No clean headroom

[hebaton]

Here’s a voltage chart that should help you.
How do your voltages on pins 1 and 6 of the preamp tubes compare to this chart?

V1 pin 1 213v pin 6 213v
V2 pin 1 180v pin 2 310v There is a 100K resistor across pins 1 and 6

That looks like the vaues I should get for V3. Is it possible the builder mixed up the physical positions ????

I am getting V3 pin 226v pin 6 240v

[hebaton]

Here is a picture of the actual board if that can help. Also, I looked trough my parts bin and found some 220K resistors I could use in series with some precision 68K I have. I am just not sure exactly hat they are. I think 1/2 watt

[cdemike]

220k mixing resistors (the resistors in question) will increase the gain vs. what you have if the current resistors measure closer to 330k (see sluckey's last post). You should be able to measure at least the one which doesn't have the 500pf peaking capacitor across it and get an accurate measurement. I assumed (perhaps incorrectly) that the amp is a recent build. If that's the case, the resistors' value shouldn't have drifted, though carbon comp resistors do drift over time, especially in higher temperature environments.

Here’s a voltage chart that should help you.
How do your voltages on pins 1 and 6 of the preamp tubes compare to this chart?

V1 pin 1 213v pin 6 213v
V2 pin 1 180v pin 2 310v There is a 100K resistor across pins 1 and 6

That looks like the vaues I should get for V3. Is it possible the builder mixed up the physical positions ????

I am getting V3 pin 226v pin 6 240v

You should have zero or very close to zero volts on pin V2 pin 2. If you meant V2 pin 6, those readings look appropriate. The picture you provided does not look like there was a mix-up between v2 and v3.

Re: RG's points about there possibly being too much clipping in V2, it might be worth checking the value of the cathode resistor V2A (pin 8 ), since a low value there will cause the cathode follower to clip early. But most Bassman/JTM45s I've played generally get into phase inverter and output section clipping before or around the same time as V2. If the pot were too linear, you'd have an amp that would go from being off or very quiet to very loud very suddenly. Obviously that can make the amp overdrive since the pot taper makes a setting or 2 or 3 electrically equivalent to a pot with a more logarithmic taper set to, for example, 4 or 5. In other words, it seems like you'd be posting about fixing an amp that's suddenly too loud or whose volume knob is too sensitive vs an amp with very little headroom.

What are you settings? Does the amp behave this way with different guitars? What if you roll the guitar's volume down? How loud is the amp when it begins to break up? Have you tried swapping preamp tubes between positions?

Unrelatedly, it looks like the cathode for V2A (pin 8 ) is pretty close to the heater tab (pin 9). That could be how the picture is appearing on my computer or the angle of the picture, though. I don't think it's likely touching anyway (I doubt you'd have sound if they were touching; you may also be blowing fuses if they were), but while you're considering working on it, I'd recommend touching up that joint. I'm also somewhat concerned about what appears to be the grid stopper on V4 touching its socket pin, but that could also be a misreading of the picture on my end. Obviously disregard if I'm seeing those incorrectly or if it's just an unfortunate angle in the picture.

Edited: Apparently "8" next to ")" gives you " "

[sluckey]

Here’s a voltage chart that should help you.
How do your voltages on pins 1 and 6 of the preamp tubes compare to this chart?

V1 pin 1 213v pin 6 213v
V2 pin 1 180v pin 2 310v There is a 100K resistor across pins 1 and 6

That looks like the vaues I should get for V3. Is it possible the builder mixed up the physical positions ????

I am getting V3 pin 226v pin 6 240v

Don't look at that voltage chart posted by Stevem. It's for a different amp that has four small tubes. Your amp only has three small tubes.

Recheck the voltage on V2 pin 2 and I bet it will measure zero volts.

Also verify the wiring to V1 socket. Looks odd to me.

[hebaton]

You are correct about pin 6, not pin 2, sorry for the confusion. The schematic calls for 270K mix resistors, I will be ordering them. IN circuit, either of them reads 165K, so I assume they are in fact 330K. Not sure what to check about V1 wiring, What is it that looks strange to you ?
Also, I have the Metroamp kit manual and looking at the voltage chart on that, there seems to be an issue with v2. According to Goearge, V2 pin 1 and 6 should be around 155 and 280
Mine is somewhat higher. would that not explain the issue ?
With PAF style Humbuckers, using only the bright channel, volume on 3, bass on 2, mids 7, treble and presence 4 to 6, Guitar volume 7 clean, 8 crunch, over that, loud and dirty !

