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

[norburybrook]

Hi,

I'll start by saying I'm a newby at all this so have read a lot here but not posted.

Ok I've built my first proper amp a JTM45 using various layouts from this forum.

It's all built and I was trying to Bias the amp properly after installing the Shuguang KT66's and playing it for an hour or so.

I can get a reading in MV across pins 1 and 8 on V5 and I've biased that to 43mv but I cant get a reading at all on V4.


I took out V4 and tightened the earth nut on the socket as I thought the ground from pin 1 might be suspect.
When putting the valve back in I decided to swap the two output valves, don’t know why but I did.


Switched on and when I switched the standby on I got the squeal from hell 😊 Ah I thought , need to swap the output Transformer primary wires, which I did. Switched on again and was again greeted with the same if not worse squeal. I then swapped the valves round again, and everything was as it was before even thought the output transformer wires were still reversed.



Any thoughts? all the voltages to the valve seem exactly as they should be, the valve glows nice and orange from the heaters, I just get no reading across pin 1 and 8 , and if I swap the valves round I get a horrible squealing.

thanks

Marcus


p.s. not the tidiest build but it's my first so excuse that.

I'd got the resistor grounding from the wrong pin in this picture I've since rectified this, it's going from pin 1 now rather than pin 8

[norburybrook]

Ok, today I swapped out the KT66's for a pair of 6L6's which were the only thing I had to hand.

I can now get a reading on V4 , so with that in mind I biased for these valves around 34mA to check things.


I've now got serious motorboating.


Does this mean my valves, or at least on of them is bad?

Put the KT66's in my bandmaster and it sounded fine.?????????????


Put them back in the JTM -rebased to 43mA on V5 but again zero reading on V4?

The amp seems to work fine, It doesn't have the clean headroom my bandmaster has , is that what you'd expect?

I'm stumped.


Marcus

[jelle]

Quite the tidy build, no worries about showing pics of that!

What happens if you lift one side of the 27K NFB resistor, and start up again?

I am trying to see if something is still not right with the phase of the feedback loop, even though I did read your posts.

If still making noise, please pull all preamp tubes.

If quiet then, put them in one by one, starting with the phase inverter tube, then the cathode follower stage, then V1.

One tube may be bad, or it is a manifestation of a underlying problem that needs attention.

BTW, the observation that the heaters of the tube glow does not say much about the operation of the tube, other than that the cathode is being heated.

Please report back,

Jelle

[jelle]

And before you do this....coffee sinking in, it is early here.... please put grid stoppers on the input grids of the power tubes. Do this first. 1.5K 1/2W should be fine.

Also, please confirm that the screen grid supply from the filter cap goes through a shared 1K 2-5W resistor, into two separate 470 ohms resistors.

Also, do not ground through the steel of the power tube sockets. Use a separate ground lug. The locking variety, like: http://www.mojotone.com/amp-parts/fiber ... ocking-Hub
and put it straight onto the aluminum of the chassis.

Jelle

[Stevem]

The output tube sockets used on a typical marshall amp grip the output tube pins in a very different way than the type of female reciver's in a stock a Fender output tube socket, in fact the Marshall style holds up better long term then the a Fender type and is more difficult to get to go intermittent than the Fender!
Since you have not stated that the Marshall is red plating that tube than a bad pin 8 to ground connection thru your bias test resistor seems to be the first thing to check out.
Note that pin 1 does not need to be wired anywhere when running the 6l6 or kt66 type tube family,, so just be concerned with pins 2 and 7 which are the heater, and pins 3,4,5 and 8.
Does V4 Light up in the Marshall with both types of tubes?

Note that many tubes from China are made with smaller pins then others and sometimes this is a issue , also some I have had where in need of resoldering a pin or two that would go intermittent .
Poor connections by means of tube socket to pins and or wire solder connections to tube socket can set up a high resistance, and a high resistance acts like a antenna that will make for squealing and oscillations that will drive you nuts!

