tag:blogger.com,1999:blog-1501574484591494853.post5101607109469198985..comments2022-10-09T14:04:43.466-07:00Comments on Blues Harp Amps: 12AX7 Tube Swaps, Hot Mics, and Other MythsRick Davishttp://www.blogger.com/profile/10785663666440700258noreply@blogger.comBlogger18125tag:blogger.com,1999:blog-1501574484591494853.post-17562986193377986192012-09-24T05:54:04.601-07:002012-09-24T05:54:04.601-07:00[Lets see how long it is before someone chimes in ...[Lets see how long it is before someone chimes in that to get good tone it takes PRACTICE. I left that part out because it is painfully obvious.] <br /><br />you got that right Rick.lol It is all about balance, I use a crystal mic when playing blues, and adjust the amp and mic to get the best sound, then mic the amp, but for Rock and roll I use a Fireball dynamic mic with a wireless sender (so we can Rock n Roll). With the fireball I need more volume on the mic and more volume on the amp to get a comparable but smoother sound. The mic, amp and speaker all combine to create the sound, [lets see how long etc, } and then capure that sound as faithfully as possible with micing up. I used to turn the amp up till I could hear it on stage, which destroyed the tone, and I used to use a line-out which bypassed the speaker. No point in installing an expensive speaker upgrade and then bypassing it. Pete.pigeon petehttps://www.blogger.com/profile/16084644103480859515noreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-50548699016605029702011-05-17T20:52:43.569-07:002011-05-17T20:52:43.569-07:00here"s my problem i have a amp made in china ...here"s my problem i have a amp made in china and want to use more commonly available tubes the preamp are 6n2 tube.which i want to go to 12ax7,i know a rewire of it will have to be done. then the power tubes are 6p1 . and i want to go to el84 ,does anyone have a simple plan that this can be done with ?thanks in advance .<br />tom<br />tominrexmont@yahooAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-78413119647459884812010-05-16T14:23:11.317-07:002010-05-16T14:23:11.317-07:00Rick,
Thanks for your posts here. I just replaced ...Rick,<br />Thanks for your posts here. I just replaced the 12AX7 in my Blues Deluxe Twee reissue with a 5751 I picked up online. I still have a nice tone with more volume, playing with a Shaker crystal mic. I also use a Shaker Retro Rocket that I like for a different style and a Shure 520DX.<br /><br />I still cannot turn this amp up beyond 4 without feedback but I think I'll be fine with that. Planning to try it out at a jam tonight so we'll see. <br /><br />Anyway, just wanted to say thanks to you and the other contributors here and leave some feedback (of the good kind) here.<br /><br />BuzzsawUnknownhttps://www.blogger.com/profile/14489509132222783171noreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-69633225845141654562010-03-27T13:23:22.293-07:002010-03-27T13:23:22.293-07:00Guys, I need tube help. I have a 66 aa165 bassman ...Guys, I need tube help. I have a 66 aa165 bassman head in a fender clone type cab w a weber 15a150(jdnewell cab 1x15). My ech removed the ss rectifier and installed a tube 5u4gb rectifier for sag. I play blues guitar and harps in bands. I had this done to have a true 2 channel amp w sag w independrnt settings,tone wise. He also insalled a bias sw. for cathode bis to fixed bias. Heres the issues: first I dont understad the simple diff of the bias switch's purpose, other tha moore or less power?. Two: My band is not loud, I can only use the volumes at about 3! Unless I turn up the vol on the guitar and bullet mics. I decided to pull the 12ay7s and ax7's out of v1,v2,v3. I can now turn up the amp volume more, but according to all these posts, I need to knoow am I hurting the amp itself? Please, I dont understad much techtalk, other than the tube swap ideas all over the internet, and the diff. between the 5_ _ series tube rectifiers. (I also use a Clark Beaufort tweed deluxe, and a magnatone w a 10" weber 10a125 spkr.) Please advise and thakks drew davis,ny dmaj135@yahooAnonymousnoreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-1197521239394618302010-02-16T17:10:21.273-08:002010-02-16T17:10:21.273-08:00Ah OK... other then simple black face component va...Ah OK... other then simple black face component values used vs silver face component values, I think what you are describing is mostly about changing the power tube's biasing circuit from a "bias balancing" circuit to a "bias level" setting circuit.