Harp Amp Secrets and Tips
by Gerald Weber original RTF verson
Before 1993, when I designed the first tube amp specifically for blues harmonica, little research had been done on how to make a tube amplifier sound best for harp.? We all know that tube circuits can be voiced to perform a certain way, but the question is, "How should they be voiced for harp?"? In addition, harp players use "industrial grade" high-impedance vintage PA microphones such as the Astatic and the Green Bullet.? These type microphones are prone to feedback.
I am gonna share some of my research with you and show you how to modify an existing tube amplifier for use with a blues harmonica; or you can build your own harp amp.
Although I had previously spent time with Kim Wilson (The Fabulous Thunderbirds) listening to various speakers and speaker configurations to determine what was best for harp; it was the internationally famous blues harp player, Paul Orta (The Kingpins), that convinced me to design the first tube amp specifically for harp. Paul brought in an old wooden cabinet National tube guitar amplifier.? This particular amp originally belonged to Jimmie Vaughan and was his first guitar amp.? It sounded horrible for guitar.? It was over-compressed without any clean tone whatsoever. Although I did not date it, I would guess it to be late 40?s.? It had a field coil speaker driven by two 6L6G?s running Class A push-pull.?? Paul had bought this amp because he liked the tone for harmonica, but he found he could not use the amp in a live gig situation.? It was not loud enough and there was too much hum. Even if the amp was miked through the PA, the hum level was beyond usable. Paul loved the tone of this particular amp and felt it to be the definitive tone for harp.? We just needed to get rid of the hum and increase the volume about five fold.
Paul asked me to analyze this amp to find the essence of its tone.? I would then use what I learned to design and build the perfect harp amp for him.
After studying the old National Amplifier, I broke its essence down into basics.? I believe these basics to be perfect for harp amp tones. In the process of analyzing this amp, I stumbled onto a way to fatten the tone and eliminate feedback (more on this later.) Here are the basics along with an explanation:
- Low plate voltage on the preamp tubes.?? The National had very low
voltage on the plates of the preamp tubes. This would be a terrible idea for
guitar but worked great for harp. ?The plate voltage determines both the frequency
response and the headroom.? With low voltage, the tube breaks up at almost
any volume and the frequency band shifts to a lower register.? Since harp
frequencies are relatively low compared to guitar, this lower voltage fattens
the notes while helping reduce high frequency feedback.? Although the National
had voltages running around 95 to 100 volts, I experimented with different
voltages and found 80 to 90 volts to be ideal for harp.?
If you have a guitar amp you are modifying for harp, you could simply increase the value of the power supply dropping resistor that feeds the plate resistor of the preamp tubes. Since preamp tubes draw very little current, the dropping resistor may have to be replaced with a much larger value.? I would use a decade box in place of the dropping resistor and by trial and error find the appropriate value resistor. Do not change the plate resistor, only the power supply dropping resistor!
- Larger than normal coupling caps.? The exact value of a coupling
cap determines how much lower frequencies get through the circuit.? Larger
values simply let more signal pass.? Through experimentation, I found the
.1 uf? coupling cap to be best for harmonica.? Of course, if you were modifying
an existing guitar amp, you would simply change the coupling caps (typically
.01uf or .02uf and sometimes .047 uf) to a .1 uf.? You would use a 400 or
600-volt rating.
- Simple two-stage design. Less is more.? If you get more stages of
gain, you are amplifying the feedback problem and you will just have to turn
down the volume. This explains why the normal channel on a blackface Fender
works better for harp than the Vibrato channel.
The National actually had only one stage of gain and a paraphase style inverter that acted as another stage of gain. A tweed Champ, tweed Princeton and tweed Deluxe are examples of amps with simple two-stage design. It is interesting to note that all of these amps work great for harp.
- Paraphase style Phase Inverter. This obsolete design has not been
used since the mid-fifties.? The paraphase inverted takes advantage of the
fact that a tube inverts the signal by 180 degrees.? A very small signal is
divided off the signal path that is driving the first output tube.? The small
signal is run through another preamp tube, which drives the other output tube.?
Since running it through the addition preamp tube inverts the phase by 180
degrees (with respect to the signal driving the first output tube), phase
inversion is achieved.
