Need some guidance on Plexi build, low preamp voltage

[munkeyboy]

So, i received a traynor yba1 that someone modded (poorly) .  Several changes to stock and a few burnt up resistors and a burnt socket.  So I just decided to rebuild it in to a plexi 50 lead. All new jacks and sockets.

I used the various sources as a guide, but ceriatone plexi 50 http://www.ceriatone.com/ceriatone/wp-content/uploads/2016/10/Ceriatone-69-Plexi50-lead.jpg was my go to layout.  I did have to change the rectifier section as this PT has no CT.   So far the power section is working. A little more voltage on the el34s than what ceriatone has (450 v 414).  But the issue comes on the preamp side.  Something is off with the power there.  V3 pins 1 and 6 are close (185v vs 190v).  But grid pins are at 70v vs 36v.   That also puts the 55v at the presence pot which seems excessive.   And turning the presence pot affects volume more than I'd think, however, I'm not that familiar with presence controls. 

But worse are V2 and V1.  Pin 6 on V2 should be 257v.  I get 140.  Pins 1,7 and 8 should be around 150v, I get 10v.   V1 the voltage is about half what it should be. 

I've been thru the docs and traced out best i could and found some issues I overlooked and fixed (missing solder and grounds), but now I'm at a loss.  I'm sure i've done something wrong, but can't see it.    Plugging in to any of the input jacks produces weak signal.  Plugging in to the effects loop produces louder clean signal (as expected).

Where should I be looking for a short or is it something else (i've tried different 12ax7s)?    I've attached a pic of the problem area.  The voltage before the 10k 3w resistor seems correct (250v).   It all goes wonky after that. 

My voltage reading vs (ceriatone voltage chart)

V1 - Pin1 76v (142v)
V1 - Pin3 0.3v (0.8v)
V1 - Pin6 72v (184v)
V1 - Pin8 0.4v (1.5v)

V2 - Pin1 10v (150v)
V2 - Pin3 0.3v (0.8v)
V2 - Pin6 140v (257v)
V2 - Pin7 10v (150v)
V2 - Pin8 8v (151v)

V3 - Pin1 185v (196v)
V3 - Pin3 70v (36v)
V3 - Pin6 181v (190v)
V3 - Pin8 70v (32v)

V4 - Pin1/8 34.3mV (35mV)
V4 - Pin3 450v (414v)
V4 - Pin4 440v (410v)
V4 - Pin5 -35v (-36v)
V4 - Pin6 451v (415v)

V5 - Pin1/8 35.9mV (35mV)
V5 - Pin3 450v (414v)
V5 - Pin4 440v (410v)
V5 - Pin5 -35v (-36v)
V5 - Pin6 451v (415v)

[sluckey]

Check for wiring errors. Check component values. You will also have to change values of B+ dropping resistors too. You're only feeding V2 with 140V B+ so no V2 voltages can be more than that.

[munkeyboy]

That's where I'm at a loss.  If i bypass the first 10k, the voltage still drops to 140v.

[munkeyboy]

Oh I think I understand you.  It must be due to the 2x 8k2 resistors.  They drop the B+ by 200v.

Thanks sluckey, let me try removing one.

[munkeyboy]

Well, that didn't work.  Made it worse actually.  I've took out one 8.2k 3w, that helped a little, but then the remaining 8.2k 3w started burning up.  I swapped in a 10k 5w.  It hasn't burned up, but is discolored.  And I tried swapping or jump other values for the 10ks around v2.  Just made things worse.  Jumping one brought down the b+ significantly.    It went from 460 after the choke and before the first dropping resistor, to 160 after it.

However removing the v2 tube restores voltage to somewhat normal values.  At this point I've tried 3 different tubes and starting to think there's a short in or around the v2 socket.

[sluckey]

Check for wiring errors. Check component values.

[John]

Did you draw an as-built schematic? This is where IMO they are invaluable for troubleshooting.

[munkeyboy]

I'm not very good at reverse engineering layout to schematics.   Getting better at the opposite, but still learning.

However, I have compared many many schematics to the layout and the wiring i have (jtm, bassman, etc.).  I can't see where I'm going wrong.  I did find issue with v3 pin 3 and 8.. at least I believe i did.  The stock presence pot is 250k instead of 5k.  Jumping the presence wire to ground with a 8k resistor (what I had laying around), fixed it.  5k pot on order.

It seems problem area is on or manifested at V2. As per layout and schematic, pins 1 and 7 are jumpered and pin 6 is jumpered to pin 1 with a 100k resistor.  However, there is 240v going to pin 6, but after that 100k resistor to pin1, it drops to 19v (and thus 19v at pin 7).   I don't know enough to understand what the possible causes would be, but that seems to be a drop way lower than what a 100k resistor would do as I'm expecting 150v or more.  I've also validated and played with that resistor value without much change.  Also pin 8 is also at 15-19v when it should be 150 or so.  BTW, removing the V2 tube changes everything.  No large voltage drop between 6 and 1.  (Of course the voltages are now in the 400's).

However, I will say that all the changes i've done to get 240v at V2 pin 6 and correct voltage to V1, seem suspect.  I've changed the first dropping resistor from 2x8ks to 2x5ks and completely removed the 10k dropping resistor to V2 and the 10k resistor to V1.  This doesn't track with any of the schematics or layouts.

