Bluesharp questions

[Big Jim]

I have a friend that wants me to build a Harp amp for him. I don't know much about what harp players dig in an amp. I found a layout for a 5F2H circuit but thats about it. He is wanting something around 10-30 watts. Does anyone have any suggestions about this type of amp. What qualities do harmonica players look for in an amp? What is the holy grail circuit for harp?
Jim

[rdjones]

I have a friend that wants me to build a Harp amp for him. I don't know much about what harp players dig in an amp. I found a layout for a 5F2H circuit but thats about it. He is wanting something around 10-30 watts. Does anyone have any suggestions about this type of amp. What qualities do harmonica players look for in an amp? What is the holy grail circuit for harp?
Jim

Pair of cathode biased push-pull 6V6s with a 10 or 12.
Look at the circuit for a Fender MusicMaster Bass.
Not a lot of gain needed, the Green Bullet,etc puts out enough to drive a fairly simple circuit.

Grail tone might be a Champ, but that's not enough power for what you want.
You probably want at least a 10" speaker.

rd

[ToneMerc]

Jim, here's my layouts of the 5F2H circuit. The natural color board is setup for dual cathodes. FWIW, a designed harp amp will usually suck for guitar. Headed out, but will post more later.

TM

[Big Jim]

Thanks Tonemerc!
Yes please let me in on any tips. I think the 5F2H is where I'm headed. My understanding is that it is built upon the Tweed Princeton Chassis, with bigger iron and a KT66 tube. Weber sells a kit, but I kind of like picking my own components. I'm basically fishing for the best deals on chassis, trannies etc. Thanks!
Jim

[ToneMerc]

Jim, here are the keys to harp amps

1. Mic compatible input impedences
2. Lower preamp voltages(100-140V)
3. Larger value coupling caps(.1 to .47)
4. Less gain is more

Take a look at some of the 50's lap steel and PA amps.

Like yourself, I prefer to pick my own parts and I generally shy away from complete kits. Is weight or size a concern for your harp player?

The 5F2H is a great start as it was designed especially for harps. I would recommend that you change the .0068 tone cap to .01 though. I'm using a Russian K-40Y PIO in that spot. The remaining caps are MOD(Brazilian made) PIO caps, the same cap is rebranded by MojoTone and called the Vitamin T.

The circuit was designed around a PT with a 360V secondary winding and 110mA HT current. I would verify that the current PT that Weber offers meets this, at on time they were shipping an undervalue PT. I'm using iron that I had Heyboer wind for me.

In addition, I would tweak the 27K string value to your liking. If you want more "grease", try something beween 33K-54K. Also, don't be afraid to try more filtering on the front end as well.

Sonically, there's most likely not much of a tube rectifer advantage as "sag" is a moot point. You could experiment with a ss rectifier instead of a tube.

An easy way to almost 30 watts would be a parallel 6V6 scheme. You will need a PT with at least a 130mA or so, with 150mA being even better. Now biased accordingly, you should be able to squeeze out 14W per tube. If you wanted only 4-6 clean watts, you could get away with stock Champ PT, most provide only about 60-70mA of HT current.

Here's version 1 of my PT. It's bit overkill on the 5 & 6.3V widing as I thought I had enough HT current to run dual KTxx tubes. On version 2, I increased the HT by 80mA. If have another made, I most likely will decrease the other windings a .5A as well.

360-0-360@ 150mA
5V@ 3.5A
6.3@ 4.5A

If you wanted to experiment with different OT tubes, maybe a single KT66 or dual 6V6, I would recommend this off the shelf OT.

http://shop.dobermanamps.com/product-p/ ... b20wse.htm

For dual KT66s I had Heyboer work me up something bigger.

Pictured; the PT referenced above, the 5H choke and a stock Champ PT with upright mounts.

Mike

[Doctordog]

I had a friend who played harp. Bought a 59 Bassman RI and didn't like it because it didn't break up like he wanted it to. But everyone's different, so I would ask your friend if he wants it to distort or be clean. I might suggest the 5E3 circuit, it can get pretty loud, built to stock it breaks up early, but there are a million mods to clean it up and change the tone if you want.

