Gold Lion KT-66 and VTA 70 resistor change question.

Hi all,
I did, search for something definitive on this before creating a post.
Just got state tax return back and wouldn't you know, its a little more than enough to buy a quad of GL KT66.
before i go buying these, I could swear I read on here that you might need to change some resistors on the VTA 70
to keep the KT 66 from running too hot.
I wouldn't want to trash a quad of tubes at that price.
Thanks in advance.

Midwestside wrote:Hi all,
I did, search for something definitive on this before creating a post.
Just got state tax return back and wouldn't you know, its a little more than enough to buy a quad of GL KT66.
before i go buying these, I could swear I read on here that you might need to change some resistors on the VTA 70
to keep the KT 66 from running too hot.
I wouldn't want to trash a quad of tubes at that price.
Thanks in advance.

Read the data sheet : https://frank.pocnet.net/sheets/086/k/KT66.pdf
At power loss > 27w the maximum g1 resistor is 100k, below that power level 250k
VTA has in stock form 270k

This won't say that a kt66 would redplate at once, it says that to ensure long time reliability one should respect these figures.

when in doubt, and ALWAYS with KT88 tubes, R29-32 must be changed to 100K,
For KT120 tubes, R2-32 must be 51K

Hi Guys... When I read the OP's comments this thought came up in my mind - "What are you talking about, Willis?"

I may have been one of Bob's first customers for the VTA ST-70 (See my original thread on this at https://dynacotubeaudio.forumotion.com/t17-kicking-it-off-here-s-mine - notice the date!). As you can see it had GL KT66 installed and still has the same quad tubes to this very day. In 12 years it has NEVER red plated... At the time Bob advised that the only thing to do differently from running the "proper" EL34s etc is to adjust bias to 450 millivolts on each. I also own a set of Mark 3 amps (also with the VTA driver boards) and did at one point run the ST-70 with the quad of KT88s in place (also Gold Lion, bias adjusted to 500 millivolts) however I do prefer to "do things the proper way, "the natural place for KT88 tubes in the universe is in Mark 3 amps" (or ST-120 if you wish).

So basically I swap between 3 sets of amps on a yearly basis - my Mark 3 monoblocks, my ST-70 and for contrast, a Yamaha A-S2000 "Sand amp". So over the past 12 years the ST-70 with GL KT66 tubes have been in continuous use for about 4 years, 3-4 hours per day on average in total. "Red Plating" is only something I have read about but never seen in real life. FWIW the Mark IIIs are far more "heavy" on tubes than the ST-70 - I have had to replace the KT88s twice in this time, however at least one time it was due to poor quality control (IMHO) - one tube became "noisy" (crackling sound) and then another from the same set went microphonic (really badly). The ST-70 is still running the original KT66s you see in the pics that I bought 12 years ago, and has never given me any reason to want to replace it. Red Plating?? What is that you speak of?

Just my 2 bits.


-- Jannie in BC, Canada

JunkyJan wrote:Hi Guys... When I read the OP's comments this thought came up in my mind - "What are you talking about, Willis?"  

I may have been one of Bob's first customers for the VTA ST-70 (See my original thread on this at https://dynacotubeaudio.forumotion.com/t17-kicking-it-off-here-s-mine   - notice the date!). As you can see it had GL KT66 installed and still has the same quad tubes to this very day. In 12 years it has NEVER red plated... At the time Bob advised that the only thing to do differently from running the "proper" EL34s etc is to adjust bias to 450 millivolts on each. I also own a set of Mark 3 amps (also with the VTA driver boards) and did at one point run the ST-70 with the quad of KT88s in place (also Gold Lion, bias adjusted to 500 millivolts) however I do prefer to "do things the proper way, "the natural place for KT88 tubes in the universe is in Mark 3 amps" (or ST-120 if you wish).

So basically I swap between 3 sets of amps on a yearly basis - my Mark 3 monoblocks, my ST-70 and for contrast, a Yamaha A-S2000 "Sand amp". So over the past 12 years the ST-70 with GL KT66 tubes have been in continuous use for about 4 years, 3-4 hours per day on average in total. "Red Plating" is only something I have read about but never seen in real life. FWIW the Mark IIIs are far more "heavy" on tubes than the ST-70 - I have had to replace the KT88s twice in this time, however at least one time it was due to poor quality control (IMHO) - one tube became "noisy" (crackling sound) and then another from the same set went microphonic (really badly). The ST-70 is still running the original KT66s you see in the pics that I bought 12 years ago, and has never given me any reason to want to replace it. Red Plating?? What is that you speak of?  

Just my 2 bits.


-- Jannie in BC, Canada

Obeying datasheets is the mfg word that "with this conditions most will work according to spec". Breaking the spec may or may not result in failure but the probability certainly increases.
If one wants to maximize the chances of zero problems a starting point is to follow spec.
Besides, there is no cost in reconfigure the amp , change of 4 resistors is all that is needed.

Bob Latino wrote:Using KT66 output tubes in my own VTA ST-70 amp with a 270K G1 resistor for R29 - R32, I have never had a problem with any KT66 ever red plating ?

In the VTA ST-120 and VTA M-125 monoblocks this G1 resistor is a 100K resistor which works well with 6550, KT88 and KT120 output tubes.

Bob

Thanks Bob, and everyone for their answers.