapping back to 6L6 Power Tubes. This will ensure uninterrupted performance and avoid running the EL34s over their rated safe operating voltage.

Right, I was just curious if that was added to the manual as more of a "cover our asses" and proceed with caution because the tunes will wear quicker type of thing, or if it's seriously putting the amp at risk to run full power with 34s

 

[MarkV User]

Well it's good you have a fallback configuration.

TBH I am a bit puzzled with this only cause so many have a V and do not appear to have an issue with the presence (me included). For sure some folks don't find the V a good solution. for their needs. Totally get that. Now @bandit2013 gets a pass... as his V certainly exhibits other strange behavior issues, probably mistreated in it's adolescent years by Randall or Doug . BTW, I'm not disagreeing with what you hear or your modifications just curious as to why?

That said presence is one of the more defining features in an amp. Case in point my Stiletto, after adding the simple Depth Mod to the presence circuit it was a game changer. It provided the capability to enhance the bass response of the presence which allowed it to be much more frequency balanced. So I for one get how a change to that would be a big deal. TBH I wasn't really aware of how complicated Mesa had made the presence on the V anyway.

Hopefully I'll get a chance to do the el34 test on my V this week.

Ok, I have two different theories, not necessarily exclusive. First idea is that the presence thing is down to component selection. Either the spread on those caps in the feedback loop or the resonant frequency and inductance of the capacitors. It doesn’t even have to be a change of parts. Manufacturing batches could be different, etc. I never did analyze the circuit or simulate it or even measure the phase shifts carefully to validate the idea.

But I saw a big part of the problem go away when I removed all the switched options in the presence circuit. Now my amp looks very much like a Mark III presence circuit in all modes. But.. the problem can still happen.

Second theory is that my C90 speaker was bad and that was responsible for most of the problem. That speaker really was defective and it only came out clearly in testing. It had some nasty bright resonances starting at low power. The presence circuit feeds the negative speaker compliance error back and then cuts out part if that feedback to boost presence. But a weird enough speaker compliance error could boost instead of cut. When I put in the BN12-300S the amp became ear safe. By the time I changed it for the 300W EVM 12L I’d already gain stages the amp and at the moment of the speaker change that amp became a glorious thing. But the problem can still happen e.g. if the tube bias is too hot.

The real cause could be something different. It is possible to figure it out, but I think just installing bias adjustments will be my next move. That and restoring some of the Channel 3 gain. By the way… some of the problems are connected. My amp has the ringing problem worse when it is biased too hot. Bandit’s troubles also started with bias that was way too hot and was burning his tubes…

I have heard glorious Mark V amps on YouTube. I know they are out there. My amp never sounded like those but now it is something very different and also great. If it was easier to work on I would experiment with the presence circuit and fix it. But it is not worth the trouble and risk. Better to run 5881s, always in Variac mode, and experiment with the maintenance friendly amps.

 

[MarkV User]

Right, I was just curious if that was added to the manual as more of a "cover our asses" and proceed with caution because the tunes will wear quicker type of thing, or if it's seriously putting the amp at risk to run full power with 34s

I think it is a real concern because when I put in EL34s in Variac mode they seem to run a little hot. I didn’t measure the bias current but could if you were interested.

 

[bandit2013]

It could be a CYA thing, more so due to the reliability issues with EL34 being an issue. It is not the plate voltage that does them in, it is the screen voltage. Depends on the tubes. In a different post, I got this answer... I do recall reading something similar.


I'm sure the issue of EL34's not getting a vote of confidence from Boogie is because of the variety of "inferior" tubes available in today's market. The days of robust, high current, durable EL34's definitely are in the past. As a disclaimer, Boogie just wants to CYA should you want to run in the EL34 mode. The same issues occur in some vintage era Marshalls. This was one of the reasons they were shipping US (domestic) versions of their 100W amps in the early 70's with 6550's and not EL34's. The 6550's were tolerant of higher plate voltages and better at withstanding mechanical failures.

 

[woodbutcher65]

I happen to be fortunate enough to have a few Sylvania 6CA7s (STR-416 if Mesa branded) that are still quite good, healthy tubes. IF I were to put them in my Mk V, with these tubes I should not NEED to use the Variac power mode. Correct? Because they can handle the voltage!

 

[MarkV User]

I happen to be fortunate enough to have a few Sylvania 6CA7s (STR-416 if Mesa branded) that are still quite good, healthy tubes. IF I were to put them in my Mk V, with these tubes I should not NEED to use the Variac power mode. Correct? Because they can handle the voltage!

Don’t do it blind at least. You can get a tube bias tester that will interpose a connector and let you measure the bias current being applied to your tubes. Infer the range that is reasonable for that tube. I suspect one issue with Mark V amps may be that the actual bias voltage and current for a particular tube varies more than it should between amplifiers. That would go a long way to explaining the diversity of results people get. I’ve come to think it is a bit nuts to shoot blind the way we do with the Mesa amps. I want to add external bias test points and two different bias pots for the two pairs of tubes.

 

[woodbutcher65]

No worries. I'm an experienced amp tech and quite well equipped, with tube testers and everything needed to service, troubleshoot, and repair just about anything that uses electricity.

 

[MarkV User]

Oh, you are more experienced than I am then. You should be able to figure out how to test it safely if there is anything resembling a spec you can rely on or if you index the measured rails and bias on the Mark V against an amp contemporary with those tubes manufacturing date. I find it a bit confusing that all the old tube amps ran hotter than the absolute max ratings on the tube data sheets. But there is a yardstick at least.

 

[woodbutcher65]

No, the amps didn't exceed the tube max specs, IF you were using the RECOMMENDED tube types. If you can FIND the elusive PhilipS/ECG/Sylvania datasheets for the 7581A, 7027A, STR387, STR415, or 6L6GC tubes, you'll find that the amps made for them may push close to the max ratings but won't exceed them. That'd be a really bad engineering decision that would result in amps that can't be used, due to a lack of tubes that can survive in them.

However, there are certainly some tubes that you just can't run in your 80s Mesa Mark II amp. They won't last long, if they even survive long enough to check the bias on them.

Giving credit where it's due, Sovtek (Reflektor factory and its various brand names) have now been in business making power and preamp tubes for several continuous decades and they've learned how to make excellent reliable tubes. Sovteks today are a much better, more reliable tube with more consistent out of the box performance than they ever were in the 90s or 00s. But still, check their ratings and don't put them in amps that exceed them.

I'm hanging on to a very lightly used quad of STR415s specifically so I have tubes I can trust in a Mark II. At the moment I don't own a Mark II. But I will again.

 

[rarebitusa]

Well appreciated those who chimed in, finally had a chance to try the el34s out. Initial take is... I do like what they bring. Is it earth shattering... hmmm no, but it definitely does provide a nice el34 vibe to the amp, no doubt.

The sparkle on Ch 1 Fat Clean was pretty sweet. Ch 2 Crunch, which I run with tube rect had a nice bright snap to it. Ch 3 was also good with heavy rhythm but for single note work there is where to me the push of the 6L6s benefit. As expected they're less dense/fat/stiff with more sag vs the 6L6s (441s outer, 440s inner pair). It does give the V a different character and it's an easy swap. If I didn't have a Badlander and a Stiletto already leaving them in there may have been a serious consideration. But swapped back in the 6L6s for now. Later I'd like to tra some other cab/speaker arrangements, for this session it was the WB G12-65 1x12 at moderate volumes.

Gotta give it up for the V:90, in some ways it's an amp tweakers dream. Maybe not every feature it has is a clear winner but **** it's fun to screw around with if that kinda thing floats your boat.