What happens if you 'jumper' the spring reverb 'send' and 'return' on a valve amp?
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What happens if you 'jumper' the spring reverb 'send' and 'return' on a valve amp?
Simple question really - if you connect an RCA phono to phono cable between the send and return and pretty much short that loop, will it explode...???
Re: What happens if you 'jumper' the spring reverb 'send' and 'return' on a valve amp?
LOL...
I HATE simple questions. A quick rule of thumb; the longer the question the shorter the answer, the shorter the question the longer the answer.
Knowing more about the amp in question would help to keep the answer more specific to your problem and keep things nice and concise. But here goes...
Your typical spring reverb tank is a passive device. It's input drives a transducer that drives a spring and that is picked up by another transducer which is connected to the output. It could be hardwired to the chassis of the amp, but uses RCA connectors because that makes it easier to swap out.
The amp will have a driver circuit for the reverb tank, which could be valve or transistor, and the return will be a simple mix with the direct signal, often no more than a resistor based summing circuit (Dave will know far better than I). The reverb send level controls the gain in the driver circuit, more level = more gain.
If you have a valve reverb driver it will either use a spare half of a pre-amp valve (typically a 12AX7) or a dedicated valve (often a 12AT7). The 12AT7 is a lower gain version of the 12AX7, and even if half of a 12AX7 is used the circuit is designed to keep the gain at similar levels to a 12AT7.
The bottom line is that in a typical valve amp you will have no problem replacing the reverb tank with a RCA link cable. But while I can't think of anything off the top of my head that works differently, for absolute peace of mind I would like to check the schematic to be perfectly sure. But we aren't talking about explosive problems.
But it would be nice to hear from the real wireheads like Dave just to make sure I'm not getting confused in my old age.
Andy
I HATE simple questions. A quick rule of thumb; the longer the question the shorter the answer, the shorter the question the longer the answer.
Knowing more about the amp in question would help to keep the answer more specific to your problem and keep things nice and concise. But here goes...
Your typical spring reverb tank is a passive device. It's input drives a transducer that drives a spring and that is picked up by another transducer which is connected to the output. It could be hardwired to the chassis of the amp, but uses RCA connectors because that makes it easier to swap out.
The amp will have a driver circuit for the reverb tank, which could be valve or transistor, and the return will be a simple mix with the direct signal, often no more than a resistor based summing circuit (Dave will know far better than I). The reverb send level controls the gain in the driver circuit, more level = more gain.
If you have a valve reverb driver it will either use a spare half of a pre-amp valve (typically a 12AX7) or a dedicated valve (often a 12AT7). The 12AT7 is a lower gain version of the 12AX7, and even if half of a 12AX7 is used the circuit is designed to keep the gain at similar levels to a 12AT7.
The bottom line is that in a typical valve amp you will have no problem replacing the reverb tank with a RCA link cable. But while I can't think of anything off the top of my head that works differently, for absolute peace of mind I would like to check the schematic to be perfectly sure. But we aren't talking about explosive problems.
But it would be nice to hear from the real wireheads like Dave just to make sure I'm not getting confused in my old age.
Andy
There is a profound African saying, "A white man who cannot dance is a victimless crime, whereas a white man with a djembe drum ..."
Re: What happens if you 'jumper' the spring reverb 'send' and 'return' on a valve amp?
Seems an odd question...
I should imagine that depending on the phase relationship of the send and return it may result in a negative feedback situation on that particular channel, reducing the gain of the channel's output.
Or if it may result in an uncontrolled positive feedback loop, creating squeals, howls, "motorboating" or even supersonic oscillations.
There would be losses in the passive spring reverb unit so removing the unit should increase the gain in that circuit beyond what it was designed for.
Not the sort of thing you would normally do.
As Zen says, hard to know without seeing the schematic.
Tim
I should imagine that depending on the phase relationship of the send and return it may result in a negative feedback situation on that particular channel, reducing the gain of the channel's output.
Or if it may result in an uncontrolled positive feedback loop, creating squeals, howls, "motorboating" or even supersonic oscillations.
There would be losses in the passive spring reverb unit so removing the unit should increase the gain in that circuit beyond what it was designed for.
Not the sort of thing you would normally do.
As Zen says, hard to know without seeing the schematic.