<br /><br />The notion that "to create a "true biased circuit" (as opposed to the advertised "biased circuit" that was actually unbiased) in the early 70's TR's,"... is actually incorrect as all tubes including power tubes have to be biased in order for them to work.<br />The biasing circuit in the stock AA-270 amp sends a differential bias voltage from that pot to each power tube in order to "balance" the idle current of the tube as a set.<br />However, there is no easy way to adjust the bias current level without changing the fixed resistor in the circuit.<br />If that fixed resistor was also a pot, you'd have the best of both worlds, Power tube current balance and power tube idle current level!!Bruce Mission Ampshttps://www.blogger.com/profile/12332665477545401048noreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-24948959511957126452010-02-16T14:36:56.508-08:002010-02-16T14:36:56.508-08:00Bruce,
The way it was explained to me is specific...Bruce,<br /><br />The way it was explained to me is specific to the Twin Reverb, but to my understanding is true for any early Silverface Fender amp that had a Blackface predecessor (and maybe this is wrong but I've read it in other places too when researching the early Silverface Twin Reverb). <br /><br />When the Silverface Era began, CBS changed the values of various capacitors and resistors in the amp to create a cheaper production cost. In lessening the values, some wires had to be re-routed (I guess to redistribute power?), which in turn created an unbiased circuit (AA-270) that was advertised as a biased circuit (AB-763). <br /><br />Now, the changes that were made were very minimal but ultimately changed the tone and overall sound quality of the amp. Historically, the Silverface TR is shot down and said to be crap, but what people don't know is that you can very easily "Blackface" the circuitry, to be identical to the original AB-763, and very sought-after, Blackface TR, which is what I did to my amp (1971 TR in near mint condition). <br /><br />As you'll notice in the AB-763 schematic layout (which is the biased circuit), there is a pot with what looks like a flat-head screw in the middle that is connected to the transformer. This would be the biasing pot. You can change the voltage going into the tubes by turning this clockwise and counter-clockwise to your liking. In the AA-270 schematic, this pot is there, but had virtually no effect or use within the amp. You couldn't change the voltage going into the tubes. <br /><br />So, to create a "true biased circuit" (as opposed to the advertised "biased circuit" that was actually unbiased) in the early 70's TR's, the capacitors and resistors need to be changed back to their original values, and one or two wires need to be re-routed. This allows you to use the pot as it was originally made for, and to have better and a more custom tone in the Twin Reverb.<br /><br />Maybe I'm wrong, I'm not an expert. But this is how it was explained to me, and I've also read this in articles about the early 70's Twins. Did this help?<br /><br />-DaveDave Eisnerhttps://www.blogger.com/profile/05638717674040093158noreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-74318343117370637502010-02-16T10:57:56.746-08:002010-02-16T10:57:56.746-08:00In regards to Dave Eisner's comments;
What doe...In regards to Dave Eisner's comments;<br />What does a "true biased circuit" mean?<br />Also, what do you mean by:<br /> "I swapped out the capacitors and changed them to their original values as they were in the '65 TR, so I have that circuit. I can take a screwdriver to the pot while playing and turning to changed the power going into the tube."<br />What circuit and do you mean? What capacitors?<br />And, do you mean change the bias voltage pot setting to alter the idle current of the power tubes... or what? Capacitors won't change the bias circuit so I am just a bit confused by what you are descibing.Bruce Mission Ampshttps://www.blogger.com/profile/12332665477545401048noreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-86311504255883778022010-02-13T13:49:59.274-08:002010-02-13T13:49:59.274-08:00Something to keep in mind is that just because the...Something to keep in mind is that just because the tube base and the pin out is the same for any given group of tubes, (specifically, base #9A, the very tubes this thread seems to be about) ...it doesn't mean you can simply swap them all around with absolutely no consequence.<br />Yes, of course some consequences very subtle and others are more severe!!