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 Post subject: Re: KT120 Oddblocks
PostPosted: 05 Oct 2012, 21:16 
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Once more I have to say my Silver Dragon 300BSE is dead quiet (zero audible). Three stage filtering with M-caps (Mundorf polypropylene) 47uf-10H choke-47uf - resistor - 100uf. Regulated DC on all the heaters. Should be easier with a PP design.

I have read a few posts where they claim the EH was a good choice for 300Bs. I must do something with the Shuguang 300B-Zs I have.

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 Post subject: Re: KT120 Oddblocks
PostPosted: 06 Oct 2012, 05:40 
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Location: Lisbon, Portugal
gofar99 wrote:
Hi Mark, since is all about Oddblocks... not too far OT. I probably would initially try EH ones. Readily available and not all the costly. I have tentative plans for a 300B Oddwatt. Not all that different from the others, but the directly heated filament does make for a few changes. There are other projects ahead of it so it may be a while. (a stereo KT88 amp and a guitar amp for starters). My main consideration is noise level. It might be a challenge to get it down into the -90dbv range that I prefer.

I´m looking forward to that one Bruce!

You really rock! :P

Happy weekend! :)

Miguel


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 Post subject: Re: KT120 Oddblocks
PostPosted: 21 Dec 2012, 05:17 
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While playing with my KT120 amplifiers in one of evenings I tried to look at signals in different parts of the build. It comes to my attention that we lose almost 50% of signal (in amplitude) in the driver-to-SIPP transfer. How?
The signal amplified by the driver stage is applied to the grid leakage resistor (grid-to-ground) Rg. The voltage across the grid-cathode of the "active" tube consists of two parts: voltage across the Rg minus the voltage across the CCS. So if we raise the voltage across Rg, say to 1V, the voltage difference should directly apply to the grid-cathode of the "active" tube. The active tube will conduct more, the CCS will raise the voltage across itself to react to the corresponding current rise. If there was only one tube the voltage across the CCS (in ideal situation of course) would be equal to the signal amplitude, but there is the slave tube! From the point of view of the "slave" tube the voltage increase on the CCS is applied negatively to its the grid-cathode, making it conduct less which partly "compensates" the additional current from the "active" tube. Both tubes are in the A-region, so they should be linear to the grid-cathode voltage. The stable point is when the Ugc-active=-Ugc-slave, so in our example of 1V it should be stable at 0.5V on both tubes (opposite sign). That is why we have the signal lose of 50%.
To prevent the waste I connected the driver's output directly to the grid and cathode (I have different windings on the power transformer for the driver and output stage, so I can have different "grounds" for the driver and the output stage), but it happened that in the case the SRPP worked wrongly (no load). I can't add an additional resistor to the driver's output (that is between the grid and the cathode of the active tube), because the resistor will be in parallel with the CCS (prevent the CCS to work properly). Now I'm in the process of finding the puzzle solution.
I know the Oddblocks do not have problems with gain, but easing the driver's stage mode of operation is good thing for such powerful tubes like KT120s.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 21 Dec 2012, 17:29 
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Hi Poty, It is true that there is an apparent loss of signal. It sort of puzzled me as well. Eventually I decided it had to do with driving two tubes in almost a totem pole mode in the output stage. It is in some ways similar to the SRPP, just has some of the connections going to external locations (the ends of the trannie). Regardless if you jump back to tubes 101.... the maximum linear output of a tube in class A mode can not exceed the difference between the grid and cathode at idle. Typically the KT120s run with about 28-33 volts on the cathodes. The SRPP can deliver easily in excess of +/- 80 volts (most will do well over 100 with the 300 vdc on the anode) so there is excess drive. A problem can arise if the two triode sections are seriously mis-matched. Then the center point of the SRPP range would be offset and full drive would not be available. Sections being this far off are fortunately rather rare and really would constitute a defective or failed tube. I would be interested to see if someone has the math behind the signal halving and would enlighten us all. It is a point I accept, but haven't investigated and to be truthful haven't worried about.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 21 Dec 2012, 19:20 
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poty wrote:
While playing with my KT120 amplifiers in one of evenings I tried to look at signals in different parts of the build. It comes to my attention that we lose almost 50% of signal (in amplitude) in the driver-to-SIPP transfer. How?
Not at all. This stage has the exact same voltage drive requirements regardless of how driven. The confusion point is where the ground is referenced. Your description of the circuit operation is correct, it's only the assumption of loss which is incorrect.

