KT88 - Single Ended Tube Amplifier Build 2018 - Part 8

BG210 - Wiring up the audio pot for volume control. Rechecking everything, powering up the amp, doing some voltage checks, and basic bench testing. Amp is preforming very well. Trying it out on a song...

[@dnewma04]

Paul (Kegger) was a good friend and I think of him often. He was a bit of a mad scientist and if you ever had the chance to see him at work in his lair, you'd know what I mean. You couldn't relax because you might lean against some 700v exposed voltage. :) I talked with him often through the process of coming up with this amp and his excitement level was always obvious. Really enjoyed this series of videos, I can sense some of the same excitement from you through the testing video and this one. Thanks for bringing some more attention to Paul's work.

[@curtisgriffin7924]

Excellent video and build. Can't wait to the tweaking video as that will give us so much info on how to tweak our amps. Excellent built amp, hard to tell the real audio quality over youtube have to hear it in person. Thanks again for the great vids

[@tremolux13]

This is a great series of videos, thank you Mark sounds good! Can’t wait for the upcoming and final videos.

[@EngineeringVignettes]

Waiting for this video to drop, thanks for uploading it. Spent the time going back through your backlog of older vids. It sounds like my early education was much the same as yours. Been an Electronics Engineer for ~25 years now with a tech career before that. And I spent my grade school days pulling tubes out of junked equipment as well. Thanks for going through the details of your progress, I understand it's geared for audiences with less experience in electronics but I follow along anyways, never know when you pick up some handy trick. I think this will be my first tube build but I will wait to see how your tweaks turn out. Nice %THD for a single ended amp, congrats! Cheers, - Eddy

[@MrBakedAlaskan]

Dig that tune, man! Thanks for the videos. I am learning a whole lot and entertained as well. Kudos!

[@danielthomas3057]

Very nice build. Great to see the amp operating in full glory. In upcoming video would like to see it's performance in both Triode mode and Ultra linear.

[@moodyga40]

Awesome mark thanks for sharing just what i want to build and you have done all the homework building it with us watching. i think i might try mine at 390 volts dc RMS

[@RobertKohut]

Nice!! Impressive power! Great video series!

[@materialsguy2002]

Thanks for this series, Mark. I am looking forwards to the detailed measurement video. Perhaps you could repeat the measurements after you do your tweaks as well, just to see if anything measurable has changed.

[@pauldow1648]

Handsome looking unit. A work of art.

[@donhiscock6933]

Exalent video Mark

[@OrganNLou]

GREAT JOB,,,,GORGEOUS AMP!!

[@johnsenchakinternetsecurit8935]

Nice Video Mark

[@rciancia]

Awesome build Mark. Only suggestion I would have is to add a 120v LED to the front so you know its on... BLUE of course. :)

[@Blueglow]

Tweaks!

[@mfr58]

That's pretty impressive, 15W, 0.5% with no global feedback!!

[@alexreeve]

turned out great!

[@RobertKohut]

Great job!!

[@THEtechknight]

I thought you were gonna flip that UL switch and see how it changed the performance!

[@basildouglas855]

Good day Mark, l complement you on this great build, well presented and a good job indeed. However in this comment I need to apologies to you for my poor attitude towards you regarding a previous video, ( I think it was a mod added to your hookup attenuator for testing amplifiers). About 2 months back. I am sorry for my remarks, they were uncalled for, disrespectful to you and totally unnecessary. My apologies for my bad behavior. Kind regards Basil Douglas

[@turnersparadise8368]

You can hear the amp despite the crappy camera mic. Sounds good, I wish I was there in person to hear it. Yeah, I think this may be my first build, but I will let you finish the series first. I am really looking forward to the tweaks.

[@Blueglow]

Just ordered me a full spectrum audio mic

[@lydiapawlak8564]

Yes; solution is to buy (4) of the 2"x2"x3/4" Bad Vibration squares from Meniscus Audio and put the chassis corners set on the squares. This also gives space for the amps natural capacitance to sound more open! Releases heat as well... The squares are corrugated hard rubber sandwiched with cork in between.

[@hollowaysteve]

Very nice indeed. Great record too. Who is it please. I notice you said it was on your ‘defunct’ record label.

[@ER-yq1lc]

Hi Mark, I'm thinking of doing this build or something similar and had a few questions for you. Working on a budget, if one were using EL34 only, would the 15 watt edcor OPT be fine? Many of the designs on the single-ended website use small signal pentode/triode tubes like 6an8 etc., could you talk a little about the differences in using those compared to a dual or single section triode for the driver? Could one use a 250k dual gang pot instead of the 100k so you could add an RCA handbook phono-stage at the input? Love your channel, thanks for sharing such great info and knowledge!

[@jeffcoulam]

Great job on the amp! Looks great, I wish I could hear what you're hearing :). I really like the song any chance you can share the artist?

[@robertferguson5120]

Hey Mark, do you have any tips or thoughts on holding down that long route of coax from back to front? It seems like it'd be sagging down when the chassis is flipped back over. Maybe some small dabs of epoxy on the heat shrink in a few spots?

