Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tuckerson on July 23, 2017, 09:53:42 am

Title: Reverb problem
Post by: Tuckerson on July 23, 2017, 09:53:42 am

Im having issues with reverb channel on my ab763 DR build,   i turn the volume up all the way and the amp is really quiet, but when i turn the reverb up it gets louder fast and it sounds terrible.  I  am getting reverb though so i guess thats a good sign,  i was checking all my wiring in the reverb circuit ,  i might go back through it again

Title: Re: Reverb problem
Post by: 2deaf on July 23, 2017, 11:19:35 am

I don't see where you connected the coupling cap. from V2B to the 3.3M resistor.  It seems unlikely that it would be under board because it would cross the wire going to the 12AT7 driver.  Also, what is the value of the resistor going from the 500pF cap. to ground?

Title: Re: Reverb problem
Post by: Tuckerson on July 23, 2017, 12:03:16 pm

I have a 1 meg resistor there

Title: Re: Reverb problem
Post by: Tuckerson on July 23, 2017, 12:13:04 pm

Here is a photo , 

Title: Re: Reverb problem
Post by: sluckey on July 23, 2017, 12:33:29 pm

Change this resistor...

Title: Re: Reverb problem
Post by: Tuckerson on July 23, 2017, 12:41:20 pm

That measures 470k on my meter

Title: Re: Reverb problem
Post by: shooter on July 23, 2017, 12:46:53 pm

I'd change it just because the yellow band is brownish, make a future repair guy happier :laugh:

Title: Re: Reverb problem
Post by: Tuckerson on July 23, 2017, 12:57:17 pm

I changed it to one that read 470m ohms and it works now

Title: Re: Reverb problem
Post by: sluckey on July 23, 2017, 01:06:48 pm

I'd like to see a picture of that 470m resistor.

Title: Re: Reverb problem
Post by: Tuckerson on July 23, 2017, 01:16:59 pm

Heres a pic

Title: Re: Reverb problem
Post by: sluckey on July 23, 2017, 01:29:39 pm

Uh, that's a 470K resistor. 470,000 ohms.

That's the value that should be there.

What kind of meter do you have that shows that other resistor (yellow-violet-brown) to be 470K and also shows this 470K (yellow-violet-yellow) to be 470m? Your meter is off by a factor of 1000. Or maybe you are misinterpreting the display?
 

Title: Re: Reverb problem
Post by: Tuckerson on July 23, 2017, 01:40:18 pm

Could be , on the 470k it showed on my meter 470m and on the 470ohms resistor it reads 470k

Title: Re: Reverb problem
Post by: sluckey on July 23, 2017, 02:58:02 pm

Quote from: Tuckerson on July 23, 2017, 01:40:18 pm

Could be , on the 470k it showed on my meter 470m and on the 470ohms resistor it reads 470k

I know. What meter?

Title: Re: Reverb problem
Post by: Tuckerson on July 23, 2017, 05:37:25 pm

html?utm_referrer=direct%2Fnot%20provided

Its a cheapo harbor freight meter,  had it for years,

CE P98674

Title: Re: Reverb problem
Post by: sluckey on July 23, 2017, 05:56:17 pm

OK that explains it. My concern was that if you were relying on a cheap meter to tell you which value resistor to put here and there that you "MAY" have some other wrong values scattered across that board.

Title: Re: Reverb problem
Post by: PRR on July 23, 2017, 08:12:36 pm

> CE P98674

https://www.harborfreight.com/5-in-1-digital-multimeter-98674.html (https://www.harborfreight.com/5-in-1-digital-multimeter-98674.html)

That's not what I would call a cheap meter.

If it is old and abused, it may not be in best health.

I've never seen a meter even try to say "470Meg". My good Heath hardly read 100Meg. Self-leakage over any surfaces may be 100Meg or less. This meter's specs say it goes to 40Meg, a good spec. So a "470Meg" reading suggests the display controller has a fault.