I have a rebuilt STA ST-70 about 8 years ago with original power transformer so I could only use EL34 and KT66 tubes. After 60 years the power transformer finally gave up. So now I have the new one that Dynakit sells so what else to I need to do the make it use KT88 tubes in it like the VTA ST-120 does?
4 posters
Upgrading my VTA st-70 so I can use KT-88 tubes
truebluephil- Posts : 17
Join date : 2013-07-01
truebluephil- Posts : 17
Join date : 2013-07-01
- Post n°2
Re: Upgrading my VTA st-70 so I can use KT-88 tubes
by truebluephil Mon Mar 06, 2023 8:32 pm
I am surprised that I didn't get any responses. Anybody know what I need to do to convert to the st-120?
Bob Latino- Admin
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Join date : 2008-11-26
Location : Massachusetts
truebluephil wrote:I am surprised that I didn't get any responses. Anybody know what I need to do to convert to the st-120?
You CAN run KT-88's in Dynaco ST-70
1. If you have an upgraded VTA ST-70 power transformer or an upgraded Dynakitparts power transformer
2. And the amp has a VTA driver board
What you do is remove the 10K resistor that is now in the R39 position and replace it with a 1K (1000 ohm). Doing this will alter the bias range of the amp to better suit KT88 and 6550 output tubes. Where you biased your EL34 output tubes at .400 VDC, you can now bias the KT88 or 6550 tube at about .450 VDC. Save the 10K resistor if you ever want to go back to EL34 tubes again
With those changes you can now run KT88 or 6550 output in your Dynaco ST-70. See where R39 is located on the photo below.
Note 1 - You now DO NOT HAVE A VTA ST-120 just because you have KT88 tubes in the amp! You just have a Dynaco ST-70 amp that can run KT88 tubes. A VTA ST-120 has a larger transformer with a higher voltage on the two RED secondary wires which give a B+ DC voltage about 60 volts higher than any ST-70 transformer will give.
Note 2 - Do not do this change if your Dynaco ST-70 has an original transformer. An original Dynaco ST-70 power transformer has a stack of one and a half or one and 5/8 inches. The VTA and the Dynakitparts ST-70 transformers have stacks above 2 inches.
Bob
peterh- Posts : 1869
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Location : gothenburg, sweden
It seems that my previous post has disappeared. Is this censorship or is it computer failure ?
Bob Latino- Admin
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Join date : 2008-11-26
Location : Massachusetts
peterh wrote:It seems that my previous post has disappeared. Is this censorship or is it computer failure ?
Your post was censored/removed because you gave INCORRECT information which I did not want sitting here up on this thread. You had said that you could NOT use KT88/6550 output tubes in a Dynaco ST-70. You CAN use these two tubes in an ST-70 IF ..
1. You upgrade the power transformer (remove the original ST-70 power transformer) and upgrade it to a modern ST-70 transformer from either VTA or Dynakitparts. NOTE - nearly all ORIGINAL Dynaco ST-70 amps have cloth covered power transformer wires.
2. You use a VTA driver circuit and alter the bias circuit to allow more negative DC to be dialed in.
Bob
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truebluephil- Posts : 17
Join date : 2013-07-01
- Post n°6
Re: Upgrading my VTA st-70 so I can use KT-88 tubes
by truebluephil Wed Mar 08, 2023 10:12 am
Thx Bob, I thought I could covert with the new power transformer. Should I add the second choke like in the st-120. Also how much power increase so I get with the KT88 tubes?
Bob Latino- Admin
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Location : Massachusetts
- Post n°7
Re: Upgrading my VTA st-70 so I can use KT-88 tubes
by Bob Latino Wed Mar 08, 2023 10:38 am
truebluephil wrote:Thx Bob, I thought I could covert with the new power transformer. Should I add the second choke like in the st-120. Also how much power increase so I get with the KT88 tubes?
If you have the original C-354 Dynaco choke rated at 200 milliamps still in there, I would replace the original choke with a new Triad C-24X rated at 240 milliamps. This is a drop in replacement. In some original Dynaco ST-70 amps the choke will, after many years of use, cause the original choke to ooze out some type of pale gray looking wax which will wind up on the inside of the bottom cover directly under the choke. You don't need the extra choke in your amp like the second choke in the VTA ST-120. One choke will suffice.
There will be a very slight increase in power if you can use KT88 or 6550 output tubes in place of the original EL34 output tubes in your Dynaco ST-70. You may not even notice the small increase in power. What you will notice is that a KT88 in place of an EL34 will give your amp a little more top end frequency extension. Consider though that, if your speakers are little "bright" right now, you might not like a KT88 in your Dynaco ST-70.
Bob
Tubes4ever- Posts : 167
Join date : 2015-07-14
Location : Star, Idaho
Bob Latino wrote:truebluephil wrote:Thx Bob, I thought I could covert with the new power transformer. Should I add the second choke like in the st-120. Also how much power increase so I get with the KT88 tubes?
If you have the original C-354 Dynaco choke rated at 200 milliamps still in there, I would replace the original choke with a new Triad C-24X rated at 240 milliamps. This is a drop in replacement. In some original Dynaco ST-70 amps the choke will, after many years of use, cause the original choke to ooze out some type of pale gray looking wax which will wind up on the inside of the bottom cover directly under the choke. You don't need the extra choke in your amp like the second choke in the VTA ST-120. One choke will suffice.
There will be a very slight increase in power if you can use KT88 or 6550 output tubes in place of the original EL34 output tubes in your Dynaco ST-70. You may not even notice the small increase in power. What you will notice is that a KT88 in place of an EL34 will give your amp a little more top end frequency extension. Consider though that, if your speakers are little "bright" right now, you might not like a KT88 in your Dynaco ST-70.
Bob
Bob,
Will the kt88 give more high frequency extension than using 6L6GCs?
Bob Latino- Admin
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Location : Massachusetts
- Post n°9
Re: Upgrading my VTA st-70 so I can use KT-88 tubes
by Bob Latino Fri Mar 10, 2023 10:00 pm
Tubes4ever wrote:Bob Latino wrote:truebluephil wrote:Thx Bob, I thought I could covert with the new power transformer. Should I add the second choke like in the st-120. Also how much power increase so I get with the KT88 tubes?
If you have the original C-354 Dynaco choke rated at 200 milliamps still in there, I would replace the original choke with a new Triad C-24X rated at 240 milliamps. This is a drop in replacement. In some original Dynaco ST-70 amps the choke will, after many years of use, cause the original choke to ooze out some type of pale gray looking wax which will wind up on the inside of the bottom cover directly under the choke. You don't need the extra choke in your amp like the second choke in the VTA ST-120. One choke will suffice.
There will be a very slight increase in power if you can use KT88 or 6550 output tubes in place of the original EL34 output tubes in your Dynaco ST-70. You may not even notice the small increase in power. What you will notice is that a KT88 in place of an EL34 will give your amp a little more top end frequency extension. Consider though that, if your speakers are little "bright" right now, you might not like a KT88 in your Dynaco ST-70.
Bob
Bob,
Will the kt88 give more high frequency extension than using 6L6GCs?
In my opinion - yes > A Kt88 will have slightly more top end than a 6L6 ...
Bob