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EZ125 Quad KT88 Project

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#41
I have seen the criss-cross done in some amps and in other they parallel them, I used to like the idea that if I blow a tube with the criss-crossed wiring I could still use the amp on the other tube, but who doesn't have a box of spares 🙂

The amp is a monoblock so not a problem splitting up the way the power is fed to the caps.

You are 100% right, sometimes if it works just leave it be, but sometimes that's too easy.

I have not had time to look at some of the operating points on the tubes, but they are out of whack for sure. The cathode follower is at the limit of cathode to heater voltage if I'm understanding the data sheet but that is all part of the few changes to the power supply dropping resistors.

I am surprised that no one has tossed in their preferences for the operating points of the 6SN7's, but the thread may be too long to read 🙂

Sandy
 
#42
Funk,

I stand corrected .....

Yes, I saw more articles dismissing cathode stripping than the other way round (I might be expected to know coming from the era - though at the time the availability of articles/publications) down here was limited and a schlepp to get from sources.)

Regarding PSU stress, again point taken, though would it not depend on the design? I worry more about the initial peak voltage before load. As said I use a choke-input filter where the peak voltage across the PSU would be quite high before a minimum load, with little stress coming on onto full load.

Still, in general, as you said.
 
#43
SandyG said:
I am surprised that no one has tossed in their preferences for the operating points of the 6SN7's, but the thread may be too long to read 🙂
Click to expand...

... and optimal values in terms of lowest distortion not so easy to just 'read' off load lines on graphs. Also as someone suggested, the general graphs in manuals are hand-drawn while Spice models are also still approximate.

Your voltage values (plate-cathode) are somewhat lower than what I used for 6SN7s, but they did not look alarmly out. Still, when next a free moment arrives I will have a look (and let you know per PM if I strongly disagree!).

Perhaps others will comment in the meantime?
 
#45
I know that amp, that's one of the Tubes 4 HiFi models. I have built one of their KT88 ST-70's and have one of the preamp's waiting to be finished.

I was going to build one of those as the power was good and decided that it was too simple 😉, and that not in a bad way, but wanted more glass and some octal's on the project even it it made no difference. One thing on the EZ125 design is that you could likely drive a few more kt88's without too much trouble.

I'm interested in your power transformer, did you have that made at Heybor? I have mine from them. The one I have is 400 CT 500ma, 105v, 6.3 CT 3A and 6.3 8A and finally the 5v 6 amp. It runs a bit hot with out use of the 5V, but not really sure if a problem at all (runs about 140F).

I think the mk4 amp used a 415vac supply for 125 and I think you are getting a bit extra juice since you are running 425vac. It's a very simple design, and by the looks very clean too.

I have not yet got back to tinker with some of the power supply values, but I did hook it up to one of the channels on the stereo and side by side with the ~40 watt Grommes 260A clone I built it sounds good, more bottom end punch which is expected.

Sandy
 
#50
kingneb said:
Yes, those units are Heybor.

Specs:
410-0-410 (820 volt) B+
70-0-70 (140 volt) Bias
6.3 volt 8 amp filament
The opt is a Heybor as well.

1.9k Primary
4, 8, and common secondary.
Power Handling 130 watts.
Click to expand...

I have the same output transformer it seems, I told him to make it sized for 150w, but from the looks of your power transformer it has a bit more thickness in the core then mine. When I get the next one I will have him make it a bit bigger as it seems to run a bit hot even when not running the 5U4's which would be another 30 watts of power dissipation.

My Heybor PT is currently
800v CT 500ma
105v Bias Tap
6.3v CT 4 Amps
6.3v CT 8 Amps
5v 6 Amps

Looks like yours has some nicer end bells too, mine were made with the end bells on top of angled brackets so they look a bit odd with the bells lifted off the laminations a bit.

Have not had a chance to tinker with the amp. Going to work on lowering the voltage a bit to the CF stage and bringing up the voltage a bit on the input's 6SN7, from what I see the bias may be a bit on the low side at 2.9 (not much) but more so that it's only drawing 1.7ma which seems low from discussion and other schematics I have seen. Those are easy changes, but just have to take it apart and swap a couple of resistors.

I also think I will extend the soft start a few more seconds just to make sure that all 6SN7's are hot and to reduce some of the differential across the relay a bit and keep some load on the power supply when the relay does pop. It's currently at about 11 seconds will add another 2 or 3 to it.

I have a Quant Assylum 400 analyzer and once I figure out this mess I'll run some numbers.

I also have some KT120's to try but I don't think they will make any more power but who knows...

Sandy
 
#51
KT120s will not produce any more power unless you set up your driver to produce more voltage swing. Since your OPT is rated for 150 watts, that sounds feasible.

Keep in mind the filament draw of the KT120 and the grid stoppers.
 
#57
Skidave - That's the place where I saw the info too, they compare different ways to measure power amps. A few different solutions show in the article. I have no had time yet to mess with the QA400, need to do a few things with the amp first then will mess with it.

On the KT120's will get to that eventually 😉 I had the power tranformer have the 8 amp capacity for the filaments for the KT120's just in case. But the KT88 provide plenty of juice and other then trying to see how much power could be squeezed out of the amp, not in a hurry to give them a try.

Pulling the amp apart for a couple of resistor changes to increase delay on the start up a bit, and then the main dropping resistor to the 6SN7's as I think that's where the work needs to be done. My thinking is the plate voltage to the CF is too high and the input is a but too low. So will play with that to see about getting something that looks better.

Sandy
 
#59
Are you giving it more input signal or less feedback? The only other reason is the KT120 may have more voltage and/or current gain in themselves.

I never used the KT120 but I may try a pair in an a two-tube push pull for 90 watts.

What is the distortion?
 
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