Need Resistor advice

S Clark · « **on:** 17 Sep 2015, 06:07 pm »

Several years back, Gary Dodd took a couple of my old Eico mono integrated amps, keep the iron, and basically threw everything else out, and made a magical pair of little mono integrated amps for my bedroom. Very wife friendly, just turn it on and dial in the two volume knobs to your liking.
Earlier this spring, I asked Gary if I could clone these amps for my daughter and got his blessing. I felt quite confident, having cloned a Dodd design with my physics classes, and this was a simpler design. I put off the project for a couple of months, and in the meantime we lost Gary... and of course, now I need some help.

I built one of the amps over the past couple of weeks. Gone back over it several times, caught several errors, redid things, and got it playing music. But the distortion is bad, really bad.
I suspect that a source might the resistors I've used. Before I started, I took it to a local shop (not an ethical group but the only one in town) and was told the original resistor were 1/2 watt.
I ordered a box of 400 resistors for just over ten bucks that covered most of the values, but they are only half the size of the ones that Gary used. His were blue, 3mm thick, and mostly 1% tolerance. The box of resistors were tan, 2mm thick, and mostly %5 tolerance. Is this the likely problem? Also, with various resistors to choose from, metal, carbon, ??? what should I use? Without my buddy Gary, I could use some advice.
This is a two 6qb5, 6ca4, and two 6dj8 design with an output transformer, and power transformer, and a choke. The output is about 14W.
Do I start over with better resistors from Mouser or Digikey?

Scott

S Clark · « **Reply #1 on:** 17 Sep 2015, 06:08 pm »

Sorry, double post.

Steve · « **Reply #2 on:** 17 Sep 2015, 10:40 pm »

Quote from: S Clark on 17 Sep 2015, 06:07 pm

Several years back, Gary Dodd took a couple of my old Eico mono integrated amps, keep the iron, and basically threw everything else out, and made a magical pair of little mono integrated amps for my bedroom. Very wife friendly, just turn it on and dial in the two volume knobs to your liking.
Earlier this spring, I asked Gary if I could clone these amps for my daughter and got his blessing. I felt quite confident, having cloned a Dodd design with my physics classes, and this was a simpler design. I put off the project for a couple of months, and in the meantime we lost Gary... and of course, now I need some help.

I built one of the amps over the past couple of weeks. Gone back over it several times, caught several errors, redid things, and got it playing music. But the distortion is bad, really bad.
I suspect that a source might the resistors I've used. Before I started, I took it to a local shop (not an ethical group but the only one in town) and was told the original resistor were 1/2 watt.
I ordered a box of 400 resistors for just over ten bucks that covered most of the values, but they are only half the size of the ones that Gary used. His were blue, 3mm thick, and mostly 1% tolerance. The box of resistors were tan, 2mm thick, and mostly %5 tolerance. Is this the likely problem? Also, with various resistors to choose from, metal, carbon, ??? what should I use? Without my buddy Gary, I could use some advice.
This is a two 6qb5, 6ca4, and two 6dj8 design with an output transformer, and power transformer, and a choke. The output is about 14W.
Do I start over with better resistors from Mouser or Digikey?

Scott

As long as the resistors do not overheat, no sweat. The problem most likely lies in the value of the caps or a misplaced lead.

Sorry about Gary.

Cheers
Steve

G Georgopoulos · « **Reply #3 on:** 17 Sep 2015, 11:52 pm »

I think the cause is the modifications you made,resistors values to 5% tol. would not have caused it,also the wattage of resistors has mainly to do with power handling in circuit.check if resistors have changed value because of lower power rating.

all the best.

S Clark · « **Reply #4 on:** 18 Sep 2015, 12:40 am »

Thanks guys. That helps a bit. I read over on DIYAudio that the brown/tan resistors are carbon and the blue are metal. Can anyone confirm that and does it matter?

G Georgopoulos · « **Reply #5 on:** 18 Sep 2015, 01:05 am »

Quote from: S Clark on 18 Sep 2015, 12:40 am

Thanks guys. That helps a bit. I read over on DIYAudio that the brown/tan resistors are carbon and the blue are metal. Can anyone confirm that and does it matter?

yes that's right,but i dont think it's a standard,also metal film usaually have 1% tol and carbon 5% tol,does it matter,not my experience,what is important is the power rating and value.

cheers...

Steve · « **Reply #6 on:** 18 Sep 2015, 03:40 pm »

Quote from: G Georgopoulos on 18 Sep 2015, 01:05 am

yes that's right,but i dont think it's a standard,also metal film usaually have 1% tol and carbon 5% tol,does it matter,not my experience,what is important is the power rating and value.

cheers...

I concur with G, Scott. And with that much noticeable distortion, we are talking a very high distortion figure, so probably a wrong value, either by heat, or misreading of a resistor color code. That happens to all of us once in a while.

