5f2h-

[stratavox]

Anyone built and used one yet? Thoughts/opinions? Looks good, and I trust Bruce's design, I'm just looking for real gig responses. Loud enough, did it "cut through", any dependability issues, etc. Thanks!

[Morten Stenbaek]

Killer tone ! But not loud

I have built a few inspired of the BC design. One just built in champ size (5w-OT) and one in with 2 6L6 in series and a 30W single ended OT. Instead of a 10". I have used 2 x 8" speakers. They both sound fantastic !! Very compressed and dirty tone. Unfortunately they are not loud enough for other than small club gigs. I can't hear my self on stage anyway. For the big version of the amp I use a PT from a Tweed Deluxe. This is the best amptone I ever had – sadly just not loud enough. And why ? the 2 6L6 and the 30W OT should be enough ! Maybe Bruce would stop by and give a few comments to this ! Please ! I

[Bruce / Mission Amps]

Mission Amp 5F2H amp

When I first started that project I was looking to come up with a decent sounding light weight harp amp and figured anyone who used small 5-15 watt amps for harp would be also running mic'd through the PA.
The first one I made used a big 125ESE, 15w SE OT from Hammond and a standard 120ma PT. That worked well so I decided that would be the min OT.

Side note:
I sent Weber one of my Hammond 125ESE 15 watt OTs to see if his Chinese company could duplicate it. The original WeberVST SE15 OT was actually a Chinese copy of that very same Hammond 125ESE OT.
I had some prototypes from Weber/China sent to me and although pretty good, I was not knocked out by it compared to the custom wound 15W OTs from my USA transformer company.
I sent one of my USA made SE OTs to Andy Fuchs to see if he liked them in his Lucky 7 and he liked them quite a bit... I don't know what he actually ended up using though.
The Weber OTs might be quite a bit better now, I don't know, I never used anymore.

Regardless, with the Weber 5F2A chassis and a Chinese KT66, my Hammond OT'd harp amp made about 8.5 watts, dead clean output but was not quite as loud as my Mission MA5E3 tweed Deluxe, driving the same sized cabinet and 12" speaker.
Yet, the SE KT66 harp amp had a nasty outrageous harp overdriven tone, that with a little tweaking in the tone stack, sounded pretty darn good and it was actually quite fun for any harp player who messed with it and I could have gone into a little production run with it as is.
Truthfully, I had hopes of using a 120ma-150ma PT, a 200ma choke input filter, the Hammond 25 watt 125GSE OT and "TWO" KT66s in dual SE, all mounted in that same 5F2A chassis, .... using the tube rectifier's socket for the other KT66 tube/socket and a solid state rectifier!
I figured I could see at least around 15-16 watts of pure class A, dual power tube KT66s and, a class A amp does not enjoy power supply sag so the solid state rectifier gives the needed, solid power supply and lower noise.
But unfortunately I had much too much on my plate at the time to do another amp design.

The actual model released by Weber was reduced down to a smaller amp and lower power.
But, in order to at least sound good, I requested from MOJO a Harvard sized cabinet, which uses the same size chassis as the 5F2A does.
That way, instead of the 8" 5F2A cabinet, at least the player had a larger Harvard sized cabinet and a decent 10" speaker!
I think I convinced Weber to use the very same Harvard cabinet I talked MOJO into building for me and I was under the impression they were buying that cabinet from MOJO for a while, until their local shop got off the ground.

So what?
Well, with the 10" bigger cabinet and a simple PA mic shoved in it, you still have what you need.
But playing out using the 10" 5F2A/Harvard cabinet and no mic, can be challenging.
If I were to do this over again, I would go back to my original vision of the 5E3 sized cabinet and a good 12" speaker, the bigger trannys, choke input filter SS rectifier and a pair of KT66s in dual SE.

As far as still not being loud enough... well, when working with local pro players, I have found it really does take about 30-50 watts with at least two 12", three 10" or four 10" speakers to be mic free in a medium sized room with guitar players using 20-35 watt amps.
One of my harp customers uses my higher powered, harp modified tweed Super 5F4 chassis, at around +32 watts, installed in a standard 3x10 Bandmaster with Alnico speakers and he is heard.
Another uses one of my tweed Bassman 4x10, harp modded amps and a pair of El34s at around 50 watts with two P10Rs and two Eminence 10" ceramic speakers and he is heard too!
Still the smaller amps are totally usable if you are willing to stick a littke mic in front and run through the PA.

[Morten Stenbaek]

Thank you Bruce !
Really nice to hear the story and your thoughts about this project. I forgot to mention some other diffs in my amp from your design.

• Traffo is the 30w SE from Torres (http://www.torresamps.com/) - don’t know how good this is compared to the Weber or Hammond
• I don’t like solid state and use 5u4 or GZ34 ( I like to get the compressed sag from the rectifier)
• I didn’t have a choke – so I made it without (no noise hell)

I do follow all your thoughts around using a mic. in front of the amp. I really liked the sound of the KT66 – just loved the sound of dual 6L6 a little better. I have not tried dual KT66 since i assume my PT is to small for this.

Any case thanks for a really great amp !

[Bruce / Mission Amps]

The object of the higher current choke in front of the OT and power tube was to keep the B+ voltage down a little while increasing the current sourcing capabilities of the B+ rail.
Look up how an inductive filter works.... choke input.
That way you can use a smaller value power tranny but trade the extra unneeded B+ for slightly more current to the OT.
So a smaller 100ma PT can still work very well in this circuit with a min 100ma choke.

