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It works!! (now some questions)

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1/3/1999 2:12 PM

It works!! (now some questions)

Well after correcting some stupid mistakes (the fuse blowing was caused by me stupidly putting the diode in the biasing circuit in BACKWARDS. Thanksfully the fuse blew rather than the tubes :O) I fired up my first DIY amp (a tweed bassman clone built from the corspe of a demolished bandmaster) and it WORKED! I still have a few problems but the hum is not too bad unless it's on 10 and the amps actually sounds pretty damn good all things considered...

Questions (for anyone who might have an idea):
1) The feedback circuit (stock tweed bassman style) literally sucks the life out of the amp; output is greatly diminished and the amp sounds like it has a blanket over it. I see sparks inside the presence pot and get an audible pop if I turn it all the way up. I'm using the amp into an 8 ohm load (the OT does 2/4/8) so I know I really need to change the resistor values but why does the feedback loop suck so bad? Should I worry about the sparks? I know the DC on the pot is normal but it sure doesn't sound pretty. Disconnecting the feedback circuit makes the amp come alive; it's MUCH louder (probably twice at least) and sounds good.

2) The treble pot acts just like a 2nd volume knob making it kind of wacky since if you want to decrease treble you also turn down the amp. Is this normal?

Thanks...

MJ Harnish

1/4/1999 11:54 AM
SpeedRacer	#'s 1 & 2 sound like wiring problems at first glance (this being a first time amp and all) DC on the presence pot is normal, but only a couple volts (or not even..) sparks are bad.. Real bad. Sounds like some B+ is getting in there somehow. Treble control also sounds miswired - check your layout against the Fender drwg lead by lead. Set -fb resistor for tap you use the most.

1/4/1999 1:57 PM
MJ Harnish	I figured the sparks weren't a good sign. ;O) I'll have to go through and double check everything. I agree that the treble pot also sounds like it's miswired so that will have to be checked too. Thanks for the tips. MJ

1/5/1999 7:37 AM
MJ Harnish	The FB problem was a mis-wiring job. I had the lead from the OT going to the wrong side of the 27k resistor. It works perfectly now. No sparks and very little scratchiness. :O) I double checked the treble pot wiring and I can't find anything wrong. I even checked the taper of the pot to make sure that I didn't have a reverse tapered pot in there but still no luck...

1/5/1999 10:33 AM

Does the feedback circuit still suck tone now that it’s wired correctly ? You don’t have to change the feedback tap to use an 8 ohm speaker on the 8 ohm tap. The 27k f/b resistor from the 2 ohm tap will feedback the "correct" voltage for 2, 4 or 8 ohm loads as long as the load is connected to the matching tap.

The treb pot would work like a volume control if the 250p cap was s/c or high value. Try a new cap.

Dave

1/6/1999 1:44 PM

Thanks for the help.

The feedback circuit seems to work fine now. My understanding is that you need to change the value of the resistor to compensate for the fact that I'm using the 8 ohm tap if I want to keep the feedback relationship the same as it was orginally designed. However, I'm much more inclined to let me ear be the judge so I'm probably just going to fool with the values for a while or hook up a pot.

As far as I can tell, I treble pot doesn't act like a volume control now that I've tested it more. What happens is that with the treble all the way down there's no output; that makes sense since all of the preamp's output is going through the treble pot. I've never played a real bassman so I don't know if this is how they work.However, it seems to work fine once you turn the control up a bit. It still works backwards (10 = 1) and I've triple checked everything and it's wired exactly as the schematic (3 different ones) says it should be so I guess I'm just going to reverse the leads.

So far the amp sounds good and is only noisy if I turn up above 7. However, it doesn't seem very loud for a 40-50w amp. On 10, the amp is not deafening at all which makes me wonder if I've got some other problem going on... I'll keep you posted as more questions come up.

1/6/1999 1:53 PM
Bruce C	That treble pot has to be wired wrong. You should be able to turn the treble all the way down and still get signal. Sounds like the "bottom" of the treble pot is grounded, which it shouldn't be. It should go to the "top" of the bass pot.

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