• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

acrosound ultralinear 2 with KT90 tubes?

P

P.T.

New Member
acrosound ultralinear 2 with KT77 tubes?

I have a pair of these amplifiers. one has the original holland el34 tubes(one is getting a bit weak) the other has telifunkin el34 (both weak and one the fillament rattles just on heater ac). The amps run 480 volts B+ and if I run el34 I will have to cut the idle current down to 50ma/tube or exceed 25w plate dicipation just at idle. The assembly manual for the amps sais I can use el34,kt88 6550 tubes. The KT90 looks like it maybe the rite thing to put in there. In any case if I put taller tubes like the kt90 in it I will have to make spacers for the cage/cover to accomidate the taller size. Has anyone used tubes other than the stock el34 in these amps? what do you recommend.
P.T.:music:
 
Register to hide this ad
hey-Hey!!!,
The EI KT90's haven't reliably made their ratings in almost a decade. I have some that do, and they're nice tubes for sure. If I had to pick a tube for that amp, I'd get winged-C Svetlana SV6550's. EH's KT88's seem to get good press too. For that matter, so has the Chinese octal EL156. So as I think about it, I might go for the 156's. Their bottle is certainly cool looking...:)
cheers,
Douglas
 
Thanks for the helpful information.
It apears the EH KT88 is a good choice. I can get them into the sockets with 1/2 air space around them and the top cover will fit. Best of all they should act like large el34 and that will be just fine. It would be cheaper to put just el34 tubes in these things but I may as well spend extra bucks and not run the el34 tubes at or beyond their design ratings(as was the practice of the original designers). The factory manual came with the amps and they wanted me to set the bias current at 72ma/tube this would put the idle dicipation at about 34 watts/tube. this is way over the specifications for the tube and expecting 60 watts rms out of the amplifier was way over specifications. The amp with usable tubes in it does sound very nice so I think they are worth restoring. I will change all the carbon resistors that run with high voltage/ and dicipation on them. Useing metal film type. Also I will replace the 2 paper coupling caps with polypropoline types. The main filter caps seem to be ok so will let them ride for now(maybe).
P.T.
 
hey-Hey!!!,
The main filter electrolytics are pretty risky too. New ones will also work better...no sense hobbling the amp.
cheers,
Douglas
 
Yes, I will have to replace the electrolitic caps also. I have a sweet leek stereo 20 with preamp. It is england,:music: 1962. It worked just fine but there was a sale on B+ filter caps so I replaced them(yes it did sound better). When listening to music its just no fun to worry about the amplifier going up in smoke. I have some very old radios that the filter caps have miraculously survived in for over 60 years and still work fine. Example E.H. Scott slr-m receivers(1945). I love radio/audio stuff(we need to have some obsessive hobbies).
P.T.:music:
 
After much research and though about this question. I have ordered 4ea JJ KT77 tubes. I intend to run them at 50ma of static cathode current/tube. With the 480 plate voltage they should sound great and give plenty of power. I just could not convince myself to put the oversized tubes in these things.
We shall see if this is as good as expected.
P.T.:music:
 
I ran EL34's in my UL-II's but they ran ballistically hot even when rebiased.

They were not happy in there.

I would use KT88 clones.
 
You will be venturing into unknown territory long term so keep a close eye for stable bias on those KT77's. I've tried them in my VRD mono blocks for about 2 weeks and they showed no signs of faultering. They had about 515V on the plates and I believe I was biasing them at 50ma.

My gut says you will be fine but proceed with caution.

Craig
 
hey-Hey!!!,
Some of the vintage stuff ran its finals over spec. The Citation II was one. Its 100 mA cathode current equated to 46 or 47 Watts shared between the plate and screen. Even GEC KT88's looked unhappy with this treatment. Try runing an example or two up to its rated dissipation some time, it is an intersting experiment.
cheers,
Douglas
 
hey-Hey!!!,
Some of the vintage stuff ran its finals over spec. The Citation II was one. Its 100 mA cathode current equated to 46 or 47 Watts shared between the plate and screen. Even GEC KT88's looked unhappy with this treatment. Try runing an example or two up to its rated dissipation some time, it is an intersting experiment.
cheers,
Douglas
 
All good thoughts to keep in mind. I am waiting on the tubes. The total heat generated is the same or more with bigger tubes like the kt88. But, then the clearance around the tubes is much less with the bigger tubes.So, getting the heat out of it is less easy. I have been listerning to the one amp with useable original holland el34 in it. I found the 5ar4 was weak. with a good one B+ is up to 517volts Line voltage here 125 ac. At 50ma cathode current the original set up is just fine dicipating the heat after an hour on time all ok. It sounds nice. I am running it into an old ar2(found at thrift store never finished plain wood on outside). With jazz music it sounds much better with the damping switch off and control set all the way ccw(.5). I have several speaker sets I can use including the original ar2s' that I got with the setup. Audio is fun to experiment with. My favorite amp for pleasure so far is a home made single ended 45 triode stereo. 6sl7 one half for each predriver. Nice 1.5 watts rms into 6 ohms. the clean sound and sound stage is great. Who knows if I will like these amps as well. Likely not the same but time will tell. With minimal damping(feedback) it will be interesting to experience.
P.T.:music:
 
This should do it for updates. I finished the rebuilds. All new electrolitics. new coupling caps. all new tubes (except the original 12au7s tested good and work good so they stayed). 6 new 2watt metal film resistors/unit. I fixed way to many bad(or non existent) solder joints. Quallity was job none. Was like an old jaguar. With the both amps done they sound EXCELLENT!! Put about 300 bucks into these things and it was worth doing. They look really nice too not beat up at all. I run the output bias at -41 volts with B+ 510 volts this gives 46 ma/ output jj kt77. After amp on for an hour. no overheating, no problems. I think the way I went with these is just fine. I encourage anyone who knows how to work on this type electronics to consider these as worthy of the effort. Happy listening to all.
P.T.:music:
 
You must log in or register to reply here.
Back
Top Bottom