Well looking at this schematic, the amp is fixed bias, way above my skill set to muck about with the power section valves. I stick to about 75% of plate dissipation and use a biasrite to measure my Marshalls.

http://www.prowessamplifiers.com/schematics/fender/Vibroverb_aa763-Schematic.html

Though looking at it, you could up the resistance of the resistor coming off pins 1&6 of the 7025 valves (12AX7's I presume). Allowing for a tad more negative feedback in the preamp section. Don't take em up any more than 20/30k (so 120K or 130K resistors instead). Also you could clip out the capacitor bypass resistor coming from pins 3&8 of the preamp valves. Though it looks quite highly rated in my very amateur opinion. 1.5K (early brit amps used 820R, near half that). Also it can lead to somewhat a sterile sound, I tried it, didn't think any gain increase was worth the sterility. That could be well different with a harp, it's a way more organic sounding instrument anyway.
(I'm terrible at reading schematics. It's very much a squint and drag my finger across the screen affair on this end. I could well be very wrong with those values)

Really you'd need to get the amp to a tech to do them things. Don't risk doing the lightning fandango. Even when it doesn't cause visible damage, it aint fun.

Other than that ask your amp tech, and I really advise using one. I would question my above guess work with him. If it were my amp, that I wasn't familiar with, I'd pay someone to do it. Suggest what I suggested, if he nay says it, go with his better judgment.

Get onto Hall Amplification for one of these.

http://www.hallamplification.com/main.html?src=%2F#2,2

Some say dropping the voltage across the amp can strip the getter (silver coating) out of your valves, shortening their lives. Indeed I've had to replace my rectifier valve today for exactly that reason (it was a no name 70's thing so who knows). The rest of em look fine though.
Also you'd go through valves quicker than normal by biasing hot. So to my mind, what you lose on one hand, you gain on the other.
That said, it seems the easiest option. The guy who sat down and explained basic valve operation to me, told me Fender amps test better on the workbench than any other guitar amp he'd had in. As well as repairing amps for Hendrix and PA's Michael Jackson. He spent 40 years designing the things for Laney then Carlsbro.
If he rates em so highly, I'd have to question the validity of any preamp mod or bias raise to one. If you're going to mod it, make sure it's reversible.

EDIT wrong link.

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Thank you, Senor Ninja!

I am a bit worried about sound sterility--you're right, a harp/harmonica is a bit more organic sounding than a guitar, but it still can veer over into the sterile side of the road with the wrong equipment.

I have thought about a Dr. Z's Brake Lite--a power brake that was highly recommended by a technician I trust--but I would have to reach behind the amp and adjust it in order to play super-clean on guitar (which is not very often, but...). I would have to try it out, due to all the nasty things I have heard/read about power brakes.

One person suggested modifying the amp to have a master volume (and I'm not one to be prejudiced against master volumes), but I don't know about that severe of a mod on an amp I like so much...it is reversible, and I could mount the knob on the back so as not to mar the appearance of this beauty.



I really like my Vibroverb--it sounds equally good for guitar and harp, which is a hard thing to find (with the exception of the breakup/dirty issue we're discussing). It also cuts through the mix well (40W 2x10 combo is plenty loud for the bands/venues I play with); I have been impressed by the Egnator Rebel amps (both the 20W and 30W model intrigue me). The Rebel 30 has independant switchable channels, so I can have one cleaner and one dirtier--one for guitar, one for harp.

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Good Vibes To Y'all!



Screamin' Armadillos
Texas Roadhouse Music
Guitar/Slide Guitar/Harp/Vocals

This sounds like a cool option, too...although I am concerned about shortening the life of the tubes, (or as you call them, "valves"...I know "the Tube" is your subway...Brits always sound more cultured...I unfortunately sound like some half-drunken cowboy, and I don't even drink! But I digress...a lot).
When you say shorten lifespan, do you mean by one-half its' normal life or one-quarter or what? I don't gig as much as I used to (work, family, etc.), so the tubes/valves/whatever should last me quite a while.

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Good Vibes To Y'all!



Screamin' Armadillos
Texas Roadhouse Music
Guitar/Slide Guitar/Harp/Vocals

Who knows, I don't think anybody has accurately made a comparison. All I can tell you about that VVR gizmo is that a lot of well respected US amp builders use it. It's use in the UK is nowhere near as widespread though it is over here. In my amp for one. I don't know any amp builder who makes tubes. I don't know of any amp maker that amp users buy tubes from. Lets face it, if you pay Mesa/Marshall/that-other-company-who-will-remain-nameless prices for tubes made in exactly the same place as all those cheaper tube sellers prices. You can do better.

Now Mr Armadillo, lets be clear about somethings.

1 running a amp hot, or in the area where tubes start to sound best, will shorten tube life. You're working them harder, it has to.

2 You already have a great way of lowering amp volume with the DrZ powerbrake. Attenuators are not the complete answer, but a lot of nonsense has been written about attenuators. They will never give the sound of a amp pushed hard. Because the speaker is not getting all the power it wants, it won't sound the same but quieter. It doesn't work like that. Speakers need power to operate like a speaker being pushed.