As noted before, I've changed from bridge to full wave rectifier section (copied the yba-1 schematic) due to the lack of CTs.  It could be the rectifier changes I did, but I'm not sure how to test properly.  However, I'm getting the voltage I expect out of that section.

Anyway, I'm not saying I don't have something wired wrong. Obviously I do, I'm just not experienced enough with circuits to narrow the problem area.  I've checked wires and values, save for pulling every resistor off and checking it individually.   If anyone could provide hints on what might cause the large voltage drop on V2 pins 7 and 1, that's would be awesome...  Not looking for a fix, just clues on how to narrow down the issue or maybe what causes voltage drops like this.  Otherwise I'll keep plugging away or start from scratch again.

[John]

Looking at the Plexi50 Superlead, V2 is a cathode follower. That's why you have that big voltage drop. With the tube out, there's no current flowing, so no voltage drop. If you look at pin 8, you have a 100K cathode resistor there, or at least you should have. Lift one leg of that 100K resistor that's hooked to pin 8, make sure it's reading 100Kohms, or close to it.

[John]

Wait. pins 1& 7 shouldn't be jumpered, should they? You're putting DC directly on the input grid. That doesn't seem right? They're not jumpered in the schematic.

[munkeyboy]

They are jumpered.   Jumpered on the hoffman plexi50 too.  http://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf   I'd love to know how that works at other builds are not this way.  Wondering what the difference is.

But funny you posted about the cathode follower.  After I posted the last message. I went back thru to trace with the highlighter again and found the 100k swapped with the 820 on the cathode.  Hoping that's it.   Getting ready to fire it up and see...

Thanks John!

[John]

Unless there's hardly any similarity between what you're building, and the schematic, Ceriatone has messed up the layout for V2. I could be totally wrong of course, but --- wait, they've just flipped the pin inputs for layout purposes, never mind.

[John]

found the 100k swapped with the 820

That should fix it.


Ignore what I said in previous post, they turned the socket 180 for a cleaner layout I guess. I was looking at the Marshall instead of Hoffman's schem. He's got it drawn the way your layout is. My bad!

[munkeyboy]

Yep, that was it.  Well i feel like an idiot the first time i missed it during the highligher trace.  Thanks you your help John!

[John]

Glad you found it!

[munkeyboy]

QQ.  With the non-centertap PT,  I should run 100 ohm resistor to ground from each heaters right?   Is there any other thing I should address with this scenario?

[John]

QQ.  With the non-centertap PT,  I should run 100 ohm resistor to ground from each heaters right?   Is there any other thing I should address with this scenario?

That's right.

[tubenit]

Does the PT have a center tap for the heater wires?  If not, then use 100R resistors to make an artificial ground.  See Hoffman's Library of Information.    What is your voltage on heater wires?

[munkeyboy]

Does the PT have a center tap for the heater wires?  If not, then use 100R resistors to make an artificial ground.  See Hoffman's Library of Information.    What is your voltage on heater wires?

Correct no CT on any PT source.   I have 3.3v on the heaters.

[tubenit]

The schematic for your Traynor YBA-1 shows a center tap for the 6.3v heater wiring.  Are you positive you don't have one?

IF I am remembering correctly,  I think sometimes center taps can be "internal" meaning inside the transformer and not showing externally?

http://el34world.com/charts/Schematics/files/Traynor/Traynor_bassmaster_yba1.pdf

With respect, Tubenit

[John]

Does the PT have a center tap for the heater wires?  If not, then use 100R resistors to make an artificial ground.  See Hoffman's Library of Information.    What is your voltage on heater wires?

Correct no CT on any PT source.   I have 3.3v on the heaters.

Measure across the filament wires/pins. Should be 6.3 to 7 vac.

[munkeyboy]

The schematic for your Traynor YBA-1 shows a center tap for the 6.3v heater wiring.  Are you positive you don't have one?

IF I am remembering correctly,  I think sometimes center taps can be "internal" meaning inside the transformer and not showing externally?

http://el34world.com/charts/Schematics/files/Traynor/Traynor_bassmaster_yba1.pdf

With respect, Tubenit

unfortunately i have the 1978 model which does not.  See page five of the this link.  http://www.lynx.net/~jc/YBA1Schems.pdf

And yes, it is 6.3 across filaments.


I'm also getting a lot of hum from the bright channel.  I used the push/pull CTS switch from Hoffman and it is not like the switch in the layout.  It seems to work for switching to bright, but wondering if i got something incorrect there.  There are only 2 extra lugs instead of the 6 i see on other layouts.  I made the assumption that the mica goes to one (right lug, looking at rear of pot) and the middle lugs goes to the other (left lug, looking at rear of pot) and the ground is already taken care of inside the pot.  But maybe that is wrong, maybe the 2nd lug should go to ground.  Any thoughts?

[John]

and the ground is already taken care of inside the pot.  But maybe that is wrong, maybe the 2nd lug should go to ground.  Any thoughts?

One of those lugs needs grounded. I'm not familiar with the fancy switching things  so I don't know which one.

[sluckey]

This is how that CTS switch/pot should be wired...

[munkeyboy]

Thanks, That's how i have it.  Prob just need to address my lead dress :)

Thanks Guys!