Mark III Red Stripe Simul-Class Power Tubes (& Help)

Hi there!

Many a time have I used this forum as a guide for various things concerning my beloved M/B Mark III Red Stripe head, but never have I registered here to actually post something. I am doing now, because I am in need of help.

My red stripe head is an American model, but because I live in Europe I have always used a transformer to get the riht voltage into the amp. Having owned the amp for well over 4 years, after the previous owner got it from the USA, the amp never failed me until a couple of months ago. The amp stopped working and the transformer within the amp was blown. I took this opportunity to change the transformer to a European model.

I changed the transformer, took the opportunity to add a tweed-mod (since the EU-Transformer had the right output) and eventually found out the culprit; a blown capacitor in the bias circuit. This screwed up the bias, overcharging the power tubes and eventually wearing out the original transformer. At least, that's what I figured out. I replaced every part that I thought is damaged, but my current power tubes are probably screwed up as well. They red-plate and still overcharge. So; I want to change them. The problem is, I have no idea what kind of power tubes I need, except that I need two pairs of two (for class A and B). Can anyone help me, or recommend me, some good tubes for my Boogie? The bias has been unmodified and is operating as a factory-standard Mark III will have done.

Thank you in advance!
Kind regards,

Richard

A set of 4 Winged =C= 6L6 to Boogie spec can be got from Watford Valves in the UK. I have a set of those in my Mark III and they are very nice sounding.

I can run a pair of EL34s in the outer sockets in my Mark III and I just use Boogie branded ones when I do but right now it a full quartet of =C= 6L6s

I am not sure if the tweed mod is significant and contributing to issues. I've not done that one.

Any set of boogie branded power valves should be fine too.

Thanks for the reply! I was also looking into placing different tubes in the inner and outer sockets, but the fixed bias can be a problem, right?

I think I need to for certain first that my current tubes reslly are broken. It seems like it from what I'm measuring and from the fact that they red-plate while the amp's parts are all set up correctly. Is there a different way to find out, before spending £170/€200 on new tubes?

Add to that that these Winged C tubes are almost twice as expensive as other 6L6s, I don't really know what to choose.

Does the redplating valve change when you move them around or if you are in Class A mode vs Simulclass. You can run pairs of tubes in the outer sockets to see if one is bad.

The MKIII manual has more notes on troubleshooting tube issues.

http://mesaboogie.com/media/User%20Manuals/Mark%20III.pdf

The Mark III simulclass (or my Puple stripe st least) is designed to allow swapping of EL34 & 6L6 in the OUTER pair. All Mesa spec valves of either type meet the bias (cold) setting of their amps. They do recommend EL34 in the outer pair but I've never had an issue with 6L6s in there.

After that I'd be chasing Mesa support which is excellent.

I had read that, so I'm fairly convinced it's a bad tube indeed, thanks.

This thing with inner/outer tubes for EL34s is also a little confusing, since different sources cite different things. However I have found that most recently people are saying only the outer tubes should be swapped.

Now there's one other thing that's suddenly confusing me; apparently I have a long head, with the Full/Half Power switch on the front of the amp instead of the back. Should this make any difference whatsoever?

I have read that some amps are incorrectly labeled and according to the schematics I have a Simul-Class with a different switch.

RichardS said:

I had read that, so I'm fairly convinced it's a bad tube indeed, thanks.
I have read that some amps are incorrectly labeled and according to the schematics I have a Simul-Class with a different switch.

Click to expand...

Scrap that. Of course it's a Simul-Class, it has a sticker on the back that says so :lol:

Only the OUTER pair of valves can be swapped to EL34s and only then in definitive Simul-class models - not the ones that say half power and cut a pair of valves. I've not read anything to suggest you can ever swap the middle ones from 6L6 without more mods but I could be mistaken.

I might send the serial number to Mesa Boogie and see if they can confirm everything.

If in any doubt just run all 6L6 (Mesa branded) which should be on spec for all MK III models. The tone difference in Class A mode is minor as most of the dirt in boogies is from the preamp tubes.

Redplating could well indicate a bias supply problem still exists.

I'd be pulling chassis and measuring what voltages exist WITHOUT tubes (inc ripple),before risking further damage to tubes or circuit.

If you've done this and confirm all supply circuitry ( bias, B+, heater ), then I'd add tubes, bring up on variac and watch voltages for anomalies.

regs, Dave

I actually did that and there were no anomalies. My gut feeling says the tube is already broken and red-plating because of that. I bought some new tubes and intend to install them next Saturday. Of course I'll check the voltages of the circuit board, as well as test the tubes before-hand the best way I can.

Thanks for your reply!

How did it go Richard ?

Behave itself and acted in a mature fashion ? lol.. regs, Dave

Oh, I never followed up ln this thread here!

It worked out splendidly! I installed a combination of EL34s en 6L6s and the amp is running completely fine again. In fact, I have a slight feeling that it's actually sounding better than before I worked on it!!

All-in-all I installed a new (European) transformer (instead of the original USA one), applied the tweed mod, cleaned up the reverb with a simple mod, fixed the bias and installed new tubes. So that's quite a lot of work, but I'm very glas that K did it and got the beast running again.

Thanks for the help you provided!

Teriffic !!

Another life saved eh??

Gr8 news re work done too. Agreed fair bit of tinkering there, but if it make yr ears happy then time and effort are immaterial..

Dave

:mrgreen: