{"id":9898,"date":"2019-03-24T17:07:55","date_gmt":"2019-03-24T21:07:55","guid":{"rendered":"http:\/\/www.engineeringradio.us\/blog\/?p=9898"},"modified":"2023-03-19T08:31:58","modified_gmt":"2023-03-19T12:31:58","slug":"tube-amp","status":"publish","type":"post","link":"https:\/\/www.engineeringradio.us\/blog\/2019\/03\/tube-amp\/","title":{"rendered":"Tube Amp"},"content":{"rendered":"<p>I have dipped my toe into the world of tube (or valve) audio.\u00a0 The first thing that I learned was that in general, tube amps are expensive.\u00a0 It seems that the least expensive amps run about $1,000 US, and from there it seems the sky is the limit.<\/p>\n<p>There are a number of less expensive Chinese versions floating around, most of the tube audio experts call them garbage.\u00a0 Myself; I am not so sure.\u00a0 There are also a lot of somewhat dubious claims made by the same experts about speaker cables, AC power conditioning, and so on.<\/p>\n<p>I was going to build a single-ended tube amp based on the KT88 design found here:<\/p>\n<p><iframe loading=\"lazy\" title=\"KT88 - Single Ended Tube Amplifier Build 2018 - Part 1\" width=\"660\" height=\"371\" src=\"https:\/\/www.youtube.com\/embed\/eNBgo9Md8IU?feature=oembed\" frameborder=\"0\" allow=\"accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" referrerpolicy=\"strict-origin-when-cross-origin\" allowfullscreen><\/iframe><\/p>\n<p>That is a whole series of videos, eighteen in all I think, on the design and construction of a single-ended KT88 audio amp.\u00a0 If you have the time, well worth it to watch.<\/p>\n<p>Then I decided that I really do not have a lot of time for that and I just wanted to try a tube amp and see if there is really that big of a difference.\u00a0 Thus, I purchased one of the Chinese designs based on the RCA 829B tube, which is kind of exotic looking:<\/p>\n<figure id=\"attachment_9912\" aria-describedby=\"caption-attachment-9912\" style=\"width: 1200px\" class=\"wp-caption alignnone\"><a href=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/FU-29.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-9912\" src=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/FU-29.jpg\" alt=\"FU-29 Chinese equivalent to RCA 829B dual pentode tube\" width=\"1200\" height=\"900\" srcset=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/FU-29.jpg 1200w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/FU-29-150x113.jpg 150w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/FU-29-650x488.jpg 650w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/FU-29-768x576.jpg 768w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/FU-29-400x300.jpg 400w\" sizes=\"auto, (max-width: 1200px) 100vw, 1200px\" \/><\/a><figcaption id=\"caption-attachment-9912\" class=\"wp-caption-text\">FU-29 equivalent to RCA 829B dual pentode tube<\/figcaption><\/figure>\n<p>That is the Chinese version FU-29, there is also a Russian <span class=\"ref_result\">\u0440\u0430\u0434\u0438\u043e\u043b\u0430\u043c\u043f\u0430<\/span> \u0413Y-29.\u00a0 The good news is that there are lots of these tubes available for not too much money.\u00a0 New Old Stock (NOS) RCA 829Bs run about $25-30 each.\u00a0 A Ulyanovsk GU-29 (NOS) runs about $10.00 (made in the USSR).\u00a0 Somewhat rare are the 3E29 tubes, which were designed for VHF pulsed radar.\u00a0 These are dual pentode tubes that can be run ether parallel (single-ended) or push-pull.\u00a0 They were originally designed for VHF transmitters, but have been put into use in HF transmitters and audio amplifiers.\u00a0 The USSR versions are long-life militarized versions and designed for aircraft radar; flying upside down at Mach 2 in -50 C temperatures 18,000 meters AMSL&#8230;\u00a0 My Russian friend tells me I am joking.\u00a0 I am joking.<\/p>\n<figure id=\"attachment_9914\" aria-describedby=\"caption-attachment-9914\" style=\"width: 1200px\" class=\"wp-caption alignnone\"><a href=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/reflector-6N3P-E.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-9914\" src=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/reflector-6N3P-E.jpg\" alt=\"Reflector factory, 6N3P-E dual triode tube\" width=\"1200\" height=\"900\" srcset=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/reflector-6N3P-E.jpg 1200w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/reflector-6N3P-E-150x113.jpg 150w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/reflector-6N3P-E-650x488.jpg 650w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/reflector-6N3P-E-768x576.jpg 768w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/reflector-6N3P-E-400x300.jpg 400w\" sizes=\"auto, (max-width: 1200px) 100vw, 1200px\" \/><\/a><figcaption id=\"caption-attachment-9914\" class=\"wp-caption-text\">Reflector factory, 6N3P-E dual triode tube<\/figcaption><\/figure>\n<p>The driver tubes and phase inverters are 6N3P-E (6N3, 6N3P, 6N3P-EV, 5670, 2C51 or 396A can also be used) which is a double triode tube, made by Reflector in Sartov, Russia.\u00a0 These tubes are also militarized long-life versions.<\/p>\n<figure id=\"attachment_9915\" aria-describedby=\"caption-attachment-9915\" style=\"width: 1200px\" class=\"wp-caption alignnone\"><a href=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/Audioromy-M828A.