5 watt unequal wear??

[EtherealWidow]

Don't tubes drift over time? I think it would depend on how long you've had the amp. Also, maybe there is in fact unequal wear since in 5 watt one tube is balancing out the OT.

 

[Don]

The power tubes in my Express are very closely matched. I've had them in my Super Reverb which I set the bias on after installing them.

 

[J.J]

Just managed to hook my 5:25 to my CompuBias. Since I usually stick to the 5W mode I never bothered to test the darn thing. For one thing it turned out that the so-called perfectly matched Mesa tubes are not matched at all. You'll see below:

Readings in 30W mode :

Left - plate 339V, Ik 31, W10.5
Right - plate 337, Ik 23, W7.7

Readings in 5W mode:

Left - plate 151V, Ik 70, W10.5
Right - plate 330, Ik 0, W0

Hi MCK, my measurements were quite a bit different from yours. Even after 11 months of playing everyday my tubes measured within a few ma of each other. I also used 5 watt mode at leat 30% of the time. My brand new set of Mesa tubes were also well matched.

In 5 watt mode, my left tube had nearly 300V with Ik about 35ma. How old are your tubes ?

 

[MCK]

Hi JJ,

I bought amp used from US so I don't know how many hours/years these tubes have. I probably will not bother changing them either since in 5W mode all is working well. Actually even in the 30W mode there is no discernible trouble. The amp always had a little more than average hum so perhaps that can be attributed to the tube mismatch.

Regarding the other post earlier on about tubes drifting, I would tend to agree with you that the drift should not be this much. In my case there is about 30% difference which is a lot. Few mA would be ok but hey. Nothing is perfect. I like this amp as an all around / go get it type amp in my living room / practice area so I'm ok with it as is but will try some other EL84s to see if hum improves.

By the way, your V = 300 surprises me. I am running the amp on 100V while it was designed to run on 115V since it is a US model. So if anything I would antivipate my plate voltages to be lower. Will hook it up to a step up transformer and measure again to see if anything changes.

Cheers

 

[J.J]

Ok, yes 100 volts input would make some difference. I have the export amp and it is set to 235 or 240V (can't remeber exactly), but my local supply voltage is often 250V so my amp would run a little hotter in terms of plate voltages. My 5 Watt plate voltage was not quite 300, I think it was 289.

I tried another non Mesa Sovtec tube I had and found that the plate voltage dropped from the 289V back to 230V or so and cathode current was higher at 46ma. The 5 watt mode uses a dropping resistor in series with the plate supply (2.7KOhm I think). It is the big gold power resistor mounted on the case near the power tube board. If a given tube at the fixed bias conducts higher than whatever Mesa set things up for then the plate voltage will drop. Your tube seems to draw a lot of current in 5 watt mode. My 5 watt power output as mentioned in previous posts is only about 1 watt, so perhaps your power output is closer to the 5 watt rating !

The 340V you see in 30 watt mode seems to agree with the 390V I see given your input is about 15% under and I am running 5% over voltage.

Cheers,

JJ

 

[Don]

On a 5:25 the plate voltage should be 382 in PP mode and 288 in 5 watt SE mode.

 

[MCK]

Do you think running a US unit on 100V hence with lower plate voltages could cause long term damage along the way? Sound wise I'm ok. It still sounds great!

 

[J.J]

On a 5:25 the plate voltage should be 382 in PP mode and 288 in 5 watt SE mode.

Is that the Mesa spec or what you have measured ?

Do you think running a US unit on 100V hence with lower plate voltages could cause long term damage along the way? Sound wise I'm ok. It still sounds great!

No running lower voltage won't cause any problems for your amp. Some will tell you it is bad for the tubes, but I have yet to see the proof. Some even use a variac to run lower voltages again.

 

[Don]

On a 5:25 the plate voltage should be 382 in PP mode and 288 in 5 watt SE mode.

Is that the Mesa spec or what you have measured ?

That's straight off the schematic.

Do you think running a US unit on 100V hence with lower plate voltages could cause long term damage along the way? Sound wise I'm ok. It still sounds great!

No running lower voltage won't cause any problems for your amp. Some will tell you it is bad for the tubes, but I have yet to see the proof. Some even use a variac to run lower voltages again.

Not a problem. I believe that Tweed mode on a Mark runs the voltage even lower.

 

[MCK]

No running lower voltage won't cause any problems for your amp. Some will tell you it is bad for the tubes, but I have yet to see the proof. Some even use a variac to run lower voltages again.

Thanks. That's what I've been thinking as well.

 

[J.J]

Not a problem. I believe that Tweed mode on a Mark runs the voltage even lower.

Don, do you know if the lower voltage used for that mode is for both the plates and fillaments, or is it just for the plates. I know that large transmitter tubes suffer greatly if the fillament voltage is lower than spec, but those tubes are expected to run for tens of thousands of hours. In contrast our Express power tubes are only expected to last around 1000 hours and probably don't have a long enough life for the low fillament voltage problems to surface.

 

[Don]

Not a problem. I believe that Tweed mode on a Mark runs the voltage even lower.

Don, do you know if the lower voltage used for that mode is for both the plates and fillaments, or is it just for the plates. I know that large transmitter tubes suffer greatly if the fillament voltage is lower than spec, but those tubes are expected to run for tens of thousands of hours. In contrast our Express power tubes are only expected to last around 1000 hours and probably don't have a long enough life for the low fillament voltage problems to surface.

According to the schematics that I've seen of the MKI re-issue, the voltage is dropped by a tap on the primary winding of the power transformer. Thsi would affect plate and filament voltage.

 

[J.J]

It would seem Mesa are not concerned...

 

[FXR]

On a 5:25 the plate voltage should be 382 in PP mode and 288 in 5 watt SE mode.

Is that the Mesa spec or what you have measured ?

That's straight off the schematic.

You have an Express 5:25 schematic or service manual? Care to share how to get one? Thanks!