Tube Amps / Music Electronics
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|4/18/2003 3:35 AM|
||Half power switch for Peavey Classic 30?|
I got another email from someone wondering if they can pull one pair of the EL84's to reduce the overall volume without changing the tone. Nope... the EL84 heaters are wired in series so if you pull one tube the others won't work.
Question for the experts out there: what would be the best way to deactivate one pair of EL84's in the Classic 30? I see 3 possibilities:
1. Interrupt the signal going to the grids. This might cause a pop when switching but AFAIK it would not change the impedance seen by the OT, etc.
2. Interrupt the screen supply for one pair of the EL84's. I've seen this done on a schematic for a boutique amp. Would this change the impedance as seen by the OT? This would involve switching high voltages unlike the other two choices.
3. Interrupt the cathode to ground connection for one pair of EL84's. Would this change the impedance as seen by the OT?
Here is a drawing showing which jumpers or resistors could be removed to kill the grids, screens or cathodes:
Any suggestions would be appreciated!
P.S. BTW one pair of the EL84's use a 100R/5 watt resistor on the screens, while the other doesn't use a screen grid resistors at all. Any thoughts on this? It seems to me that it would work better with something like 1K screen grids on all 4 tubes...
|4/18/2003 3:05 PM|
Maybe try the grid approach, removing the 47k grid resistors from the two outer tubes and installing 2M2 "tye-down" resistors from the grids to ground. I wouldn't think that the high resistance of the 2M2 resistors would affect the bias voltage, since the resistors in the bias supply are a lot lower value. You could even use 10M or greater, if you were concerned about how steady your bias supply is.
I would think that any of these choices would change the impedance seen by the OT. Plate resistance is a function of current, right? The two outer tubes will only be drawing a fraction of the current when idling that they would be drawing when in use, so it would affect the impedance seen by the OT, right? I don't think that disconnecting the grids would affect the OT as much as lifting the cathodes, because those EL84's are still drawing some current. With the cathode lifted, they're drawing no current.
Wouldn't interupting the screen supply just put the EL84's in triode mode? If you could connect the sceen grid to ground, that would seem to be rather effective because very few electrons could make the journey to the plate. But again, no current flow would affect the plate resistance, and therefore the impedance seen by the OT.
Hope all of this rambling is of some use to you Steve.
|4/18/2003 3:37 PM|
Well I think ENZO should be strolling in soon and has done some similiar mods to the 30 as well as the Cathode to fixed bias one a ways back but I personaly like the Cathode one the best. You probably won't get pops there. The Screens share those 100 ohm resistors so each pair runs off or shares one 100W5 resistor. See the connection from both screens and they flip the tubess around on the schematic ?
I'm no expert (yet) but to be quite honest I don't think you'll get the attenuation or tone you want by just removing two tubes as it's only a -3dB cut. You'd be better off with a Master Volume but then you'd get into the hacking zone but hey it's only a C-30. I wonder how power scaling would do?
Its' a good thought as many have asked about it that I've encountered but when told about the heaters they just shrug it off.,
|4/19/2003 2:22 AM|
Well I think ENZO should be strolling in soon...
I was hoping that he would! He's probably worked on more Classic 30's than anyone else outside of Peavey factory service.
The Screens share those 100 ohm resistors so each pair runs off or shares one 100W5 resistor. See the connection from both screens and they flip the tubess around on the schematic ?
Maybe I need new glasses but I see the screens of V5 and V6 tied directly to the screen supply, while there are separate 100 ohm resistors going to the screens of V4 and V7. Here's a link to the schematic:
I'm no expert (yet) but to be quite honest I don't think you'll get the attenuation or tone you want by just removing two tubes as it's only a -3dB cut.
I think that it would be more noticeable in a band situation— you are getting the perfect tone and the vocalist tells you to turn it down...
As for 3dB, when you are mixing down tracks, a 3dB cut or boost makes all the difference in the world...
I think that my preference would be interrupting the cathodes. (Now that I think of it, it is with cathode-biased EL84's that you would cut the screens for a half-power mode.)
|4/19/2003 1:34 PM|
Hey correct me if I'm wrong guys because I want to know but I see your point that the Screens are tied to the Screen supply but that connection is also tied to V4 and V7 going to the 100 ohm resistor so essentially using it are sharing the resistor. If you checked the screen voltage at the pins of all four tubes wouldn't they be the same ? In the case of EL-84's and 6l6's that's cool but El-34's would freak.
|4/19/2003 5:19 AM|
I'm going to hack my Delta Blues when I get a little more cash to buy tube sockets. I'm going to take the PCB out and wire in a Vox Top Boost preamp in there (maybe), and then I'm not too sure, but it'll be better than the ratty looking and shitty sounding PCB in there now. Point-to-point, dude!
I want to keep the labels the same, if possible, and also keep the reverb, master volume (post), and tremolo. The stock 15" speaker will probably have to go. It SUCKS!!!
PS. I might just do something "Fendery" in there, i.e., fixed bias, NFB, LTPI, and maybe a second channel. Don't know yet.
|4/18/2003 8:02 PM|
I'm pretty sure that to go into triode mode, you tie the screen to the plate.
By interrupt, SteveA may mean float or ground the screens ... not sure.
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