JTM45 Build

[rp]

On the bright side one down one to go.

3 and 4 internally strapped??? Cathode and heater? 8 and 5 IC too??? This is not a 12AX7 or close kin? Really f'ed 12AX7? Wouldn't the resistors on the tail have burned up?

[Littlewyan]

It was a 12AX7. I guess it motorboated because they were strapped but as the voltage got higher the resistance increased and so whilst the amp was on I just noticed a loss of gain.

Anyway I tried a 33K grid stopper on the grid of the PI but still same issue. Hoping to use Grandad's scope tomorrow to see where the oscillation is occurring.

[Reeltarded]

Killer!

Move that stopper to an input stage and let's hear it go! That kink in the PI is over. Excellent. Get a stopper on the brite side and lets see if it leaves or gets smaller.

[Littlewyan]

Just tried it on V2A but didn't make a difference. Just waiting for the valves to cool so I can flip it over and move the stopper to V1 .

Edit: Tried it on V1b but same problem am afraid . The treble control on the amp is useless at the moment, if I turn it to 0 then you can play the amp fine and it sounds alright but as soon as you turn it on you start getting that oscillation/blocking distortion. If I have the middle up and the presence you don't notice as much.

[guitarmike2107]

Well thats a first, never heard of a 12aX7 strapped internally like that, what type was it.

Are you sure you don't have a microphonic pre amp tube in the V1/V2

Also not all 12ax7 can go into V2 with the cathode follower unless you have raised your heater centre tap, IIRC Tungols dont like more than 180v heater to cathode difference

[martin manning]

Well thats a first, never heard of a 12aX7 strapped internally like that, what type was it.

This would just be a heater-cathode short, I believe.

[Littlewyan]

It was a JJ ECC83S. I was quite surprised myself as I've actually found JJ valves to be quite reliable. I've heard about Marshalls eating certain valves in the V2 position I've lowered the voltages in the pre amp which I believe should help. I wonder if I had that valve in V2 at some point before I lowered the voltages and thats what killed it.

Anyway V1 and V2 are definitely not microphonic. I tried the 12AX7s from my Express just in case and still had the same issue. I can still sometimes make V4 red plate ever so slightly by whacking a G chord continuously far a while. I'm not sure if this is normal as Marshalls are quite hard on their power valves and perhaps I just need to lower the bias a bit or if I do actually have an oscillation issue. I looked at various Marshalls and noticed a lot of them have got the wire from the treble wiper to the PI cap short as poss and not laid on the chassis like I have. However others are just a complete mess with regards to wiring so that throws that idea out the window.

Its just so strange that turning the treble up has that affect. If I have the middle and bass up as well its like you can hear the treble signal still having that issue in the background but the middle and bass signals are fine. Which has led me to think maybe its a connection issue on the treble pot.

Edit: I also noticed that the ground for the pre amp filter cap is close to the treble pot and treble wire, but really this shouldn't make any difference surely?

[Reeltarded]

Ryan, I want to see this amp auctioned to a gun owner.

Nope.. that shouldn't make a difference. I am sad this thing is beating you up. If it were mine it would already have bullet holes where the controls used to be.

You must be a saint..

It has to be something simple.

[norburybrook]

Hang in there Ryan .

I'm just about to go on tour to Australia and will be using rental amps and have had to make a small pedal board to take with me. I've been rehearsing using my Marshall to test out the board and I'd forgotten how good it sounds. I think I actually prefer the clean/breakup sound to my 102 ODS with a strat. I'm using it with a Marshall 4 x10 and it's just a great amp.


Good luck



Marcus

[Littlewyan]

Yeah thanks for bragging Marcus .

Yes Miles this amp is close to being thrown out right now. Unfortunately my Grandad is busy today so I can't use his scope .

Edit: Hang on a minute. The treble cap on the board is laying flat and just beneath it on the otherside of the board is the wire for the HT going from the 10K resistor to the 2 100K resistors for V1. Issue? Could explain why I read a small voltage on the treble cap.

[Reeltarded]

Nope. :\

I want to punch this amp. What a little bitch.

I wonder if it's not as simple as something in the inputs? Maybe tack a different jack in or even try tacking it in at different triodes to listen.. or maybe..

I'm lost in it. I think it has to be a component and not a mistake.

Not similar in failure mode, but I chased my tail for a couple weeks on a bad V2 socket one time that generated it's own special audible nightmare but never revealed itself until I was changing the amp to something else in anger. That is brutal.

[Littlewyan]

Ok I borrowed my Grandad's scope and traced the issue. However I'm clueless as to what is causing it. So I took some photos and have attached them. These were taken with the following settings - Normal Vol 0, High Vol 10, Treble 10, Middle 0, Bass 0 and Presence 0. I fed a 1Khz sine wave into the amp that may have been 2VPP I think. I know this is obviously overdriving the amp pretty hard as its much larger than a normal guitar signal but still the amp should cope. The sinewave on V1 was not distorted by the way and the cathode follower had a square wave on it as it normally would with a signal like that.

I should mention that the garbled mess on the other PI input also appears on the PI cathode.

[Littlewyan]

Here are other photos showing the speaker output with the treble at 5, 8 and 10.

The only other thing I can think to do is move the NFB wire to the otherside of the V4 socket like some Marshall amps have done.

[martin manning]

With the treble control turned down to 5 what do the PI plates look like? Do you need to slam it with that much drive signal to cause the problem to appear?

[Littlewyan]

To make that issue appear yes I have to slam the front end. If I move the signal generator to the 470K mixer resistor/volume pot connection it doesn't appear and the PI plates look like this.

Edit: The Treble Cap output looks like this, see the little notches?

Now this is all with a 2Vpp signal into the 470K Mixer resistor.