Stuck in testing part 1

[Willy]

Hello,

I recently completed my metro-styled jtm45 and started testing, but something isn't working out as supposed to. I got 3,31V on heaters so that should be ok. Ac between pins 2 and 8 of rectifier socket was 4,73V so that's not the case, I guess. After I installed GZ34 standby switch gave me only 280V, is this normal? Well, the real problems popped up when i set up preamp tubes and began testing: nothing going on! Zero voltages on pins 1,3,6 and 8. Standby switch had 434V with preamp tubes, but no action on preamp....

I've tried hard to find the error in circuit, but not sure where to look for it. I got Heyboer transformers (1202-55 and 784-103), PT connected to 240V, and I have 230V wall voltage.

Any help highly appreciated!

[toner]

You are probably missing a jumper on the board since voltage isn't getting to the preamp.

Leave all tubes except the rectifier out for now. Are the power tube socket voltages okay? If not, list any pins other than 1 and 8 that don't have any voltage.

If the power tube sockets are okay, use the image below and check for continuity (amp turned off) between the 3 green circled turrets and then the 3 yellow circled turrets. You could also just check for DC voltage on all of these with the amp fully on (not in standby). Let us know what you find.

[Willy]

You were darn right about the jumper: I was missing one! Must have lost it while I soldered those on the top of the board. The "non-existing one" is now between the lower right green and upper green point in that picture. Ill get back here, when I have enough time to make new calculations. Thanks for help!

[Willy]

Wohoo, almost got my amp running! Now I have good voltage values with the rectifier and preamp tubes on BUT I only had chance to try output tubes like one second. Right after I switched standby on, horribly loud soundwall came from amp and I immediately swithced it off (both volumes at 0). I wonder, what that was. Everything seems to be fysically ok, maybe something wrong with the OT or speaker section?

Here's my voltage chart without output tubes:

[neikeel]

Temporarily desolder the 27k NFB resistor from one of the turrets (bend it up gently so that it does not make contact) and retry.

If it sounds ok then you have reversed the primary connections that go to pin 3 of the output valve sockets

In this situation you are getting positive feedback into the amp, by breaking the connection you are stopping the +ve feedback. All you need do is reverse the wires on the sockets and resolder the NFB resistor.

[Willy]

Holy cow, my amp is working! I had those twisted green and orange cables going from the board wrong way around.

Thanks for the tip! Superb instructions by Metroamp too.

Luckily, I installed lar/mar mastervolume since that amp is LOUD!