Cheers and good listening Scott.
Steve

S Clark · « **Reply #7 on:** 18 Sep 2015, 04:50 pm »

It does seem like every time I go back over it with a magnifying glass and a lamp, I find something else that I've messed up. I haven't gone back over the original amp to see if I misread any of the resistors or caps. I think I'll do that this evening. I bet I've left something out, got a wrong value, or wired to the wrong leg of a tube socket. I'll just keep at it. If I don't find anything, I may be asking about how to test the transformers. After all, the were built in the late 50's.
Thanks guys. I'll keep you posted over the next few days if I make progress.
Scott

DaveC113 · « **Reply #8 on:** 18 Sep 2015, 07:09 pm »

It can be pretty easy to get color-coded resistor values wrong, make sure you have good lighting so oranges and browns, etc... don't get confused.

Bill Baker · « **Reply #9 on:** 18 Sep 2015, 08:36 pm »

Hi Scott,
It does sound as if some resistors were read incorrectly. If possible, double check and in questions with a good multimeter (ohms) both from the original and installing new resistors. Its easy to misread resistor bands.

poseidonsvoice · « **Reply #10 on:** 18 Sep 2015, 10:20 pm »

FWIW, I never trust the resistor color codes in a book, on the Internet or wherever. I trust my calibrated multimeter. With color codes you can be off by thousands.

I always measure the resistor in isolation, not within the circuit. I obviously measure current and voltages when already soldered in circuit.

I even measure the values when they come from Digikey, Mouser, wherever...why trust the vendor who may have an incompetent packing department?

Always trust a properly designed and calibrated multimeter!

Best,
Anand.

S Clark · « **Reply #11 on:** 18 Sep 2015, 10:44 pm »

Quote from: poseidonsvoice on 18 Sep 2015, 10:20 pm

Always trust a properly designed and calibrated multimeter!

Hmmm, I may have to look into getting something other than my Harbor Freight VOM. Is there a decent meter for under $50? Under $100?

poseidonsvoice · « **Reply #12 on:** 18 Sep 2015, 11:00 pm »

I don't know honestly. I use a Fluke and I am happy. But many times the price is related to how many functions it offers as well. Just make sure your unit has new batteries etc...so you don't get funky readings.

Best,
Anand.

G Georgopoulos · « **Reply #13 on:** 18 Sep 2015, 11:25 pm »

AUTORANGING meters are the best,very cheap nowdays, i highly recommend them...

DaveC113 · « **Reply #14 on:** 18 Sep 2015, 11:56 pm »

Just be careful, you can't measure a resistor if it's in the circuit and anything is in parallel with it....

S Clark · « **Reply #15 on:** 19 Sep 2015, 12:10 am »

I figured that. So to actually measure a resistor that is already in the Dodd built amp, I'd have to de-solder one end. That's not going to happen. In some cases, the spacing it simply too tight, and I don't want to mess up a good working amplifier. I'll just have to trust the color codes. However, going forward on building the second mono amp, I'll definitely measure each resistor before I install it.
BTW I've got an Autoranging meter on the way. I'll put Harbor Freight unit in my car tool kit.

Steve · « **Reply #16 on:** 19 Sep 2015, 11:41 pm »

Quote from: S Clark on 18 Sep 2015, 10:44 pm

Hmmm, I may have to look into getting something other than my Harbor Freight VOM. Is there a decent meter for under $50? Under $100?

Nah Scott. I use HF all the time (I also have Tektronix as well to compare to) and unless one measures values less than 100 ohms or 1%, HF is fine for most applications. In fact I have four so I can measure 4 cathode currents at one time. (4 meters are matched quite closely) The biggest problem I have are the leads breaking.

Resistors being off by a factor of 5-10 is what I am referring to.

Cheers and good luck Scott.
Steve

S Clark · « **Reply #17 on:** 22 Sep 2015, 03:14 am »

It seems like every time I go back over this %@&* amp, I find another mistake. Found a wrong value resistor after about 20 minutes tonight- not going to be an easy on to change, of course. Too tired tonight, I'll get after it tomorrow. I must have fixed a half dozen mistakes so far. Wow. I'm glad I don't do this for a living.

JohnR · « **Reply #18 on:** 22 Sep 2015, 10:25 am »

Hi Scott, I've only just now looked at this thread, but I gather you don't have a schematic, or DC voltage values. Normally, those would be the first things to check. However, you do have a working amp (the clonee) to compare with? Perhaps if you measured the DC voltages in the clonee, and then compared with the clone? Starting with the power supply, then the input stage, then work forward?

S Clark · « **Reply #19 on:** 23 Sep 2015, 12:53 am »

JohnR, like you suspected there is no schematic, only the prototype pair of amps that Gary built just before he came out with his own 6qb5 amps. You are the second person to recommend checking voltages, but the first to suggest an order in doing so.
But good news, the last repair did the trick and the amp is playing extremely quietly and cleanly. One down and one more to go - after all, they are mono amps. Thanks to all that have given advice and offered help here. You guys are the best.

Now for an evening of one channel music.