Is my OT fried?

[The Dude]

I tried to install those so that the value was visible from the top. Wouldn't you know it, one of those was the only one in my build where the value is not visible. :|

How about just using an ohm meter then?

[Gainzilla]

Lol

I'll definitely do that when I get home.

[Gainzilla]

Makes perfect sense.

[Gainzilla]

Ok, small update:

PI plate resistors are definitely 82k and 100k
PI grid resistors, however, are 1 meg. Not sure which would be better for this amp, though.

New voltage readings, measured from ground to each pin:
Pin 1 - 149v
Pin 2 - 13v
Pin 3 - 37v
Pins 4/5/9 - 6.4v
Pin 6 - 149v
Pin 7 - 12v
Pin 8 - 37v

I tried to also get voltages by measuring across each resistor but am not sure if I was doing it right. The values I was reading were very small (.002 and .003). This gives me 36.502 and 36.503v. By measuring across the resistors I assumed negative lead on one side and positive on the other (same as measuring resistance?). Is that right?

Thanks!

[Enzo]

Yes, that is how to measure voltage across a resistor.

Using the schematic in post #43:

The cathode voltage is developed across the resistance from cathode to ground and the current through the tube. That is 470 ohm, 10k, and 10k. 20,470 ohms, or roughly 20k. You have 37v there. I=V/R, so I = 37/20,470 = 1.8ma or 0.9ma per triode on average.

The tube bias develops across the 470 ohm resistor. So 1.8ma through 470 ohms gives me 0.85v.

The tube grids do not draw any current, not at idle anyway. So there will be almost no voltage drop across the grid resistors. So to find the voltage on the grids, measure voltage from grid to cathode. it should be about the same as measuring across the 470 ohm resistor. Roughly 0.85v. So the grids should sit at about 0.85v lower than the cathode.


1M versus 560k grid resistors? You can try both later and see if any subtle differences emerge that you find appealing. But when it comes to our DC voltages and just basic function of the amp, it doesn't matter.

[nickb]

Ok, small update:

PI plate resistors are definitely 82k and 100k
PI grid resistors, however, are 1 meg. Not sure which would be better for this amp, though.

New voltage readings, measured from ground to each pin:
Pin 1 - 149v
Pin 2 - 13v
Pin 3 - 37v
Pins 4/5/9 - 6.4v
Pin 6 - 149v
Pin 7 - 12v
Pin 8 - 37v

I tried to also get voltages by measuring across each resistor but am not sure if I was doing it right. The values I was reading were very small (.002 and .003). This gives me 36.502 and 36.503v. By measuring across the resistors I assumed negative lead on one side and positive on the other (same as measuring resistance?). Is that right?

Thanks!

Those voltages look about right now. The grids are actually at about 36V but only read 13V because your meter loads it down. The voltage across the 560K's should be zero and hat is what you measured. I wanted to you to do that since there was no guarantee that the were connected correctly.

Time to relax and enjoy , I think

[Gainzilla]

Deleting double post

[Gainzilla]

Perfect! I spent some time with this last night and here are my overall impressions:

1. Very balanced and pleasant sounding. Not an ice pick or harsh jangle to be heard (something I was particularly going for with this design. I spent years on stages next to a loud drummer, and playing too loud. My ears simply can't take the abuse of overly trebly amps. Lol)

2. Less gain than earlier in the troubleshooting process. I wouldn't call this high gain anymore, but a very nice crunch (plexi territory gain wise). Not sure if I care enough to figure out why if it sounds good and I can hit it with an overdrive for high gain tones... We'll see.

3. This is a warm sounding amp. Almost too warm. However I think my treble pot is bad. I'll replace that and see how things go. I could just a tiny bit more on top.

4. Lows could be a little tighter. They don't totally fall apart, but they're definitely a tad on the loose side (or maybe somewhere in the middle). I could probably live with that as it sounds so good. Not to mention I think in getting as much out of that power transformer as it can give (she's giving me all she's got, captain!).