[stratavox]

5E3H

Hi Bruce, and thanks to all for the replies. Bruce, I'm interested in the dual 6550 version you worked up- would you consider posting a schem (if you have one)? Conversely, I'd be happy to buy a copy if that's the case too...I could contact you through the Mission site. I just need that "something" different to try out. Thanks, and happy holidays all.

[stratavox]

Sorry...

I meant 5F2H...mea culpa...

[angerboy]

If I were to do this over again, I would go back to my original vision of the 5E3 sized cabinet and a good 12" speaker, the bigger trannys, choke input filter SS rectifier and a pair of KT66s in dual SE.

Would you consider doing it over again?

[Bruce / Mission Amps]

Yeah but I'd try for two 6L6s, SE 6550 or SE KT88 with a Bigass output tranny and maybe a 150ma PT with choke input for low voltage and high current.
I tried two KT66s in my MA5F11 V-lux, with the bigger 130ma PT and a tweed Deluxe OT... that sounded really good even though I could not reap all the benifits of the two KT66s in push pull.

I actually have a mid skill level player/customer right now who wants me to build a loudish, light weight, bedroom level harp amp (at around 5 watts) that he can mic up, but I think I'll build him a 10-12 watt 6L6GC version of the 5F2H, especially since I have one more of my tweed 5F2A/Harvard, over sized cabinets with the 10" Alnico in it.
It looks like a small 5E3.

[arnenym]

I've built 2 of them but with 2x6V6 powertubes and 1 with 6L6 Powertube. They sound good, and fat and i am completely satisfied. Right now i do a 5F2H -mod on a Harley Benton GA-5 (a Epi valve jr) to a friend. I use a 6V6 on that one. But he want more dirt and i put in a extra tube. The first tube is a cathodefollower and the second stage is a 12AX7 with both triodes in paralell. That sounds real good and i believe it sounds even better when the 125DSE 10w transformer arrive.

[sportster4eva]

Yeah but I'd try for two 6L6s, SE 6550 or SE KT88 with a Bigass output tranny and maybe a 150ma PT with choke input for low voltage and high current.
I tried two KT66s in my MA5F11 V-lux, with the bigger 130ma PT and a tweed Deluxe OT... that sounded really good even though I could not reap all the benifits of the two KT66s in push pull.

I actually have a mid skill level player/customer right now who wants me to build a loudish, light weight, bedroom level harp amp (at around 5 watts) that he can mic up, but I think I'll build him a 10-12 watt 6L6GC version of the 5F2H, especially since I have one more of my tweed 5F2A/Harvard, over sized cabinets with the 10" Alnico in it.
It looks like a small 5E3.

Funny you say that Bruce.. I made one a few months ago kinda along those lines. Dual 6v6 or single el34, choke input, hammond 125ese, larger box, 10" speaker. It looked like a small 5e3

[Karate Doc]

Base voltages

Well I finished up my 5F2H and I haven't played through it yet. Looking to see what others have got for voltages. No mods to the circuit and following the 3/30/07 layout drawing.

KT66 Pin3 288v, Pin 4 277v, Pin 8 18.9v

V1 Pin 1 150.8v, Pin 3 1.3v, Pin 6 155.5v, Pin 8 1.3v

Point A 295v

Point B 278

Point C 244v

Do these voltages seem right?

[Bruce / Mission Amps]

My first impression is that I'd like to see a little higher B+ on pin 3 and 4 and a little more current through the KT66.
Without re-reading all the thread... did you you the WeberVST 360vct power transformer or something else?

I don't have it anymore but I think my Weber prototype had around 320-330vdc on lug 3 and maybe 300-310 on the screen lug 4.

[sjhust]

Bruce, I'm planning on building one of these, but I have a couple of questions.

First, the transformer I'll be using puts out 300 volts center tapped, current rating is ok, I think this will work.

I don't have a big enough choke; as a matter of fact I just have a Champ-style choke, so I don't think I'll be doing a choke-input power supply. I'm using SS rectification. This puts my B+ around 425 volts, and that little choke isn't going to knock it down any.

I'm thinking that I could add a filter stage, use the choke between the first and second nodes, use some big dropping resistors, and take the plate and screen voltage off the third node. Based on your comments, it looks like I want somewhere around 300-325volts on the plate of the kt66. So based on 85mA plate current for 250V, from the datasheet, I'll probably need 1.5-2.2k 10W. Is this reasonable?

Thanks in advance for your time.

edit: never mind, i just figured out I better not use that choke at all in that circuit.

steven

[Bruce / Mission Amps]

No, power resistors in the B+ rail can really be a power waster.
Get yourself a few, 5 watt zener diodes and solder them with their cathode bands all facing the same direction and install the zener block in the center tap of the Hi-V center tap.... with the last cathode band to ground.
Use about 4 or 5 10v to 15v zener diodes.
Under a little load, this will drop the B+ about 30 to 40 vdc.

Another thing you can do is install a power resistor in series with each of the Hi-V AC leads to your SS rectifiers.
The average RMS voltage of your PT is about 300 volts there.
SS rectifiers have such a low V drop across them that dropping a little VAC before the rectifiers will drop the HI-V a bit and your filter caps will not charge up as high as with out them.
In A SE Class A amp, where the current through the power supply is fairly constant, it will not be in effect of the variable I2R losses of the resistors as much as a PP Class AB, who's current is going up and down with the demand. The drop across the resistor in the Class A amp will be pretty stable and at the max amount based on I2R loss.