It might be more obvious without adjusting supply while testing.  Just set for 24VDC and turn on. 
Some are not very loud, put your finger on it and you might feel the click.

1/4 or 1/2 watt in carbon film or metal film will be fine.
Check the dimensions, some of the modern parts are so tiny they can be difficult to work with in applications like this.

Quote from: Rkskaas on June 09, 2025, 08:24:03 PMI am getting 1.3v between TR4, TR5 base pins

How about resistance?

The one across D5 you should add in also.  You can see on the issue2 schematic it refers to the same note as R74, so it would have been added at the same time.

No, my mistake.  I meant from TR4 to TR5 as you suspected.
You can measure at the pad if transistor is removed.  Also you can measure at R29 and R30 where it has been installed in that other photo.

Sorry, I misread as to which part you bought 60 of.  Sounds like the opamps you have are legit.
That is where the resistor could go, but there are other spots on the flipside of the board, so measure resistance with your meter to verify it has not been installed.
They would have been 'tacked-on' in production until the new version of the board was available.  Units already in the field would have it added during warranty or other service, as there was probably a service bulletin calling for it.  Any serial # prior to U26000 may not have it yet.
Here is the issue 2 schematic where it is mentioned.

Measure resistance from base of TR5 to base of TR6.(edit, meant TR5 to TR4) 
Should be ballpark 330 ohms if R74 is there.  It was a revision added in 1986, if you don't have it you should add it in.  Measure before adding it in.

The quote about the opamp feedback loop seems suspect to me.  It's not like pin1 goes straight to R34.  The same guy in the other thread on the first page said it was no problem running it without the tank and I agree.

I think you may have an oscillation problem with the reverb opamp.  Marshalls can be picky where they specify MC1458 rather than generic 4558.  The different spec part could be responsible for oscillation which would cook the reverb transistors and that would cook the supply resistors.
You bought 60 supposed 1458 opamps because they were absurdly cheap at around 10 cents each.  That means they are likely fakes and probably came from China.  They may be real 4558 but that may not work here.
Buy some real MC1458 which are still available at places like digikey and mouser, which are legitimate industrial parts suppliers (unlike ebay or amazon which are full of fakes).
And if you did not get the MPSA06 and MPSA56 from a legit supplier, get some real ones and replace them as well.

If you put 120VAC to the 24V secondary, you risk exceeding the voltage ratings and possible arcing or other damage.
Aside from that, it would be stepped up at the primary side, either by a factor of 5 at the 120V tap, or a factor of 10 at the 240V tap.
So your 600V is theoretically correct, and there would be 1200V at the 240V tap.  Also, it would not be able to deliver much current at those voltages.

Quote from: saturated on May 23, 2025, 08:39:54 AMMight be time to read up on half wave voltage and rms etc 

Yes.  If there is any AC involved, meter can only give you RMS value.  Scope will not display RMS (except as calculated numeric readout on newer scopes).

It looks like the 2 trimmers are set up opposite each other, but that should just mean you would have to turn them in different directions, I think it should still work.

We've gotten spoiled now, used to be that all you could get was photo-copies, which were usually at best, on par with this one.   
This one if from '98, so probably taken from a copy or early tech scanner and that will be as good as it gets.
The only thing I have any issue with is the labeling of components on the 'pictorial'.  If they are not labeled on the board itself, that can make things more time consuming.

Not sure how this amp is set up, but are the jacks wired or board mount?
Sometimes jacks that have insulators get replaced with non-insulated, or the isolation washers get lost.
Then you can get weird ground loops, maybe because pot bodies are also grounded.
If you can dismount the jacks but leave the pots touching the chassis it's worth a try.  Then if it helps you just need to get insulating washers for the jacks.

The trimmer is not for bias, it looks to me like offset or symmetry adjustment but I'm not familiar with this amp.
There is a note about reducing quiescent (idle current) by adding 560 ohm resistor across one of the bias diodes.
You can check what quiescent for the power transistors is right now by measuring voltage across the 40411's emitter resistors (shown a 0R33 and 0R25) and calculating current.
It's possible with lower idle current the trimmer will do enough to get rid of the offset.  400mV or even 200mV that you have for offset is too much.  50mV is a better target, or maybe 100mV in worst case scenario.

As far as hfe vs beta, they are the same thing.
Here's a link to a good article on it, note what they say about hfe of circuit.  Usually for substitutions we need a beta greater than what the circuit requires.  Knowing beta of the transistor does not tell us the circuit gain.
https://www.ovaga.com/blog/transistor/what-is-transistor-hfe

Quote from: saturated on May 03, 2025, 02:28:35 PMTurns out that a variable resistor turned all the way to zero is a dead short 


fortunately my power supply kicked to zero 

Did you connect the supply to something other than the diagram in the picture?