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Thread: Using a bigger Output tranny?

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    Using a bigger Output tranny?

    Hi,

    one of the most popular mods in harp amps is to use bigger output trannies. My question is: Can I use any tranny that has the correct impedance on both ends or is there more I have to look out for? For example, can I put a Blackface Super Reverb tranny into a 5E7 Tweed Bandmaster without any additional mods if the speakers can handle the additional power?

    thanks!

  2. #2
    Senior Member Wilder Amplification's Avatar
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    Quote Originally Posted by Bluefinger View Post
    Hi,

    one of the most popular mods in harp amps is to use bigger output trannies. My question is: Can I use any tranny that has the correct impedance on both ends or is there more I have to look out for? For example, can I put a Blackface Super Reverb tranny into a 5E7 Tweed Bandmaster without any additional mods if the speakers can handle the additional power?

    thanks!
    What do you mean by "additional power"? The impedance of the primary against the B+ voltage sets the current draw, and the product of current and voltage across the load (i.e. the OT primary) = power. As long as they're close to identical impedance the physical size of the OT won't matter.

    The benefit to running a bigger OT is that you're less likely to run into OT saturation, which causes low end to fall apart and doesn't sound very good IMO. With an OT that's less likely to saturate you'll more than likely notice that you've got more punch in the lows when the amp is being ran hard at high volume.
    Jon Wilder
    Wilder Amplification

    Quote Originally Posted by m-fine
    I don't know about you, but I find it a LOT easier to change a capacitor than to actually learn how to play well
    Quote Originally Posted by JoeM
    I doubt if any of my favorite players even own a soldering iron.

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    Bad choice of words, sorry ... while IMHO it is a benefit of tweed amps when used for guitar that the low end is not as boomy as on Blackface amps. This keeps them out of the bass player's frequency terretory and contributes to the band sound. However, they could use a bit more low end punch when used as a harp amp. That's where the idea of usig a bigger output tranny comes in.

    I still have to learn a lot about trannies and I read somewhere that some are more "efficient" than others and therefore might put out more power on the secondary side.

    anyway ... thanks for your answer!!!

  4. #4
    Senior Member Wilder Amplification's Avatar
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    Quote Originally Posted by Bluefinger View Post
    Bad choice of words, sorry ... while IMHO it is a benefit of tweed amps when used for guitar that the low end is not as boomy as on Blackface amps. This keeps them out of the bass player's frequency terretory and contributes to the band sound. However, they could use a bit more low end punch when used as a harp amp. That's where the idea of usig a bigger output tranny comes in.

    I still have to learn a lot about trannies and I read somewhere that some are more "efficient" than others and therefore might put out more power on the secondary side.

    anyway ... thanks for your answer!!!
    In regards to efficiency, yes this is correct but felt I should add some clarity to it.

    No transformer is 100% efficient. A 50 watter might be feeding close to 80 watts into the primary for a 50 watt output, which would equate to an efficiency rating of 62.5% (Output Power / Input Power = % Efficiency).

    Some transformers are in fact more efficient than others, so it does depend on the % efficiency as to how much of the primary power makes it into the secondary.

    I agree that harp amps benefit from additional low end punch and since OT core size has a direct effect on this you're definitely on the right track there. Another way cool trick that they also benefit from is a badly asymmetrical power amp drive as well (i.e. unbalancing the phase inverter or cold biasing a class A SE amp). You can actually add an asymmetry control to the PI quite easily so that you can adjust from bone stock to badly asymmetrical. The Kinder Harp King amps feature this control...can't remember off the top of my head what they call it though.

    As an additional trick, another thing to consider if you want to add some low end is to try out a resonance control in the negative feedback loop. This involves adding a capacitor in series with the negative feedback resistor, then adding a 1M linear taper pot across it. All the way down the nfb loop is stock, then as you rotate the pot you undamp the speaker more and more at lower frequencies. The cap acts like a low value resistor at high frequencies so high frequencies never see it and will receive the same amount of negative feedback as they would without the cap there, but to lower frequencies it appears as a bigger resistor, so nfb is decreased at low frequencies, which makes more lows appear at the output. Sort of like a Presence control but for low frequencies.

