Stumbled on a stripped dynaco st-70 chassis.
Decided to put it to good use.
Should deliver about 80 watts a channel if my last one was anything to judge by.
Using a Welbourne pcb (slightly modified) with 12ax7's and 6u8a's
Plate voltage arround 600vdc at 400ma
Yes, tube life will be limited, but it's for Ebay when done.
Decided to put it to good use.
Should deliver about 80 watts a channel if my last one was anything to judge by.
Using a Welbourne pcb (slightly modified) with 12ax7's and 6u8a's
Plate voltage arround 600vdc at 400ma
Yes, tube life will be limited, but it's for Ebay when done.
Lets see, 60 watts of dissipation per tube, 240 watts total in the output tubes. The whole amp will melt if the rectifier tube doesn't blow up first! I have been known to bend some tube specs a bit (OK a lot) but I haven't found a KT88 that can deal with 60 watts. The best are EH KT88's and they glow dimly at 45 watts, brightly at 50 watts. They wouldn't last long (a few hours maybe) at 60 watts.
Power transformer? No, you are going to need a nuclear reactor.
If I had a quad of KT88's and wanted to get 80W out of them, I'd probably build monoblocks and transformer couple a 6V6 or something and run into class AB2. Might be feasible then...
I'm interested in how long this amp would run after being switched on. 10 seconds? 30? Unless of course there was a typo in the first post..
I'm interested in how long this amp would run after being switched on. 10 seconds? 30? Unless of course there was a typo in the first post..
I have a pair of mono blocks that run NOS GE 6550A's in ultralinear with at least 610 volts at 60ma per tube. I can get 100 watts rms out down to 60 Hz with the Acrosound TO-600 4K P-P load. 100 ma is too much and not necessary. However, I would be very afraid to try this with todays garbage 6550/KT88 tubes.
oh my, such negativity.
Guess none of you ever saw a Harmon Kardon Citation II then.
Granted it has superb transformers, though it does run as standard at 65wrms per channel with old rca 6550's.
The last one I had was biased at 85ma and sounded both loud and clean, with the original 6550's from 30 years ago. I still use them on a daily basis in my home system and they havn't burned up yet.
As for the rectifier burning up, check out the specs on a 5au4, it may surprise some of you. (1400v @ over 1a peak) but granted it would be a weak spot pushed so hard I may have to use schottkies.
I'll be using EI-kt90's which should handle the extra load, check out the specs on em, they are not toys. the kt88's are just a set of test tubes to make the layout look better. As for the power tansformer, it's an Antec 800va (5-1/4" dia. x 4" tall) toroidal mounted on it's edge and should fit fine.
Guess none of you ever saw a Harmon Kardon Citation II then.
Granted it has superb transformers, though it does run as standard at 65wrms per channel with old rca 6550's.
The last one I had was biased at 85ma and sounded both loud and clean, with the original 6550's from 30 years ago. I still use them on a daily basis in my home system and they havn't burned up yet.
As for the rectifier burning up, check out the specs on a 5au4, it may surprise some of you. (1400v @ over 1a peak) but granted it would be a weak spot pushed so hard I may have to use schottkies.
I'll be using EI-kt90's which should handle the extra load, check out the specs on em, they are not toys. the kt88's are just a set of test tubes to make the layout look better. As for the power tansformer, it's an Antec 800va (5-1/4" dia. x 4" tall) toroidal mounted on it's edge and should fit fine.
I don't think anyone meant to be... I think the prevailing thought to be considered is that you shouldn't be trying to build that large of an amp on a ST70 chassis. Even if you get it all to fit, the heat will reduce all component life. I am in the process of building a similarly rated amp - and I am using a 17x22 inch chassis to give the components breathing room. Personally, I am not found of tube rectifiers in hi-fi gear at all, let alone run at max specs. If you are going to push 400ma, you should go solid state for rectifiers. From what I read your rectifier tube is max rated at 415ma -- 400 to 415 is not what I would call a safe headroom margin. Again, I think this is what everyone is looking at - this amp will have a much higher chance of failure than it should - especially if you are building it to sell to someone who likely has NO tube repair experience. Ultimately your customer is likely to have an unpleasant experience with your amp - repeated/premature tube failure or a complete failure of a major component requiring repair. From a DIY perspective, you should be more responsible with your design (and ultimately your customer) and from a customer standpoint - no one wants to buy something that breaks or requires fixing / maintenance...itsmejto said:
There is a difference between "negativity" and exceeding a KT88's 46W ABSOLUTE MAXIMUM plate + SG dissipation rating by 14 watts.itsmejto said:
In any case, it is of course up to you how you build your amps, but please consider this a warning in case something blows up, that's all. reference:
http://www.mif.pg.gda.pl/homepages/frank/sheets/086/k/KT88.pdf
(and that's for the "real deal" Genalex tubes. don't expect any leeway with new production)
sorenj07 said:
There is a difference between "negativity" and exceeding a KT88's 46W ABSOLUTE MAXIMUM plate + SG dissipation rating by 14 watts.
reference:
http://www.mif.pg.gda.pl/homepages/frank/sheets/086/k/KT88.pdf
(and that's for the "real deal" Genalex tubes. don't expect any leeway with new production)
If you absolutely insist on turning this into a p!$$!ng contest :
The older kt88's though a fine quality tube are not up to modern kt90 standards like the EI or EH currently available.
Besides which, you are quoting plate currents for a single tube.
Having said all that, I built a similar amp way back in 1959 for a local dance hall driving two 18" EV speakers which had to be re-coned because the wern't powerfull enough to handle the output of that amp (mono). yes they were GE tubes at the time.
"KT90 is a beam-power pentode primarly designed for use in audio frequency power amplifier applications. Anode dissipation rate is 50W, which provides for push-pull amplifiers utilization up to 110W output per pair, with 550V on the anode. Up to 160W per pair may be achieved with anode voltage of 750V and screen grid voltage of 600V."
Dangerous, yes I agree, but feasable enough.
itsmejto,
When you start off a thread with a statement like: "Yes, tube life will be limited, but it's for Ebay when done," you give a clear impression that you have a cavalier attitude toward the fate of an amp that you intend to build and then sell. This is bound to attract criticism from responsible forum members, especially if some aspects of your plan appear to be unrealistic.
It is not a "p!$$!ng contest", just common sense. If you don't like other people's comments in a public forum, then I suggest you keep your ideas to yourself.
When you start off a thread with a statement like: "Yes, tube life will be limited, but it's for Ebay when done," you give a clear impression that you have a cavalier attitude toward the fate of an amp that you intend to build and then sell. This is bound to attract criticism from responsible forum members, especially if some aspects of your plan appear to be unrealistic.
It is not a "p!$$!ng contest", just common sense. If you don't like other people's comments in a public forum, then I suggest you keep your ideas to yourself.
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