Why not the 6l6? It sounds good to me.
Dynaco ST-70: Base Line Testing
- Thread starter dcgillespie
- Start date
Why not the 6l6? It sounds good to me.
The amp is pretty much fully rebuilt. Just original chassis and output transformers.If you are going to a 5AR4/GZ4 rectifier --- check the age of those electrolytic caps in the power supply. Especially the one in the first stage. Maybe worth it to replace them (or the can).
Hopefully someone can address my input impedance and 6l6 plate load questions.
More distortion ( i have measured) as the transformers are optimized for EL34. And why settle with less optimal tubes ?
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this is what the load line looks like for a 5881 at 430 volts and 40ma into a 4.3k UL transformer with 43% taps. that should be what its working into. The solid line is what the tube's conductance swing looks like, the area under the dotted line would be considered the safe operating area for the tube. You can see it spends most of its time outside of that zone. Red dot is the idle current at 40ma.
If you connect an 8 ohm speaker to the 4 ohm tap, that becomes effectively an 8.6k load. This is what that looks like
looks a lot better. Its easier on the power supply too. On paper it says into a 4.3k load it'll make 57 watts, into 8.6k it says 35. Realistically its not going to do that because the power supply won't let it, but the current required to try and make 57 is a lot more than 35.
just for comparison, this is what an EL34 idling at 50ma into the 4.3k load looks like
that claims 36.4 watts, and given measured performance of an ST70 thats pretty darn close once you account for transformer losses and power supply sag and all that.
If you connect an 8 ohm speaker to the 4 ohm tap, that becomes effectively an 8.6k load. This is what that looks like
looks a lot better. Its easier on the power supply too. On paper it says into a 4.3k load it'll make 57 watts, into 8.6k it says 35. Realistically its not going to do that because the power supply won't let it, but the current required to try and make 57 is a lot more than 35.
just for comparison, this is what an EL34 idling at 50ma into the 4.3k load looks like
that claims 36.4 watts, and given measured performance of an ST70 thats pretty darn close once you account for transformer losses and power supply sag and all that.
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Thank you, that is incredibly helpful; website noted! It looks more than acceptable with the 5881 on the 4 ohm tap which may seem to corroborate myself others reports of good sound in this configuration.
trobbins
Super Member
gadget73, perhaps worthwhile adding that those loadline plots don't clearly show the loadline along the zero current axis extending up to circa 800-900V Va levels, and that the time spent below the dashed line (the safe operating area) is typically below 50%, which implies the average dissipation is likely below the rated limit for the valve.
DirtyErnie
Well-Known Member
^^ this.
Peterh, do you know or have any guess how kt77 or 6CA7's would do distortion wise compared to a EL34 in a ST70? Thanks.
Yes. It increases. EL34 is optimal in ST70
Diggin up my old notes of using different power tubes in a tubes4hifi vta-70
All made with bias 0.5V and original NFB setup as VTA-70
All tubes new JJ randomly selected but matched pairs
Tube 10w 32w
EL34 0.31 0.92
EL34 II 0.38 1.46
6CA7 0.82 2.2
KT-77 0.95 1.1
6550 0.33 1.53
KT88 0.54 1.57
6L6GC 0.61 1.3
Later i changed NFB resistor to 2k which dramatically lowered dist. The above figures however i consider them
more significant as tube departure from linear performance is less "corrected" by the lower NFB.
It is only one sample of each tubepair, but from other measurments i consider them representative for
that brand ( JJ production seems quite stable and while tubes differ in bias and Gm the inherent properties
does not shift much.
All made with bias 0.5V and original NFB setup as VTA-70
All tubes new JJ randomly selected but matched pairs
Tube 10w 32w
EL34 0.31 0.92
EL34 II 0.38 1.46
6CA7 0.82 2.2
KT-77 0.95 1.1
6550 0.33 1.53
KT88 0.54 1.57
6L6GC 0.61 1.3
Later i changed NFB resistor to 2k which dramatically lowered dist. The above figures however i consider them
more significant as tube departure from linear performance is less "corrected" by the lower NFB.
It is only one sample of each tubepair, but from other measurments i consider them representative for
that brand ( JJ production seems quite stable and while tubes differ in bias and Gm the inherent properties
does not shift much.
Lavane
Lunatic Member
Inran KT-77's in my VTA ST-70 for around 10 years. They sounded good. I didn't hear any distortion. I picked them originally when I bought the amp kit after a discussion with Kegger. Now I'm using EH 6CA7's and really like them.
I’ve run a few variants but prefer RFT EL34s and will likely run them as long as possible.
I've had both the Gold Lion and JJ kT77’s as well as the EH 6ca7. They all had what I found to be enjoyable qualities but I haven't really had a normal EL34 installed in many years.
Lavane
Lunatic Member
At one point I used a set of KT-88's in that amp. They didn't sound right.
The top end and bass was ok, but the midrange didn't sound right to me.
But using those same tubes in an amp designed to use them they sound great. I thought the KT-77's had a well balanced sound from top to bottom.
Nothing exaggerated. But I do prefer the EH 6CA7's though. The low end is a little beefier but we'll controlled. Since I don't have tone controls in my mane system they worked out well.
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I'm not a fan of most KT-88s. I'm unsure if the amp setups were not ideal, but I've heard many KT-88s amps lack that "tube magic." Maybe it is their midrange resolution (or lack thereof).
Lavane
Lunatic Member
That's what I found using them in my ST-70. But they sound great in my PPP mono blocks. I would like to try some 6550's, but having to buy four for each mono block is expensive. I already had the one quad of KT-88's that I had tried in my ST-70, so buying another quad was the cheaper route.I'm not a fan of most KT-88s. I'm unsure if the amp setups were not ideal, but I've heard many KT-88s amps lack that "tube magic." Maybe it is their midrange resolution (or lack thereof).
6550s are my favorites. Those black plate TungSols (USA).
Lavane
Lunatic Member
At around $560 for new issue, and even more for NOS to retube those PPP mono blocks, I'm good right now.6550s are my favorites. Those black plate TungSols (USA).
