Wright Mono 8 Schematic & Operating Points
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 11th October 2017, 06:09 PM #41 jdrouin   diyAudio Member   Join Date: Feb 2012 Location: Tulsa, OK Thanks, audiowize. I don't have any 12-pin tubes but will contact you if I'm interested in buying the UBT-3s. I got the circuit breadboarded with 6SN7s and managed to do a voltage and listening test. Sound only comes out of the right channel, but what's there is good. The voltages are off between the channels, so I'm guessing there's a bad connection or short to ground somewhere. Haven't been able to locate it yet, so I might have to rebuild the left channel from scratch. If I switch the tubes, the outage stays in the left channel, so it's not that. All resistors measure to spec. Here's what I get. Suggestions welcome. Left B+1 -- 353 B+2 -- 320 6SN7 P1 -- 138 K1 -- 5.5 P2 -- 104 K2 -- 8 300B P -- 342 K -- 44 Right B+1 -- 353 B+2 -- 329 6SN7 P1 -- 146 K1 -- 5.5 P2 -- 185 K2 -- 5.4 300B P -- 346 K - 38.5
 12th October 2017, 07:57 AM #42 jazbo8 diyAudio Moderator     Join Date: Jan 2011 Location: In Transient Your voltage measurements are very odd, e.g., Left: P = 342 and K = 44 means, Vpk = 298V, Ip = 44/750 = 58.67mA, how is that possible unless the tube is bad?! Also if Pdiss = 298 x 0.05867 = 17.48W, then it's no where near the max Pdiss of 36W for the 300B. Soemthing is not right...
jdrouin
diyAudio Member

Join Date: Feb 2012
Location: Tulsa, OK
Quote:
 Originally Posted by jazbo8 Your voltage measurements are very odd, e.g., Left: P = 342 and K = 44 means, Vpk = 298V, Ip = 44/750 = 58.67mA, how is that possible unless the tube is bad?! Also if Pdiss = 298 x 0.05867 = 17.48W, then it's no where near the max Pdiss of 36W for the 300B. Soemthing is not right...
I believe the operating point is meant to be around 300Vak/60mA. That's more like 2A3 op points, but given the values of the PT and rectifiers he used, that is most likely it.

I traced the issue to a lead that had come off the plate 2 socket pin in the left channel, so I resoldered it and there's (rather good) audio from both channels.

The voltages around the 6SN7s have evened up pretty well except for around the left 300B. I tried swapping the 6SN7s and then the 300Bs, and the issue remains in the left channel. Here's what I'm getting now:

300B Left
Vp -- 349
Vk -- 56
Vg -- fluctuating positive DC voltage

300B Right
Vp -- 346
Vk -- 39
Vg -- stable -25VDC

I also swapped in different coupling caps incase one of those had gone south, but they test fine when out of the amp and the same thing happens with the new ones.

Must either be a problem with the wiring of the left 6SN7 or 300B, or else one of the components in that chain has gone south, possibly the little 0.047uF coupling cap between the 6SN7 triodes.

It sounds fine -- more than fine actually -- but I don't want positive voltage on that 300B grid.

 12th October 2017, 07:55 PM #44 jdrouin   diyAudio Member   Join Date: Feb 2012 Location: Tulsa, OK OK, problem seems to have been a slightly loose connection at the left 6SN7 plate 1 resistor. I tightened that and re-did the entire circuit on the breadboard. Now the results are much closer to what was expected. The right channel 6SN7 plate 1 is 5 volts higher and plate 2 is 7 volts higher than their left channel counterparts. Everything else is much closer though. 300Bs are now running at 310Vak/65mA, so a slightly larger dropping resistor after the rectifier tube, up from the 33ohm one currently in place, should bring it down to the target 300Vak/60mA. Left Rout -- 372 B+1 -- 370 B+2 -- 344 6SN7 P1 -- 148 K1 -- 6 P2 -- 186 K2 -- 5.8 300B P -- 359 K -- 49 G -- -13 Right Rout -- 372 B+1 -- 370 B+2 -- 344 6SN7 P1 -- 153 K1 -- 5.8 P2 -- 193 K2 -- 5.7 300B P -- 359 K -- 49 G -- -13
 13th October 2017, 05:36 AM #45 jazbo8 diyAudio Moderator     Join Date: Jan 2011 Location: In Transient "G = -13", that's much better, so which schematic are you working from exactly?
jdrouin
diyAudio Member

Join Date: Feb 2012
Location: Tulsa, OK
Sorry, forgot about our detour through WPA 3.5 Land. I'm currently listening to the schemo in post #35 (Wright Mono 8 Schematic &amp; Operating Points), the Mono 8 but with 6SN7s, R11 has a 470uF bypass cap, and R13 is 75K.

Also because of the breadboard's physical limitations, I've done a stereo build with a 5U4GB rectifier and a split rail power supply, duplicating all of the ground path connections in the original Mono 8.

PT is a Weber with a 660VCT high voltage winding. OPTs are Electra-Print 5K:8ohm, 100mA, 15W.

It's in my main system now and sounds really nice. Good detail and timbre. I'd guess that it's putting out 4-5W and has plenty of power and gain to drive my 91db 2-way monitors in a 14' x 27' living room.

I really need to get an oscilloscope to see what's going on inside.
Attached Images
 IMG_9538.jpg (630.1 KB, 77 views) IMG_9533.jpg (609.8 KB, 78 views)

Last edited by jdrouin; 13th October 2017 at 05:54 PM.

nerdorama
diyAudio Member

Join Date: Aug 2005
Location: Seattle

Looks like we might be using the same amp builder.
Attached Images
 DIY 6B4G amp 1 sm.jpg (131.1 KB, 74 views)

 14th October 2017, 12:43 AM #48 jdrouin   diyAudio Member   Join Date: Feb 2012 Location: Tulsa, OK Nice! That one is really spread out.
jazbo8
diyAudio Moderator

Join Date: Jan 2011
Location: In Transient
Quote:
 Originally Posted by jdrouin Sorry, forgot about our detour through WPA 3.5 Land. I'm currently listening to the schemo in post #35 (Wright Mono 8 Schematic & Operating Points), the Mono 8 but with 6SN7s, R11 has a 470uF bypass cap, and R13 is 75K.
Well if there is a negative grid voltage of -13V, then it could not be based on the schematic in post #35, since there is no connection for the back-bias. To make sure, you can check to see of there is a connection from R13 to the choke.

jdrouin
diyAudio Member

Join Date: Feb 2012
Location: Tulsa, OK
Quote:
 Originally Posted by jazbo8 Well if there is a negative grid voltage of -13V, then it could not be based on the schematic in post #35, since there is no connection for the back-bias. To make sure, you can check to see of there is a connection from R13 to the choke.
Yes, R13 goes to the choke. I didn't draw that connection in order to keep things basic, like PRR suggested. I was thinking about it, though, and in order to avoid the messy awkwardness of all those connection lines back to the PS ground rail and the 300B cathode resistor, I could draw arrows with letter designations as is done sometimes for the PT filament windings and their respective tubes.

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