Is my OT fried?

[Gainzilla]

I'll flip the board over and check tonight. My original thought was:
B1 - OT
B2 - Output Tubes
B3 - PI/FX Return
B4 - Preamp Stages

Probably a little light on filtering. Anyway, assuming it's on B3, what do you recommend trying in order to bring the voltage up? Moving the PI to B2 along with the Output tubes?

[pdf64]

The V3 LTP cathode voltage is wrong.
Let's find the fault before messing with the B+.

'Do you mean the 5v? What range should I be expecting?'

Assuming 1 to 1.5mA per 12AX7 section, then 40 - 60V.

There's a string of resistors between cathodes and ground, over 20k total.
What are the voltages as you go up the string?
As nickb mentions, where have you got the grid leak resistors connected to?
Pete

[g1]

Also, check resistance from PI cathodes to ground.

[Gainzilla]

Ok, good news and strange news...

First the good: It's alive! Even better, it sounds really good! Still have a few things to sort out... Miswired the tonestack so the treble pot works backwards (I had to wire the circuit upside down and backwards compared to other builds I've seen. It's in a chassis originally designed for a top-facing combo... Probably no big deal to most of you, but for me this was really strange).

Ok, now the weird: I checked the B+ nodes and did confirm that they're arranged like this:

B1: Output Transformer
B2: Power Tubes
B3: PI and Loop
B4: Preamp

I checked the forum to make sure there weren't any new posts, and not seeing any, did the unthinkable. I played with the B+ dropping resistors to hopefully get a little more to the preamp/PI... I replaced the 1K and 10K with a 100R and 1K... (sorry pdf64) While I was in there I reconnected the Presence circuit (yeesh... sorry). Anyway, now the PI tail is connected to the NFB/Input Cap. NFB signal meets that junction, and then is sent to the Presence pot (where it then goes to ground).

After doing all that, I started to check the voltages and ALL my B+ voltage is actually significantly lower. B1 went from 295v to 265v. Is that from the NFB circuit?

The way I have the PI cathode wired is via a single wire coming from the resistor, which goes to pin 3. I used a jumper to go from pin 3 to pin 8. When I checked the voltage at the top of the PI cathode resistor (where the wire goes to pin 3 and then pin8) it now reads 4v.

I'm more than willing to revert the changes I made and start from there if need be. LMK how you think I should proceed. Now that I've got sound coming out, I should be much more patient and wait for guidance.

Cheers,

BTW, I played it at low volume for about half an hour. The range of tones I was getting was impressive. Roll back the volume and switch to neck single coil for some nicely broken up cleans, switch back to bridge humbucker and bring the volume back up and it roars. Rich harmonics, nice chimey highs, and very balanced sounding. Pretty excited about this little guy!

[g1]

What are your PI voltages now?
Without a complete schematic, it is mostly guesswork for us as far as supply voltages etc.

[Gainzilla]

Sorry, I edited my previous post with the voltages, but you were too fast!

I didn't have time to write them down, but if I remember correctly, the plate voltage went down to like ~260v, and the Cathode was 4v. I'll get accurate values later. Let me paste together a schematic so you'll see what I've got. Stay tuned (and thanks).

[Gainzilla]

Ok, here is a schematic I cobbled together of the various bits I'm trying to scrap together. A couple of the bits were from Memory (EL84 grid resistors and power resistors), so I may have to update those values when I can actually look and see what's in there.

Any thoughts?


EDIT: New drawing that includes Power supply.


EDIT: One last part I forgot to include is that I only have one input and I put the two stages on a switch, kinda like this:

[g1]

Power supply drawing?
As you are still showing 20K worth of series resistance in the cathode circuit, and you said it is still around 4V, there is still something very wrong there.

[Gainzilla]

Ok. Pretty sure that's it (edited original schem above).

Where are you seeing the 20k? Is it the 10k tail in series with the 10k to ground in the presence circuit? If so, yep, its still there. Im pretty much trying to paint by numbers here. What could be causing that? Any idea?

[g1]

Yes, the two 10k's in series, plus the 470 in series so actual 20,470 ohm resistance.
Something is not wired as drawn. Measure resistance from cathode to ground, should be approx. 20K.
Perhaps the bottom of the 470 ohm is grounded.

[J M Fahey]

Please do not edit old schematics or voltages because you make all answers àfter that look stupid.

Repost with new`values, if you wish you may paint red or blue the new`voltages or corrections which differ from the older ones.

Thanks.

[Gainzilla]

I hear you. The original schem is still toward the top of the thread. The later schem from post #22 is the one I updated (added notation below to indicate that). Ideally I should've taken the time to pull that together and included the full scheme with my initial post. Apologies if that caused any churn or confusion.

[Gainzilla]

Ok, I think I might've found the smoking gun. Looks like the 470r cathode resistor is actually a 470k. Would that do it? :facepalm:

I've got a whole new round of voltage readings that I'll post in the morning. Assuming that sorts the PI voltage out, I would love to get that voltage back up to where it was before.

Cheers!

[J M Fahey]

Since your :facepalm: smiley doesn't seem to work, and given the kind of errors found, I suggest:




and don't worry, we've all been through that one way or the other

In fact one such error is behind highly successful RAT distortion pedal design, and is proudly explained by its designer.

[Gainzilla]

LOL Or maybe I should just make that my avatar.

Thanks for the words of encouragement. I learn a little more with each build, and find that I really enjoy it. Strangely, laying out a build, bending resistors/capacitors, soldering, etc. is kind of zen for me. I definitely don't enjoy wiring heaters though!

Anyway, thanks for your help and moral support!

Cheers,