    Of course, no capacitor is "brick wall" so I'm sure it will affect the highs SOME, but probably not enough to matter for what you'd be doing with it.
    Last edited by Wilder Amplification; 04-05-2010 at 05:36 PM.
    Jon Wilder
    Wilder Amplification

    Quote Originally Posted by m-fine
    I don't know about you, but I find it a LOT easier to change a capacitor than to actually learn how to play well
    Quote Originally Posted by JoeM
    I doubt if any of my favorite players even own a soldering iron.

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    Great suggestions, thanks!!!

    Which leads me to additional questions (just tell me to go to hell if I'm getting too annoying):

    1) Can you give me a hint on how to unbalance a PI? I have just started to figure out how these work at all but I don't feel confident enough to work this out by myself. AFAIR it's just replacing a resistor with a pot but I can't remember.

    Edit: wouldn't putting a pot in series with one of the 56k resistors at the PI create that effect? If yes, does it matter, if it's done on the plate or on the cathode side? I'm I on the right track at all? I wonder how much throwing it off would be safe.

    Don't they call it a "raw control" or something like that?

    2) The theory behind that -FB controll makes perfect sense. The amp I am planning to build will be based on a 5E7 Bandmaster. Would you say this would make more sense at the -FB loop in the preamp section or the one around the speaker jack? I assume the latter. At what cap value roughly would you start to experiment?
    Last edited by Bluefinger; 04-05-2010 at 06:38 PM.

  6. #6
    Senior Member Wilder Amplification's Avatar
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    Quote Originally Posted by Bluefinger View Post
    1) Can you give me a hint on how to unbalance a PI? I have just started to figure out how these work at all but I don't feel confident enough to work this out by myself. AFAIR it's just replacing a resistor with a pot but I can't remember.

    Edit: wouldn't putting a pot in series with one of the 56k resistors at the PI create that effect? If yes, does it matter, if it's done on the plate or on the cathode side? I'm I on the right track at all? I wonder how much throwing it off would be safe.
    On that particular amp since it uses the cathodyne/split load PI I would alter the plate resistor, but I suppose you could do either/or.

    Quote Originally Posted by Bluefinger
    2) The theory behind that -FB controll makes perfect sense. The amp I am planning to build will be based on a 5E7 Bandmaster. Would you say this would make more sense at the -FB loop in the preamp section or the one around the speaker jack? I assume the latter. At what cap value roughly would you start to experiment?
    You'd wanna do it with the NFB loop from the speaker. I'd start with like a 0.02uF. Going higher will shift the affected frequencies lower and vice versa.
    Jon Wilder
    Wilder Amplification

    Quote Originally Posted by m-fine
    I don't know about you, but I find it a LOT easier to change a capacitor than to actually learn how to play well
    Quote Originally Posted by JoeM
    I doubt if any of my favorite players even own a soldering iron.

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    Thanks

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    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    I've NEVER heard that one of the most popular mods is to use a bigger OT in a harp amp and I've been around a long time!! Where did you hear that?
    Especially with respect to old Fender tweed amps.
    I don't think you are going to notice very much low end difference with a Super Reverb @ 4K OT in that amp... the lowest note on a D harp is well above the low end, -3dB power bandwidth of any guitar amp OT and the 5E7 already uses a solid 30-35 watt OT.
    With the right stiff rectifier and B+ voltage, you might get more total power out of two 6L6s with the 4K Super Reverb OT but I bet it will be due to the impedance difference of the original 5E7 OT and the SR OT at 4K.
    But, is it a real 5E7 OT, running the proper Zed or a generic cheater Pro Reverb OT from some NET vendor? That makes a big difference too.
    Also, the 5E7 and 5F4 amps are made to run at +6K ohms at 2.67 ohm to 4 ohm load... the BF Super Reverb is 4K ohms at 2 ohms.
    Bruce

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    Well, I can't recall all the sources but I have heard it often enough that it made me think that there might be some truth to it. The term "big output iron" is used often when it comes to advertize a harp amp. Anyway, I'm not saying that that this is the case at all. I'm just collecting as much info as possible and then I'd like to try it all to see what works for me and what doesn't.

