| Posted by Dave | November 20, 2016 - 08:36 pm |
| Hi Dan -- Thank-you for your comments! Converting the front end of the HF-60 to a Williamson style front end would be a very good way to go, and in deed would provide more gain. The history of the doubled up 6CG7 gain stage is that the modification originally took life as promoting a change in output tubes, as the 6550 simply works better with the TO-330 than the EL34 family of tubes does. That was the sole aim of the modification. However, after I presented that modification, I had a number of requests to convert the input stage to triode operation, while reducing the excessive input gain of the HF-60 design. I suggest that a conversion could be made, and would actually improve HF distortion performance. Hence, the change to the doubled up triode input stage. That configuration allows for better HF distortion performance, as the output impedance of the stage is much lower than would be obtained by simply converting the original EF86 input stage to triode operation. A low output impedance means that the HF transition network will have less effect down within the audio bandwidth, leaving more of the distortion canceling NFB in place to reduce HF distortion. Also please understand that the effort was to still try and work within the topology of the original HF-60 design, so that an all out gutting as you are suggesting was dismissed from the get-go. If you change phase inverters, I strongly suggest that you either stay with a cathode coupled or phase splitter topology. The paraphase type inverter you describe is notably inferior in terms of maintaining equal response from the output of the inverter stage, both in amplitude as tubes are changed, and in phase response at low frequencies, since the "bottom" drive leg goes through an additional R/C network versus the top leg. If you do convert to such a design, the global NFB loop must still be inserted into the cathode leg of the AF amplifier section of that type of inverter stage. To insert it into the bottom (or second) 6CG7 grid would principally only apply the NFB to the bottom side of the push-pull pair. Thanks again for your comments! Dave |
| Posted by Dan Marshall | November 20, 2016 - 02:22 pm |
| Dave, I was considering the input phase inverter of the configuration where the first stage output signal is divided down by a factor of the stage gain and fed to the grid of the other tube half to provide the phase inversion. The 6CG7 cathode bypass cap is then eliminated and since you now have equal outputs from each half of the 6CG7 the effective output voltage is doubled giving an equivalent of twice the input stage gain. The unbypassed 6CG7 cathode resistor serves to further balance the first stage output signals. It would be necessary to cap couple into the 6SN7 grids. Since both 6SN7 grids are now receiving equal and opposite signals this results in twice the 6SN7 output voltage which results in a total of four times the gain. Feedback is applied to the second 6CG7 grid. The unbypassed 6SN7 cathode resistor further balances the drive signals to the output tubes, which is now considerably less critical. The drive signals are now self balancing rather than taking a stab at balance by using unequal 6SN7 plate resistors. Seems like a fair bit of improvement to me. Dan |
| Posted by Dan Marshall | November 20, 2016 - 09:34 am |
| Hi Dave, I just came across your post, which is of considerable interest as I have collected most of the parts (of better quality than the original) to build a pair of upgraded HF-60s. If changing the input tube to a 6CG7, or other similar dual low rp triode, then why not go the extra step and configure it as the phase inverter stage with the 6SN7 functioning as the driver stage; this would result in twice the equivalent input stage gain and also twice the 6SN7 gain, keeping the required input voltage at one volt. This would allow the driver stage to use equal value plate resistors, and also somewhat lessen the requirement for a constant current cathode source. Also, wondering whether you made a comparison of EL34 and 6550 with your upgrade mods installed. I have a quad of unused Svetlana Gold top EL34s and some Svetlana 6550s and JJ Kt88s. Dan |
| Posted by Dave | April 02, 2016 - 02:27 am |
| Hi Nick -- Yes it's possible, although I'm not sure what you'd gain, other than -- if correctly done -- slightly increasing the input sensitivity of the design. This would come about from changing the input stage from a cathode bias to fixed bias operation. Such a move would eliminate the normal degeneration occurring across the cathode resistor network, providing the slight additional gain mentioned. But for it to be actually realized, the NFB network would need to be adjusted to account for the increased Open Loop Gain (OLG) of the new bias arrangement, so that the original NFB factor could be maintained in spite of the new OLG level. With that adjustment, the input sensitivity would then increase. But the increase would be insignificant, as because the 6CG7 is a medium mu tube, it's gain level is relatively low to begin with, meaning that there is little degeneration occurring to begin with. Also, most of the existing cathode bias resistance is used in the bottom leg of the NFB voltage divider network, meaning that the degeneration you could eliminate is only that which occurs across the 270 ohm resistor -- or not very much at all. It would be an awful lot of work for precious little return on that work -- meaning that it would be more of an exercise in showing it could be done, rather than accomplishing any significant result because of it. If you want to try it -- go for it, and let us know how it turns out. I'm just saying that from this vantage point, any sonic improvement will likely be all due to expectation bias, rather than improvement that could pass any measure of scientific evaluation. Thanks for the interest! Dave |
| Posted by Nick | April 01, 2016 - 01:22 am |
| Hi! Dave, is it possible to use battery bias for 6CG7? how to do it? Thanks. |
| Posted by Dave | April 13, 2015 - 08:38 am |
| Hi Zekk -- Thanks for the kind words, and your interest in my HF-60 modification! If you applied to modification to an actual HF-60 with the Acrosound TO-330 output transformer, then the response of the modified amplifier itself is quite flat from easily below 20 Hz, to well over 20 kHz. And, because of the NFB level employed, the damping factor is also quite high for a vacuum tube amplifier (nearly 18 by classic calculations). Therefore, if you are finding that bass frequencies are excessive, you might also check the damping of your speakers, their location, or the LF response of your preamp before altering the components of the NFB loop. Besides the characteristics of the amplifier, all of these things can individually or collectively have a huge impact on the level of bass frequencies produced. Thanks again for your comments, and good luck with your projects! Dave |