You can not get a clean tone with a paraphase style inverter because one of the output tubes is being fed by a signal that was run through an extra tube. Any distortion introduced by this extra stage is amplified by one output tube, but not on the other. The result is a ?not so clean? tone.
There are two types of paraphase inverters: the fixed and the self-balancing. A good example of the fixed paraphase inverter would be the 5C3 Fender Deluxe. A good example of the self-balancing type would be the 5D3 Fender Deluxe.The National amp had a fixed paraphase inverter, but I opted for the self-balancing type instead. In the self-balancing type, a resistor that is common to both circuits goes to ground.? If one side is driving too hard, it phase cancels some of the signal developed across this common resistor, thus balancing everything out.? This is where I had a happy accident. (More on this later.)
- A Cathode-biased, Class ?A? push-pull output stage.? Cathode biased
amps are generally more compressed with a more singing quality.? In general,
a spongier response is achieved.? The Class ?A? push-pull output stage simply
means the tubes are driven by a phase inverter and they never go into cutoff.
I used a 250 ohm 10 watt cathode resistor with a 33-uf 100-volt bypass cap.? I like going with a higher voltage rating on the cathode capacitor because the higher voltage cap can take plenty of heat and still be within its operating parameters.
- High idle plate current in the output stage.? The high idle current
is typical of Class ?A? style amplifiers.? The output tubes on the National
were running about 60 mA per 6L6 tube. Since the National used 6L6G tubes
(rated for only 19 watts plate dissipation), I opted to run 6L6GC (30 watts
plate dissipation) at an even higher 70 mA per tube.
If you are converting a guitar amp from fixed bias to cathode bias, the cathode resistor, by developing a voltage on it, makes the tube ?think? it is operating at a lower plate voltage. Why?? Because the tube only ?sees? the difference between the cathode voltage and the plate voltage.
Let?s say you have a Super Reverb that normally operates at 445 volts, and you change from stock ?fixed bias? to ?cathode bias?.? You add the typical cathode resistor/ cathode capacitor to the output tubes and measure 45 volts across the cathode resistor. This means the tube only ?sees? the difference between the 445 volts plate voltage and the 45 volts cathode voltage.? It performs ?as if? it were operating at 400 volts.? Given this scenario, one could see how we could ?idle up? quite high compared to stock. Perhaps 60 mA per side would work fine without the tubes glowing cherry red.
- Tube rectifier.? Of course, the National amp had a tube rectifier.
I would always recommend a tube rectifier with a harp amp setup.? You need
the compression that a tube rectifier gives.? I used the GZ34, which is my
favorite rectifier tube.
If you have an amp that does not have a rectifier tube, one can be added by using a 5-volt 5-amp auxiliary filament transformer and an octal tube socket.? In this magazine, About 2 years ago, I wrote an entire article called ?Converting a Solid State Rectifier to Tube?. If you need more info on this and can?t find the ?Converting a Solid State Rectifier to Tube? article in your archived VG magazines, email me and I?ll email you the article.
How to Stop Feedback and Fatten the Tone
I told you about the self-balancing paraphase inverter and I told you I had a happy accident.? When I was designing the harp amp for Paul Orta, I thought I would use a potentiometer instead of a fixed resistor so I could dial in the exact value by ear.? To my surprise, I found that if I deliberately made the waveform non-symmetrical, the harmonica tone actually fattened up!
Here?s the other part of the accident.? I found if I connected the speaker reverse polarity with respect to which half of the waveform is bigger, then the threshold of feedback goes up.? This means the amp can be turn up to ridiculous volume levels before acoustic feedback occurs!? I decided to leave the potentiometer as a ?waveform symmetry? control so Paul could fatten his tone while reducing feedback!?
I will never forget the time Paul called me after doing an outdoor gig with his new harp amp and said for the first time in his career a sound man actually asked him to ?turn it down?. It seemed he was burying the drummer and the guitar player in the mix!
Gerald Weber is the Owner of Kendrick Amplifiers which is a great site, full info on buying, building, repairing and using tube amplifiers. He writes great articles for Vintage Guitar Magazine (where this article came from).
Hello:
I have inherited my father’s 1960′s tube Premier 71, and by chance have a friend who repairs old tube amps. It needs cap work. The leather handle is destroyed. Do you know of where I may find a leather handle that closely matches the original? I have seen the vintage Fenders, but was wondering if there is anything closer.