    Initially I wanted to convert my 5F4 to a harp amp but since it's such a great amp for guitar I decided to leave it alone and build me a new amp from ground up that I'll be using as a harp amp exclusively. So far the plan is to start with a Tweed Bandmaster.

    The decision which exact components will be used has not been made yet. I will probably use MojoTone trannies since I have had good results with those so far but I'm still open to any option as long as I stay within my budget.

    My 5F4 was built mostly from a Kit and it had a 4K Brown Vibrolox OT (MOJO770) included indeed. I wonder what the idea behind this is???

    Well, FWIW, so far my battle plan is (some mythbusting included):
    1) start with a 5E7 Bandmaster
    2) use Weber 10A125 speakers with H-dustcap
    3) experiment with various preamp tube combinations
    4) try SED winged C 6L6 or Tung Sol 5881 power tubes (my favorites so far)
    5) try lowering the preamp voltages
    6) tight filtering
    7) experiment with preamp -fb
    8) use .1 coupling caps
    9) try fixed / cathode bias switch
    10) try variable speaker -fb
    11) try unbalancing the PI
    12) install fixed presence control value and use pot location for either #10 or #11
    13) might try a different OT (although I'm pretty confused now)
    14) use Mallory 150 caps (because they are small, sturdy and sound as good as anything else I have tried)
    15) use metal filmresistors (because I am either death or there is no audible difference between those and carbon comps)
    16) install a 20k bias pot for the fixed bias section.
    17) try a 5meg input resistor ... some say they work better with high Z harp mics.

    This will probably be a long build with all the testing and trying out different combinations but I'm looking forward to it ...

  10. #10
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    yeah... mess around with the amp, its educational.
    Keep in mind that most harp players never blow a harp that goes below 150Hz.. That is right in the wheel house of just about any well made 25-35 watt 6L6 OT and will allow the 6L6s to make full power at those freqs, so don't fall for the big ass OT is what you need to sound good with harp.
    A lot of builders use a 4K OT in those amps... it is a matter of taste.
    4K sounds good I use 6k5 in the tweed 6L6 amps but I have custom wound 5K OTs too. Real subtle stuff.

    By the way, unbalancing the the phase inverter is GROSSLY over rated.
    It already is a bit unbalanced anyhow but don't go crazy because G Weber says so... he is a bombastic bloviator.

    Unsolder the 6L6's 220K grid load resistors from their eyelets and remove them. Change them to 120K and solder them at one end each but keep the bias point lifted.
    Install a 250K L trim pot between the two resistors and feed the bias voltage at the center lug of the trim pot.
    Another way is to make the 56k tail resistor of the PI (under the 1k5 bias resistor in the cathodyne driver) a 50K pot wired as a variable resistor with a 10K resistor to ground.
    That really messes up the balance too.

    Other mods in your list that I think are over rated:
    2
    8
    9 ... easy to see which you like best. Don't under bias these power tubes like it would be in a guitar amp.
    11 and 13, you all ready know my feelings.

    I'll assume you are not Italian, a step child of the Harmonicats and or play a bass harp and turn blue....
    Attached Thumbnails Attached Thumbnails 4b08280f1447c.jpg   bm-4.jpg   action2.jpg  
    Last edited by Bruce / Mission Amps; 04-06-2010 at 07:39 AM.
    Bruce

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    Thanks for your input!

    With all that info available on the web it's really hard to tell the difference between fact, myth and personal taste. Even pro builders sometimes get into arguments about this. Some take Gerald Weber's "harp amp secrets" as the holy bible ... I prefer to take it with a grain of salt. All I can do is to use my experience from previous builds and find out the rest myself. I might not have the experience of others but I do have a lot of patience and I am willing to learn and tinker ...

