Eico HF-85 Dropping resistor

[k**rkleclaire]

Hi,
I have an Eico HF-85 preamp.
The volume is low in one channel.
Checking V3-A it appears there is no voltage drop after the 100K dropping resistor on the plate. Y= 234v, and after the resistor, it's 231v, basically the same value
What could be causing this?

on V4-A the values are Y= 234v and after the resistor 169v this is inline with the schematic

Any idea where to look?

 

[GordonW]

Just to check- have you tried swapping the tube that is in that position? Probably either a bad tube, bad/dirty tube socket, or a bad cathode resistor, causing that.

Naturally, swapping the tube is going to be really easy, compared to the other options.

Regards,
Gordon.

 

[k**rkleclaire]

Yes swapped the tube., cleaned the socket, cathode resistor 2.2k seems fine

 

[gadget73]

Is the heater actually lit? If the tube is good but the heater circuit is open for some reason it does this.

if its a 12ax7, is the heater wired properly? 4 and 5 jump together, 9 to the other side of the heater line. If the jumper between 4 and 5 is open it will only light half of the tube.

Only so many things can cause this

 

[k**rkleclaire]

you are correct, only 1/2 the heater lights... I re-flowed the solder joints. No luck

 

[gadget73]

ohm check the heater, see if it passes a basic sanity check. If the heater is good, it must be a bad connection. I've had tube socket contacts crack between the terminal and the bit that grabs the pin. it can be extremely hard to see if it happens down inside the socket.

 

[k**rkleclaire]

I get 22.5 ohms on all heater connections, pins 4+5 and 9 to ground. I also get continuity from the top and bottom pins of the socket. Also getting 6.3 VAC on the connections. Thanks for your help.

ohm check the heater, see if it passes a basic sanity check. If the heater is good, it must be a bad connection. I've had tube socket contacts crack between the terminal and the bit that grabs the pin. it can be extremely hard to see if it happens down inside the socket.

 

[Mr Boochie]

Grab the tube that has the questionable heater contacts and jiggle it. Does the heater light on and off when it moves? If so, you prolly need to replace that socket.

If not, is this the last tube in the heater string? You may have heater line issues upstream from the bad tube.

The bad triode side should have no voltage across its cathode resistor if the heater is truly out.

 

[k**rkleclaire]

Thanks, but whatever tube I put in the socket only 1/2 lights up,,,I'll try and giggle it.
I'll check your other suggestions in the morning. Thanks

Grab the tube that has the questionable heater contacts and jiggle it. Does the heater light on and off when it moves? If so, you prolly need to replace that socket.

If not, is this the last tube in the heater string? You may have heater line issues upstream from the bad tube.

The bad triode side should have no voltage across its cathode resistor if the heater is truly out.

 

[gadget73]

I get 22.5 ohms on all heater connections, pins 4+5 and 9 to ground. I also get continuity from the top and bottom pins of the socket. Also getting 6.3 VAC on the connections. Thanks for your help.

Is that on just the tube itself, or on the heater circuit? I was meaning just the tube, out of circuit to make sure one side isn't open

22.5 ohms across the heater supply in the amp seems high. Normally the transformer side is an ohm or two.

 

[Mr Boochie]

Thanks, but whatever tube I put in the socket only 1/2 lights up,,,I'll try and giggle it.

Bad socket.

I like the brown Belton Micalex type.

 

[k**rkleclaire]

Thanks, everyone, for your help.
I believe Mr Boochie is right, it must the socket, the strange part is some tubes work in the socket ( heaters light, voltages correct) others do not, even though in other sockets they do.
So I'll replace the socket. Onward

 

[Mr Boochie]

Thanks, everyone, for your help.
I believe Mr Boochie is right, it must the socket, the strange part is some tubes work in the socket ( heaters light, voltages correct) others do not, even though in other sockets they do.
So I'll replace the socket. Onward

Good luck!

 

[TriodeLuvr]

Thanks, but whatever tube I put in the socket only 1/2 lights up,,,I'll try and giggle it.

I wouldn't giggle it too much. Most tube sockets have a poor sense of humor.



Jack

 

[gadget73]

Like a clown? Do I amuse you?