Got my Nesstone 6550's and installed them. Could not get bias below 1.4v, not close to the recommended .500v. Put by KT66's back in and biased easily to .450V. Any suggestions? Built my VTA ST-70 back in 2013.
4 posters
6550 replacing KT66 in VTA ST-70
Brap- Posts : 231
Join date : 2013-11-28
Age : 68
Location : Plainfield, illinois
peterh- Posts : 1833
Join date : 2012-12-25
Location : gothenburg, sweden
get new kt88 ( or even better EL34 )Brap wrote:Got my Nesstone 6550's and installed them. Could not get bias below 1.4v, not close to the recommended .500v. Put by KT66's back in and biased easily to .450V. Any suggestions? Built my VTA ST-70 back in 2013.
6550 needs grid resistors < 50k , above that ( vta has 270k) all bets are off.
Brap- Posts : 231
Join date : 2013-11-28
Age : 68
Location : Plainfield, illinois
thanks -- makes sense. I'll check into KT-88's (like the look) but am partial to Gold Lion and hard to find since the crisis and VERY pricy. Love the sound of my KT-66's but they are coming up on 9 years old so looking for replacements. Could not find any so I went the 6550 route without doing ore research on internal changes required. What's the best "flavor" of KT-88's out there other than GL?\ in VTA ST-70?
peterh- Posts : 1833
Join date : 2012-12-25
Location : gothenburg, sweden
The best flavor is EL34. It's also the best economic and widely available. In addition itBrap wrote:thanks -- makes sense. I'll check into KT-88's (like the look) but am partial to Gold Lion and hard to find since the crisis and VERY pricy. Love the sound of my KT-66's but they are coming up on 9 years old so looking for replacements. Could not find any so I went the 6550 route without doing ore research on internal changes required. What's the best "flavor" of KT-88's out there other than GL?\ in VTA ST-70?
works fine with the VTA-70.
In my i run JJ KT88 but that is a side effect of customer returning these and a cannot
sell them as new. Normally EL34 works fine.
Last edited by peterh on Wed Mar 30, 2022 5:02 pm; edited 1 time in total (Reason for editing : Spelling)
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Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
The bias system in the VTA ST-70 is set up to bias EL34, KT66, KT77 or 6L6 output tubes. If you want to use KT88's or 6550's in a VTA ST-70, all you have to do is reduce the value of R39 on the driver board. R39 controls the "range" of the bias system. All VTA ST-70's are shipped with a 7.5K 1/2 watt resistor for R39. This is fine for the four output tubes mentioned above. To set up the amp for 6550 or KT88 output tubes, REDUCE THE VALUE of R39 to about 1000 ohms (1Kohms). This change will allow you to dial in more negative DC voltage which is more appropriate for 6550 or KT88 output tubes. R39 is appropriately placed on the BOTTOM of the driver board between the two 100 uF caps that stick up from the bottom rear of the driver board. See photo below.
Bob
Bob
Last edited by Bob Latino on Mon May 09, 2022 8:59 pm; edited 1 time in total
Brap- Posts : 231
Join date : 2013-11-28
Age : 68
Location : Plainfield, illinois
Thanks much Bob. Will probably opt to get EL34's or KT 77 so I can swap without making chassis changes.
truebluephil- Posts : 17
Join date : 2013-07-01
I have used EL34, Kt-66 and Kt-88 and all work well in the amp. But the Kt-66 tubes are the best sounding tube for this amp. if you listen to Blues and Jazz its a perfect tube.
Happy listening
Happy listening
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Brap- Posts : 231
Join date : 2013-11-28
Age : 68
Location : Plainfield, illinois
I fully agree on KT-66 with the VTA ST-70. Truly enjoy mine. I will stay with these, just waiting for the price to get back to "normal". $90.00 each is a bit stiff for GL's.
truebluephil- Posts : 17
Join date : 2013-07-01
Brap wrote:I fully agree on KT-66 with the VTA ST-70. Truly enjoy mine. I will stay with these, just waiting for the price to get back to "normal". $90.00 each is a bit stiff for GL's.
I was lucky as as saw this shit show coming with Russia so I bought 8 of them. Hopefully this will go away soon and return back to normal.
Brap- Posts : 231
Join date : 2013-11-28
Age : 68
Location : Plainfield, illinois
So are these your personal stash or are you inclined to sell 4 of them?
truebluephil- Posts : 17
Join date : 2013-07-01
Brap wrote:So are these your personal stash or are you inclined to sell 4 of them?
sorry they are for my personal supply
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