5F2A Built -- Works but..

[jeru]

5F2A build (Weber Kit) initially completed, still finishing up the cabinet. Fired it up for the first time last night, and no major errors it seems. Good tone, low noise floor, etc. -- except for one thing.

When I hit a note loudly, or not even that loudly with the volume near the top, there is a loud BLAT/FART sound through the speaker. Given the type of noise and when it occurs, any advice on where to look first?

Thanks all.

[tugboat]

Wish I could help you. I got lucky and my 5F1 kit was perfect the first time I turned it on. Aside from the obvious check the values at each connection and checking for cold solder joints I'm not sure where to point you. Luckily a 5F2a doesn't have a whole lot of those.

This sounds like something Kap'n should know how to fix...

[ziess]

Tried a different speaker? What tubes? Is the speaker installed in a cab?

[jeru]

Tried a couple of different speakers (same noise).

Seems to be blocking distortion. Have a few things I want to try, will post again once I've done some problem solving.

[jeru]

OK - Made a few changes.

1) Changed PT primary from Brown (120V) to Blue (125). Heater AC went from 7.06 to 6.75 -- still too high,
but not exactly sure how to get that closer to or ideally under the 6.3AC I want.

2) Switched the red 660V PT secondaries to the red/white 600v ones.

3) Swapped out the stock 6V6 for Tung-Sol reissue.


Things are MUCH better now. Farting out is gone. Amp sounds great. Tried it with both the
copper cap and Sovtek 5Y3GT. Cleaner and more headroom with the copper cap. Low noise,
nice sounds.

Measurements, with Copper Cap and Tung Sol RI 6V6:

6V6 Cathode Resistor = 481 ohms
Voltage across Cathode resistor = 18

Plate current = 18/481 = .0374 - .002 (assumed screen current) = .0354 A

6V6 voltage Plate to ground = 322

SO, plate dissipation = (322-18)*.0354 = 10.76

Voltage at A = 333v
Voltage at B = 295v
Voltage at C = 257v

Good, right?

So, the next (last?) question is, do I need to do anything about the high heater voltage..?

[tugboat]

Great news! How does she sound?

[chuckocaster]

TTIUWP...

[jeru]

Ayep. 5F2A in a temporary stand that I rigged up, hooked up to a single 12" in a 2x12 cab. Also, combo cabinet in progress. Pics are crappy, taken on a tablet. Will post a couple nice ones when it's all done.

[cdw2000]

Nice job! And good fixes, voltages and cathode current sound to be right in the ballpark.

BTW, I used that Weber PT in one of my own projects and ended up doing as you did to get the voltages down.

The heater voltage is probably ok. This typically occurs for vintage amps anyway since line voltages tend to be higher now than when the older amps were built.

If you really want to get the heater voltage down, you could insert power resistors in series with the two green filament wires before the connection to the first tube. Nominal heater current for the two tubes in parallel should be 0.75 amps (best to measure this to get more accurate calculation), therefore dropping resistance should be (6.75V - 6.3V) / 0.75 amps = 0.6 ohms.

Divide that by two and use two 0.33 ohm resistors, one on each green wire coming from the transformer. This will maintain hum balance. Each resistor will dissipate .225 watts, so best to use a 1/2 watt or higher power rating.

Keep in mind that this is approximate. As heater voltage goes down, so will the heater current - except that the heaters will run a little cooler so their resistance will go down, tending to make the heater current rise. Therefore it is hard to make an exact calculation.

At the end of the day, I would just leave the voltage at 6.75V.

[jeru]

CDW - Thanks for the suggestion. I have sought advice on this elsewhere, and the consensus seems to be that it'll be fine, and best to leave it as-is.

[Don]

That's not even 8% over spec.

[jeru]

Finished the cab..! Photos Below.

[jeru]

[chuckocaster]

Looking good Josh!

[Don]

That looks great!