    Quote Originally Posted by Bruce / Mission Amps View Post
    Other mods in your list that I think are over rated:
    2
    8
    9 ... easy to see which you like best. Don't under bias these power tubes like it would be in a guitar amp.
    11 and 13, you all ready know my feelings.
    Those 10A125-O Weber speakers get great reviews so I might give them a try. I have the standard ribbed cone versions in my 5F4 and I like them very much. The only other 10 inchers I have compared them to directly so far were various Jensen RI speakers and Eminence Blue Alnicos but the Webers are miles ahead of those IMHO. YMMV

    The .1 coupling caps are subtle for sure but smaller ones cut off some bass frequencies that I don't want to miss. I even use them in smaller and brighter guitar amps like the Super.

    There might be a lot of BS on my list. However my usual routine is, if that is possible, to use a temporary backpanel made of cheap plywood and install a switch there that allows me to switch between the modded and the unmodded component. This way I can jump back and forth and compare the results directly. This is very enlighting sometimes.

    oh, I forgot ... I'll try a solid state rectifyer as well.

    Quote Originally Posted by Bruce / Mission Amps View Post
    I'll assume you are not Italian, a step child of the Harmonicats and or play a bass harp and turn blue....
    Well ... not exactly
    Last edited by Bluefinger; 04-06-2010 at 11:20 AM.

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    It is not common to use overrated OTs in harp amps. I would still use the SR OT if you have one, just because it's a decent impedance match (5.333K:2.67ohms), it's only marginally overrated, many repro 5E7 use a 35W OT these days any way (probably why Mojo went with the Vibrolux tranny in the 5F4 kit).

    Massively overrrated OT will just result in a more sterile tone.

    I would not unbalance the PI unless balance can be restored 'on the fly' by the use of a pot/switch. As Bruce says, unbalanced PIs are greatly overrrated. It kills fidelity, can warm up a harsh/bright amp, doesn't usually give you any more power. Unbalancing power tube plate current (+/- 10mA?) is the simplest way to achieve this in your amp, how about a bias balance pot & mark the pot at the point of balance for reference?

    I wouldn't "fix" the presence control either.

    Try colder than normal bias in fixed mode, say 10-15mA. Sovtek 5881 are my favourite tube for 3x10" & 4x10" amps, I find the SED has poor note separation and can go somewhat blurry. If the PT can take the extra heater current try EH KT90 (better headroom, punchier dynamics), bias as for Sovtek 5881 (they usually bias up in a broadly similar range). Power tubes that sound good in a 2x10" don't always work so well in a 3x10" or 4x10", and vice versa.

    5Meg input resistor gives better touch response & front to the note, irrespective of mic used...excepting the Shure 520DX dynamic, which just feeds back more. Xtal, ceramic, CM, CR, Hi-z dynamics (other than 520DX), lo-z dynamics with an in-line imedance transformer all work well with a 5 Meg input load.

    Lowest note on an A harp is 220Hz, you want the amp to reproduce an octave below for full harmonics, bear in mind that many players also like Low F (176Hz) & similar harps...in short the amp/speakers should shelve the low end around <90Hz mark...the difference between that and 100Hz is tangible. Nothing worse than picking up a Low F or, G harp and have your 1st position low notes come out all hollow, tinny & monotonic because the amp only reproduces the higher harmonics.

    "With all that info available on the web it's really hard to tell the difference between fact, myth and personal taste. Even pro builders sometimes get into arguments about this." - most of the guys producing "harp" amps have a quite personalised sound in their heads, subsequently many veer off the 'middle ground' kind of sound you typically get with a Fender...it just depends which side of the middle ground you like to stand. If you like cathode bias (Meteor), you're not going to buy a Sonny Jr 410 or a HarpKing - conversely if you need an amp to deliver short punchy phrases & really throw out chromatic, stiffer fixed bias might be preferable. Old & reproduction Fenders are still the most popular amps used for harp on stage.

