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 Post subject: Re: KT120 Oddblocks
PostPosted: 04 Sep 2017, 16:53 
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Hi, It seems I lost my comments so I'll try again. Thanks for the kind comments. Nice build and good selection of components. One of the things I wanted in the design was to have amps that would have exceptional performance and not break the bank. These when properly constructed have been known to embarrass ones with high pedigrees in side by side listening tests. The use of premium components is not required, but will push the amp to an even higher level. The two power levels were included mostly so lower power tubes like KT88s and such could be used. With KT120s the actual sound ought not be much different at either level. It has been my experience that class A push-pull tube power amps seem to sound pretty much the same over a wide range of input power (dc) levels.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 07 Sep 2017, 21:48 
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Location: NE Ohio
Greetings,

It's an honor to receive a reply from the designer himself. One thing that I didn't mention that might be observed from the photo is the use of a SMPS for the filament supply. I followed this with a simple -C-L-C- circuit to eliminate residual noise. Of course, the DC resistance of the inductor had to be quite low - I think 25 mOhm. When building the Heran headphone amp in early '16, again I used a 12V SMPS, but placed a voltage regulator afterward for the same purpose. One advantage of the use of a SMPS is that it relieves the power supply transformer of about 25 W in the case of the KT120 amps, which is like installing a larger one by the same margin. My experience with preamp mods, is that bigger transformers result in better sound. Going 4- and 5-times the expected load is not a crazy thing to do, in my experience.

Comparing Mr. Heran's posted circuits of the KT88 and KT120 monoblocks gives me the idea that with 3 switches per monoblock, this amp could work with KT77, 6L6GC, EL34, etc, as well as KT88 and KT120 tube types. The off/on switch for the resistance that sets the tube current via LM317 regulator might be replaced with a SPDT with central off position, providing three choices of resistance. Likewise, another SPDT/center off switch could be used to provide three choices of feedback. I'm guessing that a third such switch would probably be required to adjust the resistance between the first two capacitors in the B+ voltage supply. This would allow the user to keep the output tube voltage closer to the target of 450, because changing the current via the regulator would necessarily change the voltage drop through the first (and subsequent) resistor(s). I have the switches, resistors and small-valued caps on hand to effect this change, but not yet the tubes. I will report back on my experience.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 08 Sep 2017, 10:23 
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Hi, in general the use of multiple tube types will work. However the best loading for the KT88s and KT77s (for best distortion vs power output) is in the 6500-8000 ohm range. For the KT120s it is in the 3400 ohm range. While any of them will work at either range, the ones that are the primary tube type (KT88s in the 8K and KT120s at 3.4K) will work best. Another thing to guard against is tube dissipation. With unregulated power supplies the B+ will increase as the current drops. It may put the filter capacitors at risk.

I use SMPS all the time. Not only for heaters, but in solid state designs. Any time I need a symmetrical supply for low power I use Meanwell DC to DC converters. Cheap and work great. The 8-18 in to + and - 12 out is really handy (DCW03A-12). It is the size of an ice cube. If you want 100-240 vac to 12 vdc the PM-05-12 is the way to go. delivers a regulated 400 ma. These greatly simplify designs. BTW they provide over 1000 volts of isolation from source to load in most models.

I also use Potrans AC to DC SMPS for heaters (12 vdc at 3.5A)

I don't feel that it is "impure" to use things that work well and do the job desired even if they are not purist methods.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 09 Sep 2017, 13:23 
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Joined: 06 Nov 2015, 13:45
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Location: NE Ohio
Hello Mr. Heran,

Following up on the topic of using low output tetrodes and pentodes in the KT120 monoblock amps, I have to admit that impedance matching is something I didn't think about. Fortunately, the Edcor CXPP70-3.5K output transformer has 4- and 8-ohm taps. If the 8-ohm tap is optimal for driving an 8-ohm speaker with a KT120 tube (with turns ratio estimated to be 20.9), then the 4-ohm tap (turns ratio about 30) should work to reflect the 8-ohm load across the transformer as 7.2 kOhm impedance seen by the output tube, in this case 6L6 type, for instance. So no worries, so long as the speakers truly are 8 ohms impedance. My VMPS RM30s are 7.2 ohm in the mid range ribbons (purely resistive), while the impedance of the three bass drivers is 10 ohm resistance, but with some inductive reactance. The tweeter ribbon impedance is nominally 8 ohm - again purely resistive. So I think that at least in my case at least, switching to the 4-ohm tap will do the job when using 6L6, EL34, KT66, etc.