V2A cathode. Do you mean the bypass ? if so 820 OHMS, looks good.

I do think what I am getting is preamp clipping, not power section... but after writing that I just realised, changing the PI to a 12 at7 did make a difference...

[sluckey]

Not sure what to check about V1 wiring, What is it that looks strange to you ?

Your pic kinda makes it look like pins 2, 3, 4, 5 are all connected. Surely it's just the camera angle.

[cdemike]

Thanks, yup that doesn't sound right. Just wanted to make sure we could rule in or out the pot's taper or other easily-changed factors. A JTM45 should be plenty loud around 2, but especially with those settings, I agree there's something up unless one of your speakers is dying. Also my apologies about the typo -- I meant V2B, not V2A. The cathode resistor for the cathode follower (the resistor connecting to V2B's cathode) should be 100k. In that case the concern would be that the cathode follower's resistor might be low, though the color code checks out, and carbon comps tend to drift up, not down, over time resistance-wise. So that seems unlikely but would cause that issue. If the mixer resistors are indeed measuring individually 165k each, that might cause a subtle increase in gain, but it'd be subtle, and 270k would reduce the gain equally subtly. Individual measurements on the board should leave you with isolated reads, so no issues with reading in-circuit, especially measuring the bass channel's mixer resistor since it doesn't have peaking cap.

I also agree with sluckey on V1's wiring looking strange. I'm only seeing 8 pins, which I'm guessing is either some weirdness in the camera angle or a broken pin 4. Either way, it appears that anode on pin 6 and grid on pin 7 are in contact (big safety hazard if so...), and the cathode appears to be in contact with the heater with pin 3 bridging pins 2 and 4/5. I doubt the amp would be playable at all (and if the anode-grid contact is happening, its a true danger to you or anyone else who plays it).

[maxkracht]

What's with the black lump of a NFB resistor?

[LOUDthud]

Need to check that Volume pot taper. Set the pots to half rotation. What is the resistance from the center terminal to ground ? Check it again with the wire to the center terminal disconnected.

[hebaton]

No worries about the V1 wiring, just a camera angle thing.
I do'nt get what you're asking about a "black lump" can you indicate on the board pic ?
THe volume pot reandings are 115K and 135K as requested.

On reading resistor values in circuit. When a pair of identical rsistors aconnectd together at one end, is it not true that the resistance reading will be half of the actual value ?

[maxkracht]

I do'nt get what you're asking about a "black lump" can you indicate on the board pic ?

Sorry, I believe this is the global negative feedback resistor. If that resistor is open or too large a value your amp will distort earlier than normal. Is the presence control working normally?

[hebaton]

Ho, This is a precision Visay resistor. 27K as required. there was a a carbon comp there that was way out of spec, so I used what I had on hand. The leads beinbg to short, I had to display my incompetance in making it fit ! But I assure you it is solid if not pretty !

Something I need to doulble check ! Looking inside the amp. from under the sockets, THe power tube plates would be the 3rd pin counting right from the key correct ?

[sluckey]

Looking inside the amp. from under the sockets, THe power tube plates would be the 3rd pin counting right from the key correct ?

That would be correct sometimes. It depends on whether the key is pointing up or down, or whether you are north or south of the equator.

If you want to be right all the time then say, "When the socket is viewed from the wiring side, the power tube plates would be the 3rd pin counting clockwise from the key."

[hebaton]

Looking inside the amp. from under the sockets, THe power tube plates would be the 3rd pin counting right from the key correct ?

That would be correct sometimes. It depends on whether the key is pointing up or down, or whether you are north or south of the equator.

If you want to be right all the time then say, "When the socket is viewed from the wiring side, the power tube plates would be the 3rd pin counting clockwise from the key."

Thank you ! I sometimes ( quite often ) have a hard time getting the proper English sentence together. Iam french, we have a way of making simple thing complicated...