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    Hi,

    Quote Originally Posted by MWJB View Post
    It is not common to use overrated OTs in harp amps. I would still use the SR OT if you have one, just because it's a decent impedance match (5.333K:2.67ohms), it's only marginally overrated, many repro 5E7 use a 35W OT these days any way (probably why Mojo went with the Vibrolux tranny in the 5F4 kit).

    Massively overrrated OT will just result in a more sterile tone.
    So many opinions ... I don't have any useable left overs at this time so I will have to buy one anyway. If I decide to start with a kit amp I might just use what's included and see how it behaves.

    Quote Originally Posted by MWJB View Post
    I would not unbalance the PI unless balance can be restored 'on the fly' by the use of a pot/switch. As Bruce says, unbalanced PIs are greatly overrrated. It kills fidelity, can warm up a harsh/bright amp, doesn't usually give you any more power. Unbalancing power tube plate current (+/- 10mA?) is the simplest way to achieve this in your amp, how about a bias balance pot & mark the pot at the point of balance for reference?
    It depends how I will like it and if I think that there is a setting I prefer or if it has to be adjustable. I might keep it inside the amp and mount it directly to the board.

    Quote Originally Posted by MWJB View Post
    I wouldn't "fix" the presence control either.
    I rarely ever adjust the presence control at all. I seem to have a favorite setting that I hardly ever touch. If I decide that the unbalance pot or the variable -fb makes sense for me and I need to have that adjustable without opening the amp, then the presence control will be the first one that has to go.

    Quote Originally Posted by MWJB View Post
    Try colder than normal bias in fixed mode, say 10-15mA. Sovtek 5881 are my favourite tube for 3x10" & 4x10" amps, I find the SED has poor note separation and can go somewhat blurry. If the PT can take the extra heater current try EH KT90 (better headroom, punchier dynamics), bias as for Sovtek 5881 (they usually bias up in a broadly similar range). Power tubes that sound good in a 2x10" don't always work so well in a 3x10" or 4x10", and vice versa.
    Haven't tried Sovtek 5881s yet. My local supplier lists two types of them, 5881/6L6WGC and 5881WXT. Which ones are the ones you would recommend? So far I really like the Tung Sols, especially in Tweed Pro and Super amps.
    I have stayed away from EH power tubes since I don't like their 6L6 tubes at all (too harsh and brittle). I think in an amp that size I don't need extra headroom anyway.

    Quote Originally Posted by MWJB View Post
    5Meg input resistor gives better touch response & front to the note, irrespective of mic used...excepting the Shure 520DX dynamic, which just feeds back more. Xtal, ceramic, CM, CR, Hi-z dynamics (other than 520DX), lo-z dynamics with an in-line imedance transformer all work well with a 5 Meg input load.
    I am using Shure CM and Astatic Crystals so I'll give it a try ...

    Quote Originally Posted by MWJB View Post
    Lowest note on an A harp is 220Hz, you want the amp to reproduce an octave below for full harmonics, bear in mind that many players also like Low F (176Hz) & similar harps...in short the amp/speakers should shelve the low end around <90Hz mark...the difference between that and 100Hz is tangible. Nothing worse than picking up a Low F or, G harp and have your 1st position low notes come out all hollow, tinny & monotonic because the amp only reproduces the higher harmonics.

    "With all that info available on the web it's really hard to tell the difference between fact, myth and personal taste. Even pro builders sometimes get into arguments about this." - most of the guys producing "harp" amps have a quite personalised sound in their heads, subsequently many veer off the 'middle ground' kind of sound you typically get with a Fender...it just depends which side of the middle ground you like to stand. If you like cathode bias (Meteor), you're not going to buy a Sonny Jr 410 or a HarpKing - conversely if you need an amp to deliver short punchy phrases & really throw out chromatic, stiffer fixed bias might be preferable. Old & reproduction Fenders are still the most popular amps used for harp on stage.
    I hear you. Actually my tweed pro is a fabulous harp amp. I would use it as it is in no time at all but it's my favorite guitar amp as well so I need something else for harp and use that as an oportunity to try a couple of new things. The first step will be to build something that suits my own playing. That way I hope to learn about the effects of the various options in tuning the amp and might be able to help other players with their needs as well. One step at a time ...