Thanks,
Mike L.
http://www.tubesandmore.com or http://www.angela.com are sites that specialize in restoring old tube amps. They have several different styles of the old “Dog Bone” leather handles.
Keith
Where are you?? I can get some handles and may have a Premier handle kicking around. If it is the leather one that goes under two metal cover caps take the old pieces to a boot and shoe repair shop. They can approximate the shape from the rotted leather (maybe) and cut two or three pieces and stitch them together into a thicker piece I have had to do this myself and use a Mexican shoe repair shop. I am within 3 hours of Mexico in sunny hot (will it ever cool off hottest August and July ever) Phoenix Az. As for the cap job, if you were close or willing to ship I can only say ask around your area. Find some old burnt out musicians that have been there a while like myself who know some of the shop and repair people’s history. I am an anal SOB who would look at what caps are bad check the rest, and try to find a cap that preserves the original sound. Most techs just stick either orange drops or what they can get cheap in an amp. Most techs do not have 2/3rds of Mc Intosh’s Danish audio analyzer console and $10,K in other sound and distortion analyzing instrument on their bench either. Hopefully you might be OK with just the filter caps swapped out, and make sure nothing else is too leaky. Some of those caps that leak a tad, but not enough to cause any real problems can make for an awesome blues guitar or harp amp sound. I finally put aluminum foil and paper with bee’s wax caps in my mom’s old Supro 1624. The Q was not audiophool stupid high, and they do not dry out. They are the low end cap in the Amp Ohm Cap line. The sell silver foil, or copper foil in oil with paper, and silver leads. (STUPID MONEY EXPENSIVE) Best wishes, and I hope you get it back to a better state than it is now.
I still say boot and shoe repair shop. You will get new leather, and they have sewing machines that will sew it into the layers. An old leather handle found on line will likely look good then crumble away in short order, unless it comes from one of the miracle climate areas in the country. I tried luggage repair, but it was very expensive, and they had no real interest in doing the work right. The population of Mexican immigrants here keeps cowboy boot repair as well as normal shoe and boot repair shops alive, so there is enough competition they gladly make any leather handle shape I need for around $25. Tubesandmore is across town in Tempe Az. It is about a 20 minute drive. They only do will call order pick up from 3:00-4:30 PM but ship world wide every day. Angelina or Angela has more of the Fender stuff and is pretty pricey. Premier was a neat company. I have one or two of their amps, and a snare drum the marketed under the name Olympic. Tubesandmore essentially has what Angela has but you pay less for it. They are also a great place to buy new old stock USA, UK, German,, or Holland made tubes. They also sell the new Russian, Slovak, and Chinese stuff. Oh yeah Groove Tubes makes a 6L6GC in the USA now from roughly 80% USA parts.
You’ve probably replaced yours by now, but if anyone else needs a handle, contact Bob Schell at http://www.brookwoodleather.com/
He made a dead-on copy for my ’50s Premier 71 handle (with the arrow-shaped ends). Super fast service, and reasonably priced. His work is amazing.
dc
Looking for some insight into an old Japanese build amp and its applicability to being used with GB mic. Its Teisco Melody using 6AU6 drivers and 6RA5 outputs with a small 6 inch speaker. Very little info on the web, and a total nob on these tubes. I won’t have a chance to test this item out before purchasing due to meeting the owner some several hours drive at a Timmy’s.
I don’t know about the 6RA5. I am assuming that the tubes are about what you’d find in a radio (Teisco made radios), so the power output will be low, just enough to drive the 6 inch amp.
It will probably start out clean and get dirty as you turn up the volume. It is most likely a neat practice amp and will work well with the GB.
If you like the sound you can mic it through the PA and the amp becomes a neat little effects box to give you some tube overdrive and a little protection from feedback by tweaking the volume controls on the amp and the PA.
Keith
Picked up the Teisco Melody last night and unfortunately one of the axial caps (a 80MFD x 150WV) is broken right at the cap (unable to solder back on), so the task of trying to track down replacements is on. There’s nothing in Vancouver, BC that I can purchase from local suppliers (with the exception of a couple of radial 100MFD x 160WV or 100MFD x 250V).
