Power brake tames feedback

[Marc]

The basic question I have is why does a Marshall powerbrake tame the feedback in a Spitfire clone?

I had a guy over last night who brought a power brake that we used on the first click from maximum (least amount of braking) and it kept the amp from feeding back until the gain was set significantly higher-from 9 o'clock to 12 o'clock.

I had changed the preamp to a progressively lower gain tube ending up with a 12 Au7 in the preamp, a 12AX7 in the PI. I was thinking that the first stage coupling cap in the spitfire could be made larger (from the .002) to help fatten up the sound, but I'm wondering if the 0.1 isn't a bit too large? It would help if I could play the harp with out being threatened with divorce.


This guy wants me to build him the ideal harp amp, but wants a clean sound not a crunched blues sound. He was using a super reverb until it was stolen. He had many mods and tweaks done to the amp, but has no idea what they were.... Now he wants an amp that will cut through at stage volume, has minimal compression and distortion and that doesn't weigh much more than 30 pounds.

Is it heresy to suggest a head and cabinet? I 've seen most harp amps are combos. I have built a bunch of guitar amps but they just don't have the tonal qualities that a (this) harp player wants.

I see from another thread here that the 5C4 super is a good starting point. Perhaps with a cathode biased preamp. Are there any other standard amp schematics that can be suggested as a starting point?

Any and all thoughts are welcome. This maybe one of those projects I pass up....

thanks!
Marc

[MWJB]

This guy wants me to build him the ideal harp amp, but wants a clean sound not a crunched blues sound. He was using a super reverb until it was stolen. He had many mods and tweaks done to the amp, but has no idea what they were.... Now he wants an amp that will cut through at stage volume, has minimal compression and distortion and that doesn't weigh much more than 30 pounds.

...Phew! He wants a lot. "Ideal" is a relative term.

Getting & keeping it around 30lbs will be very tricky, for most people's idea of stage volume you will still need relatively hefty transformers, to keep things clean you will need multiple speakers (more weight - 4x Eminence 102 will keep things relatively light in that respect). 50lbs-ish is more realistic. Why not aim for another Super Reverb/Concert style amp if that was what he liked before? Be sure he really means "minimal distortion & compression"...a lot of guys say that but have second thoughts when you give them clean (not that harp amps REALLY do clean).

My preference would be a 6G12 style preamp (1MA bass pot, 50KA volume pot with a 470K dropping resistor from the treble pot, replace 10K fixed middle with 25KL pot), run preamp tube at 180-190v on the plates (12AX7), power tubes at 495-525v. 0.1uf coupling caps in preamp & PI coupling, 100uf 25v cathode bypass in preamp, 5meg input resistor at Normal #1, wire filters a la Super Reverb but use 2x220uf 350v at the mains, 2x100uf 350v at the screens (each cap bypassed with 220K 2W). Replace 15K dropping resistor in bias circuit with 1k in series with a 50K trim pot wired as variable resistor, 100K load resistor instead of 56K. Bias to 15mA as a starting point. Play with 5751 in preamp or 12AT7 in PI if you need cleaner.

5C4 will not meet ANY of the first three criteria. If you want stage volume, little distortion and minimal compression...you want fixed bias and a solid state rectifier.

Head & cab is a great idea, both my big harp amps are head & cab.

Not familiar with the Spitfire clone, are you actually reducing feedback with the brake, e.g. are you getting significantly more volume from the amp (dB), rather than just having the volume point to a different number? If you're not actually getting more voltage at the speaker, you're not getting more power.

[MWJB]

Had a look at a Spitfire schem now, mostly EL84 amps are fine with .022uf coupling caps, some EL84 amps can go overly dark with 0.1uf. Don't know if that 180pf cap accross the volume pot is a good idea for harp, or whether you would want to try some negative feedback if you were going to persevere with this kind of design. Not keen on the bridged triodes for the input tube either, although Sonny Jr has used this feature, I'd A/B against a regular, unbridged triode, or even try a cathode follower first stage, with slightly higher voltages (100K plate resistor) and a 12AX or 5751 in V1? A single 10" is probably your best bet for limiting feedback.

[Marc]

So as I learn...

