Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on March 19, 2014, 02:44:09 am
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This morning I'm not able to remember ( :sad2:) if the mute in a fixed bias amp can be done
disconnecting grids of final tubes from signal and bias and connecting it to to ground
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I'm quite sure is possible to reach mute disconnecting cathode from ground (fixed bias amp)
but at the moment my brain refuses to collaborate about grid solution :think1:
Many Thanks
K
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This morning I'm not able to remember if the mute in a fixed bias amp can be done disconnecting grids of final tubes from signal and bias and connecting it to to ground
You can't do that! The output tubes will quickly melt.
The Silvertone 1484 has your solution. Simply short the two control grids together. Since the two grid signals should be equal amplitude but 180° out of phase, they will cancel each other, effectively muting the amp. Silvertone called it standby.
http://el34world.com/charts/Schematics/files/silvertone/silvertone1484.pdf (http://el34world.com/charts/Schematics/files/silvertone/silvertone1484.pdf)
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Grazie Steve
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and, BTW, is correct to disconnect cathodes from ground ?
about that, I remember that there are schematics with fuses between cathodes and ground, and if I remember well, you can play a PPP amp (with 4 power tubes) also if the fuse of a pair melts
Franco
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and, BTW, is correct to disconnect cathodes from ground ?
That will work.
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The standby in the Silvertone 1484 and 1485 doesn't work perfectly as they aren't exactly 180 degrees out of phase, but they are close enough to provide a standby function. In fact the Silvertones when in standby will give a little sound out in response to a strong signal, which is kind of cool as a "found sound" effect that is only useful as a novelty.
Kevin O' Connor's TUT books provide some great info on the various standby options and the pros and cons of each and different ways to accomplish them. I forget at the moment which book it was...might have been TUT2.
It is best when putting in a mute function to mute the input and the output at the same time from a noise standpoint.
Greg
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Grazie ancora Steve e Grazie anche a SoundmasterG
Franco
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This morning my brain seems to compute and collaborate with me (I hope not in a deviated way :icon_biggrin:)
Thinking to the LarMar PPIMV I've draw this solution for a mute
I need (I hope) this is correct because I need this mute solution for a particular PSE project
Can someone give a look and confirm that this method is usable ?
Thanks in advance
K
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That will work. But I'm curious. Why do you want to mute at the power tube grids? A simple SPST switch between the volume control wiper and ground is very effective.
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That will work. But I'm curious. Why do you want to mute at the power tube grids? A simple SPST switch between the volume control wiper and ground is very effective.
Muting only at the volume control still leaves the power tubes on and trying to amplify so there would still be hiss out of the speakers. Muting the power tube grids, or disconnecting their connection to ground via the cathodes would essentially shut them off in addition to shutting the signal off along its path by using your method.
Greg
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I understand that. But all the power amps I'm familiar with are so quiet that you have to put your ear in the speaker to hear any hiss.
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Ciao Steve e SoundmasterG
@ SoundmasterG
Ditto
@ Steve
Ditto but the real reason is that I'm planning with a friend a 40W PSE amp that uses 2 x KT88 and 2 x GU50 tubes
as he want the max versatility, we arranged a double inverted MV control so with the pot at 12 we have 50% power from KT88 tubes and 50% power from GU50 tubes
rotating the pot (dual ganged pot single shaft) increase the % of power delivered by one kind of tube and decreases the power delivered from the other kind of tube but this way you can't have KT88 tubes pushed at max at the same time of GU50 tubes pushed at max
So, trying to have a way to obtain also this feature, I disconnect signal from a KT88 and a GU50 tube and invert (via switch) the function of the MV of the GU50 tubes that now increases at the same time of the KT88
this way we have at max power the stated 40W (supported by PT & OT that are rated for this max power) but obtained from a KT88 pusched at 100% and a GU50 pushed at 100% instead of 40W obtained from 2 x KT88 at 50% and 2 x GU50 at 50%
My choice was to disconnect grids from signal because if I disconnect the cathode I'm pretty sure the reflected impedance required changes and I don't want to swap also the secondary of the OT
I hope my explanation is comprehensible
Ciao
Franco
p.s.: the attached schematic is incomplete and capacitors must be used on the KT side, because the amp will be KT88 fixed bias (on the other side no cap is necessary because the GU50 will be cathode biased)