It is better to think about the problem this way. Let's assume you have a normal PP output stage with the midpoint ground (at the cathodes) and driven by a simple concertina phase shifter. Now everyone "knows" that the concertina phase shifter has a slightly less than unity gain right. Well, in reality, that's only "partly" right. In this configuration, to drive the output stage, the grid of the upper tube is connected to the plate, the grid of the lower to the cathode load. When we look at the upper drive with reference to ground, the signal only swings slightly less than the input. When we look at the cathode, we see the same thing. So we think in terms of driving the output stage with the same voltage as the input to the concertina and the concertina having unity gain.

However, lets look again at our reference point. Instead of using ground, lets look at the voltage between the grids of the power stage. Now, because the two are 180º out of phase, the voltage we measure is almost twice the input to the concertina. If we are looking at the voltage on the upper grid with reference to the lower we now see that the concertina must have a "gan" of approximately two (6dBv). Therefore, to drive the PP power stage, the difference between the grids must be twice the required single tube grid swing.

Now going back to the oddblock configuration, the grid of the lower tube is held at ground. Therefore when we measure the voltage of the upper tube grid, it requires twice the voltage to get full power because, again, the difference between the grids must be twice the required single tube grid swing.

This just goes to prove, once again, that there is no such thing as a free lunch. The single tube drive with a CCS on the cathodes is indeed an elegant solution to simplify the circuit, but the laws of physics still prevail and force us to make sure we have twice the drive between the grids. The 180º phase shift trick trades additional complexity for reduced driver stage voltage swing.

You'll may note that this same trick (change of reference point) is used in US household power delivery. The supply to the main panel is 240vct. By referencing across the entire transformer secondary you get 240v, by referencing either side to the center tap (which is usually at Earth ground via a ground spike) you get 120v.

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 Post subject: Re: KT120 Oddblocks
PostPosted: 22 Dec 2012, 16:41 
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Hi, Correct, I was having a senior moment. It is the difference between the grids. This is part of the reason in the newer schematics that there are matching resistors and capacitors in the "grounded" grid tube's circuit. Otherwise the impedances to ground would be wildly different. Even now they are not as the SRPP output impedance is ignored.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 22 Dec 2012, 17:20 
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Joined: 24 Oct 2010, 07:05
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Hello Suncalc,
I agree with you about general case for phase splitting (or shifting? - I don't know what term is better suited for the case). But when you spoke about physics you made a mistake. Like in your example about concertino we use active circuit for getting two opposite signals (CCS). Then we are allowed to assume that the active circuit will output 2 opposite signal with initial (input) amplitude (like the concertino does). I already mentioned, that we can easily achieve the goal - just move the input signal ground directly to the cathode of the active output tube. This change will disable the local negative feedback with the input signal and still allow the slave tube to get full input signal swing with opposite phase.
If we used a common driver design we'd be OK. In the Bruce's design the solution faces a problem though - the SRPP must have non-zero load. After moving the input signal ground wire directly to the cathode we connect the SRPP output to serially connected CCS (almost infinite dynamic resistance by design) and the grid leakage resistor. It brings problems with SRPP (it becomes not SRPP at all). Connecting a resistor between the grid and the cathode bring a problem to the SIPP circuit - the resistor bypasses the CCS.
Bruce wrote in the post just before yours that SRPP doesn't have any problem in delivering double amplitude, but I have several improvements in mind:
1. Easing the driver's stage output load (better distortion level and headroom).
2. Making input sensitivity better (additional 6dB of gain) for passive preamplifiers for example.
3. Eliminating a questionable complex negative feedback in the output stage (predictability in the load impedance for the driver stage and - I think - better distortion level).
I also remember some modifications Bruce made to the KT120 version in the driver stage to maximise the output amplitude of the stage to prevent distortions. So I assume there is some future limitations for the stage (I accept that my build works well though).
Regards,
Vladislav


