JTM 45 first build, can' get a reading on V4 to bias, V5 OK

[norburybrook]

Do I ground them to the same point the bias resistors are grounded to by adding a length of wire?

Neither end of the grid stopper resistors will connect to ground. Unsolder the wire connected to pin 5. Connect one end of the resistor directly to pin 5. Connect the other end of the resistor to the dangling wire you just unsoldered. Repeat for other tube.

does it make any difference having the bias resistor across pins 1 and 8 then to ground or going to ground after bridging the pins?

Either method will work just fine, but just to be perfectly clear... It's fine to connect the 1Ω resistor between pin 1 and 8, but you must connect pin 1 (not pin 8 ) to ground. The important thing here is that the resistor must be between pin 8 and ground, regardless of how the resistor is physically mounted. Pin 1 is not used by the KT66 so it's simply being used as a convenient tie point, similar to the way you used pin 6 as a convenient tie point to mount the screen resistors.

Either method will also work if you ever plan to use EL34s.

Anyway before proceeding with that here are my V3 PI values:

Those voltages look fine. Be aware that the voltage on grid pins 2 and 7 cannot be accurately measured due to the very high impedance of the LTP bootstrapped grid circuit. The actual voltage on the grid will be slightly less than the voltage on the cathode pins 3 and 8. I find it more useful to measure the voltage at the common node rather than the voltage on the actual grid.

Thanks for that, that could have been embarrassing

Ok, so at the moment all my voltages seem fine for all my valves at all points.

is it possible I've just got a crap valve?

I put the Kt66's in my bandmaster and they seemed to work however, it's just weird that when I swap them round I get the terrible squeal.


Right I'll post back when I've properly installed the grid stoppers.


Marcus

[sluckey]

I put the Kt66's in my bandmaster and they seemed to work however, it's just weird that when I swap them round I get the terrible squeal.

Are you saying that swapping positions with the KT66s IN THE BANDMASTER also squeals?

Based on the fact that when you put 6L6s in the Marshall you were able to get a normal bias current reading on V4, I'd have to say that particular KT66 is bad. How does the amp sound with 6L6s? Have you proven that NFB phase is correct?

[norburybrook]

I put the Kt66's in my bandmaster and they seemed to work however, it's just weird that when I swap them round I get the terrible squeal.

Are you saying that swapping positions with the KT66s IN THE BANDMASTER also squeals?

Based on the fact that when you put 6L6s in the Marshall you were able to get a normal bias current reading on V4, I'd have to say that particular KT66 is bad. How does the amp sound with 6L6s? Have you proven that NFB phase is correct?

No the KT66's in the bandmaster sounded fine. I didn't leave them in for long as I wasn't sure on the bias voltage.

When I put the 6L6's in the Marshall I quickly biased them to 35mV and then plugged a guitar in. I got a terrible sort of machine gun /square wave type distortion when I hit the strings hard1 so switched off.

the NFB phase is fine, I now know the squeal from that particular scenario


Marcus

[sluckey]

When I put the 6L6's in the Marshall I quickly biased them to 35mV and then plugged a guitar in. I got a terrible sort of machine gun /square wave type distortion when I hit the strings hard1 so switched off.

Hmm, so there is likely another issue as well. I would not call the KT66 bad just yet.

So, I would put the 6L6s back in, bias a little cool, and trouble shoot the 'machine gun' symptom. Once that is resolved, put the KT66s back in and see if they behave well.

BTW, NFB problems are not always a loud squeal, especially if there is a presence control involved. I would disconnect the NFB loop from the speaker jack until everything else is working.

[Stevem]

And to use 34s the bias adjust pot and dropping resistor will need to get you some 10 volts less bias then what you are placing now to pin 5.

[JazzGuitarGimp]

Looking at your PI voltages, I see you haven't reported a voltage for pin 5. Is pin 5 tied to pin 4 ?

[norburybrook]

Looking at your PI voltages, I see you haven't reported a voltage for pin 5. Is pin 5 tied to pin 4 ?

yes it is.

just putting in the grid stoppers as we speak.

Marcus.

[norburybrook]

Progress

I fitted the grid stoppers.

I fired up the amp in it's working state. Still no bias resistor reading.


Swapped the valves over...Squeal city again.


removed the NFB wire from the speaker output ......no squealing, and I could get a bias reading from V4 at last

amps plays fine though there's a loud hum.


So what does this mean?


Marcus

[jelle]

I think it would be good to take it to a tech. Where are you located?

[norburybrook]

I think it would be good to take it to a tech. Where are you located?

South London. Techs are very expensive here. I've not seen many times of reading this forum when taking to a tech has been cited, may I ask why in this case?

I was hoping to sort this out myself with the help of you guys here and in doing so learn in the process, something I've already done over the last few days.


I have access to a multichannel oscilloscope and tone generator if that's any help.


Marcus

[Reeltarded]

It means your output primaries are swapped with each other.

[norburybrook]

It means your output primaries are swapped with each other.

I've swapped my output primaries so they were not squealing with the 'working' config but no bias on V4 reading.

how would swapping the valves round affect the OT primaries phase?


just to make sure I swapped them , and re connected the NFB cable, same result......... Squeal.

I'm returning them to where they were.



marcus

[sluckey]

removed the NFB wire from the speaker output ......no squealing, and I could get a bias reading from V4 at last

amps plays fine though there's a loud hum.

That should mean that the NFB phase is wrong and reversing the OT primary leads usually cures this condition. There may be something wanky about how you have the NFB connected. Can you post the layout/schematic that you used? And maybe a hi rez pic showing where it connects to the board?

just to make sure I swapped them , and re connected the NFB cable, same result......... Squeal.

Does this mean that you swapped the OT leads that were connected to pin 3 of the output tubes and nothing else?

[JazzGuitarGimp]

I think it makes sense to forget about the OT phase for the time being, and get the V4 bias issue resolved first. There is a possibility that there really isn't any current flowing through V4. I suggest you discharge all the caps, and measure the OT primary's DC resistance, one half at a time. Measure from the tube socket to the center tap. These two resistances will be similar, but usually not the same. Record the resistance for each half by wire color. Then, AND DO THIS WITH EXTREME CAUTION. Use clip leads to connect meter leads so that you do not have to be probing around in a hot chassis. What you want to do, is measure the DC voltage across each half of the primary with the amp at idle. Measure for each color (usually blue and brown), at the tube sockets. So, if your primary colors are brown, blue and red (center tap), measure the DC voltage from brown to red, and from blue to red. Then report back with the DC resistance and DC voltage for each color. Please be very careful as you will be dealing with 500 or so volts when measuring. If you find there is no voltage across V4's primary, swap in the 6L6's and try again.

[norburybrook]

removed the NFB wire from the speaker output ......no squealing, and I could get a bias reading from V4 at last

amps plays fine though there's a loud hum.

That should mean that the NFB phase is wrong and reversing the OT primary leads usually cures this condition. There may be something wanky about how you have the NFB connected. Can you post the layout/schematic that you used? And maybe a hi rez pic showing where it connects to the board?

just to make sure I swapped them , and re connected the NFB cable, same result......... Squeal.

Does this mean that you swapped the OT leads that were connected to pin 3 of the output tubes and nothing else?

I'll do that in the morning.

Yes I swapped the OT leads and nothing else.

I used the metro amps instructions which to be honest were very helpful to a newb.

I augmented with the Triode electronics layout ,and