Attenuators/Solo volume increases

Si_Si_ Frets: 384
Since I'm back in the world of valves I'm back to my age old problem of getting a solo volume boost out of a vintage Marshall. I've just picked up a late 70s JMP and love the sound of it but can't get any volume boost for solos since I'm already running the pre-amp on full.. boosting with a pedal won't increase volume.

I was thinking, would it be possible to mod a Marshall Power Brake with a bypass footswitch? have it attenuate for rhythm sounds, and bypassed for lead (or even have a 2nd level for the lead sound).

I know you can buy things like the Webber attenuator with this feature already but was curious as I've used a marshall before and it worked well. 

Just wanted to think of a few options before I decide to either Mod the JMP or sell it.
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Comments

  • ICBMICBM Frets: 71963
    I'd need to check the Powerbrake schematic to see if modding it would be easy or possible. You could do it with a loop-switching box if the switch and cabling inside was heavy-duty enough for speaker current though - just put the Powerbrake in the loop.

    But... it's potentially risky for the amp. You're introducing the potential for failure leaving the amp open-circuit, and even the momentary break in the connection when the switch is operating is hazardous if the amp is actually producing high power at the time - ie if you're switching on the fly and you don't have the strings muted.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • Si_Si_ Frets: 384
    Probably not a good idea then....
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  • ecc83ecc83 Frets: 1591

    I rarely disagree with ICBM but I do not think any harm would come to the amp due to load switching.

    I know of this being done to amps from one to two hundred watts at full power and for many,many cycles, never a problem.

    I have a theory that the interwinding capacitance of the op transformer is sufficient load for the few mSecs of the open circuit to prevent the voltages rising to damaging levels.

    An additional safeguard would be a 1mfd or so foil capacitor across the switch contacts.

     

    Dave.

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  • ICBMICBM Frets: 71963
    edited October 2013
    That may be true for 'certain' :) modern amps, but I've seen plenty of blown OTs on old Marshalls that were almost certainly caused by loose contacts in the appallingly bad impedance selectors they used.

    If you think about it, the result is analogous to a car ignition circuit... OT = coil, bad contact = points, spark gap = wherever the weakest place in the OT insulation is. I metered the voltage spikes on the primary side of one - before I realised what the cause was - and they were in the KV range. The amp was not going continuously open circuit either - just briefly, under the vibration from the cab.

    I'm not sure if a cap across the contacts would help... ignition circuits have them too.

    I definitely wouldn't risk it with anything old and valuable that hasn't been tested and proved safe under those kind of conditions, anyway.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • You could run the amp at 8 ohms, but into a parallel load of one 16 ohm resistive load and one 16 ohm speaker with the attenuator in the loop pedal. That way there is always a load, possibly an incorrect load but it should be enough for any slight switching delay.

    Alternatively when I had that amp I put an FX loop in the amp and stuffed a boost pedal in it. Probably easier. If you use the hole for the low input and fit a stereo (insert cable style) Jack for the send and return you can always return the amp to stock.
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  • Si_Si_ Frets: 384
    I think modding with an FX Loop is my only real option.
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  • ICBMICBM Frets: 71963
    Si_ said:
    I think modding with an FX Loop is my only real option.
    If you're running it with the MV lower than full you could mod it to take it out of the circuit, using the Low input (if you don't use that) for the footswitch... no permanent mod to the amp.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • Si_Si_ Frets: 384
    Defo running it with the master lower than full.. you know what 100w vintage Marshals are like :)

    Might be better if I get a 2nd MV fitting in the low input instead, and make that foot switchable.
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  • ICBMICBM Frets: 71963
    edited October 2013
    Si_ said:
    Defo running it with the master lower than full.. you know what 100w vintage Marshals are like :)

    Might be better if I get a 2nd MV fitting in the low input instead, and make that foot switchable.
    Still means drilling a hole, and some more complex stuff inside.

    You can make the MV switchable to a fixed increase rather than full-up by adding a resistor between the ground end of the MV pot and the actual ground; then use a simple footswitch to bypass it. The value of the resistor determines how much boost you get - you might want to do it with a trimmer.

    I would definitely avoid doing anything that involves holes, especially in the front panel, unless the amp is trashed already. Early 2203s are one of the great 'sleepers' in vintage Marshalls, in my opinion... it would be daft to ruin the value at this point.

    Actually I think early 2203s - I had a particularly great '76 to work on a couple of years ago - are one of the best amps Marshall ever made, equal to the Plexi in many ways and more versatile.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • Si_Si_ Frets: 384
    Ah, never thought of doing it that way.. that's a much better way.
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  • You could look at the Badcat Unleash or Weber do a footswitchable attenuator. Not much cash if I remember.
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  • ecc83ecc83 Frets: 1591
    ICBM said:
    That may be true for 'certain' :) modern amps, but I've seen plenty of blown OTs on old Marshalls that were almost certainly caused by loose contacts in the appallingly bad impedance selectors they used.

