6DN7 tube HiFi amplifier mods and FAQ. PART 2 // Trying to make it sound even better!

[@MikeFreda]

The build video of this amp: kzbin.info/www/bejne/ZqTZo5aYjpeggqM

[@matthewf1979]

Antique Electronics Supply, Tube Depot and Mojotone are fantastic retailers of parts one would need to build an amplifier like this. I love the idea of using dissimilar tubes that aren’t popular. Really cuts down on the wiring and sockets.

[@MikeFreda]

Thanks for the comments. Yes, all those companies are great resources for obtaining parts to build tube amps. Cheers.

[@Nirky]

Thanks for a very informative tube amp video series. Especially the Cricut label method! How is the high frequency response? Reason I ask is I built an amplifier based on a tube I'd never used before, 6CY7 triode. It sounded good, but after I listened for a while I realized I was missing the very top end of the spectrum, say 16k and above. Have you done a response sweep with this amp?

[@MikeFreda]

Thanks for your comment. I have not done a sweep on it yet. I will on the next video. This tube is designed for a television and it’s my understanding that it is capable well above 20,000hz. I believe in a audio circuit the limiting factor would be the output transformer so I would not scrimp on them if you are looking to make a good sounding amp. Cheers.

[@Nirky]

@@MikeFreda Changing out the opt was the first thing I did. I think all or most tubes, at least those commonly used in audio applications, are capable of 20 kHz. The question is, at minus how many dBs? One can have a spec of 20Hz-20 kHz , but if at 20kHz it's down 3 dB, it's not going to sound very good.

[@cgoroo]

hi Mike, thanks for the updated vid. how did you measure AC hum at the end of the vid? (leads across the output transformer?)

[@MikeFreda]

Thanks. They way I do it, I ground the input (or you can even turn the volume to zero) and turn my multimeter on and lowest AC setting and put it across the speaker terminals. You need a dummy load or just have them still connected to your speaker. You are basically looking for any AC voltage on the output that is being generated from the amp itself.

[@doubledigitdreams]

Im really enjoying your tube gear build videos, they have inspired me to attempt your preamp build. Have you thought of making more audio diy content?

[@MikeFreda]

Thanks for commenting. I’m glad you like my amp content. I know my channel is a bit random but I do have a bunch of videos coming up all about tubeamps. Planning, designing, purchasing the parts and filming is time consuming. I wish I could do it faster.. that’s just the nature of these types of videos. Cheers.

[@lego40404]

I have 120v coming in from a Variac but it goes through a CL-70 before fuse. So I have about 118v. Wall voltage can vary depending on season, I’m in Buffalo now so I’m not to familiar with the fluctuations. I know from living in NYC my whole life my voltage could vary from 125-115. I have some AT7’s and AU7’s to roll

[@pmc9941]

Great video...again!!

[@MikeFreda]

Thank you.

[@lego40404]

I like the choke idea Mike but that is a expensive addition, same for those caps. I used a pair of Clarity caps only because I had them, I was going to order Solens. I was thinking about bumping up the mains as well, my PT has two options, 275-0-275 or 315-0-315. I went with 275 like yours, my B+ is about 10 volts less then yours. So I may try the 630 Vdc now. I do need to get a higher efficiency speaker set for this though. This amp was running a rebuilt/restored 1970 AR 2ax pair and it did sound really nice. I do have a 4S Universal pre in front the amp too, I have a 12ax7 in it but may swap to a 12AU7, it did sound warmer with the au7

[@MikeFreda]

Thanks again for your comment. Yes, a choke is a bit extra money for a slight gain in performance. I'm curious on what your incoming AC voltage is. I'm typically at 120VAC. If you are down to 114-115VAC that may be the discrepancy. I wouldn't worry too much about a 10V difference. 12AX7 are a bit harsh for hifi. Most people like a tube with less MU for a preamp. Great for guitar amps but harder to get sounding good for a hifi preamp. Cheers.