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-9915\" src=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/Audioromy-M828A.jpg\" alt=\"Audioromy M828A push pull tube amp\" width=\"1200\" height=\"900\" srcset=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/Audioromy-M828A.jpg 1200w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/Audioromy-M828A-150x113.jpg 150w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/Audioromy-M828A-650x488.jpg 650w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/Audioromy-M828A-768x576.jpg 768w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/Audioromy-M828A-400x300.jpg 400w\" sizes=\"auto, (max-width: 1200px) 100vw, 1200px\" \/><\/a><figcaption id=\"caption-attachment-9915\" class=\"wp-caption-text\">Audioromy M828A, power transformer and output transformers<\/figcaption><\/figure>\n<p>The Audioromy M-828A amplifier seemed like a good compromise between price, power, and workmanship.\u00a0 I ordered the amp from Amazon and it took about a week to arrive.\u00a0 The first thing I did was take it apart and look at it.\u00a0 I was expecting poor workmanship and cheap components, etc.\u00a0\u00a0 Overall, it seems to be pretty well-made. There are two printed circuit boards; one for the power supply, the other for the front end before the two power amp tubes.\u00a0 The power supply uses solid-state diodes, which some view as a compromise to a tube amp design.\u00a0 There are also several power supplies on one board; 460 VDC B+ for power tubes, 220 VDC screen supply, a -25 VDC grid bias supply, 12 VDC for the audio switching relay, +6 VDC for the driver\/phase inverter filaments.\u00a0 I like the idea of DC filament voltage on the driver tubes.<\/p>\n<figure id=\"attachment_9922\" aria-describedby=\"caption-attachment-9922\" style=\"width: 1200px\" class=\"wp-caption alignnone\"><a href=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/audioromy-M828A1.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-9922\" src=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/audioromy-M828A1.jpg\" alt=\"Audioromy M828A underside\" width=\"1200\" height=\"900\" srcset=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/audioromy-M828A1.jpg 1200w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/audioromy-M828A1-150x113.jpg 150w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/audioromy-M828A1-650x488.jpg 650w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/audioromy-M828A1-768x576.jpg 768w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/audioromy-M828A1-400x300.jpg 400w\" sizes=\"auto, (max-width: 1200px) 100vw, 1200px\" \/><\/a><figcaption id=\"caption-attachment-9922\" class=\"wp-caption-text\">Audioromy M828A underside<\/figcaption><\/figure>\n<p>This amp is configured for push-pull and rated at 30 watts per channel.\u00a0 I will test all of that plus measure THD, frequency response, and so on.<\/p>\n<p>There is no manual, which I find a little bit annoying.\u00a0 Also, there is a lack of a schematic diagram or any instructions on biasing and balancing the tubes when they are replaced.<\/p>\n<p>Being thus annoyed, I did some deep diving on the intertubes and found that some people had posted on how to re-bias and re-balance the thing after tube replacement.\u00a0 There were also several modifications suggested.<\/p>\n<ol>\n<li>Replace the input potentiometer with something a little more substantial.\u00a0 It does seem to be a little bit cheap and I do not like the notches in the volume adjustment.\u00a0 I will do this mod.<\/li>\n<li>Replace the coupling caps with oil-filled units.\u00a0 Not so sure about this one, but I might try it just to see if it makes a difference.<\/li>\n<li>Install a bias regulating circuit using an LM317 voltage regulator between the output tube cathode and ground.\u00a0 This seems like a good idea.<\/li>\n<li>Roll (replace) the input and power tubes with better versions of US-made or Russian-made tubes.\u00a0 The input tubes are 6N3P-E tubes from Reflector (Sartov, Russia) which are already pretty good tubes.\u00a0 I might replace the FU-29&#8217;s with a set of GU-29&#8217;s at some point.<\/li>\n<\/ol>\n<p>There appear to be several schematic diagrams with slight variations based on the changes in design over the years.\u00a0 Several designs have different input and phase inverter tubes.\u00a0 Some have different power supplies, still, others show no anode resistors or a cathode resistor.\u00a0 This is the diagram for the amp that I own, which was produced circa 2018 or so:<\/p>\n<figure id=\"attachment_10020\" aria-describedby=\"caption-attachment-10020\" style=\"width: 1169px\" class=\"wp-caption alignnone\"><a href=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/04\/m828A-1.