Problem with Bassman Build

[philster]

Hi Folks
Learned a lot from lurking around these parts and have finally gotten my hands wet with my first build, a 5F6A Bassman. Have reached the power test stage and the valves warm up nicely but it keeps burning out the 4K7 resistor between the output tubes and the power filters (the one between the B and C filters). I've been checking , double checking and checking again and can't for the life of me see where I've gone wrong - can anyone point me in the right direction before I strip the thing down and start again?
Thanks in advance,
Phil

[Firestorm]

A shorted C cap will do that, but more likely you have a short in the PI supply circuit. Double-check your work.

[philster]

Will check it now - thanks for the advice

[kwijabo]

what do you mean by burning out? and how soon after power up does it burn out? does it burn out without tubes in it? what power rating did you use? a power rating of 1W should be enough, but i usually people use >2W.

P = I^2 / R, so to max out a 1W resistor you'd be pulling 68amps or more through that 4.7kohm resistor, which is a problem most likely caused by a short. like phil said, check your phase inverter circuit and the power supply cap.

[philster]

Once I flick the stand by switch the resistor starts to smell and brown after a few seconds. Pretty sure it only happens with the tubes in - I've tried swapping out the preamp tubes so it's definitely not a tube. The resistor was a 1 watt - this I replaced with a 2 watt and it does the same, so it's definitely shorting somewhere.
I've pulled the board out of the amp and can see no evidence of a short or mistake with the wiring runs. I'm not happy with the tidiness of my wiring so I'm going to re-do lot of the runs from the board and redo the PI socket and them re-test.
Thanks for the advice,
Phil

[kwijabo]

also this thought just crossed my mind. i once ordered 1k resistors from Mouser. instead of measuring or confirming color band, i read the sticker on their bag and put them in the amp. turned out they were 1ohm, not 1k! Mouser incorrectly put 1ohm resistors into a 1k bag.

so since this problem persists even when you change that power supply resistor, i would suspect that resistor to NOT* be the problem. check the resistors around the PI, especially the plate, and make sure they are what they are advertised.

*edit: i meant to say it is not the actual component.

[rdjones]

The most likely place for a short is around the cap downstream from the resistor.
Even if there is a short at the tube socket the PI plate loads would 'take the heat' and burn before the dropping string.

Are you using a variac or bulb limiter ? Don't keep burning up components !
If you can bring it up slowly to half voltage you should be able to see where voltages are not correctly proportional.

rd

[philster]

Update...spent last night stripping out the board and re-doing the wiring I wasn't happy with. Put her back together and the resistor burning is now sorted. My final issue is no sound from the input.
I've taken some voltages off the valves - does this look right? (I'm on 240v here in the UK)
V1
1-210v
3-1.9v
6-189v
V2
1-27v
6-390v
7-26v
V3
1-300v
3-21v
6-270v
7-3.6v
8-21.9v
V4
3-561
4-559
6-560 (V5 the same)
rectifier
2-56.3
4-218
6-218
8-56.3

When I touch V2 and V3 with the meter I get taps coming through the speaker, so assume the problem is before V2?
Cheers
Phil
[/url]
IMAG0866 by Philmandoo, on Flickr[/img]

[Reeltarded]

Good deal. Yes. Just go over everything before the point where you last hear disturbance while probing.

Not familiar with the amp. Missing voltages on V1? Odd comparison against later sections.

Close now! 10... 9..... 8..

[philster]

Got it!!!! I really am an idiot - can't beat these troubleshooting threads for revealing the depth of my stupidity!
I had the 820 and 10k resistors swapped around before V2 - now the amp plays great and will spend ten minutes annoying the neighbours before I get round to biasing.
Thanks for all your help and your patience - this is my first amp in the bag
Cheers
Phil

[Reeltarded]

Congrats Phil! (you idiot!!!)

Working on amps is my favorite thing. Such a good feeling when they crank up that first time!

[Structo]

Glad you kind of have it sorted.
But not so fast.

I'm curious, did you measure those voltages at the tube socket pins?

All DC voltages with black probe to chassis ground?

Because your readings are almost double what they should be.

Did you use the proper rectifier tube? (GZ34 or 5AR4)

Perhaps you have the primary wiring of the power transformer incorrect?

It's normal to hear pops in the speaker when probing tube voltages.

When you say you had the 820R and 10K resistors backwards, do you mean the 820R and 100K?

What output transformer did you use?

Many times it is the under board wiring that we make mistakes on.

I am happy you are making guitar noises with the amp now but those voltages still concern me somewhat.

By the way, not sure what schematic you are using but here is a real clean copy of the 5F6-A

[philster]

Thanks Reeltard!

Structo
Yes, measured at the socket pins with black to ground. Think I may have used AC on the meter - would this account the difference? I'll go recheck them later.
I followed the Ceriatone layout and used the Fender schematics to check - the ceriatone adds a few "modern" safety items but is pretty much identical to the Fender.
Yes, i meant the 820r and 100k resistors, sorry! Doh! Thanks for the clean schematics - better than mine - they look like a bad photocopy!

Cheers
Phil

[philster]

Structo
Right - messes up the earlier reading. On the power tubes I'm getting 440v, which is more like it, right?
When it comes to bias, I did the transformer shunt thing and it's currently 20mA. Is that ballpark for a pair of 5881's?
Cheers
Phil

[rdjones]

Caution is urged when following a Ceriatone layout, they are notorious for errors.
Definitely use the schematic for actual circuit connections and use the layout only as a guide for physical placement.

Voltages and idle seem more realistic now.
I like to watch for redplating in subdued light until everything's settled in.

rd