Just my .02

JB

[David Root]

I am working on a 5E7 Bandmaster build modified for blues harp. V1 is a 12AY7 in parallel with 33K plate and 530R cathode, bypassed with 44 uF. I am looking for about 120V plate. V2 is a 12AX7 arranged 5E7 style with V2A gain stage feeding V2B cathode follower into the 5E7 tonestack. V2 is acting squirrelly and needs rework.

In place of the 5E7 normal channel I took a 5879 stage from a '57 GA-40 and added a tweed tone control, with both Fender and Gibson style stages feeding into the standard 5E7 cathodyne thru 220K resistors to (more or less) isolate them. I am now playing with the B+ string. Need 265V B+ on the 5879.

I am having problems with the amp and have switched it to fixed bias. I may change the PI to LTP. It was cathode biased Class A2 but the plate voltages were too low to get 40W out of it. I can get close to 440Vp with about 55% Pa bias, 35 mA cathode current with Tungsolski 5881s. The PT can support 6L6GC or KT66 power section. The OT is 4K rated 40W. Both are Allen Amps iron, good stuff, made by M-C and Heyboer respectively.

Generally you use lower gain stages and higher coupling and bypass caps. I have .22 coupling caps for example. There are some tricks in the harp amp literature to control acoustic feedback which is pretty important.

[C Moore]

Forget who wrote it, and just give it a read. If it does not help, I apologize. If you do get a little use from it, then all the better.
Best

http://www.harpamps.com/weber/Guitar51.htm

[C Moore]

Forget who wrote it, and just give it a read. If it does not help, I apologize. If you do get a little use from it, then all the better.
Best

http://www.harpamps.com/weber/Guitar51.htm

[Big Jim]

Thanks for all the info folks,
I will take a little time and do some research. I explained to my friend that time spent on the front end usually pays dividends on the end result.
Its kind of different for me being a guitar player, to know what a harp player likes. I let him plug into my 5e3 yesterday, and he said he wanted MORE distortion. I consider that little amp to be pretty gainy, so I dont know. Luckily there is no deadlines on this build.
Peace, Jim

[David Root]

Just an update which might be useful. I have reworked my amp so it is only a Bandmaster from the tonestack on. It now sounds good. I am using a Microvox harp mic, the cabinet is a tweed Bandmaster w/ three Weber 10", two plug Alnico, one ceramic, all 20W rated each.

Wall AC was about 121V when measurements were taken.

V1 is an old used longplate Mullard out of a '58 Heathkit o'scope, in parallel, 33K plate, 680R cathode w/ 44 uF bypass. 200V B+, Vp 93V, Vk 2.2V, current 3.2 mA.

V2 is an RCA 5879, B+ 270V, Vp 111V, Vk 0.82V, Vs 26V, set up exactly like the mic input of a '57 GA-40. Current 0.37 mA.

V1 and V2 feed into V3 via 220K resistors. V3 is a '71 Mullard 12AT7, in parallel, B+ 231V, Vp 129V, Vk 1.39V, current 2.04 mA. 680R Rk, 50K plate. 44uF cathode bypass.

V4, the PI, follows a std. but plate coupled Bandmaster TS, and is a '60 GE JG-5751, V4A the recovery stage is 297V B+, 215 Vp, 1.09 Vk, 1.6 mA current, 51K plate and 680Rk, 22 uF bypass.

V4B, the splitter, 297B+, 244 Vp, 51.6 Vk, 56K top and bottom w/1K5 cathode, std. fender cathodyne splitter, 0.92 mA current.

V5, V6 are Jan CHS Sylvania 6L6GA from the '40's, 415 Vp, 408 Vs, 5K6 Rs, 64.6 mA cathode current, at 68% Pa. Plate current is 31 mA/tube avg. Bias -44.3V. Grid resistors are 1K5.

V7 is a '70s GE 5R4GB, used to get the power tubes' plate voltage just barely into the OK range.

I am looking at the 5879, may futz with its set-up, maybe not.

I hope this may be useful. I found the key is to bias on the warm side in the preamp tubes prior to the TS.