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    "So many opinions ... I don't have any useable left overs at this time so I will have to buy one anyway." Not really, everyone who builds harp amps tends to use adequate transformers, Harp Gear amps used Mercury Magnetics who claim a 20% increase in current handling.

    "Originally Posted by MWJB
    I would not unbalance the PI unless balance can be restored 'on the fly' by the use of a pot/switch. As Bruce says, unbalanced PIs are greatly overrrated. It kills fidelity, can warm up a harsh/bright amp, doesn't usually give you any more power. Unbalancing power tube plate current (+/- 10mA?) is the simplest way to achieve this in your amp, how about a bias balance pot & mark the pot at the point of balance for reference?

    It depends how I will like it and if I think that there is a setting I prefer or if it has to be adjustable. I might keep it inside the amp and mount it directly to the board."

    This won't be very practical, should you find yourself needing to cut better on stage, you can't pull the amp's back panel off during a number. Nobody builds an amp with a fixed, unbalanced PI, they can all be set to stock at the turn of a knob.

    5881WXT, they are a bit on the stiff side for guitar, but will give best projection & detail of the 6L6/5881 variants in a 3x10"/4x10".

    "I think in an amp that size I don't need extra headroom anyway." Well by virtue of being 3x10", it will give away a little volume to a 4x10", so unless you are running at very high plate voltages the amp will stil be in the mid-size category...but I love the way 3x10" sound.

    "The first step will be to build something that suits my own playing." How would you characterise/describe that?

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    My experience with harp amps is that, not only do SMALLER OT's sound BETTER because you can achieve core saturation easier, but also because the resulting compression really makes the harp punch, and smooths it out due to the byproduct of bandwidth limiting. It's one of those instruments that is UBER-cool, but overamplified without peak limiting, it can cause ear fatigue and get annoying pretty quickly.

    Bandwidth limiting of both the high AND low ends in a harp amp is also a good thing because of the mic itself. Too much low or high end, and havoc can be wreaked in a hurry when the harp player wraps their hands around the mic and converts the nice cardioid pattern into an unstable omni.
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    Quote Originally Posted by MWJB View Post
    Not really, everyone who builds harp amps tends to use adequate transformers, Harp Gear amps used Mercury Magnetics who claim a 20% increase in current handling.
    Hmmm ... I might get the Mojo Bandmaster tranny or maybe the Super Reverb unit ... but I will probably start with the regular Bandmaster tranny (MOJO769).

    Quote Originally Posted by MWJB View Post
    This won't be very practical, should you find yourself needing to cut better on stage, you can't pull the amp's back panel off during a number. Nobody builds an amp with a fixed, unbalanced PI, they can all be set to stock at the turn of a knob.
    Probably ... I still don't know what to expect from this control so I'll decide as soon as I have done some testing. At this time I am just leaving room for some options in my battle plan. In the end I will probably either put it in place of the presence control or dump it completely ... I really can't tell yet before I even heard it's effect.

    Quote Originally Posted by MWJB View Post
    5881WXT, they are a bit on the stiff side for guitar, but will give best projection & detail of the 6L6/5881 variants in a 3x10"/4x10".
    ok, I might give them a try. They are pretty cheap compared to others so what the heck ... they are 30 watters, right?

    Quote Originally Posted by MWJB View Post
    "I think in an amp that size I don't need extra headroom anyway." Well by virtue of being 3x10", it will give away a little volume to a 4x10", so unless you are running at very high plate voltages the amp will stil be in the mid-size category...but I love the way 3x10" sound.
    And that might be just the size I need. We do retro stuff with a moderate drummer and upright bass. I used to play with a harp player who was great but the Bassman he used was too loud (I know they have volume controls but nobody wants to use them). That's overkill for what we do. I have a feeling that the Bandmaster is just right.