Had a look at a Spitfire schem now, mostly EL84 amps are fine with .022uf coupling caps, some EL84 amps can go overly dark with 0.1uf. Don't know if that 180pf cap accross the volume pot is a good idea for harp, or whether you would want to try some negative feedback if you were going to persevere with this kind of design. Not keen on the bridged triodes for the input tube either, although Sonny Jr has used this feature, I'd A/B against a regular, unbridged triode, or even try a cathode follower first stage, with slightly higher voltages (100K plate resistor) and a 12AX or 5751 in V1? A single 10" is probably your best bet for limiting feedback.

Thanks for your detailed responses!
This particular clone has the 180 bright cap on a toggle switch that allows it to be in or out of the circuit along with a .001 cap that can be toggled in to cut the highs. (on-off-on). It was way too bright with the 180p cap in circuit. The first coupling cap is acutally a .002. bumping it up to a .02 is not something I've tried, yet.

As I read and think about this design challenge, I'm leaning more to the Brown concert...I've heard one of those and liked the sound and I can't say I've heard (in person anyway) a 5C4. I love the tremolo, but if harp players don't use it I'd be inclned to leave it out. I haven't asked if tremolo was important to him or not.

I like the idea of a cathode follower on the input since gain is not all that important for this guy's 'dream rig' He likes to use a delay pedal and also has a specialized harp preamp, the Holmes Harp Commander II. With that pair of pedals he could drive the cathode follower enough to push the amp, maybe.
I also agree that he needs something in the 40 watt range. But not being technical at all it will be something like, build it and see if I like it....

I don't understand why a single 10" would be less prone to feedback. Is it simply less sound pressure reaching the mic? he was sure he didn't like the sound of a 15" or a 12". I'm thinking a pair of 10's would be good, especially because I have a pair that need to be used. (good design reasoning, eh?)

I'm going to keep a list of preferred mods and amp circuits to determine a common thread to get started on a design to present to him. I figure in the next few months I'll have a better idea of where to go.....

Thanks!
Marc

[badams]

Build him a Princeton Reverb with a line out. They're only about 15 watts but the line out into a PA will take care of that nicely
Beautiful clean sounding amps

[MWJB]

I don't understand why a single 10" would be less prone to feedback. Is it simply less sound pressure reaching the mic? he was sure he didn't like the sound of a 15" or a 12". I'm thinking a pair of 10's would be good, especially because I have a pair that need to be used. (good design reasoning, eh?)

With a small amp like the Spitfire (or badams suggestion for a Princeton), runing high currents on the output tubes, a relatively directional 10" makes it easier to keep the mic out of the way of the speaker to avoid feedback. 12" & 15" will be more sensitive to feedback and reduce output before feednback...all other factors being equal.

2x10" - a possibility, but best to have one speaker above the other vertically rather than side by side, for same reason as above & better on stage monitoring. Keep them parallel rather than series.

For a big amp like a Concert, 4x10" is hard to beat. The Concert tremolo adds quite abit of gain, tremolo genreally takes the front off the note with harp so rarely preferred for stage work, maybe OK for recording.

[badams]

I wasnt really thinking about the speaker in the Princeton MWJB.
I use one of the old gray plastic enclosure Micro Hush pedals to kill feedback.
Everybody has their own opinion on tone. Like the guy who wants a clean sounding amp I prefer it to. To me theres nothing musical about distortion beyond just the outset of it. Most people seem to prefer the heavy distortion?
To me it sounds like they're playing through a two hole harp. It drones on and gets real old real quick. As with guitar distortion is a good way to hide sloppy playing. A Blackface or Silverface Princeton Reverb will overdrive but you really have to push it. I use 12AT7's with maybe a 12AU7 thrown in from time to time and a 5Y3 rectifier. It sounds great with guitar or harp to my ears and to anyone else whose heard it. I've asked allot of people their opinions while listening to bands with overdriven harp and I can honestly say that the best response that I got from any of them was its alright. Most flat out said they didnt care for it . It seems to me that the only people who truly like the sound of heavy distorted harp are those who play harp that way? I dont play to please harp players.

