Mesa Boogie 12AX7 Valve problems

[rumblefish]

Hi Basschat ,

A couple of years ago I borrowed a friends Mesa Boogie MPulse 600 bass amp and was so impressed with the EQ and the power that I purchased one on Basschat a few weeks later.The amp has served me well but on my first gig at the start of this year the amp lost all volume and I ended up utilising the keyboard players set up to get through the gig.

I have a background in electronics and obtained all the relevant circuit diagrams for the M Pulse and studied them in detail.My initial conclusion was that the 12AX7 valves in the V1 and V2 positions required replacement.I ordered some Tung Sol 12AX7 valves and fitted them but still the amp only produced a low level of sound.

I then contacted one of the top amp techs on the south coast and he checked the amp out out only to advise that both of the Tung Sol valves were faulty.These valves were replaced which he supplied and the amp was then operating all OK. On the next gig the amp failed in the same way again and I returned the amp to the amp tech who advised me that the valve in the V2 (cathode follower) had failed again.

To cut a long story short after some checking on the internet and contacting Mesa Boogie Service I have found that although you can buy a 12AX7 valve at a number of price points (£8 to £50),it appears that only the proper Mesa Boogie spec valves will operate in the V2 cathode follower position.

The moral of this story is that with Mesa Boogie amps it is best only to buy only Mesa Boogie sourced valves or you can compound amp trouble shooting even more.

Has anyone else in basschat land had this problem in the past with Mesa Boogie amplifiers???

[Beer of the Bass]

I'm not too familar with the Mesa, but there is a known issue with some cathode follower circuits (not specifically in Mesa amps) where the voltage between the heater and cathode is higher than some 12ax7/ecc83 types can tolerate. The Tung Sol has a reputation for failing in cathode followers in other amps, as do most of the other current production Russian valves. It may be that if you can check the data sheets to find a 12ax7 with a higher maximum heater-cathode voltage than the Tung Sol you'll be OK, though buying from Mesa does at least save you the hassle of searching those out.
The JJ ECC83s is one that has worked OK for me in the cathode follower position of another amp, FWIW.

Edited April 13, 2017 by Beer of the Bass

[rumblefish]

Hi Beer of Bass,

thx for the info,I wish I had known about this problem as the supplier (one of the top valve suppliers in the UK) of the Tung Sols had no idea that there had ever been a problem with utilising a Tung Sol as a the cathode follower. The MB M Pulse 600 uses an HT of +390v and 12v on the heater so I guess that is where the problem occurs with the Tung Sol Cathode to heater max voltage being 180v according to the data sheet.

[Beer of the Bass]

Some US sellers of the Tung Sol caution against using it in cathode follower applications, so I'm surprised that hasn't filtered through to the UK suppliers. In a typical cathode follower (whether of the AC or DC coupled type), the cathode will be at a lower voltage than the HT voltage, though still potentially high enough to cause issues. Anecdotally, the ones to avoid are Russian made valves with a spiral filament, most of which come from the same factory despite the different names on them. That leaves JJ from Slovakia, the Chinese manufacturers or older European or American valves.

[MoonBassAlpha]

That sounds like a poor piece of design. And a good way to tie users in to buying their valves only

[Billy Apple]

I don't understand any of this tec stuff, but have had JJ's fitted in 2 amps with no problems.

[molan]

I always use;

https://www.ampvalves.co.uk

Great service and Stu is a top valve amp repair guy.

I'm sure he could help when choosing valves for a Mesa.

[Steve Browning]

Rob. I have 3 spare Mesa 12ax7's. You are very welcome to one.

Of course I could have just texted that to you but this way everyone can see what a great bloke I am. :-)

[andrew_actium]

That is a most peculiar fault on the 12AX7 and if the HT is indeed 390V then that is not too great. I've designed a few valve amps and tend to have the voltages down at 240V max on the ht. As I have an M-Pulse 600 I will check this and modify the ht voltage if necessary which is a very easy thing to do. 

I have had one fault with mine in the past which was embarrassing as it was a paid bass dep job I was doing and ended up going through the bands PA.

The fault was pretty fundamental as it was in the power supply relay cct which switches in after the valve heaters have come up. There is a capacitor after one of the voltage regulators in this section which is underrated on working voltage and failed and in doing so blew the regulator too.

Bearing in mind these amps were pretty expensive cost cutting on a 30 pence component seems stupid.

All updated now so won't happen again.

These amps are otherwise very well made and really easy to fix, I have a D800 aswell which wouldnt be as easy if the PSU or Class D amp went.

The tone ccts for you electronics guys are opamp gyrators which is a nice touch.

 

Edited August 16, 2020 by andrew_actium

[rumblefish]

Hi Andrew

Thx ,some interesting information regarding the fault on the 12AX7 with the MPulse 600.

Regarding your your fault: 

" The fault was pretty fundamental as it was in the power supply relay cct which switches in after the valve heaters have come up. There is a capacitor after one of the voltage regulators in this section which is underrated on working voltage and failed and in doing so blew the regulator too."

Last year whilst setting up for  gig at the Half Moon Putney my M Pulse 600 had the same fault,luckily the bass player in the other band let me use his MB 400+.

Now always bring a class D as a backup although I still love the sound of the MPulse.

 

[thodrik]

Any decent 12AX7 should do the job in term of preamp tubes. Of course, different types of valves operate differently. 

My Big Block 750 currently has JJs. Sounds fantastic. Mesa tubes are really good though. My Walkabout has only needed two tube changes in 10 years of use. It sounds like the M-Pulse 600 has a very different circuit though which might make it a more complicated amp than the Big Block or Walkabout.

I think that the 'buy Mesa tubes for Mesa amps' is more for their all valve amps. The Mesa amps are fixed bias and the Mesa tubes are specifically designed to work according to that bias setting. This makes Mesa amps very easy to fix when a power tube fails.Rather than having to get a tech to install and rebias the tubes, you can just remove the old Mesa tubes and install the new ones. Of course you could use non-Mesa tubes but you would probably need to get a tech to recommend a set that will work.