buschfsu

fender champ bias aa764
« on: March 13, 2019, 11:54:24 AM »
hello all,  just built a blackface champ and am a little stumped.  my B+ voltages are fine  355v and the cathode voltage on the 12ax7 is 2v but the 6v6 cathode resistor is dropping 83 volts.  its supposed to be  19.  the sound is bad very choppy almost like an intermitten solder joint.  ruled those out though.  the only thing not on the schem is im running diodes to rectify instead of a tube rec.  if the B+ is ok could the solid state rec be providing more current than a tube one?

https://elektrotanya.com/fender_champ-amp_aa764_sch.pdf/download.html

my only thought is to buy another output iron ,  tried a different 6v6 already
PRR said...
OHM'S LAW. Not as an abstract thing: you should be able to glance at a small circuit and "instantly" know I and V, the way a soccer player glances at the ball and knows the angle and kick


PRR

Re: fender champ bias aa764
« Reply #1 on: March 13, 2019, 03:37:29 PM »
Grid resistor to ground is not really connected.

Grid cap is leaky.

Layout is VERY fussy on the AA Champ. You may be screaming supersonics. I "fixed" this by gently chopsticking the tone pot wires to happier routes.

I don't know why everybody rushes to blame the output transformer.

Matador

Re: fender champ bias aa764
« Reply #2 on: March 13, 2019, 04:18:15 PM »
I was going to suggest hard-grounding the grid of the 6V6 with a wire probe as a test to see if the cathode voltage suddenly drops.

Re: fender champ bias aa764
« Reply #3 on: March 13, 2019, 05:25:39 PM »
Just a quick question about feedback across the transformer just like we see here , and in most other tube guitar amps , the feedback wire from the secondary is often unscreened and has to pass back a few stages so it can have some incidental capacitance along the way , Ive seen it a few times in vintage marshalls where a transformer was replaced and the feedback wire was left untidy ,it can osscillate .

Seeing as we have a low impedence drive from the output transformer ,why not screen the feedback wire on its journey back to the cathode of the driver tube? it might help prevent it radiating into unwanted places

That reminds me of something else I wanted to ask here , I saw a thing on Glassware ,John Brooksies site about locating your feedback wire from the speaker  voice coil instead of the terminal block at the amp output , so a 'sense' wire that carries virtually no current along with + and - from the secondary , sorry I dont have the link to post to the article , It seems  plausible  that if you take your feedback at the speaker it can better correct the actual drive voltage of the speaker see's ,the extra capacitance of screened cable ,isn't it virtually invisible to a 4 or 8 ohm secondary or even a few hundred ohms cathode tail resistor ?





buschfsu

Re: fender champ bias aa764
« Reply #4 on: March 13, 2019, 07:54:13 PM »
Cathode resistor was 4.7k instead of 470. Simple math could have told me that. 

Now i have oscilation when the nfb is connected. Sounds good when removed.   Chopsticking sounds good
« Last Edit: March 13, 2019, 08:34:58 PM by buschfsu »
PRR said...
OHM'S LAW. Not as an abstract thing: you should be able to glance at a small circuit and "instantly" know I and V, the way a soccer player glances at the ball and knows the angle and kick

PRR

Re: fender champ bias aa764
« Reply #5 on: March 14, 2019, 05:50:53 PM »
> feedback wire from the speaker  voice coil instead of the terminal block at the amp output , so a 'sense' wire

Wrong thread.

A HI-FI amp can claim Damping Factor of 1,000--- at the terminals. At the far end of 50 feet of lamp-cord you really have DF=20. Does it matter? Not much; but hi-fi fanatics worry about everything.

Most Guitar Amplifiers damp very little. Some may be DF=40. Many classic Fender tube jobs (including AA-Champ) run DF near unity. The late Ampeg VT-40 and a bunch of other no-NFB amps run DF=0.1.

Further: guitar amps don't normally run long speaker lines (acoustic latency).  Frequently under 2 feet.

The technique is valid, but dubious in most practical cases, and generally Not Wanted in G-Amps.

Re: fender champ bias aa764
« Reply #6 on: March 14, 2019, 07:06:37 PM »
Cheers Paul,

And the screening of the feedback wire , is that of any advantage in Buschfsu situation  ?


andyfromdenver

Re: fender champ bias aa764
« Reply #7 on: March 15, 2019, 01:52:42 PM »
Cathode resistor was 4.7k instead of 470. Simple math could have told me that. 

Now i have oscilation when the nfb is connected. Sounds good when removed.   Chopsticking sounds good

glad you caught your error.  You need to reverse the leads on the output transformer primary (or secondary if only a single tap) to eliminate the squeal.  I'm assuming (sorry to assume :p) the oscillation you describe is a horrible high volume sound on power up.

easiest, bestest advice: make the wiring look EXACTLY like fender's.  I was helping a friend build a champ and it was noisy as hell cause he proceeded to wire when I was unavailable to mentor.  His preamp wiring was a hot nasty mess and resulted in a noisy amp.  we moved wires with a chopstick and he learned a valuable lesson that day!
andrew
2018 current fav movie: Black Panther
2018 current fav book: Foundation ( I. Asimov)
1997-2018 fav artist: Erik Satie


 

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