We are on the same page regarding the rise in B+ voltage when the anode current is decreased for 6L6 tube. As I am using an unregulated power supply, the idea is to add a SPDT/off center position switch to allow three values of resistance between the first two capacitors in the B+ supply. This should allow the output tube B+ to stay reasonably close to the target 450V. The driver tube B+ voltage may change as well, but I would rather not add another switch and resistors to adjust this as well. We'll see if this becomes necessary. The KT120 tube is really spectacular with very low noise, seemingly flat response, openness, reliability, etc. The motivation for moving to different tube types is driven by nothing more than curiosity.

I installed a timer relay in the circuit to avoid the no/low current, high-voltage on the driver tube at startup. (I recall that you discussed this issue somewhere, as well as in the last post). The idea is to give about 25 sec for the tubes to heat up, as the current flows near instantaneously from the SMPS when the amp is switched on. You will argue that 25 sec is cutting it close, and I agree. I would have preferred closer to a minute, but 25 sec relays are what I had on hand. They seem to work fine.

You enlighten me on the topic of getting a symmetrical PS from a single power supply and a DC-DC converter. I may have to question you more closely if the issue crops up.

As always, many thanks for your helpful suggestions.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 07 Nov 2017, 01:35 
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Joined: 07 Mar 2017, 03:14
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Hi,
I'm planning to use Duelund RS as coupling caps on the signal path. They are rated for 400 Vdc. Are these ones suitable for their places?

Anyone tried Duelund RS caps? What do you think about them? Are they worth the money?


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 Post subject: Re: KT120 Oddblocks
PostPosted: 07 Nov 2017, 17:53 
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Hi Somehow I missed the questions in the De Forest post. Yes you can use output transformers that way. I have done it in prototypes to test out various turns ratios. Yes you can use 6L6s that way, but beware of max dissipation ratings. With 450 volts on tap it is easy to cook the tubes. If you use the LR8 regulator in the driver circuit the B+ to the tube will not change. If you don't you should verify that the B+ to the driver is under 325. A target value is 300. It you go higher it is difficult to get the heater to cathode situation correct. You can not exceed 200 volts between any of them with new tubes. My experience is that with age this frequently drops to closer to 100 volts. The heater lift voltage is critical in keeping everything safe. The tube will fail if you don't get it right. Again watch out for seriously mis-matched triode sections as this will shift the mid point and hence the cathode of the upper section.

My original amps use a 30 second delay and this seems fine. They still work great after 8 years of use. 25 seconds is probably OK. As you noted more is better. The tubes ought not be harmed by too soon an application of B+, but the filter capacitors might as it could be over their ratings.

400 volt coupling caps are fine. The typical voltage there is about 150 so even lower ratings should be fine. I would not go below 250 though as a mismatch in the two sections of the SRPP can swing the center point considerably. Up to 25% is OK but I would replace any tubes that have less equal triode sections.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 08 Nov 2017, 04:10 
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Thank you, Bruce. If I change 220R regular metal film resistors on the signal path to tantalum non-magnetic resistors, do I have a considerable improvement on the sound? Because, we accept different brand and quality of caps on the signal path will have different effects on the sound. I don't see a reason why resistors on the signal path can not have comparatively less but similar effects.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 08 Nov 2017, 06:48 
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ozapaydin wrote:
...tantalum non-magnetic resistors...


Question to anyone.... are those as crappy sounding and non-linear-with-DC as the capacitors? :confused:

Cheers!

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 Post subject: Re: KT120 Oddblocks
PostPosted: 08 Nov 2017, 08:52 
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Hi, I don't know if there will be any difference. Never tried them. The driver stage is the one that is most sensitive to changes that can alter the sound. The output stage is mostly influenced by the output transformer. For use as grid stoppers in the output tubes, I doubt there will be any change as they are voltage driven and there is virtually no signal current involved. I use mostly KOA Speer metal film ones and they seem to be fine. I see no harm in trying the other resistors....but I don't seen any particular reason to do it either.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 08 Nov 2017, 12:13 
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:up: :D :D :D
Geek wrote:
ozapaydin wrote:
...tantalum non-magnetic resistors...


Question to anyone.... are those as crappy sounding and non-linear-with-DC as the capacitors? :confused:

Cheers!


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