[@lego40404]

Any updates on the amp lately Mike? I’ve been running the amp on the 315-0-315 for awhile now and changed out the 12ax7 to a 12au7. I haven’t done anything else but I do want to upgrade the OT’s, hope supply chain has gotten better

[@rickg8015]

Hi.. Cool project. I love the aesthetic.. The PSU choke is a good upgrade. You might want to try getting a stiffer power trannie too that runs cool. Since you already upgraded the coupling caps to Calritycaps you might want to replace the cathode bypass caps in the output stage to Nichicon audio grade or Elna Silmic II 35V.. Lessen their value too anywhere from 220uF to 330uF.. 470uF might slow things down too much.. Please do replace those Cathode (white coffin) power resistors in the output stage too with non-inductive wirewound types from Dale, Mills, etc.. Kiwame 5W carbon film would be nice there too.. CCS can vary in sound signature, some can sound a bit on the solid state zingy so try and listen.. Just my personal thoughts.. Happy listening.. BTW, if your garage speakers dip below 4 Ohms your SE amp would really have that current sag on heavy bass passages..

[@MikeFreda]

Thanks for your constructive comments. Yes, this amp is a work in progress. I calculated the bypass cap to a 5hz cut off but going smaller may help the amp perform a bit better since my speakers can't really go that low anyway. I'm using 8" full range speakers which have a pretty flat response and are easy to drive. The PT could be upsized a bit to give me some reserve. It doesn't run that hot (compared to some guitar amps I have) The choke helps a lot. The only issue in running a larger PT is that the voltage will be a bit off and you will need to adjust some resistor values to suit. The next video of this amp will incorporate some of these mods and a CCS to see if it performs better. I suspect it will. The Morgan Jones style of CCS is pretty transparent to my ears. Cheers.

[@rickg8015]

@@MikeFreda Oh yeah, the amplifier tube section of the 6DN7 is lowish in Rp.. No need to change the cathode bypass cap value (5Hz cutoff is ideal..) Although sometimes you can get away with an 8Hz cutoff with tiny bookshelf speakers. Some designs I see use 0Hz cutoff, that it usually sacrifices the quality of the component inside the current loop (pricier and harder to find high quality parts in higher values. Quality is more critical in SE than in PP, IMHO..) Nice, Morgan Jones CCS👌🏼, you are good to go.. If your 8” FR speakers are efficient enough and have a benign impedance then you might need to upgrade the Power Tranny down the line.. You also might want to check out Edcor’s GXSE10-5K OPT’s as another alternative for builders..

[@newtronix]

Is it worth putting in a soft start circuit or delay for the ht to stop any cathode stripping on start up?

[@MikeFreda]

These are hearty tubes meant for TV’s. Adding a special circuit for a soft start is not really necessary in this amp. Adding a thermistor wouldn’t hurt at all. That’s what I did. You could also install tube rectification but that may require a different PT and different/lower value caps in the PSU. Cheers.

[@newtronix]

@@MikeFreda haha yes and being ex tv tubes i guess there's loads available for little cost. Not exactly a prized Western Electric 300b! As long as they sound good though! 😊

[@newtronix]

@@MikeFreda I did look at this vid before the soft start one you did also! Thanks for posting your vids anyway.

[@rayleung9775]

There is a local feedback on the input stage, Ck is bypassed.

[@andydelle4509]

I would never buy an "audio grade" capacitor unless it was from a recognized manufacture like Panasonic, Nichicon, Wima, ICC. A lot of these so called "audio grade" capacitors are low cost junk caps with fancy labels heat shrinked on. Buy good capacitors from established manufactures like I listed and only from reputable distributors, Digikey, Mouser, Newark, Allied. And since this not really a HiFi amp but a basic bench test amp, why do you need fancy coupling caps? Sprague Orange Drops or equivalent are just fine. You seem to have a good understanding of electronics and do quality work. Don't get caught up in this Audiophool BS! It's beneath your skills and knowledge.

[@MikeFreda]

Thanks for your comment. Yes, I would agree to purchase recognized brand caps from a reputable seller. As for this amp not really being HiFi... just because it doesn't use known audiophile tubes doesn't mean it does not sound good. This power amp is quite the sleeper! As for fancy coupling caps, yes, I probably could have gone with the proper rated Orange Drop or similar cao but for the slight extra cost and the placebo effect, I think they do sound better! lol Cheers.

[@andydelle4509]

@@MikeFreda Oh no, I didn't mean to imply the amp doesn't sound good. I have built quite a few tube amps as well. I also like the way you used TV tubes for audio. These are low cost and easy to find but also very good audio performers in some cases,