3 Attenuators don't break amps, people who don't know what their doing with attenuators break amps. Use good, thick, well maintained speaker cable going into and out of the attenuator. Don't turn the attenuator for maximum attenuation and everything on the amp to 10. Get a bit of balance, 7/8 on the volume control has long been my favourite area. Pushed enough to sound good. Them old amps can lose response when their on 10. Marshalls are famed for it.
Why would anyone as respected as Weber, DrZ or Marshall make a product for their amp that damaged the amp. It's just not good business is it?
On attenuators, make sure all cables, so going from the amps output to the attenuator, and then from the attenuator to the amp. Are good thick speaker cable. Speaker cable is different from instrument/guitar cable in that instrument cable needs to be shielded and quiet but doesnt have to handle much power. Speaker cable needs the opposite. It needs to handle loads of power, but doesn't need to be quiet. Noise picked up on instrument cable is usually quite a small amount of noise but is amplified at the amp resulting in it being loud when it comes out the speaker. Noise getting into a speaker cable isn't going to be amplified at all, all the amplification has already been done. So it's not noticeable through the speaker. So you go amps speaker out to attenuator input. Attenuator output to speaker.
That's about all you need for the correct use of attenuators. Had over a year out of my Marshall powerbrake and not blown my jcm800's up.
Oh yeah, turn the amp off or into standby before adjusting the attenuation level, just to make sure.


4 Master volume controls are never a good idea. That is not fact, that is nothing more than my very biased opinion and should be dismissed as such.
Reason I say it is because a master volume won't get you the power tube crunchiness/breakup that you want without running the amps master volume in that area and the channel volume on full. They allow you to push the preamp tubes whilst not pushing the power amp tubes. That little box that you can sometimes see on ebay. The one that you see incorrectly listed as a attenuator that sits in the effects loop of a amp is nothing more than a master volume control. It sits in the effects loop right slap bang between the pre amp and the power amp, just to do that. Stop all the pre amp signal getting to the power amp. For £30, if you have a non master volume amp that has a effects loop and want a master volume control. It's worth it, not for anything else though.
You won't go through power tubes with a master volume control, but you won't have power tube breakup. There is good argument that you will push preamp tubes as hard as if you were running the amp at max though.

All in all, if you have the DrZ use it. Get a good speaker cable made up by your tech. Make sure you ask for speaker cable, guitar cable won't do the job and will damage your amp if used as a speaker cable. Also you could get the cable from the speakers themselves extended. So the DrZ can sit nicely on top of your amp and allow you easy access.
If you get fed up of lugging it around and want a slightly better sound (and I mean slight) the VVR is good. Though that is comparing the VVR to my Marshall Powerbrake which isn't the most advanced attenuator out there. If DrZ, THD or Weber attenuators perform just as well as the VVR, I wouldn't be surprised.

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I haven't yet bought the Dr. Z Power Brake Lite, which is a little smaller than the average attenuator. It mounts on the inside of the speaker compartment. Here's a link:

http://www.drzamps.com/products/brake_lite.html

I've spoken to a few guys who use them, and most like them very well. I just wish I could try one out on my particular amp for a little while (even just an hour) and see how it truly responds to what I'm laying down musically.

I spoke to the good doctor himself (Dr. Z) and he said it would cure what ails me, but I guess there must be some Missourian in this Texas boy*, cuz I just need to hear/see for myself before I'll pull the trigger.

(*In case you don't know, Missouri is known as the "Show Me" state, and the denizens of that area are known for requiring extensive evidence before trusting any supposition.)

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Good Vibes To Y'all!



Screamin' Armadillos
Texas Roadhouse Music
Guitar/Slide Guitar/Harp/Vocals

For that amp, I'd chose the VVR, atleast you can discretely mount it on the front panel. Lose one of the input sockets on the channel you don't use. Or if you use both channels lose the input socket you like least.

It's easy to see when the tubes going bad. The grey/silver getter coating will literaly strip from the top of it.

Like this



A lot cheaper. Please bare in mind that is a brand new JJ el84 next to a old 70's Eurotubes80 EZ80 that I got 2nd hand. Who knows what life it had before I got my hands on it. You can clearly see the missing getter coating off the top of the tube. That was the only tube to suffer that fate in seven months hard work, the others are fine. Theres a whole science of assessing the operational condition of a tube, simply by looking at the getter. The VVR works well for me, is geared up to handle upto 50watts RMS on either fixed bias (your amp) or cathode biased (my amp) amps.
The only problem with it is you either need to be handy with amp innards, suicidaly gungho, or have a decent tech to fit it. I see it as a far better prospect, to spend £35 on one of them, than £160 on a very small attenuator that has to be mounted in the amps cabinet. That is my only real gripe about the DrZ attenuator. Attenuators dissipate power as heat, so good airflow is important to their continued good operation.
Now whether that is applicable in this case or not, I don't know, I'm only presuming so.

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