Any advice on where I can buy replacements would be most welcome as I can put in the radials now to get the amp up and running but would like to get axials as they look more like stock units.
Jim
I replying to your wanting more info about Multi vox amps. I purchased a “lot” of vacuum tubes and radios from an old guy that’s giving up the hobby. Within the lot I purchased is a Multi Vox P.A. Single ended push pull with 2 7591′s and 3 12ax7′s (tubes are labeled Multi Vox with what I think is Sylvaina’s yellow stamp). It has a Mic “Z” adaptor, hum balance and bias adjustment.
Outputs are 2,4,8,16 ohm’s and 70 volts, 35 Watts R.M.S. Inputs are Tuner, Phono, P.A. and Mic.
Model EA35-6 Made in Montreal, Canada. I have put the bottom cover back on yet there is at least 10 Mullard mustard .022 tuning caps dated 1965 within.
Jonathan
i have a premier twin 8 “widow’s peak” version with the gold foil grill and i’m trying to find the gold foil grill any info will help thanks
gary
Art stores have metallic sheets for use in art projects. If you can find a gold colored one that is the right size, it will look like the original. You can try the big online art supply houses, but there is so much stuff on these sites that I could not find anything quickly for you.
Keith
Hi,
Could you tell me how to contact Mic Kaczmarczik?
I have an amp repair service shop in Edmond and
would really like to ask him some questions.
Thank you for your time.
Best Regards
Ron Thomas
Edmond, OK
I only communicated with Mic once many years ago about using his usenet archives. I found his email through his university job, but I don’t know if he is still there.
Keith
Help, Picked up premier model 88N. Someone cut all the wires out of the audio output transformer. I know there is not a schematic. But if I could get some close up pictures it would help ALOT. Thanks guys for any help. I don’t know why someone would do this. Jim
i love my amp,a”4-10 deluxe” by Blues Pearl but i need a back up. i’ve been doing some research and it seems most people would agree 8in. speekers work best to achieve a great blues sound but it’s a pain to always have to mike it.i’ve seen amps with 2,3,6 and 8 8in.configureations but no 4′s.it seems to me (for my purposes)4 8′s,2 6v6′s and a weak preamp would be ideal.there must a good reason why even boutique harp amp manufacturers and kit cos.don’t offer such a set up.i’m considering having one built or modding an old ampeg gemini two but before i do i’d like to know why it might not be such a good idea.any thoughts?
There is an old Valco, Silvertone, or Danelectro (can’t remember which) that had eight 8-inch speakers. It came apart like a suitcase with 4 in each half. There is a legend that it was Little Walter’s favorite amp. The Gibson GA-90 had six 8-inch speakers, and is supposed to be a great harp amp.
Keith
i appreciate your speedy reply! i spoke with a buddy (bass player not harp) who insisted that with 8in. speakers,i would no doubt get the tone i want but they have no business on stage as no matter how many speakers in the cab.,i’d still have to mike it or go direct.something about limited power to smaller speakers.i’m still on the fence about this…as you know (as a working musician)TONE is the priority not convenience or practicality.i have also heard those silvertones and valcos make great harp amps but might need some reenforcent even in small to medium rooms.maybe i’ll save my dimes a little longer and buy a boutique harp amp with a 12 and two eights?…man that a lotta dimes…a lotta cheap gigs…yeah, i said it!
I have read a lot of good reviews of VHT tube amps. I want to buy a new amp that I can get a dirty sound from. There is a VHT special 6 1×10 guitar combo amp for $229 and a VHT special 6 1×12 guitar combo amp for $329. Which is the better harmonica amp? Is the 1×12 worth the xtra $100 dollars. Should I get a delay pedal also or is it unnecessary? Let me know of any better alternatives in this price range. Thanks
On the MASCO gallery:
“The MA-60 had four 6L6′s four 6J7′s one 6sj7 two 6sc7′s and two 5v4 rectifiers.”
I think you might have slipped a digit on this amp. I have a 1951 MA-60, and the tube layout is four 6J7′s at the input, 6SC7, 6SJ7, 6SL7GT, 6SN7GT, two 807 power tubes, and two different rectifiers- 5Y3GT and 5R4GY. It has four inputs, a phono input, and a booster jack in the back.
