Modding an Orange Rockerverb 100 for harp

[rf7]

I know this is crazy, but a client wants me to do it. I was going to bump up the coupling cap values to allow more bass through, and maybe drop the plate voltage at the first stages of the preamp. This guy wants to be able to run harp through the dirty channel with the gain cranked and no feedback.

Has anyone tried this with a high gain big tube amp before? Any suggestions?

[J M Fahey]

Agree with your diagnostic: your customer is crazy

run harp through the dirty channel with the gain cranked and no feedback.

Only possible with a 2 mile long microphone cable.

Truth is *all* killer harp amps are real old Tweed era amps, or same vintage PA amps, with basically flat frequency response (very little eq if any) , gain enough to barely reach clipping but no more, and bass and treble limited by their cheap construction.
The end result was good ... or we are used to it because that's all there was available in the "golden era".
Frequency limiting and shaping came also from the very poor dispatcher type microphones used, plus using them in the terrible acoustic conditions between cupped hands and the like.
With due respect, changing any ingredient in that classic recipe will mean failure of the dish.
jm2c

[MWJB]

JM Fahey wrote: "Truth is *all* killer harp amps are real old Tweed era amps, or same vintage PA amps, with basically flat frequency response (very little eq if any) , gain enough to barely reach clipping but no more, and bass and treble limited by their cheap construction."

I certainly wouldn't go as far as to call this a universal truth.... 99% of harp players use fixed-bias, post '57 circuits in their stage amps (late tweed 4x10" bassman is the most popular by a country mile, followed by early 60's brown/blonde amps & BF amps. Sonny Jr, Harpgear, Kinder instruments are all fixed bias & have regular levels of gain compared to a Fender Normal channel, when fitted with 12AX7 preamp tubes). Many of those old PA's have a funky sound but much of interest in them is from people choosing to "interpret" Little Walter's words on harp amps (he never actually mentioned any known amp brand by name). They can make cool recording amps (many used for recording are modified), or low volume stage amps, but won't usually hold their own on a regular electric stage.

Typical frequency response of popular harp mics was 10Khz to 18Khz, only in the last 10yrs have the Sure CM/CR (5Khz) become prominent with the cessation of Astatic crystal manufacturing. Even as early as the 50's harp players were using regular dynamic, hi-z, vocal mics. By the early 60's the Sure 545 (industry standard at the time) was the "one" to have (after LW was photographed with one, futher popularised by Butterfield), along with the 585. Resurgence of bullet style mics was really a revisionist thing. There were no "harp mics" until recently, the more popular mics were all intended for voice.

Back to the original Q - ALL harp amps feed back. If you can turn your amp fully up, without feedback, you can never utilise the potential power. Subbing low mu preamp tubes is a common trick, but has its drawbacks (softer dynamics, limited frequency response). Split load plate resistors or tacking additional resistors from volume pot wiper to ground to adjust sweep are also options. Some of the loudest amps I have heard & played through for harp were Fenders that could only be turned to "3" on the vol pot.

Dirty channels are usually avoided by harp players.

Bias cool/cold & have him play through it...plate currents in single figures, mA-wise, are not unheard of in 6L6 fixed bias amps used for harp. You might also explore deliberatey unbalancing the output section, by using unequal plate currents, or by unbalancing the PI tube if such a thing has an obvious route.

You might well be flogging a dead horse...especially if the amp has 12" speakers. It's no coincidence that the most popular harp amps run multiples of 10"s.

[rf7]

Thanks for the responses gentlemen! Of course he's using a 4x12 cabinet. Dead horse, here I come!

[Bruce / Mission Amps]

I agree with almost everything MWJB just said... except the part about the bandwidth of popular harp mics!
My Chicago harp amp is most certainly NOT a warmed over tweed Fender amp... and depending on what speaker configuration is chosen, even though the volume pot goes up to 12, with most decent hot harp mics it will start feeding back like a howling wolf with the volume up past 9-10 unless you know how to hold your mic!!
Mission Harp Amps - Blues Harmonica Amplifiers - Home

[Bruce Saunders]

I saw Col. JD Wilkes of Th' Legendary Shack Shakers using the rig described at a London gig just the other week (Orange Rockerverb 100, Marshall 4x12 Cab). As they're on their European tour the band were using the support band's amps, and it was a very feedback prone rig. JD had to adjust it several times during the course of the set. He was using a JT30 shell with a push button on-off switch fitted in the top. I don't know what element was inside however.

[rf7]

Well I did it, and now he's getting bass feedback. What a surprise-- Not!

[MWJB]

It's not "bass feedback"...it's probably just "feedback", if you have dropped the idle current the amp just feeds back *at* a lower frequency, rather than feedsback '*because of* low frequency. You were never going to eliminate feedback. Has he noticed no difference to tone/ease of use?

[rf7]

Oh yeah, the tone is much better with more bottom end.

[MWJB]

Good work.

[rf7]

Why thank you!