Using a bigger Output tranny?

[Bluefinger]

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!

[Wilder Amplification]

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.

[Bluefinger]

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!!!

[Wilder Amplification]

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.

[Bluefinger]

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?

[Wilder Amplification]

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.

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.

[Bluefinger]

Thanks

[Bruce / Mission Amps]

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.

[Bluefinger]

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 ...

[Bruce / Mission Amps]

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....

[Bluefinger]

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 ...

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.

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

[MWJB]

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.

[Bluefinger]

Hi,

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.

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.

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.

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.

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 ...

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 ...

[MWJB]

"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?

[jrfrond]

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.