Express Build

[ampbldr2]

My Express Build.

-Mojotone Cloth 22 AWG Push Back Wire and Mojotone Pots.
-NTE wire for Power section
-Mouser and Digi-Key resistors and caps.
-AES for power caps and IEC and some other parts
-Faceplate was a 3ply Pickguard from amazon. May replace in future.

Purchased Chassis, PT/OT and boards from RJ. Took me some
time to get the proper hole alignment for the PT in order for the CAP stack to fit
flat over the head screw that gets in the way when laying the cap stack flat
and siliconing to chassis. Ordering from mouser and digi-key was challenging to
procure all the parts. Backorders, Min 10 and 100 on some parts was very discuraging.
My last build was my Rocket in 2015 and did not run into this.

I have a mixed bag of wire from valvestorm, NTE, and Mojotone cloth. I am
not sure I'm crazy about the cloth wire and may replace later in time.
The heater wire turned out great with pre-tinned stranded wire from Valvestorm.
It's like a solid spider web of wire and tie-wrapping adds to its strength.
I also like the NTE wire as you can push back any shrinkage from heat and
still looks clean. The push back wire at 22awg is very thin and is not as clean
and the pre tinning is thin and can wear down to the copper very easily.

I like to leave a bit of extra wire so I coiled and tie wired where I could
in the power section. Not sure if others disagree with this technique?

This is the first time I had to cut a hole for the IEC mains and bought a
Nipper tool from AES which worked great. Wear gloves would be advised as the tool
is difficult to work with.

I am near completing the build waiting for a resistor order from Digi-Key
that comes Monday.

(20210710)
received some more mouser and digi-key parts and got alot done today.
I had to redo the posts for the power section. The posts I had in there
were too short and i had some 1/2 inch so I replaced them which was an
improvement in the amount of space below. Wires no longer touch the bottom
turrets.

I have to wait til august for the large Choke resistor from Mouser so I am
going to use the arcol 1K/25w

[romberg]

Nice! I like the faceplate. Looks pretty cool. Have you fired it up yet?

Mike

[ampbldr2]

I received my resistors late yesterday so I had time this morning to finish wiring it up. I did slow start on a variac using dim bulb test. I took it out of dim bulb and biased the output tubes. When I tested fully tubed with a speaker I have a few problems.
1. In Standby position I am getting some hum through the speaker.
2. in On position it is very noisy and does not sound right. Scratchy and noisy. I tried different tubes in all v1-v5 and still the same.
3. When I turn the presence pot CCW it starts squealing really bad. Also the presence pot is scratchy when rotating. My presence wire is laying along the chassis.

All my B+ voltages look to be normal.
B+1 = 403Vdc
B+2 = 385Vdc
B+3 = 307Vdc
B+4 = 289Vdc
B+5 = 274Vdc
Bias = -36Vdc

Anyone suggestions?

[ampbldr2]

I thought I had the wiring of RJ's OT primary wiring correct but looks like Red is start and Brown is finish and the Blue wire is the CT that ties to the Standby switch.

I reversed the Blue and Red and now when I turn on I get a huge squeal because my bias is way off. I had to panic and shutdown. I am checking bias not and the minimum I can get is -60mv across the 1ohm resistors with a plate around 403Vdc. That puts the outputs too hot around 25 watts.

I will see if I can lower that 220K in the bias to allow my trimmer to come down. I used a 25k trimmer. I can also see if I have a 50k trimmer lying around.

Wondering what others had done when running into this bias problem?
Also hope I didn't hurt the Primary of the Output Transformer?

[pompeiisneaks]

If you swap leads and get horrible squeal it means you have positive feedback instead of negative feedback, swap the OT leads again between red and brown on the Power tubes.

Presence pots always have DC on them, but due to the non common 'tweaking' of them it's usually not a big deal... set it and forget it.

~Phil

[ampbldr2]

Hopefully i explained properly that i had the OT primary blue and red wires in backwards. I fixed this and now my bias pot does not come down enough to set properly. Will see if I can sort out tomorrow.

[pompeiisneaks]

I think so, it sounded to me like you wired it 'wrong' and now you've wired it 'out of phase' with Negative Feedback making it Positive Feedback.

~Phil

[pompeiisneaks]

To say that differently, in the picture you show, red is CT and that should be connected to B+, the blue and brown should go to your power tubes. If you connect blue and brown opposite of how they should be, it makes the negative feedback wire into positive feedback causing that squeal. Just leave red where it is and swap brown and blue to opposite power tubes and it should be fine.

~Phil

[brewdude]

The diagram shows blue as center tap?

[ampbldr2]

yeah RJ has Blue as the CT on the OT.
I did swap Primaries on the EL34 pin 3 and that did the trick. I also had to go with R29 at 150K.
All good now. Get to sound test later today.

Thanks,
Todd

[pompeiisneaks]

The diagram shows blue as center tap?

Sorry typo...
In the amp I just built, and as far as I can recall MOST used red as center tap... but this is def not that case.
~Phil

[ampbldr2]

Another question though.
I have a brand new set of EH-EL34s and when I bias with the 1 ohm this is what I am getting. Seems low on V5 and not very well matched.
I will try moving tubes in other locations and recheck measurements to see.

Is the idle current on V5 look acceptable?

V4 403Vdc * 41.1mVdc = 16.56w at idle
V5 403Vdc * 26.6mVdc = 10.72w at idle

[pompeiisneaks]

No that is incredibly poorly matched... I'd swap tubes and see if the low current follows the tube, or is related to that side of the power section of the amp. I.e. there may be a problem with a solder connection or component on that tube... maybe the 1 ohm resistors has one that's way off, or some other thing, etc... but something's making that one tube not conduct as well, and it's either the tube OR the components tied to that socket, or the socket itself.

~Phil

[ampbldr2]

Thanks Phil,
It was the tubes. I bought a Quad of EH-EL34's and one side was way off. The other 2 tubes were close.

These are my new readings
v4 402Vdc * 36.89mVdc = 14.83w
v5 403Vdc * 35.73mVdc = 14.40w

My 25kl pot for biasing is fully CW and wont go any more so I can not get the tube wattage higher.
I will run cold until I can figure out what resistor needs to go into R29. Right now R29 is 150K
I'm thinking 120K I could try next.

[pompeiisneaks]

Thanks Phil,
It was the tubes. I bought a Quad of EH-EL34's and one side was way off. The other 2 tubes were close.

These are my new readings
v4 402Vdc * 36.89mVdc = 14.83w
v5 403Vdc * 35.73mVdc = 14.40w

My 25kl pot for biasing is fully CW and wont go any more so I can not get the tube wattage higher.
I will run cold until I can figure out what resistor needs to go into R29. Right now R29 is 150K
I'm thinking 120K I could try next.

Good to hear, I think you want to go the other direction with R29. That resistor creates a voltage drop, and you're trying to make the voltage even 'smaller' negative. i.e. say -38 go to -35VDC. if you increase the resistor, it will lower the negative voltage coming into it. You can even double check by seeing the max negative voltage with the amp all the way CCW and it should be higher. You go 'colder' the more negative the voltage is into it. Then you get 'hotter' by lowering that voltage.

Decreasing that resistor should increase the voltage going into it. You might try the original 220k and see what it gives you, or just something in between...

~Phil