<br />********************************<br />Almost all triode tubes discussed here (dual or otherwise) are set up to operate in single ended Class A mode. That means they are drawing MAXIMUM plate/cathode current at idle.<br />A good example of a glitch I've seen is when a customer is experimenting with preamp tubes and installs a tube such as a 12AU7, which has a very low impedance and draws much more quiescent idle current then the 12AX7 it replaced, ... and all in an cheaply made amp circuit, designed to just barely support the original 12AX7 in the first place.<br />Most of the time these amps are built around a very very cheap price point and use inexpensive parts with low power rated supporting resistors on a clock radio style printed circuit board. <br />The original 12AX7 could be running it's supporting plate or cathode resistors right at it's 1/8w to 1/4w rating and when trying to support a tube such as a 12AU7 and it's subsequent higher idle current, those low wattage parts can and do fail. Sometimes they get so hot the come unsoldered from the PCB!<br />Another side effect, which also effects the amp tone as much as the "tone" of the tube, is that the extra current these other tubes demand, create additional voltage drops across the resistors in the B+ rail nodes. The nodes are used to deliver the proper B+ to the original tube as the designer had planned.<br />Now those "delivered" plate load voltage nodes are all different too.Bruce Mission Ampshttps://www.blogger.com/profile/12332665477545401048noreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-66835854843391772722010-02-13T10:49:01.743-08:002010-02-13T10:49:01.743-08:00I really like this post, Rick. In addition to wha...I really like this post, Rick. In addition to what you said, the circuitry must be a true biased circuit. <br /><br />When Fender sold the company to CBS (i.e. the start of the Silverface era), CBS changed a number of values of the smaller capacitors in that were originally Blackface amps to reduce production cost. <br /><br />What a lot of people don't know is that CBS Fender advertised these amps as having a true biased circuit, when actually they were not. The slight changes in capacitor value does not allow for a true biased circuit. For example, my '71 Twin Reverb has a sticker on the inside panel with tube values and it actually says "AB763 Circuit," when it actually should read "AA763" or "AA270." <br /><br />Now, it doesn't really matter because I swapped out the capacitors and changed them to their original values as they were in the '65 TR, so I have that circuit. I can take a screwdriver to the pot while playing and turning to changed the power going into the tube. It's pretty cool, but like you said, when you bias them too high the sound waves square off and it sounds pretty awful.Dave Eisnerhttps://www.blogger.com/profile/05638717674040093158noreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-48717663173776288472010-02-13T06:08:35.734-08:002010-02-13T06:08:35.734-08:00Interesting post. I find many harp players will re...Interesting post. I find many harp players will recommend tube swapping, not so much for tone change, but just to cool down the feedback monster. My CM bullets are hot, and while it's not a problem with my Champ, stepping up to my Deluxe or reissue Bassman I need lower gain preamp tubes, as even using a volume control at the mic will make these amps scream at relatively low amp volume settings. <br /><br />I agree with you that tone is not improved with a mere preamp tube swap. But does using lower gain tubes, such as the 12AY7 or even the 12AU7 in an amp designed for the 12AX7 cause any problems to the amp circuits?Big Ernie Fullernoreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-82189822641380574372010-02-12T23:32:03.736-08:002010-02-12T23:32:03.736-08:00Well the HG2 is basically a Princeton with a few t...Well the HG2 is basically a Princeton with a few tweaks. My mistake I should have typed 5F2A not H.Paul "Kingley" Routledgenoreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-65609327604750865292010-02-12T18:22:37.980-08:002010-02-12T18:22:37.980-08:00Somebody has to point out the obvious. Is the HG2...Somebody has to point out the obvious. Is the HG2 a 5F2H or a tweed Princeton?Joe's Blues Bloghttps://www.blogger.com/profile/00878016113359073133noreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-40062680202266455852010-02-12T08:52:03.759-08:002010-02-12T08:52:03.759-08:00Joe, yer killin' me here....Joe, yer killin' me here....Rick Davishttps://www.blogger.com/profile/10785663666440700258noreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-14500504661183778952010-02-12T08:51:35.090-08:002010-02-12T08:51:35.090-08:00Drew, the 12AY7 tube is GREAT in circuits for whic...Drew, the 12AY7 tube is GREAT in circuits for which is is designed. This article is about swapping tubes into guitar amp circuits specifically designed for the 12AX7 tube. There is a common misconception that all the 12A tubes are interchangeable. They're not, and there are tonal consequences -- however slight they may be -- for using them in place of a 12AX7 without also making other changes to the amp.