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 Post subject: Re: KT120 Oddblocks
PostPosted: 22 Dec 2012, 17:48 
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Hi, Good discussion. The design goals were simplicity and quality output. With the present KT120 mono blocks I believe all that has been achieved. They are however essentially the top limit of the design. SRPP stages have some finite limitations, most notably the heater to cathode breakdown voltage. It is possible with multiple power supplies to get around this, but the complications don't appear to warrant it. I figure that 45 to 50 wrms is pretty much enough from most speaker systems. If you need more, then based on efficiency and such I recommend going SS. Really big tube amps pretty much have to go the class AB route and have multiple additional stages. A consequence of this is normally the need to use more global NFB than I find acceptable. (yes I know that U/L is local NFB). With the KT120 amps delivering under 0.1% distortion at 1 watt and not reaching 1% until past 10 (typically closer to 15) with a bandwidth of 10HZ to 35K and S/N of -90db. I see no particular need to alter the configuration. Great bang for the buck.

A sort of added thought. I chose the SRPP for the driver intentionally. As a voltage amplifier (not current) it is quite linear and has a large output capability. A feature that is overlooked but of prime importance to me is its ability to reject power supply noise. I really hate hum and noise and the SRPP and push pull output help in keeping it quite low. Even though both the SRPP and SIPP require a few more components than a SE version of either stage, I figure the end results justify those applications and that there is no simpler design that could.

BTW, I use a passive preamp about 95% of the time.

Good listening
Bruce

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 Post subject: Re: KT120 Oddblocks
PostPosted: 26 Dec 2012, 06:20 
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Bruce, I have a wild idea of "power SRPP". :) What if we put 2 KT120 on top of each other? Would it be possible to drive 8 (or even 4) Ohm without a transformer? Yes, there should be huge output capacitor, but it seems now it's easier than huge transformer. :)
Side thought about your added thought. With its around 6 dB of PSRR, SRPP loses the battle (about PSRR) with plain balanced design (which in some implementations has more than 20dB of PSRR). Both designs use 2 tubes/stage. SRPP has some more output capabilities, but using balanced design we automatically get two opposite signals to drive the output stage (which in the case does not have to be SIPP and does not reduces the amplitude). I know that convertion a single-ended signal to balanced poses some additional tricks, but it's always preferable to have balanced capability first hand and the paradigm of SIPP could be used here (in the driver side) also. But!!! I agree that this would be really different amplifier with its own problems and constrains.


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 Post subject: Re: KT120 Oddblocks
PostPosted: 26 Dec 2012, 20:46 
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Hi Poty, I suppose you do a SRPP with KT120s. But other than as a sort of exercise, it would probably be so inefficient as to not worth the effort. A SE EL84 might deliver more output. :)

As for rearranging the stages etc in the Oddwatt, it would as you noticed be a different amp. Hard to tell how it would work and sound. There are lots of choices in topology for amps and each has weaknesses and strengths. The strengths of an Oddwatt are simplicity, few non-linear components in the audio stream, low noise, clean sound and low distortion, the weaknesses are low efficiency, the need for a bit more input signal than many other types and it also places a few requirements on the driver tubes that may not occur in other designs.

Are they the be all amps for everyone, certainly not, but they will serve many folks quite well and out perform others that are significantly harder to build and more costly.

Good listening
Bruce

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