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Power tubes dying?

Kuntry09

Member
Messages
79
My amp just doesn't seem to have the same sound and feel that it had when I first got it, about a year ago now. When I purchased the amp ('66 Bassman) the previous owner had Mesa SRT 425 power tubes in it. The amp hadn't been played in a very long time so I'm really not sure how old they maybe. I've already replaced the preamp tubes not too long ago because one of them had become microphonic. I have experienced noise with the power tubes one time about 3 weeks ago but that may have been due to one of the tubes being loose from being moved around. the reason why I'm wondering if the power tubes maybe dying is because it seems like I have to turn the amp up more to achieve the same overdrive as I was getting when I first got it. The amp has been modded to be more lke the original blackface circuit and the bass channel is now a overdrive channel. The amp is also running at 8ohm instead of 4ohm that the amp likes to see. Is it possible that the tubes maybe burning out or am I just fooling myself? :bonk
 

RussB

low rent hobbyist
Messages
11,157
I replace power tubes in every used amp I purchase, just to be sure
 

zenas

Member
Messages
8,872
First off how does one mod a 66 Bassman to be more like an original blackface ?
Every single Fender amp I've seen from 1966 was a blackface.

Anyway back to the problem it could be tubes first I would swap in some good preamp tubes and try that. 9 times out of ten it's a preamp tube getting weak. (I know you said you just changed them but that don't mean a thing with tubes)

Also since this is a 66 have the electrolytic caps ever been changed?
 

Hwoltage

Member
Messages
9,625
When you first got the amp, what was the speaker ohm rating?

Per the tubes/speaker, it's important to keep in mind that the speaker ohm rating plays a very large part in setting the bias of the output tubes. Basically, you can set the output tubes to a certain bias with a certain speaker ohm rating (your amp was originally set up for a 4 ohm load) but if you change to a speaker with a different ohm rating you will change the bias of the output tubes. This means that you can change the current draw of the output tubes by changing the ohm rating of the speaker, and by changing the current draw of the output tubes you change the point at which the output tubes will begin to break up. Going to a higher ohm load will make the output tubes less likely to break up at a certain volume setting, than they were at the same volume setting with a speaker of a lower ohm rating. Conclusion: If the amp was set up with a 4 ohm load, it will not break up as quickly with an 8 ohm load.

As for your output tubes potentially burning up, the Mesa SRT 425 output tubes evidently are 6L6 tubes. From what I can gather per the Bassman schematics of that era (AA****, AB**** etc) those amps were originally stocked with 6L6GC tubes. 6L6GC tubes can handle higher levels of voltage and current than 6L6 tubes, so despite the 8 ohm speaker load on the amp lowering the current draw (the amp was set up for a 4 ohm load which will draw more current, the 8 ohm load forces the tubes to draw less current) it is still possible the 6L6 tubes that exist in the amp right now may be forced to operate at voltage and current levels that exceed their maximum specifications.

If you decide to replace the output tubes, replace the 6L6 tubes with 6L6GCs, and have the amp re-biased by a professional, but be sure to have the amp biased with the speaker load you intend to retain for the duration. You could probably get away with running an 8 or 4 ohm load with 6L6GCs but if you go back to a four ohm load, do not run 6L6s. Of course any tube change that is made should be followed with a professional re-bias, as stated.
 
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Kuntry09

Member
Messages
79
First off how does one mod a 66 Bassman to be more like an original blackface ?
Every single Fender amp I've seen from 1966 was a blackface.

Anyway back to the problem it could be tubes first I would swap in some good preamp tubes and try that. 9 times out of ten it's a preamp tube getting weak. (I know you said you just changed them but that don't mean a thing with tubes)

Also since this is a 66 have the electrolytic caps ever been changed?
Let me rephrase myself. Yes it is blackface already, but the circuit was modded so that is was similar to the earlier "classic" blackface models. I believe the original circuit in mine was the AB165? I don't have it on hand so I maybe wrong. I do have some old tubes I have on hand for spares so I'll trying swapping out the whole set and see. I've tried swapping them one at a time to see if there was a difference but it didn't seem like it. And yes, I believe the caps have been replaced.

When you first got the amp, what was the speaker ohm rating?

Per the tubes/speaker, it's important to keep in mind that the speaker ohm rating plays a very large part in setting the bias of the output tubes. Basically, you can set the output tubes to a certain bias with a certain speaker ohm rating (your amp was originally set up for a 4 ohm load) but if you change to a speaker with a different ohm rating you will change the bias of the output tubes. This means that you can change the current draw of the output tubes by changing the ohm rating of the speaker, and by changing the current draw of the output tubes you change the point at which the output tubes will begin to break up. Going to a higher ohm load will make the output tubes less likely to break up at a certain volume setting, than they were at the same volume setting with a speaker of a lower ohm rating. Conclusion: If the amp was set up with a 4 ohm load, it will not break up as quickly with an 8 ohm load.