And yes, I have a TON of pics, in case anyone is interested in seeing this amp. Also, I’m wondering if this amp is an anomaly or something, because I’m finding very little info on it. I can find plenty on the MA-17, 25, 35, and 50, but next to nothing on the MA-60.
Anyone know the correct ohms of the 3-inch Jensen P3VH speakers/tweeters used in the Premier Model 71 amps from the mid-1950s? Any schematics showed up for one of these amps? Thanks.
Anyone tried the new 2-6V6 Fender Pawn Shop amps for Harp??Like to hear informed opinions. thanks, randy
Need help- playing through a sonny jr 4-10 and getting a clicking noise when i really tweek a note. I’m going through a reverb pedal and a shure 707A. What’s up. thanks
I’m geared up to try one. Only shop in my town who’s carrying them is still awaiting delivery of them. But he HAS gotten the Greta desktop amp in, and I tried that. I’m seriously considering getting one to use like a preamp head on my Kalamazoo Model 2.
Hi,
Am I right in assuming that most of your site is related to harp and guitar amps?
I am a bass player and have an amp repair and service business here in Edmond,OK.
I had a customer bring in 3 old vintage Fender amps which I serviced and did any needed repairs and got him going very well. One of his amps was a 59′ Blonde Bandmaster that was immaculate. He has taken care of these jewels.
Anyway, he gave me a 67′ SF Bassman with the AB165 circuit. I did a cap job, replaced all three transformers with new Mercury Mag trannys and modded the hum balance as a true bias. I tested and kept the original pre amp tubes and installed a pair of Winged C 6L6′s. Right now I play it thru my old SVT 8-10.
This cab I removed and plan on having the originals 10′s re coned thru Ted Weber Company. The tens in it now are cheap cast speakers and my SVT head really did a job on them. Learned my lesson about the stuff sold by some eBay sellers. At low volume they sound good but the coils are slapping here and there with amp at 4 and my bass up pretty loud. Anyway I have acquired to old Eminence 15s and I’m going to build my own cab just for this bassman.I had read that the AB165 wasn’t really liked as a bass amp. My BF in 65 with 2-12 cabs was incredible. So why the dislike as a bass amp for the AB165?
Also what circuit are you referring to when doing a bias mod on these to be like the 63 bassman? I have looked a the 6G6′s and I see no bias control pot.
Can you set me straight on this mod?
Thanks much for a very informative site.
Regards
Ron Thomas
Edmond, OK
Hi, I inherited an old Peavey Classic 30 tube amp. I have ordered a green bullet mic, will this work well together?
Thanks for your help
It should work well. There are some Shure bullets that are low impedance that might not work. If it has a guitar type plug then it would work fine. The tube Peaveys are underrated. I think they have a good sound. They might have too much gain for a microphone, but you can swap out the 12AX7 tubes for 12AU7 tubes and lower the gain and get a nice response out of it.
I agree! Peavey Classics are some of my favorite amps. I’ve been craving a Delta Blues 115 or 210. To me, those are the best in the line for harp. They’re still gainy, but like you said, the preamp tubes can be swapped.
I have a premier model50 I am thinking of adding
a line out to it. Anybody know how to wire resistors
to output jack? I would like to be able to keep
the speaker functioning to use as a monitor.
Thanks,
Mike B
I have a old Magnavox tube amp that I am trying to make into a harp amp it has a 12AX7,2 6v6,s and a 5Y3 tube I am willing to do what ever it takes to make this the best it can be for a harp amp anything I can do to improve it the reason I bought this amp is that I have read in different articals that these are very good tube displacement to use on harp amps I have not tried it out yet to see what it sounds like the way it is,because I am new to all this and need to install an input jack,I am not sure where it would wire in? I have a book about old tube amps it has a schematic of a Fender Princeton 5F2 it is spot on down to every resistor cap everything looks the same wired the same,anything I can do to improve this amp?
Try swapping the 12AX7 tubes with 12AU7 tubes. This will mellow it down and help fight feedback. You will get a smoother sound with less distortion, which may sound better with harp.
