Metropoulos Forum

Hi everyone - I'm building a 100w Plexi superbass and I'm getting the following (low) readings from v1,2,&3 they are generally about 1/2 what they should be


V1 1-83v, 2-0v, 3-1v, 4-3.1v, 5-3.1v, 6-154v, 7-0v, 8-1v, 9-3.1v

V2 1-92v, 2-0v, 3-1v, 4-3.1v, 5-3.1v, 6-179v, 7-92v, 8-0v(?!), 9-3.1v

V3 1-136v, 2-6v, 3-20v, 4-3.1v, 5-3.1v, 6-130v, 7-7v, 8-20v, 9-3.1v

V4,5,6,7 seem to be ok @514v on pins 3,4,6 @ 3.1v on pins 2&7 and -41v on pin 5 (w/o tubes installed)
No fuses blown either
I can't find anything that's causing a 0v read on V2/8... my guess is that may be causing the low readings throughout all three tubes. (?)8
I've triple checked everything - wiring, components/values, locations etc. - doesn't mean I haven't overlooked something several times, though.
Any ideas or suggestions what to check/ where to look for the problem would be greatly appreciated.
Thanks, Joe

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Replace circled resistor with a 10K (brown/black/orange), got a 100K in there now dropping your PI and preamp voltages way too much for starters. Also voltages are off in addition to that (for example pins 1 and 6 of V1 should both read approx the same since it's a shared cathode on that tube). Looking close at the pics, looks like V1 and V2 may not have the heater pins 4 and 5 jumpered together (as they should be), rather the heater only running to pin 4 alone?

Thanks! - I replaced the resistor w/ the right one and the voltages are making more sense now running a little high,though - I still have to place the jumper on V1&2 (I may have read this wrong, but i thought the instructions said it wasn't needed on those two tubes - that's why they weren't jumped)but I still have 0v on V2pin8 - the chart says I should see 169v there - anyone see anything that would cause this?

Thanks again for your help, Joe

jz wrote:... - anyone see anything that would cause this?

Yes - not having pins 4 and 5 of V1 and V2 tied together WILL cause this. Recheck the instructions if you wish, but all preamp tubes must have pins 4 and 5 tied together for the heaters to be able to heat up both triodes in the 12AX7 bottle properly (thus allowing both triodes to work properly and draw current). Jumper those pins!

+1

It's pins 3 and 8 that only get jumpered on V3 only. V1 also gets 3 and 8 jumpered on earlier circuits that use the shared cathode circuit on V1. But pins 4 and 5 MUST be jumped on all 3 and from your pics it's clearly evident that you only have them jumped on V3.

Yeah, I read it wrong The instructions specified that both wires from the twisted pair didn't have to be jumpered - you can use one for the jumper. - Sorry, I'm still a noob learning how these things work. All seems ready to go to the next test stage. Thanks for your help - especially for explaining why what I had wrong was causing the problem - You guys rock! I'll post some pics & sound clips when it's done -

Thanks! Joe

jz wrote:Yeah, I read it wrong The instructions specified that both wires from the twisted pair didn't have to be jumpered - you can use one for the jumper. - Sorry, I'm still a noob learning how these things work. All seems ready to go to the next test stage. Thanks for your help - especially for explaining why what I had wrong was causing the problem - You guys rock! I'll post some pics & sound clips when it's done -

Thanks! Joe

What that means is that one of the red wires gets stripped long enough to go through BOTH pins 4 and 5 and serve as a jumper, while the 2nd red wire on those pins doesn't have to jump over to the next pin because the first one already jumped the two pins together.

Yeah - I understand now - Thanks!
One more thing before moving forward - w/o the power tubes in, I want to make sure this is ok-
a couple voltages are a little low-
V1 pin 6 is 190v chart says 213v
V2 pins 1&7 are 163v chart says 168
V3 pin 7 is 12v chart says 27v pin 8 is 35v chart says 42
and the lowest I can get the bias set is -41v
All the other readings are a bit higher as the instructions indicate they should be before the power tubes are installed.
Will these other readings adjust closer with the tubes in or do I have something else incorrect going on here?

jz wrote:Yeah - I understand now - Thanks!
One more thing before moving forward - w/o the power tubes in, I want to make sure this is ok-
a couple voltages are a little low-
V1 pin 6 is 190v chart says 213v
V2 pins 1&7 are 163v chart says 168
V3 pin 7 is 12v chart says 27v pin 8 is 35v chart says 42
and the lowest I can get the bias set is -41v
All the other readings are a bit higher as the instructions indicate they should be before the power tubes are installed.
Will these other readings adjust closer with the tubes in or do I have something else incorrect going on here?

Without the power tubes installed you don't have full load on the supply yet. When you install the power tubes that will load the initial B+ down some which will pull everything else down with it.

Also don't put too much faith into the voltage chart. Voltages will vary sometimes as much as 10-20%.

Now on your bias supply...swing your bias pot fully in the direction that gives the HIGHEST negative number on pin 5 BEFORE installing your power tubes. Meaning if when measuring your pin 5 voltage you measure -41 in one direction while measuring -51 in the other, rotate it in the direction that gives you -51. This sets your power tubes up fully cold, then you can open up the bias from there. Doing it the other way will send them into red plate territory as soon as you take the amp out of standby.

Ok - got it working now and it sounds like it should - no pops,squeals, all tone & presence controls work correctly. Surprisingly quiet for being out of the head box. However, two inputs are not working. 1 & 2 bottom inputs are dead. (the two closest to the bottom of the chassis) they make no sound at all - no crackle sizzle - nothing - even if the guitar cord is wiggled- no signal at all. It looks to be wired right and the resistor values (1m on the jacks and 68k on the board) look right. Any suggestions would be greatly appreciated - Thanks, Joe

jz wrote:Ok - got it working now and it sounds like it should - no pops,squeals, all tone & presence controls work correctly. Surprisingly quiet for being out of the head box. However, two inputs are not working. 1 & 2 bottom inputs are dead. (the two closest to the bottom of the chassis) they make no sound at all - no crackle sizzle - nothing - even if the guitar cord is wiggled- no signal at all. It looks to be wired right and the resistor values (1m on the jacks and 68k on the board) look right. Any suggestions would be greatly appreciated - Thanks, Joe

Post up some pics if you can. That would be the best way to help you out. Glad to hear everything else is good to go.

Nevermind I see the issue...move the green wire that runs from the bottom input jacks to the board down one terminal on the bottom input jacks. Here's a layout showing how they should be wired -

http://www.wilderamplification.com/sche ... utjack.pdf" onclick="window.open(this.href);return false;

You can see that the two green wires that go from the jacks to the board actually wire up to the bottom input jack, but with the green jumper wire between the bottom and top jacks. On yours you have the board wires going to each end of the jumper across the jacks, which won't work.

Its ALIVE!!!!!! Man, does this thing sound good! Pretty good amount of gain too! Thanks for the help getting me through the final stages here. Now I have to build some cabs to go with it. I'll get some pics and sound clips up in a couple weeks after the break in is done. Thanks again, Joe