[@RulgertGhostalker]

just got done watching the whole 8 part series, and thought of a good idea for the amp's chassis base if you want it open on the bottom; and that is fitting a decent hardwood picture frame, what would be picture side, up...where the amp chassis footprint is the size of the otherwise would be glass, ( But Then Oh Wait, the picture frame notch is on the wrong side..there has got to be some molding profile that could be 45-ed and would work ??? )

[@robharley9838]

Really enjoying the build Mark! I understand the practicality of the felt pads, I find them useful myself on projects. But ouch! I had to wince when I saw those feet. Hope you can find a way to use the bottom panel holes to mount proper feet for that beauty. Maybe some elastomeric mounted discs or some cones. :)

[@Blueglow]

Just ordered some black felt... never see either once it's sitting down but I do like the black over white

[@robharley9838]

I hear you. And it is nice and clean to have the chassis hug the table top. Just wondering if a little space underneath by way of some taller feet wouldn't provide an air circulation benefit. Really glad you're diving deep on this, I learn a lot of things. Thanks for what you do.

[@Blueglow]

@@robharley9838 Very little heat under the chassis. Only the KT88 cathode resistor putting off any heat. Everything else putting off heat is above the chassis.

[@robharley9838]

You're right of course, I get it. The thermal situation under there is not extreme. Perhaps it's just aesthetic sensibilities that are getting the best of me. Love the work Mark, you've got it goin' on. Best

[@russellesimonetta3835]

I,d put a bottom on it with vents. Then you can afix some feet on it also protects the innards when moving.

[@spyridonnassopoulos8281]

THANK YOU....

[@KleyDeJong]

Does anyone know who the artist is performing St. James Infirmary towards the end? Great version.

[@thisisjohnrichardson]

+1, anyone know who's cover this is?

[@stevenyork6712]

hi mark are you going to build a preamp for the 807 which i and my brother simon have built. they are working fine but i feel a preamp would give them a bit more input drive.my be it could serve both amps keep up the good work steven

[@Blueglow]

its on my project list

[@desmondroche7805]

Congratulations ! Beautiful build, as usual. Having watched all your previous build videos, I can’t help but ask, how does it sound compared to the SE 807 amp ? Perhaps the time to compare them is after all the tweaks are done on the SE KT88 amp ?

[@Blueglow]

vocals on the 807 are best I've ever heard. lows and highs on this are amazing, overall I'd pick this one

[@desmondroche7805]

Great. Now I know which one I’ll try to build !

[@tonup80]

Great job Mark! I wonder if you could replace the list of tubes listed on the schematic like el34, and etc with out any mods? Also which version of St James infirmary is this? I can't seem to find it online.

[@KleyDeJong]

I'd love to know too!

[@erikvantuinen2187]

Good videoes. Not sure where or how to bias the output tubes on a Sherwood s5500 2 tube amp? Need help.

[@dandee5018]

I had build a set of Altec Model 19s in solid Cherry.. with Altec parts.. but I built my own crossover from the Altec Schematic ..I think this amplifier would be a perfect match... It looks like a fairly simple build with point to point wiring.. which is the only way I would build one.. where do I get the BOM ??

[@akrsriv]

Hey mark, are you using any phono preamp with this? if yes, then which one.

[@SciPunk215]

Great work. Thanks for sharing!! I'm really confused about the phrase "line level". Why does everybody disagree on what voltage range constitutes "line level"? Why is it a range?? I'm also curious why we design amps that require a pre-amp? Is there a reason why we don't include an additional gain stage? Also, how many amperes does this amplifier draw from the wall when under load?

[@1959Berre]

Line level used to be 1V in the early days of telephone (hence "line"). Today it can have any value in the range of 150 mV - 1V

[@diabolicalartificer]

Why no cap between the pot and grid resistor?

[@danielthomas3057]

A quality job for sure. A bottom cover, though, is a good idea for safety. And a fine amplifier like this deserves to have proper feet on it rather than little pieces of double sided sticky tape. Just my humble opinion.

[@1959Berre]

Being a blues addict, I am glad you did not paint it yellow.

[@joemartin6439]

Fast fast fast you leave me in the dust

[@loverforever4473]

Nice sound i want build tis Amp for electric guitar and cigar box guitar

[@and1424]

Hello! Thanks for sharing. I built this kt88 amp. I have 220vdc in plates of driver, 2.6v in cathodes..but I get no more than 2.8watts per channel. My power transformer is 350-0-350. You believe that B+ being low might be the cause? I have 347vdc in plates of kt88. Thanks for your help!

[@jdsalinger73]

Your B+ seems pretty low to me. You should have around 400V with a 700VCT power transformer. Is there a problem with your output transformer? Did you follow the schematic?

[@RobertZovko]

More than excellent characteristics, 15W for 0.5% thd. (UL) and in triode mode?

