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    plug in problem for KT88

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    sailor

    Posts : 269
    Join date : 2011-04-04

    plug in problem for KT88

    Post by sailor on Tue Apr 19, 2011 7:33 pm

    I had a couple of new KT88 and I tried to plug them in the original Dynaco 70 sockets but they would not go. Are the sockets on the older units made for smaller pins. The old EL34s will plug into my new amp no problem.

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: plug in problem for KT88

    Post by Bob Latino on Tue Apr 19, 2011 8:57 pm

    Hi,

    The pins should be the same size - about .090 inch (90 thousandths of an inch). The KT88's you are using may have more of a squared off tip on the end and maybe you should taper them a little with a file.

    I would be careful trying KT88's in an ORIGINAL ST-70 with the original PA-060 power transformer. The original PA-060's are somewhat undersized for the amp to begin with. KT88's draw more current than EL34's and at the very least your amp will run noticeably hotter with KT88 output tubes. Another issue is that the bias system on original ST-70's is not set up for KT88's and a KT88 will bias probably way over to one side of the bias pots. If that does happen and you are using the original ST-70 bias system you will have to lower the value of the two 10K resistors on the 7 lug terminal strip.

    Bob

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: plug in problem for KT88

    Post by sailor on Wed Apr 20, 2011 10:07 pm

    Thanks Bob I was only going to put them in long enough to see how they sounded in a dyna 70 compared to the single ended amp they are presently in. They are penta SC's from china and I had noticed that the old mullards were very tapered on the tips and how clean the solder job was compared to the new tube.

    wolverine

    Posts : 60
    Join date : 2010-02-20

    Re: plug in problem for KT88

    Post by wolverine on Thu Apr 21, 2011 11:29 am

    I agree with Bob....in fact, I have expanded the bias ability on all of my Dynaco's by taking out the 2 10k resistors on the 7 lug strip and putting in 4.7k 3 watt resistors in their place. Also pulling out the 2 10k biasing pots and putting in 25k 3 watt pots in their place. This way I never worry about what tubes I want to roll, and biasing can be accomplished very easy on tubes other then the EL-34's. I have replacement power transformers in all of my units I had made at Heyboer in Grand Haven, Michigan. Kevin also sells replacement power transformers and I have one of his, I think Kevin keeps those in stock and they are certainly very nice transformers, and Kevin is a great guy to deal with, he will help you make correct choices and get you into the exact parts you need. Also, I have two of Kevin's output transformers and they sound every bit as great as any cloth lead original.
    wolverine

    Disles1

    Posts : 17
    Join date : 2010-12-21
    Location : Calgary, Alberta

    Re: plug in problem for KT88

    Post by Disles1 on Thu Apr 21, 2011 2:47 pm

    sailor wrote:I had a couple of new KT88 and I tried to plug them in the original Dynaco 70 sockets but they would not go. Are the sockets on the older units made for smaller pins. The old EL34s will plug into my new amp no problem.

    I would also check that you haven't squashed/pushed down the innards of the tube sockets. I did that when I built the ST-120 and it caused quite a problem. A couple of the KT88 pins had caught the edge of the socket innards and as I pushed in the tube it took the innards with it. I had to replace the inner pins as I had almost completely squashed a couple down into the tube socket. Embarassed

    sailor

    Posts : 269
    Join date : 2011-04-04

    Re: plug in problem for KT88

    Post by sailor on Thu Apr 21, 2011 8:02 pm

    By the way, this is the amp that I am trying to figure out what to do or not do to it.
    I took Bobs advise and filed down the tips a little. With careful wiggling they went in. Turned the bios as low as it would go. It really wasn't as low as it should have been. Fired it up for a quick listen. The difference was at the extremes. The bass was a little better, the highs were a lot more detailed and a little more volume at the very top end. I liked it better than the el34. I had the same results on my other amp.
    As far as the B+ voltage problem for the KT88s I am using diodes in stead of the 5ar4 because my 5ar4 died in the testing process. Because of this I am running at about 500 volts on the B+. So that is about right for those tubes. I assume the major problem with the transformer would be the low ma. but my speakers are 99 db/1 watt so I would only need about 5 watts output out of the tubes.

    I guess my next question would be: Does a tube use about the same ma. of power to produce the same watt output in triode and pentode modes?

    tubes4hifi
    Admin

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    Join date : 2008-11-30

    Re: plug in problem for KT88

    Post by tubes4hifi on Thu Apr 21, 2011 9:27 pm

    your listening of the KT88s is exactly right, they have more bass and treble, but many people love EL34s for their really smooth midrange.
    The problem with KT88s in a Dyna ST70 is that they draw 2 amps of filament current for each tube, that's 8 amps total plus the tubes on the driver stage. EL34s draw 1.5 amps of filament current each, that's 25% less, or better said, the KT88s are sucking 33% more power out of the transformer than it was designed for. That's why a popular replacement tube for the EL34s are KT66 or KT77 tubes.
    If you haven't tried big bottle 6CA7s, that's my preference for a stock ST70

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