Pics and Clips of a new amp

[rfgordon]

This is a harp amp I've just finished for a guy--he wanted one with reverb, so that's what I built. Never seen a dedicated harp amp with verb, but I can't claim to know too much about the harp scene.

The circuit (sorry, no gut shots--if I open it up again before delivery, I'll take some) is kinda like an old 5D4 with a D-style one-tube reverb stuck in the middle. Low gain, as you might expect, but that helps reduce feedback problems. The controls are simple, as the picture shows: Volume, Tone, Reverb and Wave. The Wave control is the feedback killer that Gerald Weber writes about in one of his books--works like a champ when tested it with my ultra-cheezy Radio Shack omni mic.

A guitar player would call this a "dark" amp, but that's better for harp. The first 12AX7 (which is a very nice vintage tube) has both sides in parallel with an RP of 220k (82 VDC) and an RK of 680 ohms and a CK of 100mF. It really don't do clean.

The cheesy clips are recorded into the PC with my Radio Shack mic and the Windows Sound Recorder. That's why they're only 60 seconds long.

The one clip with a bit of echo has that added in the Windows program. Extra cheese at no extra charge.

The covering is SkinTex Chroma color-shifting vinyl from YourAutoTrim.com. My customer chose that and the grill cloth. Nice combo. The vinyl shifts from green to purple, depending on the angle. Mucho expesivo at $69 a yard!!

The speaker is a 15" Eminence Delta Lite neo mag. Big fat low end, good for harp.

I must say, it chaps my noogies to have to put Fender tilt-back legs on there, but that's the only kind out there at the moment, and the customer wanted tilt-backs, so on they go.

The guitar is one a friend built me: maple over chambered korina, Bolivian rosewood neck with ebony board and Rio Grande pups. Sustain monster.

Anyway, there ya go.

[rfgordon]

Here's the other clip. BTW, the Vol and Tone are dimed, guitar straight in via my favorite Klotz cable.

[skyboltone]

Great Rich:
I kinda like it. Probably like you say, if it was a song and dance man, it would only know one song and one dance but they'd be good'ns.
Oh, I almost forgot. Can you switch the guitar so it has either inner or outer coils in series or parallel? Get some Stratification?

[jimipage]

I really like that "greasy" clip

[rfgordon]

Thanks for the kind words, y'all. The guitar has had the middle pup replaced with a Duncan Fat Cat, and it's a bit brighter than the Rio Grande humbuckers (Texas neck & BBQ bridge). For the most part, this guitar is less bright, due to the influence of the rosewood neck, which extends under the top to the bridge/tailpiece.

[David Root]

Ah, the all-American nutrition pyramid: sugar, alcohol and GREASE! I liked the extra grease track too. Did you try a slide on it? I bet it would sound super swampy!

I dabble in harp myself, and as it happened I read that article of Gerald Weber's about the wave control with interest very recently. I have been planning a harp amp for some time now and am torn between the 5D4 Super and the 5E7 Bandmaster, but with cathode bias, as basic circuits. I want to do a two channel pre with a 5879 pentode as channel two, to have a Fender and Gibson style preamp, but I would also like to get 40W out of the power section. That will be quite a trick I think with cathode bias & 6L6GC/KT66 power tubes.

I'm curious about your choice of 220k plate resistor. I checked out your parallel 12AX7 in TubeCad and it indicates a "theoretical" gain of 85 with that 220k plate resistor. However, its point on the loadline is skewed towards the g zero bias line (about -0.6 bias @ 250V B+) so in practice it wouldn't get anywhere near that gain, nor would it "do clean" too well, especially with HBs.

[rfgordon]

I tried several combinations of values till I hit on what I thought sounded best. The 220 k RP allows for the potential for a large voltage swing, which is necesary, since a Green Bullet will hit this thing with as much as 3 volts. I wanted the plate voltage low so it'll grind hard. I went with both sides in parallel in order to lower the output impedance and, since this and the PI are the only gain stages, preserve as much gain as possible thru the Vol/Tone controls.

To be perfectly honest, I never look at those tube charts. I just go with what sounds right.

[David Root]

......that's the test that counts. I was curious because I've done some reading, preferred amps, old schematics etc. and checked out the harp forums (fora?) and never seen the 220k plate resistor, except on a PI. I will have to try that myself.

Have you tried higher than 5D4 power tube grid resistors? I have seen up to 470k in old schematics, vs. the 220k standard. That would help a little in the gain department, assuming cathode bias.

[rfgordon]

David,
I put 1k5 5watt resistors on the control grids of the power tubes, so in a way the power section is a bit modernized. The grids have 1k5 grid stoppers, too.

One reason I use both sides of a 12ax in parallel is that, and I forget where I saw them--didn't save them (Doh!) was that, according to somebody's charts (well, I guess I DO look at some charts!), the 2nd order harmonic curve is different for both sides vs one side. Lots of the Matchless amps use both sides with a 220k RP.

This thing is now tuned to be dimed for the grind, but not be too loud for most venues in our area.

We'll see how good my intuition is next week when the customer starts playing thru it!

[David Root]

I didn't know that about parallel operation. I've used that before for the usual reasons--more gain and much lower impedance and I like the tone too.

Let us know how the live show test goes!

[rfgordon]

Yeah, you betcha!

This was a good amp to build, because it let me test out several conepts for future amps.

First off, I really dig the combination of cathode bias, low voltage 6L6s with a 15" speaker. The harp guy gets the Eminence Delta Lite (one reason I picked that one is that the speaker terminals are banana plugs, and in order for the feedback killer to work, the speaker has to be in proper relation to the mic. By using banana plugs, he can swap that (depending on what mic he uses) in a few seconds--much faster than spade terminals. I also tried it with a Weber Chicago 15" and I really like that one.

This one also showed me a bit more about the old tweed circuits, since I used the self-balancing paraphase inverter on it. For guitar, it does tend to compromise the clean, and it doesn't offer the same gain as a LTP, but it helps the amp grind sooner. A lot of the pickers I talk to bemoan the fact that their amps don't "give it up" till they're way too loud for most situations.

So, after the amp that's in process now (a 6V6 powered Rocket-inspired amp), I'm thinking of doing another one for the 15" Chicago. It'll be simple--Volume, Tone, Reverb. V1 will be both halves of a 12ax7 in parallel (I may put a local NFB switch on it), then into a Blackface style reverb with a big tank, then a gain/mixer stage, and maybe the old school PI. Cathode bias 6L6s. Prolly end up wicked for slide. Might cover it in brown gator tolex.

I'm still hoping to get a Harley rider for a client--I'd love to do a H-D themed amp in orange & black carbon-fiber textured vinyl, with the H-D emblem on a custom grill cloth. Maybe a diamond plate face. Sorry, waxing artistic there for a second....

I'm hoping to be able to have a few amps done for sale by the Washington DC guitar show in November. My dealer and I are going in on a booth. There hasn't been a guitar show in this region in a very long time, so I'm hoping there'll be lots of action. That, and it's right at the beginning of Christmas shopping season! mmmmmmmmmm, cash flow...

[jimipage]

schematic? pretty please?

[rfgordon]

If you want a schematic of the Echo-Harp, the best I can offer is to draw it on paper and snail mail it to ya. I don't have the time or patience to spaz my way to one in a grapics program. Now, if anyone wants to get the paper copy and do it up in the computer, I'm all in. I'll send my Two Nob Trainwreck schematic also. Just PM me your snail mail address. Any voluteers?

[jimipage]

PM sent

[rfgordon]

To those who volunteered to digitally render some schematics, a gazillion thanks! I will endeavor to get them out to y'all this week.