I've been mulling over a few performance enhancements for my KT120 Mk. 2 amp. The major addition I wanted to add was an Enhanced Fixed Bias (EFB) output stage regulator. While I was in there, might as well throw in a few power supply upgrades I've been contemplating also.
The basic design of all the iterations of my "KT120 amp" is:
Sliding bias up or down without changing DC balance is doable with individual bias adjust pots, but each change in bias requires resetting DC balance--a tedious and cumbersome adjustment. So with the addition of EFB, I decided to add a bias adjust pot on the EFB board itself, where one single pot is used to set bias for all four output tubes. Then, each individual adjust pot that previously was used for both DC bias and balance is now only needed for DC balance. This is working well in that I can set DC balance once via individual adjustment of each of the four pots, and then slide bias up or down by adjusting the bias pot on the EFB regulator board.
The test procedure then is basically the following:
As stated in the Mk 2 thread, the Mk. 2 amp (with no EFB), measured with the following distortion numbers at 50 watts output, both channels driven, and at 60 mA quiescent current per tube:
But, I don't like idling the KT120's at 100 mA each. That's a little too hot for my tastes. So I think a reasonable compromise here is to find the idle current that will deliver < 1% distortion at full power output from 20 Hz to 20 KHz. That idle current was determined to be 85 mA per tube. Plate voltage was measured at 465V, which idles each tube at 66% of max plate dissipation. Distortion at this bias setting is:
Now, since I have a quad of new production Tung Sol 6550's available, I thought I would measure distortion with them in the amp. Minimum distortion reading was found to be at 77 mA and 475V plate, at 50 watts output, both channels driven. Quiescent dissipation is 87% of max. Distortion readings are:
My gut says that if I were to convert the output stage to pentode mode (instead of UL) I would be able to achieve the same distortion numbers at lower static dissipation levels. But there is a tradeoff...usually more feedback is required with pentode output stages to obtain the same damping factor as with less feedback but in UL mode. And more feedback represents its own set of issues that need to be dealt with.
So overall, the numerous iterations of this amp I've built over the last 6 years have lead me to what I believe is about the best this topology and configuration has to offer. For sure this is the best overall performing amp I am personally able to create. I keep saying that as I build each amp, as I learn a little something new with each build. But I think also because of the off-the-shelf parts used, this near world class performance within the stereo chassis configuration is within reach of the normal DIYer.
My thanks again to Dave Gillespie for inventing the EFB regulator and making it public!
I will leave you with the Mk. 3 schematic as well a few pics of the amp and the EFB board. This is a really pretty amp with a polished aluminum chassis that is clear anodized and with wood accents. The unique thing about this chassis is the transition from front face plate to top plate is a clean 90 degree angle made from one solid piece of aluminum. It makes for a very clean look.
Amp:
EFB and negative bias board:
Schematic attached as PDF document.
The basic design of all the iterations of my "KT120 amp" is:
- Two channels on one chassis with one power supply for both L and R channels.
- Mullard-like topology: triode voltage amp, 6SN7 cathode coupled inverter, UL connected output stage.
- KT120 power tubes.
- Triode Electronics clones of the Dynaco A431 output transformers (these have 33% UL taps)
- UL operation.
- B+ voltage of 475V supplied to center taps of output transformers.
- Fixed bias operation.
- The Mk. 3 utilizes an Enhanced Fixed Bias (or EFB) output stage regulator whereas the Mk. 2 does not.
- The Mk. 3 utilizes a snubber on the high voltage secondary winding for some power supply noise suppression whereas the Mk. 2 does not.
- The Mk. 3 swaps the positions of first and second banks of capacitors in the high voltage supply before and after the smoothing choke to reduce transient ringing within the L/C interaction of these components. In the Mk. 2, the first filter cap was an equivalent 275 uF and the second filter cap (after the choke) was an equivalent 110 uF. In the Mk. 3, these capacitor banks are reversed.
Sliding bias up or down without changing DC balance is doable with individual bias adjust pots, but each change in bias requires resetting DC balance--a tedious and cumbersome adjustment. So with the addition of EFB, I decided to add a bias adjust pot on the EFB board itself, where one single pot is used to set bias for all four output tubes. Then, each individual adjust pot that previously was used for both DC bias and balance is now only needed for DC balance. This is working well in that I can set DC balance once via individual adjustment of each of the four pots, and then slide bias up or down by adjusting the bias pot on the EFB regulator board.
The test procedure then is basically the following:
- Set bias to 60 mA (that's what I used on the Mk. 2), set power output to 50 watts both channels driven, then measure distortion.
- Find the lowest distortion bias setting at 50 watts output, both channels driven.
- For giggles, substitute in new production Tung Sol 6550 tubes (because I have a quad laying around), and find its lowest distortion bias setting at 50 watts output, both channels driven.
As stated in the Mk 2 thread, the Mk. 2 amp (with no EFB), measured with the following distortion numbers at 50 watts output, both channels driven, and at 60 mA quiescent current per tube:
- 20 Hz: 1.35%
- 1 KHz: 0.42%
- 20 KHz: 2.0%
- 20 Hz: 0.95%
- 1 KHz: 0.25%
- 20 KHz: 1.25%
- 20 Hz: 30% decrease
- 1 KHz: 40% decrease
- 20 KHz: 37% decrease
- 20 Hz: 0.8%
- 1 KHz: 0.15%
- 20 KHz: 0.71%
But, I don't like idling the KT120's at 100 mA each. That's a little too hot for my tastes. So I think a reasonable compromise here is to find the idle current that will deliver < 1% distortion at full power output from 20 Hz to 20 KHz. That idle current was determined to be 85 mA per tube. Plate voltage was measured at 465V, which idles each tube at 66% of max plate dissipation. Distortion at this bias setting is:
- 20 Hz: 0.82%
- 1 KHz: 0.17%
- 20 KHz: 0.95%
Now, since I have a quad of new production Tung Sol 6550's available, I thought I would measure distortion with them in the amp. Minimum distortion reading was found to be at 77 mA and 475V plate, at 50 watts output, both channels driven. Quiescent dissipation is 87% of max. Distortion readings are:
- 20 Hz: 0.97%
- 1 KHz: 0.165%
- 20 KHz: 1.15%
My gut says that if I were to convert the output stage to pentode mode (instead of UL) I would be able to achieve the same distortion numbers at lower static dissipation levels. But there is a tradeoff...usually more feedback is required with pentode output stages to obtain the same damping factor as with less feedback but in UL mode. And more feedback represents its own set of issues that need to be dealt with.
So overall, the numerous iterations of this amp I've built over the last 6 years have lead me to what I believe is about the best this topology and configuration has to offer. For sure this is the best overall performing amp I am personally able to create. I keep saying that as I build each amp, as I learn a little something new with each build. But I think also because of the off-the-shelf parts used, this near world class performance within the stereo chassis configuration is within reach of the normal DIYer.
My thanks again to Dave Gillespie for inventing the EFB regulator and making it public!
I will leave you with the Mk. 3 schematic as well a few pics of the amp and the EFB board. This is a really pretty amp with a polished aluminum chassis that is clear anodized and with wood accents. The unique thing about this chassis is the transition from front face plate to top plate is a clean 90 degree angle made from one solid piece of aluminum. It makes for a very clean look.
Amp:
EFB and negative bias board:
Schematic attached as PDF document.
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