Mod50 down... need help ASAP!

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PM33AUD

PM33AUD

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Hello,

I've sent a message to Ian, but maybe I can get some help in the interim here. Seems a bias on TS6550's at 45ma is NOT acceptable (contrary to how some other Mod50 users stated is OK) regarding plate dissipation and now I have a blown output transformer after a tube went :doh: (click here to see the 6550 carnage) Where can I find a replacement OP transformer ? I need to get this amp up and running yesterday since I sold my other amps to get this guy and we have recordings to finish. I have only done a DC check on the primary and B+ to one tap seems to have blown 'open' which would explain why the fuses aren't blowing.

As a side note, I normally have biased amps measuring plate voltage and cathode current to get Pd, but figured the manual saying the 6550s were OK to 45ma and some other users' reports were good enough for me to be lazy and not have to crack the amp to do it properly. Unfortunately this laziness is costing me dearly :((

After this is all over, I'd like to start a new bias values spreadsheet for Mod50 users so that they can still bias tubes from rear panel but give max ratings per brand tube to avoid something like this in the future since there are many tubes where bias current isn't adequate for safely operating. I know the manual has started this (with the warnings on 6V6 types) but it needs to be expanded IMO.

Phil
 
Phil:
sorry for your trouble. 45mA for a 6550 is well within its ratings. 70% dissipation at 500VDC is 58mA. I do believe you simply had some bad luck with tubes, which is certainly not uncommon nowadays. The plate voltage in the MOD50 is 500VDC. 45mA idle current is actually only about 50% of maximum dissipation being a 45 watt tube. Our recommended setting really quite conservative. Even when a tube does short/fail, it is not very often that the glass cracks/breaks. That would certainly lead me to suspect some not so great tubes.
I don't have any spare MOD50 transformers right now but, the JTM45 output transformer from Mojo Musical Supply has almost identical specs and should fit the existing bolt pattern and sound pretty much the same. They are both made by Heyboer transformers to essentially the same specs.
 
Thanks Bruce,

I am upset as I've never had such bad luck with ANY tube, ever, than these 2 sets of TS6550s. Apparently, they are not rated for 500V plate voltage (speculation by other members - haven't checked myself).

Bottom line: will I be able to tell the difference in the trannys? If you think no, then I'll order it right up.

Phil
 
Really? Not rated for 500 volts? I have not been able to locate the actual specs for the Sovtek/TS tube. Can you tell me where to find the info? How did you verify they are not rated for 500 volts? I assumed, as you did, that they were on par with every other 6550. I have never seen a 6550 rated less than 600VDC. I would not call that tube a 6550 if it does not have a voltage rating close to the "original specs". Fact is, the specs can be almost meaningless nowadays. A Sovtek 5881 is actually "rated" at 400VDC on paper yet every amp maker for the past 20 years+ has been exceeding that with no consequences. Reliability is often not related to the specs at all and is more a product of mechanical and physical quality and construction. The fact that the glass broke on yours, and apparently caused a severe "abnormal" failure and maybe damaged your amp, would be an indication that those tubes are just not made well as opposed to having exceeded the specifications. I would suggest not using those tubes again. I do know of countless players using 6550 tubes (maybe not TS?) in amps with 500+ volts with excellent reliability. VHT, Bogner, Deizel, Soldano, old Marshalls etc...etc... understand the quality of today's tubes is really not what it used to be, though there are many good reliable tubes made today. Also, when you see an old brand name come back (TungSol, Gold Lion, Mullard etc.) after many years of extinction, it is just a name and really may have nothing to do with the original tubes from 20++ years ago. I have confidence that the Mojo output transformer will sound very much the same as the original. I have never done an A/B comparison but we did use them in the "amp building class" amps and were really good. As I said, both are made by the same manufacturer to essentially the same specifications. They do have the mechanical specs available at Mojo if you want to verify fit before ordering one. FYI, in all the years of making the MOD50, I have never seen a MOD50 output transformer fail. Congratulations, you are the first !!
 
bruce egnater":1ga5t5t0 said:
Really? Not rated for 500 volts? I have not been able to locate the actual specs for the Sovtek/TS tube. Can you tell me where to find the info? How did you verify they are not rated for 500 volts? I assumed, as you did, that they were on par with every other 6550. I have never seen a 6550 rated less than 600VDC. I would not call that tube a 6550 if it does not have a voltage rating close to the "original specs". Fact is, the specs can be almost meaningless nowadays. A Sovtek 5881 is actually "rated" at 400VDC on paper yet every amp maker for the past 20 years+ has been exceeding that with no consequences. Reliability is often not related to the specs at all and is more a product of mechanical and physical quality and construction. The fact that the glass broke on yours, and apparently caused a severe "abnormal" failure and maybe damaged your amp, would be an indication that those tubes are just not made well as opposed to having exceeded the specifications. I would suggest not using those tubes again. I do know of countless players using 6550 tubes (maybe not TS?) in amps with 500+ volts with excellent reliability. VHT, Bogner, Deizel, Soldano, old Marshalls etc...etc... understand the quality of today's tubes is really not what it used to be, though there are many good reliable tubes made today. Also, when you see an old brand name come back (TungSol, Gold Lion, Mullard etc.) after many years of extinction, it is just a name and really may have nothing to do with the original tubes from 20++ years ago. I have confidence that the Mojo output transformer will sound very much the same as the original. I have never done an A/B comparison but we did use them in the "amp building class" amps and were really good. As I said, both are made by the same manufacturer to essentially the same specifications. They do have the mechanical specs available at Mojo if you want to verify fit before ordering one. FYI, in all the years of making the MOD50, I have never seen a MOD50 output transformer fail. Congratulations, you are the first !!
Click to expand...

Yes, I do feel special in some respects :thumbsup:; what do I win?! But reports that the TS6550s can't handle this power were only reports from other forum members... I haven't dug up the data myself. Considering it was two sets of these tubes with problems (from different suppliers, different dates), I will never use them again and will pay the little extra and put SED tubes in the amp (already have some EL34s on the way). I did do a quick DC check on the OP transformer and one leg of the primary to B+ measures open. I'll order that puppy up now.

Thanks Bruce,
Phil
 
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