12DW7/7247 wired parallel

dscottguitars · 26th January 2012, 05:46 PM

Hi, My Fender Concert ('83 Rivera) has the input tube V1 wired in parallel. I came across some 12DW7's and wondered if that would produce a 'unique' sound. Can one use this tube, w/a 12AU7 and a 12AX7 in a parallel circuit?

Thanks,

Daniel

Loudthud · 26th January 2012, 06:12 PM

The biasing will be messed up. The 12AU7 side will pull the plate voltage so low that the 12AX7 side will be in grid leak territory. You would need to bias each half separately to get the plate voltages close enough to tie together.

dscottguitars · 28th January 2012, 01:43 AM

Thanks, I'm not sure I understand everything. With a low plate voltage on the 'x' side that would breakup quicker like a distortion pedal, yes? It sounds like the sound would be distorted but I didn't know if that would be a 'good' distortion. It can't hurt anything can it?

Daniel

Loudthud · 28th January 2012, 02:05 AM

Grid leak biasing is an old technique used to bias preamp tubes. The cathode is basically grounded and the tube runs wide open except the plate voltage gets so low that current starts leaking out the grid. This causes a voltage to form on the grid that tends to bias the tube colder, enough that it will pass a signal. It won't hurt anything but your guitar volume pot will be scratchy. If you are using a stompbox effect, the results will be unpredictable depending on exactly what components are on the output of the effect. The distorted tone will be at a low volume so you will have to crank the gain to get some volume. If you like the distortion, a little re-arrangement of the resistors between the jacks and the tube, and an added capacitor will isolate the grid voltage from the guitar and/or stompbox.

dscottguitars · 28th January 2012, 02:42 AM

Thanks again, I now understand what grid leak voltage means/does by your explanation. The cap really made it apparent to me. I would need to add a small cap in series to block the DC voltage leaking, yes? That seems similar to a coupling cap-decoupling. It has a large resistor now, 1.5K and 47K on the plate. The stock was 680 for the resistor, I increased it to get more headroom on the first channel. I don't know much about the parallel wiring of the tubes, I think there's more current from what I've read. I may try it to use for low volume distortion while practicing.

Daniel

26th January 2012, 05:46 PM	#1
dscottguitars diyAudio Member Join Date: Mar 2011	12DW7/7247 wired parallel Hi, My Fender Concert ('83 Rivera) has the input tube V1 wired in parallel. I came across some 12DW7's and wondered if that would produce a 'unique' sound. Can one use this tube, w/a 12AU7 and a 12AX7 in a parallel circuit? Thanks, Daniel

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26th January 2012, 06:12 PM	#2
Loudthud diyAudio Member Join Date: Nov 2007 Location: Near Dallas Texas USA	The biasing will be messed up. The 12AU7 side will pull the plate voltage so low that the 12AX7 side will be in grid leak territory. You would need to bias each half separately to get the plate voltages close enough to tie together.

28th January 2012, 01:43 AM	#3
dscottguitars diyAudio Member Join Date: Mar 2011	Thanks, I'm not sure I understand everything. With a low plate voltage on the 'x' side that would breakup quicker like a distortion pedal, yes? It sounds like the sound would be distorted but I didn't know if that would be a 'good' distortion. It can't hurt anything can it? Daniel

28th January 2012, 02:05 AM	#4
Loudthud diyAudio Member Join Date: Nov 2007 Location: Near Dallas Texas USA	Grid leak biasing is an old technique used to bias preamp tubes. The cathode is basically grounded and the tube runs wide open except the plate voltage gets so low that current starts leaking out the grid. This causes a voltage to form on the grid that tends to bias the tube colder, enough that it will pass a signal. It won't hurt anything but your guitar volume pot will be scratchy. If you are using a stompbox effect, the results will be unpredictable depending on exactly what components are on the output of the effect. The distorted tone will be at a low volume so you will have to crank the gain to get some volume. If you like the distortion, a little re-arrangement of the resistors between the jacks and the tube, and an added capacitor will isolate the grid voltage from the guitar and/or stompbox.

28th January 2012, 02:42 AM	#5
dscottguitars diyAudio Member Join Date: Mar 2011	Thanks again, I now understand what grid leak voltage means/does by your explanation. The cap really made it apparent to me. I would need to add a small cap in series to block the DC voltage leaking, yes? That seems similar to a coupling cap-decoupling. It has a large resistor now, 1.5K and 47K on the plate. The stock was 680 for the resistor, I increased it to get more headroom on the first channel. I don't know much about the parallel wiring of the tubes, I think there's more current from what I've read. I may try it to use for low volume distortion while practicing. Daniel

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