I really would like your help on my 300B project

[dhaen]

Ralph,

I think, maybe that you have the hot and cold (inner and outer) connections of your input connector on the amp breadboard swapped over.

That's my guess....

 

[fdegrove]

GUESS.

Hi,

I think, maybe that you have the hot and cold (inner and outer) connections of your input connector on the amp breadboard swapped over.

I am even 90% sure that's the culprit.

Cheers,

 

[Ralph]

When I said I measured 0,0 Ohm when interconnect in place, that was wrong (DMM was on wrong scale). I measure 9,7 Ohm. I'm not for 100% sure what you want me to check.

Ralph

 

[dhaen]

Ralph,

Please check that the "inner" wire of the input cable goes to V4 grid, and that the "outer of the cable goes to the earth (chassis) connection.

I think they may be reversed.

 

[fdegrove]

reversed leads.

Hi,

Ralph,we suspect that inside your amplifier the ground wire is connected to the hot side of the RCA input and vice versa.

Measure with your DVM between hot side (+) of the RCA and chassis ground.

If you measure 0 Ohm there then reverse the connections at the input and measure again.

What I don't get is how you measure the 65K input resistor correctly though.

Cheers,

 

[dhaen]

Frank,

What I don't get is how you measure the 65K input resistor correctly though.

I was thinking that the measurement conditions may have changed: One taken with the plug connected, then disconnected.

 

[fdegrove]

CHANGES

Hi,

I was thinking that the measurement conditions may have changed: One taken with the plug connected, then disconnected.

Provided the preamp is connected at the other end you would have a short.

We'll find out soon enough....

Cheers,

 

[Ralph]

I really appreciate your help guys. I do measure 65K Ohm between (+) of RCA and any groundpoint. (without interconnect) The (+) of the RCA is connected to pins 2 and 7 of the 5687 (grid).

Pin 8 is connected to the earth (heater center tap). I put 12,6V on pin 4 and 0V on pin 5. That 0V isn't directly connected to the earth directly, but through pin 8.

 

[dhaen]

Ralph,

What do you read with the cable connected?

 

[Ralph]

With pre connected, I read 9,7 - 9,8 ohm between RCA input (+) of poweramp and earthpost.

 

[dhaen]

Now disconnect the other end, and measure.

 

[Ralph]

John, the measure are repeatable the same. I measure 9,8 Ohm at the RCA (+) on the poweramp with the preamp connected. When I unplug the preamp, the figure rises to 65K.

 

[dhaen]

Ralph, I want you to disconnect the cable at the preamp end, then measure at the amp.

 

[Ralph]

John, I did the exact same thing, when I disconnect the cable at the preamp end, I read 65K.

 

[Ralph]

Wait, let me check another preamp, a Linn Wakonda.

I measure 239 Ohm with cable connected, when disconnecting the preamp side, I read 65K again.

 

[dhaen]

Ralph,

It must be that the 2 connections of the cable are swapped. This must be at one end or the other. Probably in the equipment (it's vry difficult, though not impossible) to have a swap-over in a plug).

 

[fdegrove]

CABLES

Hi,

It must be that the 2 connections of the cable are swapped. This must be at one end or the other. Probably in the equipment (it's vry difficult, though not impossible) to have a swap-over in a plug).

Are these cables homemade?

If so,disconnect it completely and measure the hot (+) at both ends.
If the DVM does not respond the wires are reversed.

Fishy stuff....

Cheers,

 

[Ralph]

Cables are fine, and not homemade. I checked them, no problems found.

I double checked all the wiring, nothing found yet, will check again. But are we looking in the right place, isn't it suspected for the measured input impedance to change when a preamp is connected?

 

[dhaen]

isn't it suspected for the measured input impedance to change when a preamp is connected?

Yes, that's true. But when you reported a very low resistance between the signal and ground, it looked like a wiring fault in this area.
Since then, it's drifted a bit, and I'm unsure at the moment.

Are you saying that the measured resistance is different with the 2 different preamps?

 

[Ralph]

John, definitly different between my WAD preamp (9.8 Ohm) en my Linn Wakonda (239 Ohm). Let me check the wiring again and come back to you later, ok?