[cws5]

I received a DV336 from a fellow head-fier last week, FedEx dropped the box (which broke a 6AS7), and the right channel is now dead except for buzzing.
 
I took voltage readings. I'm hoping that one of you tube amp DIYers with more experience than me can give me a few ideas as to the source of the problem.
 
Here's the relevant data from the readings with a sin wave feeding the amp:
 
INPUT TUBE (6SN7)
G2 .35v ac
P2 .028v ac; 7.2v dc
C2 .04 ac 24v dc
 
G1 .35v ac
P1 .32v ac; 88v dc
C1 .04v ac; 2.6v dc
 
As you can see, insufficient voltage is hitting the plate of one half the input tube (only 7v, versus 88 for the other half).
 
The OUTPUT TUBE is not showing any differences between halves that are not attributable to the input tube problem. That is, both plates are hit with 165v, etc. Also, both tubes light up properly (no problem with the filament voltage). And I've tried two different 6SN7s, both of which give me the same readings.
 
Here's a bit more info to help think about the plate voltage problem:
 
(1) the resistor between the socket's plate lead and the capacitor leading to the B+ reads the same for both working and non-working channels (27k ohms);
 
(2) because I do not get a zero reading but get 27kohm, there is continuity from B+ to the socket plate lead;
 
(3) at the capacitor I'm reading the full 165v.
 
So why in the world is only 7v hitting the socket's plate lead?
 
Your help is very, very much appreciated.
 
 

 

[digger945]

Quote:

 
(1) the resistor between the socket's plate lead and the capacitor leading to the B+ reads the same for both working and non-working channels (27k ohms);
 
(2) because I do not get a zero reading but get 27kohm, there is continuity from B+ to the socket plate lead;
 
 
 

The pictures I have seen show 2 resistors between B+ and each plate. Do you have a picture of yours?
 

 

[cws5]

Many thanks for replying, digger965. Here's the image:
 

 
Note the resistor between the large black cap in the lower right hand corner and pin #2. I'm measuring full B+ at the junction of that cap and the wire from it to pin #2, but am measuring only 7v at pin #2.

 

[digger945]

With the amp turned off, and everything discharged, and making sure you measure to make sure everything really is discharged, measure the resistance from pin 2 to ground and pin 5 to ground.
 
 
Was the tube left in the socket during shipping?

 

[cws5]

I discharged the caps to the point of a nominal zero reading (it's difficult to get them absolutely discharged). I then took the readings you suggest from pin 2 to ground and pin 5 to ground. The readings are identical: the resistance reading slowly rises from about 30k.

 

[cws5]

Oh, I forgot to answer: the tube was NOT left in the socket during shipping.

 

[digger945]

Have you measured with the tubes out to compare?

 

[cws5]

Yes, I've measured with tubes and without tubes; the measurement is the same in both cases.
 
Given the buzzing in the bad channel, do you figure there's some sort of grounding problem with the channel?
 
Also, I'm curious what your train of thought is about possible causes. That is, feel free to give me the reasons behind your suggestions for my taking readings, if you like.

 

[digger945]

Without the amp here in front of me, it's tuff to say.
 
Measure the voltage drop across the 2 plate resistors, putting one lead on each side of the resistor.
 
Do the same for the cathode resistors.

 

[cws5]

Okay, done: 30k ohms across each of the plate resistors, and 1k across each of the cathode resistors. So, still no difference between channels. And just for the heck of it, I measured for the output tube: 1k across the cathode resistors. All measured with tubes in and caps fully discharged.

 

[digger945]

oops.
 
What I meant was to measure the DC voltage across said resistors, with the amp turned on and tubes in place.
 
Do you have another amp that you can check the tubes in, an amp that uses these tubes?
 

 

[cws5]

Ah, voltage drop. Right. My mistake.
 
I've just measured with amp plugged in and tubes in place. The two channels are significantly different for both the plate and cathode resistor voltage drop.
 
Good channel:
plate 79v
cathode 2.6v
 
Bad channel:
plate 160v
cathode .29v
 
 
I understand why you ask whether I can see if my tubes are good. I don't have another amp that uses these tube types. But I can say that I've tried two different input tubes, so it's unlikely the cause is a bad tube. I'll have a second output tube early next week, as I bought a cheap one to double check that the problem isn't somehow a bad output tube.

 

[cws5]

Another thought: this is a direct coupled amp, so no cap between the stages. So do you think that a bad output tube could possible screw up the voltage readings taken at the front tube?

 

[digger945]

Yes. That's why I asked for the voltage drop at all 4 cathode resistors, which will give the current in each circuit. I suppose that a short in the output tube could pull the voltage down.
 
Another test would be to turn the amp off, and let discharge, then apply the test leads to the suspect plate to be ready to measure, then turn the amp on and notice the initial voltage, and then the drop as the tubes begin to conduct.

 

[digger945]

Your bad channel currents do not equal. Assuming correct measurement, directly at the resistor leads at the resistor body, plate resistor current is 5.3mA and cathode is 0.3mA.
 
The good channel matches, at 2.6mA.