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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 18 May 2017, 06:25 
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This is more interesting. Hi-Frequency roll-off can be the result of stray capacitances in different parts, but first of all you should check the Spectralab options. Just to check - connect the input and the output with help of R1-R3 directly, without Groovewatt.


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PostPosted: 18 May 2017, 12:42 
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Hi, For testing purposes I equalize the input signal to look like a cartridge and then see how flat the output is. Yes it does introduce another variable, but it is a known value and works well for me.

I use mostly 1/4 watt carbon film resistors in this design. 1/2 watt is fine as well. There is virtually no power dissipation involved except in the power supply and the 20K cathode resistor in the output stage. It should be a 1 watt. I find that in builds where I used metal film resistors the end result was slightly (a db or so) quieter, but seemed to be about the same amount "brighter" sounding than all carbon film builds. Neither is a game changer, but worthy of mention. Do not use carbon comp resistors unless you like lots of hiss.

I recently discovered by accident....that if you want a little hotter top end you can change the 22k grid stopper in the second stage to about 18K or even 15K if you go much further than that the top end gets really energetic. At 18K there is about a +0.5 db at 20K HZ. With 15K it is about +0.75 db there. At 1K it is about 3.5 db and rising above the audio band significantly.

Good listening
Bruce

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PostPosted: 18 May 2017, 15:10 
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poty wrote:
This is more interesting. Hi-Frequency roll-off can be the result of stray capacitances in different parts, but first of all you should check the Spectralab options. Just to check - connect the input and the output with help of R1-R3 directly, without Groovewatt.

Measured both cables separately.
Attachment:
39K_response.png

Attachment:
10k_response.png


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PostPosted: 18 May 2017, 16:10 
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Sorry, what did you measure?


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PostPosted: 19 May 2017, 00:46 
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poty wrote:
Sorry, what did you measure?

I measured cables that was made for this tube amp measuring with Spectralab using this connection:
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PostPosted: 19 May 2017, 03:50 
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If you spoke about measuring just cables without Groovewatt in the middle - then you have something wrong with your test setup, because cables could not do so much attenuation.


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PostPosted: 19 May 2017, 21:49 
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Hi, Typical builds of the preamp are flat within about 0.5db from 25HZ to about 19K. Better builds with selected components extend that from 20HZ to 25K. This is all with a load of 50K and 100PF.

Good listening
Bruce

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PostPosted: 22 Aug 2017, 21:37 
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Hello Bruce. I was studying the groovewatt schematics, and have not been able to understand the passive equalization on the latest build. On the past ones, the values of the capacitors and resistors matched the calculation of a generic passive RIAA network. On the latest build, not only the series conected 27nf capacitor and 10k8 resistor are inverted (I expected to see the capacitor conecting to ground), but I cannot reproduce the RIAA calculations to match the component values. Can you help me understand? Has it something to do with the first stage output impedance (ECC803s SRPP)? Thanks!


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PostPosted: 23 Aug 2017, 03:23 
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Any passive circuit depends on source and load impedances. The Bruce's circuit is no exception. In calculations the source impedance should be connected in serial with the source signal and the input connection of the passive circuit, the load impedance - accordingly - in parallel with the output connection of the passive circuit


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PostPosted: 23 Aug 2017, 15:10 
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Hi, Which component of the resistor and capacitor in series goes to ground and which goes to the hot side doesn't matter. The values for the RIAA are first computed and then they are adjusted to match the circuit input and output impedances. The most significant being the anode impedance of the driving stage. That impedance becomes part of the value for the input resistor to the EQ section. It is dependent on the actual tube type and circuit configuration. Without going back to my notes I believe the value for the input resistor is around 89K and the anode impedance of the first stage about 24K as shown. If you use a cathode bypass capacitor to get a little more gain the value changes and the input resistor needs to be adjusted accordingly. The 470K grid resistor on the next stage is also included in the calculations and changing it will require re-computing the EQ. The actual value for the 27nf cap was selected to match the remainder of the circuit. The calculated value is 29nf but that causes the actual real world response to deviate from RIAA. This is a classic case of when the number crunching gets you close, but not actually on target. There are variables that we either can't model well, or the tolerances for components are against us. Essentially this is where folks like me jump in and make the thing work right. 8-)

Good listening
Bruce

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