5. Presence circuit is either not working properly or is just very subtle. Again, after I get the treble pot working I should have a better idea what the highs are doing.

Overall, this was a fun build and I learned so much! By far my most ambitious build to date. Thanks for all the great guidance and patience! If anyone is interested in hearing it, I'll be happy to post some clips.

Cheers!

[Tom Phillips]

...3. This is a warm sounding amp. Almost too warm. However I think my treble pot is bad. I'll replace that and see how things go. I could just a tiny bit more on top...Cheers!

In what way do you think the treble pot is bad? I suggest that you measure it with an ohmmeter as your troubleshooting approach. The pot itself just contols the high frequency content that is passed through by the tone stack caps. The pot itself does not create treble.

[Gainzilla]

Hi Tom,

Thanks for the advise! I'll definitely check with a meter before I do any swapping. Here are the symptoms:

- Starting at noon and rolling the treble clockwise the tone thins out (not just adding treble, but sounds more like rolling off everything else). At around 2 o'clock. it gets scratchy and then intermittently drops out until about 3 o'clock.
- Starting at noon and going counter-clockwise, the volume gets significantly louder and sounds like it's turning up the mids.

Everything looks like it's connected right. Let me know if anything sounds like it's going somewhere wonky.

Treble Pot:
Pin 1 - Connects to Treble Cap
Pin 2 - Goes to FX loop send
Pin 3 - Connects to TS Cap 1, and also goes to Bass Pot pin 2

Middle pot:
Pin 1 - gets a signal from Bass pot pin 3
Pin 2 - Connects to TS Cap 2
Pin 3 - Grounded

Bass Pot:
Pin 1 - No Connection
Pin 2 - Connects to Treble pot pin 3
Pin 3 - Connects to Mid pot pin 1

[Tom Phillips]

The symptoms you describe don't seem like a pot issue. Regarding the connections it would be best if you just post the schematic.

[Gainzilla]

Sorry, it was buried way back in the thread. This is the schem I'm working from. Pretty much a FMV.

Checking the pot value right after the kiddo goes down.



Thoughts?

[Tom Phillips]

My first thought is that your tone stack is different from the usual Fender. I think that the .02uF caps sound good in a Fender stack but you are also using 500k Pots and a different wiring sceam. I must be missing shere you discuss this in the thread. Can you point me to a post number?

[Gainzilla]

I don't think the tone stack has been discussed, other than I might've mentioned that I had to go over it a bunch of times. My chassis is an old kitty hawk top mounted combo chassis that I'm using in a front facing combo cab. Because of where the power plug is punched and pre drilled transformer holes I had to build this reversed and flipped. This was a huge deal for me as I've just never seen that before, and I wanted to minimize errors.

To my eye this looks most like a plexi TS, but with tweaked values. And actually while waiting for my 500k pots to arrive, I actually have more typical plexi 250k/1M/25k pots.

I swapped out the pot as there did seem like there was something weird going on. Loos like maybe some solder or clipped wire end got in there. The new pot works, but the general behavior still remains. It feels like the controls interact in a more exaggerated way.

[Gainzilla]

I don't think the tone stack has been discussed, other than I might've mentioned that I had to go over it a bunch of times. My chassis is an old kitty hawk top mounted combo chassis that I'm using in a front facing combo cab. Because of where the power plug is punched and pre drilled transformer holes I had to build this reversed and flipped. This was a huge deal for me as I've just never seen that before, and I wanted to minimize errors.

To my eye this looks most like a plexi TS, but with tweaked values. And actually while waiting for my 500k pots to arrive, I actually have more typical plexi 250k/1M/25k pots.

I swapped out the pot as there did seem like there was something weird going on. Loos like maybe some solder or clipped wire end got in there. The new pot works, but the general behavior still remains. It feels like the controls interact in a more exaggerated way.