    Quote Originally Posted by MWJB View Post
    "The first step will be to build something that suits my own playing." How would you characterise/describe that?
    Hard to say ... I have not found my perfect amp yet. I do not like RI Bassmans because they are too aggressive sounding. My more or less stock Tweed Super sounds good but is a bit too spongy for a live situation (although it's useable). My Tweed Pro with a vintage Jensen C15N is by far the best. It's got the right size, has good definition and still sounds dirty. I might as well build me another Pro but I would like to try a different speaker configutration as well. I do play chromatic as well and like good cut and definition without being obnoxious but I love that Masco type sound of the Meteors as well and I'm a die hard Little Walter fan. I really need to try different options to know what I really want ...

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    Well there's no one amp that covers the Little Walter sound, but there are a few things that are pretty common to many amps of the period. I'd go with a larger cathode resistor at V1, say a shared 1K or 2.2K per triode. Cathode bias...will be handle by your switch, also try cathode bias without the power tube bypass cap. Most of the amps from LWs golden period would not have had NFB, perhaps your variable NFB will do the job, or a NFB defeat switch? Removing the global NFB will cut available volume before feedback a little, but the brassier tone might make up for it enough to be livable with. You probably won't like the cold bias idea, but I'd still build in the possibility as your cathode bias switch will have the hi current side of things handled.

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    Quote Originally Posted by MWJB View Post
    Well there's no one amp that covers the Little Walter sound, but there are a few things that are pretty common to many amps of the period. I'd go with a larger cathode resistor at V1, say a shared 1K or 2.2K per triode. Cathode bias...will be handle by your switch, also try cathode bias without the power tube bypass cap. Most of the amps from LWs golden period would not have had NFB, perhaps your variable NFB will do the job, or a NFB defeat switch? Removing the global NFB will cut available volume before feedback a little, but the brassier tone might make up for it enough to be livable with. You probably won't like the cold bias idea, but I'd still build in the possibility as your cathode bias switch will have the hi current side of things handled.
    Thanks!!!! Just because it worked for LW does not mean that it's gonna work for me and I'm not that much of a copy cat so I'll give all options a fair chance ...

  19. #19
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    Quote Originally Posted by Bluefinger View Post
    ...
    1) start with a 5E7 Bandmaster
    2) use Weber 10A125 speakers with H-dustcap
    ...
    Bluefinger,

    Just to comment on this. I built a 5F6A clone, but I used a Weber 5E7M chassis and the 5E7 cab with the sig10s speakers. (Used the stock iron too). Make sure the top speaker will clear the OT. The particular way Weber had the holes for the OT, I had to remove the bellcover to clear, but I've seen other 5E7s with the OT in a slightly different mounted position that may have allowed for better clearance. You'd hate to have the chassis all completed and then find out it wont clear the speaker.

  20. #20
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    Thanks for the hint ... usually I check this but it would definitely be annoying if I forget to do so ...
    Funny you mention this. I already had similar thoughts, just the other way around. I thought about using a 5F6 chassis forthe 5E7 so I can have a full BMT tone stack plus an additional control for variable -FB, power tube unbalancing or whatever I decide to use it for. Mojo can do custom cutouts but I might have a custom cab built by a friend anyway ...

  21. #21
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    A friend of mine near Boulder, Dan Treanor, has a Weber 5E7M amp that is stock except for speakers. He used a mix of Vintage Series Webers. That amp sounds great. I've played it a few times and liked it as much or more than some of the boutique amps. So my only suggestion would be to build it and play it for a while before modifying it. You may like the sound.

