JTM45 rectifier question

[SPeter]

Just finished my first build. It is a Metro/Valvestorm JTM 45. I started the test phase this morning and right at the beginning something was not right.

I read 740V AC to the ground on the rectifier pin 2 and the same on pin 8. I measured the voltage between the two pins and it was 5V so I thought oh well lets keep going and installed V1, V2 and V3..

The V1, V2 and V3 all read about 30% higher voltage than the manual specifies.. it also mentions that up to 10% higher is OK but not 30%. All the other voltages were in the ballpark at least so I decided to install the power tubes.

I plugged in a speaker. When I fired up I got a huge fart sound of the speaker so I shut it down immediately. I pulled the output tubes to go through the voltages again now I read 0 on pin 1 on V1 v2 and V3.

I'm looking at my work and I can't find what is wrong. I suspect that my power transformer isn't right (it is a brand new classictone I got in a JTM set from triodeelectronics).

I took some closeups of the build, needless to say I could use some help...
https://photos.app.goo.gl/G9chwnWvAFvTkXWz7

[martin manning]

Assuming you can power it up with all tubes (including the rectifier) removed, what do you get for AC volts to ground from the rectifier socket pins (2, 4, 6, and 8), and what do you get for AC volts from pin 8 to pin 2? Check AC volts to ground from power tube socket pins 2 and 7 too.

[Roe]

swap the OT primaries to avoid the fart. the voltages should be high w/o tubes and the classictone has higher voltages than the metro PT. 440-450v is historically correct

[SPeter]

Swapping the OT priamries did the trick!!! Thanks!!! I would have never found it alone because I followed the BYOC manual which is based on the same OT I have and it shows this backwards.

Now onto biasing. V4 is reading 0 at the pin 8 while V5 is reading fine around 45mV and it is changing by turning the bias pot. I'm checking the connections right now but everything seems fine so far.
I'm gonna create a voltage chart to see what is going one.

[SPeter]

So I could get a reading on the V4 and biased it to 40mV but the amp sounded bad distorted with my guitar at a very low volume (did not want to crank it yet).

So checked the readings again and realized that the V4 has some issues. V4 P4 rreads 5V, P8 reads zero. V5's P4 value creeped up to 46 from the bias value I set at 40.

Uploaded the voltage chart as I can't embed it to the post:
https://photos.app.goo.gl/FfU9rkXHzMs4nkhy9

Any ideas to check?

[SPeter]

Just a follow up it seems that the brown 470 5W ohmite resistor on that V4 is dead. The other one on V5 measures right at face value while this one shows infinite resistance... I don't have an other one to try so it has to wait. At least I found the problem... (well hopefully)

[martin manning]

I read 740V AC to the ground on the rectifier pin 2 and the same on pin 8. ...I pulled the output tubes to go through the voltages again now I read 0 on pin 1 on V1 v2 and V3.

This was measurement error of some kind then?

[Roe]

voltages look ok but try the 1k flying resistor with these high voltages

[SPeter]

I read 740V AC to the ground on the rectifier pin 2 and the same on pin 8. ...I pulled the output tubes to go through the voltages again now I read 0 on pin 1 on V1 v2 and V3.

This was measurement error of some kind then?

No, the rectifier still reads that high. The V1, V2, V3 at zero was my mistake. Their readings are what I posted in the voltage chart.

[SPeter]

voltages look ok but try the 1k flying resistor with these high voltages

Sorry I'm not so familiar with the lingo (yet), you mean replacing the 470 to a 1k on both V4 and V5?
Can I do 470 on V5 and 1k on V4? The reason I ask is that I only have one 1k right now LOL

Is it possible that I killed the resistor when soldering it in? It is a 5W so it should be able to handle some heat but it is also very low rating...

[Roe]

voltages look ok but try the 1k flying resistor with these high voltages

Sorry I'm not so familiar with the lingo (yet), you mean replacing the 470 to a 1k on both V4 and V5?
Can I do 470 on V5 and 1k on V4? The reason I ask is that I only have one 1k right now LOL

Is it possible that I killed the resistor when soldering it in? It is a 5W so it should be able to handle some heat but it is also very low rating...

the flying resistor is placed between the 470R screen resistors and the 8k2 (10k) dropper (power resistor) between screens and phase inverter nodes. The screen resistors should be fairly balanced. 470R and 1k is not a good idea

[SPeter]

the flying resistor is placed between the 470R screen resistors and the 8k2 (10k) dropper (power resistor) between screens and phase inverter nodes. The screen resistors should be fairly balanced. 470R and 1k is not a good idea

Gotcha, the BYOC layout ( http://byocelectronics.com/brit45instructions.pdf ) shows this flying resistor thing I think. What does this do?:

Flying resistor by cvt01, on Flickr

[Roe]

It decreases screen current and voltage. original 45s had it. you can even use el34s with this resistor. it increases tube life but reduces power slightly

[SPeter]

I got the replacement 470 ohm resistor, dropped it in and I could bias the amp and played it an hour last night. I'm in love so far!

Still waiting for the 1K flying resistor to come in.

I also installed a LarMar master volume pot but did not have time to test it much.

Tonight I will redo the voltage chart just to close this now that everything seems to be working as it should.

[martin manning]

See if you notice some tonal change when you add the flying resistor. With that you effectively have 2470 ohms on each screen statically and 1470 ohms dynamically. I’m interested in your voltages when you get them. You are running 8k primary impedance, correct?