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Dynaco ST70 modification to use KT66

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Posted on May 28, 2024 at 21:49:41
tudo99@yahoo.com
Audiophile

Posts: 3
Location: San Jose
Joined: May 28, 2024
 Hi. I am new to this forum. I recently brought Dynaco ST70 with no modification as using 7199 driver. I like original sound not modifcation using 12AU7 or VTA board sound. Recently I tried reissue Gold Lion KT66 for 15 minutes and I am in love this sound better NOS Mullard EL34. I read manual on Dynaco said ok to use KT66 but I am afraid of burning power supply. What is best modification to use KT66 without chaning much orginal design. Some advice said replacing 15K to 250K (I might remember wrong). Please provide schematic or picture in details so I might try or ask people modification

 

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RE: Dynaco ST70 modification to use KT66, posted on May 28, 2024 at 22:38:25
6bq5
Audiophile

Posts: 4592
Location: SF Bay
Joined: August 16, 2001
 Welcome to the asylum!

The KT-66 is the MO Valve/Mullard Kink-less Tetrode (KT) version of the 5881, or 6L6GC (yes, the 5881 is the small form factor version by Sylvania...)

The 6L6 has less plate capacity (voltage) than the EL-34/6CA7 250-500 for the 6L6GC versus 450-800 for the EL-34
- this from the RC-30

In operation, a pair of 6L6GC typically yield ~30Watts, AB1 Push-pull
and 50 Watts for a pair of EL-34s in an AB1 PP circuit

Pin-outs are the same, but I would lower the Plate voltage and bias if you want to run 6L6s in a Stereo 70.
This way you'll get more than a few hours of run-time...

So have at it-

I have nothing to add on the driver circuit for this amp-
there are lots o' options, especially if you have good 7199s


Happy Listening

 

RE: Dynaco ST70 modification to use KT66, posted on May 29, 2024 at 05:09:35
tubular.well
Dealer

Posts: 224
Joined: March 4, 2013
 You need to reduce the grid resistors to < 100k . This is to avoid
the KT66 to start drawing grid current and subsequently redplate.

Apart from reduced reliability and increased cost you gain nothing. Dist
is increased and power reduced.

 

RE: Dynaco ST70 modification to use KT66, posted on May 29, 2024 at 18:27:41
tudo99@yahoo.com
Audiophile

Posts: 3
Location: San Jose
Joined: May 28, 2024
 Hi tubular.well (D). Thanks for your suggestion. Can you show where on schematic to replace resister?

 

RE: Dynaco ST70 modification to use KT66, posted on May 29, 2024 at 18:29:43
tudo99@yahoo.com
Audiophile

Posts: 3
Location: San Jose
Joined: May 28, 2024
 Hi tubular.well. I don't understand what you do mean on KT66. I have very good RCA 7199

 

RE: Dynaco ST70 modification to use KT66, posted on May 30, 2024 at 10:27:24
Tre'
Industry Professional

Posts: 17618
Location: So. Cal.
Joined: February 9, 2002



.
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Dynaco ST70 modification to use KT66, posted on May 30, 2024 at 21:59:44
6bq5
Audiophile

Posts: 4592
Location: SF Bay
Joined: August 16, 2001
 please remember that the KT 66 is a VERY different tube from an EL-34
Yes, they are both Octal based Pentodes, Power tubes -
but electrically there are some important differences:
1: the KT 66/6L6GC has the G3 tied to the Cathode on pin 8. Not so in the EL-34
2: yes the rest of the pinouts are the same
3: the max plate voltage is different:[Class A amp PP] KT-66/6L6GC= 270; ERL-34= 800
While the two tubes both fit and will operate, for a while, the ST-70 was designed for EL-34 output tubes and has voltages, bias, grid settings to reflect that - considerably above the design specs for the KT-66/6L6GC...
Happy Listening

 

RE: Dynaco ST70 modification to use KT66, posted on May 31, 2024 at 08:47:25
Triode_Kingdom
Audiophile

Posts: 10199
Location: Central Texas
Joined: September 24, 2006
 "please remember that the KT 66 is a VERY different tube from an EL-34"

And in addition to everything you noted, the optimum loading and gain are substantially different. Not a good swap.

 

RE: Dynaco ST70 modification to use KT66, posted on June 2, 2024 at 05:08:25
PakProtector
Audiophile

Posts: 12393
Joined: May 14, 2002
 As I recall from the KT66 data sheet, the max value for fixed bias is 250k.

However, the likely tubes to go in there are not the NOS MOV/Genelex...and so I would concur with the move that value to something more like 100k. That said, I would also contemplate lowering the matched 47k plate loads of the inverter...

The basic circuit is good, but change one thing and it is best to go and adjust the rest of the circuit as it is all what I would call it 'connected'.

Douglas



Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: Dynaco ST70 modification to use KT66, posted on June 5, 2024 at 07:32:23
rage
Audiophile

Posts: 803
Joined: December 17, 2010
 even with the wrong load I think 6l6gc sounds better than EL34. :)


I would look to lower B+ voltage however... choke input would be nice - small chassis!

 

RE: Dynaco ST70 modification to use KT66, posted on June 25, 2024 at 23:50:40
ivan_terrible
Industry Professional

Posts: 220
Location: Ivangorod
Joined: April 26, 2015
 "KT-66 is the MO Valve/Mullard Kink-less Tetrode (KT) version of the 5881, or 6L6GC"

No it's not.
THE KT66 is the production version of the KT8, a failed transmitting valve.
It has a totally different anode design compared with the 6L6 and a much larger cathode, with a bigger heater.
It was made only by GEC UK.

The anode shape and large cathode make the electron path very different from the 6L6/807 which worked very well in RF transmitters to 50Mhz.
The KT8/KT66 design struggled at anything over 10-15Mhz, but is better for audio they say.

The original 6L6 is a top cap less 807 with slightly lower anode quality.
Both are originally US beam tetrode designs.
You can get easily 50-100W+ from a pair of 807.

The 807/6L6 has aligned grids which means it has very low screen grid current compared with the non aligned EL34 - which is a PENTODE.

The original EL34 was the EL37.
The prototype EL34 was the 9 pin EL60.

FYI.
The 807 is known for producing high levels of IMD and odd harmonic distortion which is hard to tame and has several rugged versions inc the STC 254-255B and the 5933.

The top cap versions will run happily at way beyond the "safe" region of the EL34 which is strongly NOT recommended to exceed Va of 500-550V.
The 807 will run happily up to the undocumented figures of 835-900V very well in AB2.

As a result of the non aligned grids the EL34 will have the screen glow quite easily under and square wave high power test session, and has both a much higher gain than the 6L6 and a different ultra linear transformer tap ratio as a result.

If you want to use 6L6GC - a very reliable 30W Pa it will need twice the voltage drive.
TBQH when I swopped out the 807 on my amps to the KT8 (aka KT66), I couldn't tell any difference in sound or measurements.

The KT66 is an IMD pump, and tends to produce lots of THD
If you like higher levels of IMD this one is for you!

 

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