[norburybrook]

Thanks guys, appreciate the help, as someone new to all this it can be a bit overwhelming when things don't work. However I'm sure I'll learn a lot more form sorting this out than if it had worked first time

@jelle I'll have to take all this in and go through what you said slowly

I'm still painting by numbers a bit so I need to work out where those parts are physically.

I've done a continuity test an all the grounds seem fine testing that way.

The output tubes seem to be grounded this way on most Marshall layouts I've seen.


@Stevem, V4 glows very nicely with al the valves tested.

The pins on the valves are perfect, very snug fit in the sockets.


When I switch the KT66's around I get a terrible squeal/oscillation with one particular orientation, the other way round and things seem fine though I get no bias voltage across 1 and 8


Thanks guys

Back to the bench!!!

Marcus

[jelle]

What are the voltages (to ground) on all pins of the power tube sockets?

Also, the grid stopper is located as the one in the picture. Look for the resistor below the green 470R resistor.

[JazzGuitarGimp]

Is it possible you have both ends of your bias test point resistor tied to GND at V4? If so, you won't get a voltage reading there. You might try using an ohmmeter and measure from pin 1 to pin 8 on each of the output tube sockets. You should see the value of your bias test point resistor (most use 1R, but I didn't want to make an assumption). But if you have zero ohms at V4, then there's your problem.

[norburybrook]

Hi,

The resistance is measuring the expected 1R across both resisters so that's fine.

OK voltages to ground:

V4

1:
2: 3.19 AC
3: 450. DC
4: 454
5: -53 vDC
6: 452
7: 3.19 AC


V5

1:
2: 3.18 AC
3: 447 DC
4: 452
5: -53 vDC
6: 455
7: 3.19 AC


I don't have any resisters of the right value to put in as grid stoppers at the moment .

I thought I try pulling the valve I suspected as being suspect but turning on at a low volume just made a horrible machine gun noise!!! I thought if it had been fault it might have ben running in single end mode and pulling the valve wouldn't change much............ooops.......



I haven't tried any of Jelle suggestions yet, just the voltages as a start.


Thanks again guys much appreciate the help.

Don't know if this is relevant but, I noticed when trying the amp out in its 'working' state the treble knob didn't do very much. I would have thought it would roll off the top end as you wound it back but it just seemed to lessen the bass when turning it up.


Here's another pic if that helps


Marcus

[Stevem]

Why are you using pin 6,as it is not hooked up to any section in the tube!
What is your voltage on the plates of the phase inverter section, as I see you only have 400 volt rated caps there?
In fact for now let's disconnect the input cap to the PI section and just deal with the output stage and power supply?

[jelle]

Or start pulling tubes....

[norburybrook]

Why are you using pin 6,as it is not hooked up to any section in the tube!
What is your voltage on the plates of the phase inverter section, as I see you only have 400 volt rated caps there?
In fact for now let's disconnect the input cap to the PI section and just deal with the output stage and power supply?

Thanks guy's, bed time here over the pond. I'll attack this again tomorrow.

I'm using pin 6 because all the layouts I have all use pin 6.

I basically followed the metro amps JTM45 build instructions , with some help from some other layouts I got from this forum ;Ceriatone, triode electronics etc


I'll do Jelle suggestions tomorrow morning and post back.

Cheers

Marcus

[norburybrook]

Morning chaps,

Ok I've ordered some 1.5k 3w resistors from maplins and am picking them up later today. I realise when yo do this a lot you'll actually have supplies of a lot of regular value resistors to hand. I have to order them!

This was all they had hence the wattage. Do I ground them to the same point the bias resistors are grounded to by adding a length of wire?

edit; just looked at the photo and that does indeed seem like how it's done.


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?

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


V3

1: 253 DC
2: 27
3: 43
4: 3.14 ac
5:
6: 252
7: 28
8: 43
9:3.14 AC


Does this shed any light?


Marcus

[sluckey]

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.

[M Fowler]

1R from ground to pin 1 then resistor lead to pin 8.