As for your output tubes potentially burning up, the Mesa SRT 425 output tubes evidently are 6L6 tubes. From what I can gather per the Bassman schematics of that era (AA****, AB**** etc) those amps were originally stocked with 6L6GC tubes. 6L6GC tubes can handle higher levels of voltage and current than 6L6 tubes, so despite the 8 ohm speaker load on the amp lowering the current draw (the amp was set up for a 4 ohm load which will draw more current, the 8 ohm load forces the tubes to draw less current) it is still possible the 6L6 tubes that exist in the amp right now may be forced to operate at voltage and current levels that exceed their maximum specifications.

If you decide to replace the output tubes, replace the 6L6 tubes with 6L6GCs, and have the amp re-biased by a professional, but be sure to have the amp biased with the speaker load you intend to retain for the duration. You could probably get away with running an 8 or 4 ohm load with 6L6GCs but if you go back to a four ohm load, do not run 6L6s. Of course any tube change that is made should be followed with a professional re-bias, as stated.
I've been running the amp at 8 ohm ever since I got it but I'm assuming that is was biased at a 4 ohm load. I'm not very sure to be honest. The tech knew what ohm load I was running so he may have accounted for that.
 

Jerry Glass

Member
Messages
870
Per the tubes/speaker, it's important to keep in mind that the speaker ohm rating plays a very large part in setting the bias of the output tubes.
You might want to retract this statement. The OT only reflects the AC load from the speaker; the DC load, in a quiescent state, only factors in the DC resistance of the OT primary.

Kuntry09: There are any number of reasons that your amp could be losing drive and the 8 ohm load is most likely not the culprit. Since you amp has been modified, several unknown variables have been introduced which makes diagnosis from a distance unlikely. I think your best bet is going to be to involve your tech.
 

Hwoltage

Member
Messages
9,625
You might want to retract this statement. The OT only reflects the AC load from the speaker; the DC load, in a quiescent state, only factors in the DC resistance of the OT primary.
Thank you. I suppose it would have been more accurate to say that changing the load on the secondary affects the operating conditions (load line) upon which the quiescent bias point operates. The OT does reflect the DC load on the secondary: The impedance ratio of the transformer (turns ratio squared) is multiplied by the load on the secondary to determine the impedance value the plates see on the primary at the initial stages of matching an OT to a set of tubes/voltages etc. Agreed? :)
 
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Kuntry09

Member
Messages
79
Thanks for the replies guys. I talked with my tech yesterday and came to the conclusion to go ahead the replace my speakers (which I had been planning on doing) to get the correct ohm match THEN look into replacing the power tubes....if that is even the problem.
 

Hwoltage

Member
Messages
9,625
Jerry Glass is right in that there is no telling how the amp is set up. You really shouldn't be running 6L6 tubes if by chance the amp is still set up for 6L6GCs. An 8 ohm speaker over a 4 ohm isn't really going to change the current much, more the overall sound. I tend to over analyze a bit. ;)

These guys are right, you should find a tech who is willing to service it, and make sure he's aware those tubes are 6L6 and not 6L6GCs.
 

RussB

low rent hobbyist
Messages
11,157
The Mesa STR 425 is the Russian wafer base 5881. A rather lame tube IMHO


 

MasterEvan07

Where's that buzzing coming from...?
Gold Supporting Member
Messages
4,474
I was having an issue where all of the sudden the volume would drop and everything would get really distorted...like a pedal dying distorted, not good break-up distorted. But I'd turn off the amp and come back later and it would be ok...it's done this twice now and it seems substantially buzzier than normal - do I have a dead or dying tube? Everything is lit up in the back, so it appears normal but I already ran through my cables and pedals and they aren't adding any uncommon noise...?
 

zenas

Member
Messages
8,872
The Mesa STR 425 is the Russian wafer base 5881. A rather lame tube IMHO


Dammedest thing with those Sovtec 5881s. I got a set in a 1965 TR I bought. They were put in by a "tech" who gave the amp a "clean bill of health" for the seller. Anyway after I recapped it I went ahead and used the 5881s and dammed if they don't sound good in the thing.
But in a TR most of us never get the power tubes cooking anyway. So for now they're staying.
 

Blue Strat

Member
Messages
30,243
If the amp sound/feel has changed over time the first thing to check is the tubes. Power tubes will certainly have the effect you've mentioned when they get weak.

Why spend more money on speakers, or anything, than tubes which you'll ultimately need anyway?
 

pdf64

Member
Messages
8,145
I'm wondering if the power tubes maybe dying is because it seems like I have to turn the amp up more to achieve the same overdrive as I was getting when I first got it
It could just as easily be a pre-amp tube or a faulty component.
A power amp tubes get a mushy tone/feel, and can't push out the sound pressure level that they used to.
So, is the issue lower overdrive level, or lower sound pressure level?
Pete
 






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