  22. #22
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    I have already built a couple of those amps for myself and other players (actually 5F4s and 5E5As) and just because I like this circuit so much is why I choose to go that direction. Still I think that it's usability for harp can be improved with a view tweaks. Trying a couple of mods is more a passion than necessety for me so there's no way around it

  23. #23
    Senior Hollow State Tech Bruce / Mission Amps's Avatar
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    Quote Originally Posted by Bluefinger View Post
    I have already built a couple of those amps for myself and other players (actually 5F4s and 5E5As) and just because I like this circuit so much is why I choose to go that direction. Still I think that it's usability for harp can be improved with a view tweaks. Trying a couple of mods is more a passion than necessety for me so there's no way around it
    I've built a fair number too.
    I built one special 25-30 watt 5F4 harp amp for some pro players I know and sent it down to be used in the back line for the live shows at the Blues Music Awards, now always called the Handy Awards.
    With respect to harp playing, I had a hard time getting the E series tone stack to really sound good. It has a lot of midrange.

    And speaking of Dan Treanor, I played with Dan a couple weeks ago here in Denver... he used a friends Masco 18 that was rebuilt by Jim Rossen (found on WeberVST BBS) and then I modded a bit more.
    Very nice tone and Dan is pretty darn good to.

    http://bluesharpamps.blogspot.com/20...pped-into.html
    Bruce

    Mission Amps
    Denver, CO. 80022
    www.missionamps.com
    303-955-2412

  24. #24
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    Ok, I have finally decided to try all those harp mods on my beloved 5F4 without messing up it's usability as a guitar amp. After spending the whole day in the workshop, trying all kinds of things of things, here's my conclusion:

    o) I increased the filtering to 100F, 47F, 22F and 33F. This tightened things up considerably but voltages stayed pretty much the same. It is definitely an improvement for the harp. For guitar it's different but not necessarely better or worse. I have to get used to it but I assume I can live with it.

    o) There was already a switch in the amp, than disconnects the preamp -fb loop. It's great for guitar but not so for harp because headroom is reduced by a fair amount. I only engage it when use with guitar.

    o) I tried the suggested cap and pot at the global -fb loop to get more bottom end but it didn't give me very audible results, although I tried all kinds of values. I removed it again.

    o) I used the ground switch to disconnect the global -fb loop. This one works great with harp and guitar. I like it! On and off is all I need, no need for a pot.

    o) I dumped the presence control and removed the corresponding .1 cap completely. The 2x10" speakers are bright enough for all applications. I won't miss it a second.

    o) That hole was used for an unbalance pot for the PI. While that sure is not a MUST HAVE in a harp amp, it gives the breakup quite a different character. I find this very useful to go from sweet to gritty.

    o) replaced the Tung Sol 5881 RI tubes with TAD 6L6. Now it doesn't work so hard and starts to feed back later.

    Things left to do:

    cathode bias (where the heck can I put that witch?)
    lower preamp voltages

    since I really like that amp as it is right now I might leave those for a future amp.

    Thanks to all your tips this is finally a great harp amp. Next friday it will be put through it's paces ... we'll be playing with Steve Guyger and he will have the questionable honor of playing it live for the first time with it's current setup.

    all the best

    Herbert

  25. #25
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    Cool, I look forward to an update.

  26. #26
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    The more I play around with this amp, the more I like it. At least I have debunked the myth that a good harp amp is a bad guitar amp. It kills for both applications. I get to like the tighter filtering for guitar as well. Now I am really curious what Steve thinks about it.

  27. #27
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    Before the 1990's there was no such thing as a "harp" amp, amplified harp was just recorded on amps, principally designed for guitar but manufacturers also expected mics to be plugged into them, PA's in the modern sense didn't arrive until the late 60's. Ergo, any amplified harp recorded prior to the 90's (everything by Jacobs, Horton, Butterfield) was recorded on 'non-harp specific' amps.

    Of course, some amps just turned out to more suitable for harp than others, but even dedicated harp amps still share a round 95% (some have more than this) of their components with amps used for guitar. Some completely stock amps make awesome harp amps. It's analageous to mugs...a tea mug is only a tea mug when it has tea in it, fill it with coffee & it's a coffee mug :-)

    A 2x10" amp tends to have more common factors that suit getting the best out of both harp & guitar than many other amp configurations.

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