Tim
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- Tim Gillett
Frequent Poster - Posts: 2707 Joined: Wed Jan 30, 2013 12:00 am Location: Perth, Western Australia
Re: What happens if you 'jumper' the spring reverb 'send' and 'return' on a valve amp?
JM-1 wrote:...if you connect an RCA phono to phono cable between the send and return and pretty much short that loop, will it explode...???
Hopefully not... but I expect it will become extremely distorted at best, and at worst you risk destroying the front end of the receiving amp.
Most of these spring tank systems need a reasonable amount of power to drive the springs and they are often driven by a small power-amp output stage, but the pickup transducer is relatively insensitive and requires a fairly high gain recovery amp.
It would be a foolish thing to try for no obvious benefit... so why are you asking?
H
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- Hugh Robjohns
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Posts: 43706 Joined: Fri Jul 25, 2003 12:00 am
Location: Worcestershire, UK
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Technical Editor, Sound On Sound...
(But generally posting my own personal views and not necessarily those of SOS, the company or the magazine!)
In my world, things get less strange when I read the manual...
(But generally posting my own personal views and not necessarily those of SOS, the company or the magazine!)
In my world, things get less strange when I read the manual...
Re: What happens if you 'jumper' the spring reverb 'send' and 'return' on a valve amp?
Of course you're right, there not being an active loop in the first place. (blushes) But not sure "nothing" would happen. Wouldnt there now be a duplicate of the sent signal, which would either sum with or cancel the original signal depending on whether the signal was in or out of phase, or somewhere in between?
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- Tim Gillett
Frequent Poster - Posts: 2707 Joined: Wed Jan 30, 2013 12:00 am Location: Perth, Western Australia
Re: What happens if you 'jumper' the spring reverb 'send' and 'return' on a valve amp?
Actually I have no intentions of doing this. But an amp tech actually did tell me to try it 
On my amp's reverb 'loop':
Input impedance: 310ohms
Output impedance: 2575 ohms
So I'd be asking for trouble...
It was fun to see your reactions though - especially the one to use plutonium connectors. Now all I need is a bolt of lightning and a flux capacitor...
On my amp's reverb 'loop':
Input impedance: 310ohms
Output impedance: 2575 ohms
So I'd be asking for trouble...
It was fun to see your reactions though - especially the one to use plutonium connectors. Now all I need is a bolt of lightning and a flux capacitor...
Re: What happens if you 'jumper' the spring reverb 'send' and 'return' on a valve amp?
Perhaps THE classic circuit is the Fender? If we were allowed attachments I could show you, but as it is you will all have to go and hunt up the schematic of the Super Reverb AA763 I am looking at!
The drive is provided by paralleled sections of a 12AT7( ECC81)and thus there is about a watt available. Now, I believe Fender tanks to have an input Z of 8 Ohms? If so the drive voltage is some 3V rms.
The recovery amp is 1/2 a 7025 (aka ECC83)and has the usual 100k anode load (and is fed from 400volts! They did not bloody care then did they?) This means a gain of around 60times, about 36dB.
Sooooo! If you plug the RCAs together that wee 7025 will try to produce 180volts rms or over 500V peak to peak!(actually with a 400V supply it might manage half of that!) but despite these horror voltages I doubt the valves would suffer. The 7025 signal goes to a pot and then a ~12dB attenuator so any level you like could be passed on to the rest of the amp.
"Worse things happen at sea", or so my old mum used to say.
Dave.
The drive is provided by paralleled sections of a 12AT7( ECC81)and thus there is about a watt available. Now, I believe Fender tanks to have an input Z of 8 Ohms? If so the drive voltage is some 3V rms.
The recovery amp is 1/2 a 7025 (aka ECC83)and has the usual 100k anode load (and is fed from 400volts! They did not bloody care then did they?) This means a gain of around 60times, about 36dB.
Sooooo! If you plug the RCAs together that wee 7025 will try to produce 180volts rms or over 500V peak to peak!(actually with a 400V supply it might manage half of that!) but despite these horror voltages I doubt the valves would suffer. The 7025 signal goes to a pot and then a ~12dB attenuator so any level you like could be passed on to the rest of the amp.
"Worse things happen at sea", or so my old mum used to say.
Dave.
#They did not listen, they are not listening still...Perhaps they never will?#