    If you think about it, the result is analogous to a car ignition circuit... OT = coil, bad contact = points, spark gap = wherever the weakest place in the OT insulation is. I metered the voltage spikes on the primary side of one - before I realised what the cause was - and they were in the KV range. The amp was not going continuously open circuit either - just briefly, under the vibration from the cab.

    I'm not sure if a cap across the contacts would help... ignition circuits have them too.

    I definitely wouldn't risk it with anything old and valuable that hasn't been tested and proved safe under those kind of conditions, anyway.

    Well, I bow to your vastly greater experience in this field IC of course, but!

    The amps being switched were not all modern ones. I remember a 100W Marshall and aVoxAC30 head being A/B'ed. And WTGR, a "fault" condition is not the same as a proper, fast switch.

    The "Kettering" ignition system is a bit of a red fish if you will excuse me! The ratios are vastly different. 1000:1 as against maybe 50:1 for an amp tops? Then, ignition coils go from a high direct current in the coil to an open circuit and back again, really nothing like the circuit we are discussing here. 

    I do not of course advocate that the OP do anything that might damage what might be a valuable amplifier but my experience with what must be 1000s of cycles on various amps leads me to think it is  a pretty safe practice. 

    (We HAVE to A/B many, many speakers ICBM to find the ones you hate most!).

    Dave.

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  • martinwmartinw Frets: 2149
    tFB Trader
    I've done dual, foot-switchable master volumes on Marshalls a few times. Works a treat.
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  • ICBMICBM Frets: 71963
    edited October 2013
    ecc83 said:
    The "Kettering" ignition system is a bit of a red fish if you will excuse me! The ratios are vastly different. 1000:1 as against maybe 50:1 for an amp tops? Then, ignition coils go from a high direct current in the coil to an open circuit and back again, really nothing like the circuit we are discussing here. 

    Yes, but that just means the induced voltage spike is in the KV range not the 20KV range, which is what I found. Even a KV spike is not good news for an old OT. I know the LT current in the coil is DC too, but at low frequencies in full distortion the peak current in a speaker circuit is probably not far off the same, and if the circuit happens to open right at the peak...

    I may be being overly paranoid but I really don't like the idea of switching speaker loads on the fly! It's notable that at least one of the companies who make purpose-designed amp switchers go the trouble to mute the amp inputs during the switch operation too. Whether out of similar paranoia or experience, I don't know.

    ecc83 said:
    (We HAVE to A/B many, many speakers ICBM to find the ones you hate most!).
    Ha :).

    Although I'm beginning to think you might be right with the ones for the ID series. They do seem to sound 'worse' through 'better' speakers.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • I use the dual volume Ultimate Attenuator.. It's exactly what you need. I run various plexi style amps but also a Friedman brown eye and this handles all of them. I highly recommend contacting Mark Gregg at Magus Innovations and get him to build you one.
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  • ecc83ecc83 Frets: 1591
    ICBM said:
    ecc83 said:
    The "Kettering" ignition system is a bit of a red fish if you will excuse me! The ratios are vastly different. 1000:1 as against maybe 50:1 for an amp tops? Then, ignition coils go from a high direct current in the coil to an open circuit and back again, really nothing like the circuit we are discussing here. 

    Yes, but that just means the induced voltage spike is in the KV range not the 20KV range, which is what I found. Even a KV spike is not good news for an old OT. I know the LT current in the coil is DC too, but at low frequencies in full distortion the peak current in a speaker circuit is probably not far off the same, and if the circuit happens to open right at the peak...

    I may be being overly paranoid but I really don't like the idea of switching speaker loads on the fly! It's notable that at least one of the companies who make purpose-designed amp switchers go the trouble to mute the amp inputs during the switch operation too. Whether out of similar paranoia or experience, I don't know.

    ecc83 said:
    (We HAVE to A/B many, many speakers ICBM to find the ones you hate most!).
    Ha :).

    Although I'm beginning to think you might be right with the ones for the ID series. They do seem to sound 'worse' through 'better' speakers.
    Well, you get kVs on anodes under normal* reactive speaker loads anyway but I can see where you would be worried with "priceless" old kit! Meantime I shall have a ponder and see if I can come up with a "safe" topology. Something with fast and "slow" relays perhaps that mean the amp is never unloaded even for a microsecond?

    ID speakers? Yes, it seems a fruitful area of investigation to find the best ones.

    *And several kVs with the brutal Z loads I used for pink noise "destruction" tests. (they didn't BTW, get destroyed that is!)

    Dave.


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