Resistance/Voltage checks

[bob lennon]

I am building the 3.0 and have done a resistance check. 18 is around 240k which is within spec. of 249k, but 4 is around 209k. There are 249k resistors here and the reading across them is about the same so do I need to change the resistor at 4.

I also have had some other discrepancies at 30, 31, 32 at around 215 ohms and h2, h4, h5, h7 also 215. h3, h6 around 13k.

[fullheadofnothing]

There is probably nothing wrong with the resistor; it's possible that something has been mis-wired and you have a parallel connection to ground. Pictures could be helpful here.

As for your other values:

[T]he values signified with a * are going to vary from ohmmeter to ohmmeter because these terminals are connected to the filter capacitors, which try to charge themselves off the battery in the meter, causing a fluctuating reading. If the circuit is connected properly these readings will wander in the tens or hundreds of KΩ or higher range. What you want to watch out for is a zero reading at one of these terminals, which would indicate that something is mis-wired.

[bob lennon]

There is probably nothing wrong with the resistor; it's possible that something has been mis-wired and you have a parallel connection to ground. Pictures could be helpful here.

As for your other values:

Sorry for messy grammar, the resistance across the 249k resistor at 4-5 is about the same as 4 to ground. I can post pics later when there is more light as it is early morning here.

[bob lennon]

There is probably nothing wrong with the resistor; it's possible that something has been mis-wired and you have a parallel connection to ground. Pictures could be helpful here.

As for your other values:

[bob lennon]

Thanks for your reply, I studied the circuit a bit and looked at the solder points which started to look a bit untidy as I had been working on it 'til late, so I tidied them up a bit around the problem and it now measures around 235k ohms, which is kind of within spec I think so I will progress to the voltages now!

[bob lennon]

My voltages seem to be more or less spec., but 4 is reading around 0.35v when it should be 0v. Is this o.k.?

[bob lennon]

Switching the tubes changes voltage at 4, starts above 1v and gets lower as it heats up, there is a slight hum coming from the transformers, cannot tell which though if this is not good.

[corndog71]

Maybe I'm nitpicking but I see several connections where wire leads are poking through holes and were probably loose when soldered.  If possible I try to create a little hook with wire leads to ensure a solid connection before soldering.  Also, with no power applied, check all connections for any loose wires / unsoldered connections.  It's not uncommon to miss something.

[Paul Birkeland]

I would also add that the shielded twisted pair wires are run against the chassis, while yours are floating well above the chassis.  We strongly recommend building the kit as directed and shown in the manual for the quietest and most reliable operation.  The same can be said for resistor and capacitor leads that poke through terminal strips and tube socket connections without being trimmed back.  Eventually, they can cause some problems.

[bob lennon]

Maybe I'm nitpicking but I see several connections where wire leads are poking through holes and were probably loose when soldered.  If possible I try to create a little hook with wire leads to ensure a solid connection before soldering.  Also, with no power applied, check all connections for any loose wires / unsoldered connections.  It's not uncommon to miss something.

I see, that is good point to point practice perhaps. I have since done a once over and will check things again in more detail.

[bob lennon]

I checked and tidied the solder points and re-routed some power lines. The resistance measured at 4 is still out at around 36k. Again, 30, 31, 32, h2, h4, h5 and h7 are 109k, h3, h6 are 13k ohms. All other resistances are good. I swapped the 0.1 Dayton caps for Audyn to see but made no difference, I melted the power switch so had to swap that out as well.

[bob lennon]

I just tried removing the 249k ohm resistor and measured it to be good, so I have resoldered it and the reading was better so I tried switching it on. As before the voltages at 4 and 18 read higher at first, then start to slowly go down, 18 starts around 0.05V and goes down to around 0.02V. 4 starts under 1V and goes down after a minute to under 0.5V and less, all other voltages are normal. I measured the resistance at 4 after this and it was about 250K ohms. The transformer hum I noted previously was not apparent this time.
I am wondering if I can hook it up and take it for a spin yet?

P.S. Voltage at 4 goes down to less than 0.08V

P.P.S. Checked ground by disconnecting black wire at 9 then terminals 4, 18 and 23 read high ohms, 0.3 ohms connected at 23.

[bob lennon]

I couldn't find anything seriously wrong so I tested it and found there to be some hissing and crackling on startup that persists in one channel, I tried switching the tubes around and the noise also switched, the stock tubes are Sylvanias.

[bob lennon]

I am hearing a low hum from the transformers (not from speakers or the jack output) and high pitched noise from tubes at about the same level but hum is more penetrating, are these noises typical?

I checked the housing grounds on transformers and chokes by scraping a bit of my paint job off and they were all grounded. I made some loop back plugs to test for line noise, I thought I heard a hum with volume pot from 1 - 4 O'clock, but with some twiddling it turned into wiper noise.

There is low constant noise in one channel, unrelated to volume and about the same level as the wiper noise that changes channels when I change the tubes around, I have just bought some other nos tubes but I was wondering if I should send in the questionable tube from the kit for you to test?

[Doc B.]

Let the amp break in for a while and let us know if the tube noise goes away or persists. If your voltages are within spec the transformer should be fine and perhaps just needs its mounting nuts tightened up a bit.