copperheadroads 11/11/2017 4:26 PM
Measuring output of a tube amp ?
I am here with my 68' Custom deluxe reverb on the bench & fixing the TIC in the tremolo & so i decided to bias the amp . & tried it & the amp just don't seen very loud anymore ,
I tried measuring VAC across a 100 watt 8ohm load resistor while using an Iphone signal generator to generate a 1khz signal & I'm only getting about 1.24vac .... am I doing something wrong ?
 
xtian 11/11/2017 4:35 PM
You’re on the right track, but you need to know when the output sine wave starts to clip or distort. I do that with an o-scope. Then vRMS squared divided by load = watts.

So you’ve recently biased the amp. What are your bias measurements and what plate voltage do you have? 6V6GTs?
 
copperheadroads 11/11/2017 4:50 PM
This amp has 445v & 438v & it was originally biased at about 40milivolts
 
Randall 11/11/2017 4:59 PM
you are at an automatic disadvantage without knowing what the output of your phone is.
 
g1 11/11/2017 5:47 PM
And you have to be using some measuring device that is good at 1Khz. Some meters won't but will read 100Hz.
 
copperheadroads 11/11/2017 6:29 PM
Maybe the amp is just fine & i'm use the loudness of 50 watt Traynors & Marshalls lately & I wonder whats with bias , as this thing will only had the bias pot that goes between about 29v & 41v on the meter ,I biased it by ear at about 38-39 ...Maybe the meter is buggered i was using Single bias probe .
 
Enzo 11/11/2017 6:49 PM
Why assume it is the output stage? What if the phase inverter is compromised? Or an earlier stage.
 
J M Fahey 11/11/2017 10:47 PM
Quote Originally Posted by copperheadroads View Post
I am here with my 68' Custom deluxe reverb on the bench & fixing the TIC in the tremolo & so i decided to bias the amp . & tried it & the amp just don't seen very loud anymore ,
I tried measuring VAC across a 100 watt 8ohm load resistor while using an Iphone signal generator to generate a 1khz signal & I'm only getting about 1.24vac .... am I doing something wrong ?
**Where** are you injecting the I Phone output?

FWIW phone/MP3 player earphone out is usually between 100mV and 200mV RMS
YMMV ...A LOT !!!!

A typical tube power stage needs between 1V and 2V RMS injected straight at the phase inverter if LTP and 30 to 50V RMS if Cathodyne so NO WAY a phone injecting there will do.

Fine if you inject at preamp Guitar input, set Master to 10, all tone controls from 5 to 7 and rise volume until amp starts buzzing which is "the poor manŽs scope" ..
 
Mick Bailey 11/12/2017 2:13 AM
Biasing doesn't dramatically alter the amp volume unless it's biased so negative the tubes cut off - way outside the range of adjustment on perhaps any amp. My recollection is that there's no bias adjustment pot (assuming here it's the reissue) - how are you rebiasing?
 
alexradium 11/12/2017 2:41 AM
any smartphone output can drive a guitar amp to clipping from the guitar input,and you hear the clipping is coming when the amp starts buzzing/whistling.
Is the normal channel the same?
 
Dave H 11/12/2017 2:24 PM
Quote Originally Posted by J M Fahey View Post
A typical tube power stage needs between 1V and 2V RMS injected straight at the phase inverter if LTP and 30 to 50V RMS if Cathodyne
I'm always seeing that said about the cathodyne. It's true but in almost all cases the other half of the duel triode is used as a gain stage preceding the cathodyne which means that for a single 12AX7 the LTP has about half the gain of a cathodyne + gain stage.
 
copperheadroads 11/14/2017 6:30 PM
Is there a reasonable signal generator that i can buy (basically compact ) if your saying the iphone app will not work & i was injecting the signal at the input .
 
olddawg 11/14/2017 7:33 PM
The generator app should be fine. You are doing something wrong or have an issue. You should be able to measure the output of the iPhone across the input jack. There should be plenty. It can drive a line level input to saturation.
 
copperheadroads 11/14/2017 8:01 PM
the amp was on 10 & the tone controls were up pretty high & i didn't get an increase in voltage as I slowly turned the amp down some .
Meter was hooked up with probes on either side of the 8 ohm dummy load resistor set to VAC & test leads from the dummy load to the speaker wires with the wires removed from the speaker & but still plugged in to the amp i ended up with 1.24vac .
 
J M Fahey 11/14/2017 8:32 PM
OK, you injected headphone output signal at guitar input, set all controls at 5 or 10 as needed and got 1.25V at speaker out.
ThereŽs some 5 or 6 gain stages and many elements between them.

So now you post schematic here, preferrably as a graphic and not a pdf, and weŽll suggest test points and what you "should" measure at each of them.