[MWJB]

I hear what you're saying, it's courses for horses and everyone needs to play off a tone that they enjoy listening to. However, many players generate distortion right at the harp, so the amp (or even SM58 & PA) can sometimes be irrelevant, I know guys who won't get a clean sound out of ANY amp. Also a broad term like "distortion" covers a lot of ground, yes a late BF/SF Princeton is generally a "middle of the road" sort of tone, but few amps are really clean, many just distort in a different way, (different degrees of crunchy harmonics vs smooth & greasy, punch vs compression) but distortion, or coloration, is pretty well always there to some degree.

Trust me, guys like Piazza, Clarke and Little Walter (his more overdriven stuff like "Watch Yourself", "Rocker" etc.) aren't/weren't hiding anything when playing with a distorted tone.

[badams]

First off Bro. Please dont get defensive by my the way I communicate with people. Theres really no malice meant. I just dont talk to grown men like they're little girls whos feelings are tender. Also I dont expect you to agree with my opinions. I'd be shocked as hell if you did.Of course they can get a clean tone if they want to. Just dont bear down so hard on the reeds. If you're trying to impress me with your knowledge of things harp,amp sound or music then please dont. It aint working. Know what I mean? If there ever was a know it all then its gotta be me.
PS
I wish I knew half as much as you do when it comes to working on amps.
No brown nose . Just truth

[MWJB]

That's cool with me.

Cheers.

[badams]

PS
I wouldnt call my Princeton middle of the road. If there ever was a clean sounding amp then its gotta be it. I was just playing it with the volume on 10
via an electric guitars humbucker pickup and there wasnt a trace of distortion.
Cleaner than a whistle. One of my harp mics would overdrive it but still....its a fairly hot pickup.

[MWJB]

I hope my comment didn't come accross as suggesting that the amp (with harp) was "so, so" or not inspiring/didn't sound good. What I meant was, it's a good datum line for an electric sound... not too crunchy, not too saggy, not too greasy, just balanced.

[Marc]

MWJB & Badams,
thanks for your comments. I hadn't thrown the PR into the design touch stones. I think it has some of the generally accepted design specs (gads!) that are standards for harp amps, the PI, reverb and the 10" speaker. I've heard many over the years and few that were in need of a lot of tone help. With the exception of the flubby bass and higher levels.
I have a bassman style combo amp (JD Newell made I believe and one of the baffles I have for it is 2 10" speakers on a diagonal. The combo would be heavy, so it may not be what my customer wants. I'm still leaning towards a head and cabinet amp for that reason and the hope that it can help with containing the feedback. Of course as one gets past the half century mark it's not only heavy amps that are a drag. You end up having to stand in front of the speaker to just hear what sound is coming out! Ha Ha

I was also thinking that a buffered effect loop would also be a good design element. It would not be in keeping with a short signal path but it might allow better control of the out board effects, primarily delay but who knows what else.

on with the muse....

Marc

Marc

[MWJB]

"I was also thinking that a buffered effect loop would also be a good design element. It would not be in keeping with a short signal path but it might allow better control of the out board effects, primarily delay but who knows what else."...I don't think that this is going to be a good idea for a harp amp, in line effects are generally preferred.

[Marc]

"I was also thinking that a buffered effect loop would also be a good design element. It would not be in keeping with a short signal path but it might allow better control of the out board effects, primarily delay but who knows what else."...I don't think that this is going to be a good idea for a harp amp, in line effects are generally preferred.

It turns out that the loop was a short lived idea. This guy wants to put all his effects infront of the amp.

Here's what deisign parameters I've suggested so far:

Portable
About 30 watts
A low gain pre-amp (relative to a guitar amp)
Single channel pre-amp with sensitivity control - not exactly sure what I'm thinking there
Bass and treble tone controls - maybe baxandall, I haven't decided
Separate gain and master volume controls
Anti-feedback control - a local feedback loop at the first stage
negative feedback control (sometimes known as a soul control)
Line out jack
Multiple output impedance

I was also mulling over the possibility of borrowing the simple compressor from Fred Nachbaur's design and incorporating that.

I'll try to put together a schematic in the next week or so. Now that school is about to start I can't keep running off to the beach every weekend.

Marc