<br /><br />BTW, the 12AU7 -- which I don't recommend using in amps at all -- was a great tube in some old vintage HiFi amps. They used two of these low-gain tubes in series for a lush tone.Rick Davishttps://www.blogger.com/profile/10785663666440700258noreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-91509696040045861952010-02-11T13:02:10.329-08:002010-02-11T13:02:10.329-08:00It's important to observe, that the first prea...It's important to observe, that the first preamp slot in Bassmans, Bandmasters, Pros, Supers and Deluxes SHOULD be a 12AY7. That's how they were manufactured, shipped and used in the 50s - and using a Bassman Reissue with a 12AX7 in the first slot is a recipe for harp feedback.<br /><br />As regards those old tweeds - I wouldn't change a damn thing, tubewise. Just make sure you have killer NOS tubes. After that it's down to the mic you use, your technique with the mic and whether or not you know how to dial in an amp.Ev630noreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-17223506533808653002010-02-11T10:28:30.738-08:002010-02-11T10:28:30.738-08:00To get good tone, you've got to practice.To get good tone, you've got to practice.Joe's Blues Bloghttps://www.blogger.com/profile/00878016113359073133noreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-53861644323497240762010-02-11T04:40:32.936-08:002010-02-11T04:40:32.936-08:00Paul, thanks for visiting my blog and posting a th...Paul, thanks for visiting my blog and posting a thoughtful comment.<br /><br />The 12AY7 tube in a custom harp-specific amp does not have the liabilities it may have in a guitar amp, such as a Fender. The HarpGear Rock Bottom's circuit has been designed to accomodate the 12AY7 tube. The problems may occur when this tube is used in a circuit designed to support a 12AX7.<br /><br />In a small amp (the subject of this article), a 56% reduction in gain is significant and unneeded. If the goal of the tube swap is to reduce feedback, turning down the volume on your hot bullet mic will indeed give you a better result.<br /><br />Swapping in a 12AY7 will not give the player "Chicago tone and breakup." You know that. Using expensive NOS tubes won't do it either. Getting an amp voiced correctly for harp -- especially a guitar amp -- is a bigger job that requires patience and a good ear. There are no instant plug-n-play fixes.<br /><br />Again, thanks for visiting my blog.Rick Davishttps://www.blogger.com/profile/10785663666440700258noreply@blogger.comtag:blogger.com,1999:blog-1501574484591494853.post-35751846162708345852010-02-11T02:53:35.241-08:002010-02-11T02:53:35.241-08:00I do agree with a lot of what you have said Rick. ...I do agree with a lot of what you have said Rick. I do have a different view point on some of the things you mention though.<br /><br /> would always use a 12AY7 in the pre-amp of most harp amps (not all though). This is simply because I have found over the years that a good 12AY7 (NOS RCA for example) gives a deeper thicker tone and adds a lot of warmth to the overall tone, especially in Fender amps. <br /><br />I totally disagree that turning down the volume on the mic with a CR/CM element gives the same result as a tube swap. That is just so far removed from my own personal experience.<br /><br />Also If someone is using a crystal element for example then the mic is not working at it's optimum output to generate the capabilities of it's full tonal spectrum, if the volume is turned down on the mic. <br /><br />A 12AY7 can live perfectly happily in a 12AX7 preamp slot with no problems to the circuitry. Which I think is worth mentioning. .<br /><br />It's interesting that you recommend that people buy a HarpGear HG2 (a fantastic amp by the way). That amp is based on a 5F2H circuit and uses a 12AY7 in the preamp, as does the HarpGear Rock Bottom. <br /><br />I would always advise people to try a 12AY7 if they have feedback problems, along with other things such as tone settings and mic cupping techniques.Paul "Kingley" Routledgenoreply@blogger.com