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-10020\" src=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/04\/m828A-1.jpg\" alt=\"Audioromy M-828A schematic diagram\" width=\"1169\" height=\"826\" srcset=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/04\/m828A-1.jpg 1169w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/04\/m828A-1-722x510.jpg 722w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/04\/m828A-1-150x106.jpg 150w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/04\/m828A-1-650x459.jpg 650w, https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/04\/m828A-1-768x543.jpg 768w\" sizes=\"auto, (max-width: 1169px) 100vw, 1169px\" \/><\/a><figcaption id=\"caption-attachment-10020\" class=\"wp-caption-text\">Audioromy M-828A schematic diagram<\/figcaption><\/figure>\n<p>Full schematic here: <a href=\"https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/M828A_full.pdf\" target=\"_blank\" rel=\"noopener noreferrer\">https:\/\/www.engineeringradio.us\/blog\/wp-content\/uploads\/2019\/03\/M828A_full.pdf<\/a><\/p>\n<p>After all my investigations were finished, I put the amp back together and plugged it in.\u00a0 I then ran my known CDs through it and it sounded a bit rough.\u00a0 I was a little bit disappointed until someone said that it takes about 10 hours or so for a tube to break in.\u00a0 I connected it to my speaker test load (8 ohm, 50 watt resistors) and let it run for a day.<\/p>\n<p>What a difference a day makes.\u00a0 The second listen to the same CD proved to be much, much better.\u00a0 There is definitely some coloration from the tubes.\u00a0 A side-to-side comparison between my solid-state Kenwood VR-309 amp and the Audioromy M-828A has the tube amp sounding much richer.\u00a0 There is no real way to say it; it sounds full while detailed and clean all at the same time.\u00a0 Playing through my homemade speakers, which are mid-range deluxe, stringed instruments sounds very detailed.\u00a0 You can hear the pick hit the strings on an acoustic guitar.\u00a0 You can hear the bow scrape across the strings on a cello.\u00a0 It is unlike any amp that I have ever owned.<\/p>\n<p>I am enjoying very much listening to Dave Mathews and Tim Reynolds Live at Luther College CD as I am typing this.<\/p>\n<p>Now, I don&#8217;t know what the difference between this amp and the $10,000.00 version of the same tube amp made in Canada, other than the $9,500.00 difference in price.<\/p>\n<p>A few comments about this amp and the 829B push-pull amp design.\u00a0 First of all, since the screen grids are connected internally, there is no way to run this tube in ultra-linear mode.\u00a0 Usually, ultra-linear mode involves taking feedback from the output (anode) or the output transformer and feeding it into the screen of the power tube.<\/p>\n<p>Secondly, it is widely commented that these amps are notoriously difficult to bias and balance.\u00a0 One or both sides of the output tube will red plate due to over current.\u00a0 I am hopeful the LM-317 bias regulator circuit will take some of the difficulty out of this.\u00a0 With an ordinary push-pull amplifier, the balancing issue is taken care of with matched tubes.\u00a0 Since both tubes in this push-pull circuit are in the same envelope, getting a matched pair is not likely.\u00a0 So, the tricky act of balancing the two outputs from the same tube will have to be carried out each time the tubes are replaced.\u00a0 That being said, hopefully, a set of those Soviet tubes will last for a long time.<\/p>\n<p>One thing that I did do is make a bunch of voltage measurements and noted them on the schematic diagram.\u00a0 If there are ever any problems with this unit, having a set of base voltage measurements should go a long way toward troubleshooting and repairing it.<\/p>\n<p>Finally, while the 829B is a rather exotic tube, it likely does not perform to the level of an EL86 or KT88 single-ended design.\u00a0 That being said, I have no problems with purchasing this amp and I am enjoying the toob audio sound very much.<\/p>\n","protected":false},"excerpt":{"rendered":"<p>I have dipped my toe into the world of tube (or valve) audio.\u00a0 The first thing that I learned was that in general, tube amps are expensive.\u00a0 It seems that the least expensive amps run about $1,000 US, and from there it seems the sky is the limit. There are a number of less expensive &hellip; <a href=\"https:\/\/www.engineeringradio.us\/blog\/2019\/03\/tube-amp\/\" class=\"more-link\">Continue reading <span class=\"screen-reader-text\">Tube Amp<\/span><\/a><\/p>\n","protected":false},"author":4,"featured_media":9911,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[102],"tags":[319],"class_list":["post-9898","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-audio","tag-audiophile"],"_links":{"self":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/9898","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/comments?post=9898"}],"version-history":[{"count":20,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/9898\/revisions"}],"predecessor-version":[{"id":11059,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/posts\/9898\/revisions\/11059"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/media\/9911"}],"wp:attachment":[{"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/media?parent=9898"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/categories?post=9898"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.engineeringradio.us\/blog\/wp-json\/wp\/v2\/tags?post=9898"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}