[@Blueglow]

will test both next week. just tested triode today

[@RobertZovko]

I'm running between 400-410V B+ and a 500ohm cathode resistor to get 8.2W that's running in UL, in triode mode around 5.5W (no visible clipping).

[@samuel_towle]

Hello again. @17:50 where you are showing voltages on your schematic, looking at the 33 mfd. cap in the feedback circuit your showing 442 volts on a 400 volt capacitor. Is that a typo on the cap rating or should that be increased to a 500 volt capacitor?

[@samuel_towle]

In part 9 I saw it was a 450 volt rated capacitor, and it's in the Plate supply for the driver, not feedback. Still might be a bit hot for that cap though.

[@gizmothewytchdoktor1049]

mark. i have to ask....no d.c. blocking audio safety cap on the rca input before the audio pot?

[@Blueglow]

Trying to build it the way Keggar & others did in the original design. All those are tweaks but not required. Tried to stick to the basics and make it simple.

[@gizmothewytchdoktor1049]

k :-) got it. simple wins. me...i am just paranoid. too many years doing p.a. work.

[@ampdoc]

Dear Mark, kind of a Mayday, Mayday, Mayday! The schematic of the actual amp (PS is ok) on the website shrunk to a point that the values are indiscernible: the interstage cap now looks like 22uF instead of 0.22uF. The dot is not visible anymore, and there's no zero before it either.

[@Blueglow]

I'll fix this weekend.

[@harbselectronicslab3551]

you can buy the "Loop" side of Velco separately at the store......just cut that into strips if you want black.

[@Blueglow]

good idea

[@jtfoto1]

Such a shame I am in Australia running 230V imput. Would love to know how to adjust components for that voltage.

[@MrMannakin]

You would need a different power transformer is all.

[@christopherweber1905]

Do I need a additional preamp for this amp?

[@ashutoshsrivastava8056]

Hi Mark, I was trying to build this amplifier. I wired it and then tested it but found that the sound is very less. Also, vocals are very subdued. While measuring the voltages across, I found that B+ is around 465V, voltage at 33uF cap positive is around 434 V, but at plate of 6N1P, i am getting 186 volts at pin 1 and 177 volts at pin 6. I am planning to change 39K resistance with a 31/32 k resistance. I also used 180 Ohm resistance with LED for biasing. The voltage i am getting at the cathode of 6N1P is 2.97 V. should i try to use 160Ohm Resistance or 2.97 is fine? Regards

[@Blueglow]

2.97 seems ok but feel free to play around with that value to see how it affects sound

[@ashutoshsrivastava8056]

@@Blueglow But is plate voltage of 6N1P too low. The amplifier is giving an auditorium like sound with male voice fully subdued..

[@ashutoshsrivastava8056]

Forgot to mention that my speakers are 6 Ohms, and i have used 8 Ohms tap of audio transformers. My 8 Ohms drivers are in transit..

[@christopherredfern6472]

@@ashutoshsrivastava8056 ive just built this amp and have the same 186 volts - did you find the answer ?? - ive never understood why this guy mark never replies or helps out like he ought to chris

[@akrsriv]

@@christopherredfern6472 My amplifier was getting feedback and interference from the volume control knob. I replaced the volume control with different one where connections were far away and the problem was sorted out. The volume is now great. 186 V is good for listening to vocal music. First try removing volume control from your circuit to pinpoint the issue. Ashutosh

[@chrisvinicombe9947]

Don't most modern sources have a 2.3V output? Both of my CD and DVD players do. How would it handle that? Sounds good with that vinyl 😁

[@Blueglow]

Would work fine, mine have to use volume knob a bit. 2.3v is a good bit

[@chrisvinicombe9947]

Thanks for the reply. Yeah I have to use the -20db muting on my vintage amp or the volume control is a switch. I don't know why digital sources need to be so loud really.

[@1mctous]

@@chrisvinicombe9947 So they'll drive an amp to clipping without a preamp. You can use a passive device (source switch/volume pot) and eliminate a gain stage as I have.

[@1mctous]

That old Audio Research apparently has a good MM phono stage. The KT-88s seem to counterbalance the Ortofon's (2M Blue?) somewhat harsh treble.

[@chrisvinicombe9947]

@@1mctous I forget that not all speakers are easy to drive.

[@rubenproost2552]

How can we say an amp sounds a certain way if distortion is very low. Doesn't every amplifier that approaches zero distortion sound the same..? Or could there still be a different interaction with the speaker? Damping factor, asymmetric damping, etc. I noticed this amp does not have the output transformer in the negative feedback loop.

[@Blueglow]

No, far from true. Way more than distortion determines the sound / color of an amplifier.

[@swinde]

Why pick a song based on over driven distorted guitar?

[@rylanjensen9270]

Music.. we need a link.

[@loverforever4473]

But you must make a more Audio jac and make a mixer . Trible. Bass . Volume

[@joemartin6439]

Love watching u but too stupid to understand much

[@sun_korea]

Schade-feedback is goood

[@medicwheat69]

I am telling you I want that record. Lol. Seriously.