Different rectifier tube in Mark V

[The Mink]

Hey folks. I took my Mark V to the shop after a total power failure and smoke smell, the whole deal. He thought it was going to be a specific diode issue that he’s seen in various Mark Vs, but it turned out to be an intermittent rectifier tube. He replaced it with a NOS Westinghouse 5U4GB and all is well. In fact the amp seems to sound better than ever… is that weird?

I’ve heard lots about rolling tubes in various positions, but what about the rectifier?

Anyway, that tube must have been on the edge for over a year, because I thought I was falling out of love with the amp but I think the tone was just gradually deteriorating.

Thoughts?

 

[domct203]

The Rectifier tube is used as a diode, so it either works or doesn’t work, they don’t (noticeably) slowly wear out like a power tube.

Sounds like yours took the fuse out when it failed.

If you spend all your time in 10W or 45W with the tube rectification selected a change in Rectifier tube can absolutely effect the feel and perceived response of the amp, but it wouldn’t be as dramatic as changing preamp tubes or power tubes. While the amp uses silicon diodes when in 90W mode, I believe the Rectifier tube is still in the circuit, but it will not influence the tone and feel as the silicon diodes are tracking much faster. A Rectifier tube failure in 90W will also take out the fuse.

Tone and response is very subjective, all that matters is that you are happy with your amp, and it sounds like you are.

Glad to hear it was a simple fix and you are up and running!

Dom

 

[bandit2013]

A bad rectifier tube can ruin your day. I have gone through many in the Mark V and oddly enough I don't run the 45W or 10W modes if at all. Rectifier tube will always be in the circuit regardless of what power mode you use. It is running in parallel to two silicon diodes when using 90W power or CH3 on 45W. CH1 and CH2 have the option to use the Rectifier or silicon diodes. 10W, you have no choice as it will be running the Rectifier as the class A circuit needs that voltage reduction.

You can remove it when using 90W power and it will still operate. (turn off the amp before removal). I would not recommend leaving it out though. If the tube shorts out which is usually the fault mode, it will blow the fuse due to the excess current drawn by the faulty rectifier tube. Sometimes the getter flash gets consumed and will loose vacuum and result in issues. I have had a few failures of the 5U4GB but never had any smoke to go with it. That usually tends to happen when you burn out the screen resistors. The ones on the class A circuits will fail first.

I have probably run my Roadster for many more hours to years without the need to replace any of the Rectifiers in that amp yet. I do make use of them too. Also, the same would go for the MWDR. Why the Mark V can be problematic with the 5U4GB is probably due to the quality of the tube itself.

 

[steve Rhenius01]

The Rectifier tube is used as a diode, so it either works or doesn’t work, they don’t (noticeably) slowly wear out like a power tube.

Sounds like yours took the fuse out when it failed.

If you spend all your time in 10W or 45W with the tube rectification selected a change in Rectifier tube can absolutely effect the feel and perceived response of the amp, but it wouldn’t be as dramatic as changing preamp tubes or power tubes. While the amp uses silicon diodes when in 90W mode, I believe the Rectifier tube is still in the circuit, but it will not influence the tone and feel as the silicon diodes are tracking much faster. A Rectifier tube failure in 90W will also take out the fuse.

Tone and response is very subjective, all that matters is that you are happy with your amp, and it sounds like you are.

Glad to hear it was a simple fix and you are up and running!

Dom

I’m wondering about using a GZ34 as a sub. Will its heater current draw be too much for the mains transformer? Otherwise it seems a better tube.

 

[tbonesullivan]

I’m wondering about using a GZ34 as a sub. Will its heater current draw be too much for the mains transformer? Otherwise it seems a better tube.

"Better" depends on what you are looking for in a rectifier tube. The GZ-34 has a 1.9A current draw at 5V for the filament, while the 5U4G has 3.0A current draw at 5V. So, definitely not going to run into issues there. The max current for the GZ-34 is 250mA DC, while the 5U4GB is 275mA, so I don't know of that is an issue.

Now, the GZ-34 is the least "squishy" of the Rectifier tubes. It has the least voltage drop at load of all the rectifier tubes, at 10 volts. The 5U4GB has a 50 volt drop. This is why mesa uses the 5U4GB rectifiers: maximum squish.

 

[steve Rhenius01]

Thanks, that’s useful and understandable. Would not wish to upset the careful planning implicit in the design.

 

[domct203]

Thanks, that’s useful and understandable. Would not wish to upset the careful planning implicit in the design.

Changing rectifier tubes will have an effect on how the amp feels. Yes the GZ34 has less sag than the 5U4G, but it’s still sagging a bit compared to silicon diodes.

If you find the amp a bit too loose using the 5U4G but too stiff with silicon diodes, the GZ34 could be a good compromise.

With that said, if the 5U4G has been delivering the goods don’t fix what ain’t broken.

Dom