Keith
You might want to try some lower values instead of 68k for those series input resistors . I’ve had good luck with 10k and 33k at #1 and #2 input jacks . This puts a bit of drive on the pre-amp tubes that’s lost with the 68k ones . I use a large 1 or 2 watt carbon type for the sound quality I prefer , but you should experiment with your own ears . I’ve seen people use no resistor here at all also . Very spanky with a weaker mike . Try one of each you have around with strong clips and then you can solder in what you land on as the best one for you and your mics !
ok I will but does,nt that swap make the amp allot more quiet? it only has one 12AX7 tube do you think I should wire in 1 more so each 6V6 tube has a seperate preamp?
this amp only has one 12AX7 two 6V6,s and one 5y3 I have read that it is really good for a harp amp the chasis has pleny of room to add another tube or tubes one thing I am wondering is if this amp has a self balancing phase inverter also is the 5Y3 tube the rectifier and can it be swapped out for a GZ34 rectifier tube I read they are the best for harp
I will try that soon as I get the input jacks installed thanks
hi again I am working on a different amp now it is a Electromuse Model 10A It has a strange tube line up they are 6F6GT,then it has a 8 pin tube that does not have the letters/numbers then it has a 6F6,and a 5Y3Gt question is this a model 10a and what tubes interchange with the ones I listed here. All the schematics I can find don,t have these tubes in them. so I am wondering if this is a different model or these tubes will interchange like the 6F6 is it the same as a 6V6 same with the 6F6GT is it the same as 6L6GT I am trying to get this amp working again the tubes light up but it has a loud buzz (might be a multi cap?)any advice?
Jim,
I saw your posts at one of the forums. The Magnavox turned out to be a puzzle.
The 6F6 is a like an older version of the 6V6. Push comes to shove you could use a 6v6. The 6F6 was probably a metal can. The GT just means Glass Tube, so they are the same tube.
The strange tube is probably a 6sl7, 6sj7, 6sn7 or something like that. These are old style preamp tubes with a sweet sound, but without much gain. Probably a 6SJ7 tube. Look at the oldest champ circuits.
Buzz is 99.9% electrolytic cap. This amp is old enough that the caps all have to be dried out so change all the big caps. It can sometimes (rarely) be one of the fat cement style resistors or very fat resistor with high watt and low value. These resistors sometimes crack or change their value over the years. It is cheap and never hurts to change these resistors to get the amp back into spec.
I found a schematic for this electromuse as it turns out they were made by Valco the ones with the electromuse name did not have model numbers on them but i think it is same as a Valco model ,10A anyway the tubes in the schematic are the one by the output transformer by it,s self is a 6J5GT,labeled preamp 2 on schematic mine is a 6F6GT the next one below that tube in the schematic is a 6SH7GT, labeled preamp 1. mine has a tube without markings,but I think I see a 7 on it,so it is probably a 6SH7GT, the next tube is a 6V6GT,in the schematic,labeled power amp,mine has a metal 6F6 tube. the last one in the schematic is a 6X5GT,labeled rectifier,mine has a 5Y3 in that place. I guess my question is are the tubes in my amp interchangeable with the ones in the schematic. do the tubes in the schematic do the same function as the tubes that are in my amp?
I could be just random tubes that someone plugged in and the amp never worked with them. The 6F6 would not be any kind of replacement for the 6J5. The 5Y3 and the 6X5 are both similar kinds of rectifiers, although I don’t remember if the pins correspond.
I doesn’t look like the tubs would match. Check the circuit underneath the chassis and see if the wiring looks the same as the schematic to make suer that you indeed have the same amp.
Keith
Any luck on finding a B 160 schematic? I’ve seen 6l6 around but not much about the ones that have 7591′s.
Thanks.
I am just starting out, having alot of fun, looking for that sound ! I inherited a Fender Blues Deluxe from my son; I think it’s from the 90′s. Someone has suggested swapping the pre-amp tubes, and getting a Weber smooth cone, Signature series Speaker. The tube swap looks pretty straightforward, but the special Weber Harp Speaker is 6″, and the Fender it’s replacing is 12″. Also , the Fender is 40 or 50 Watts, and the Weber is 15 Watts.
There is a Weber 12″ Speaker, also smooth cone, for 25 Watts. What would work ?
Should I just buy a lower Watt Amp made for Harp ?
Any suggestions for me ?
Thanks,
Eddie