Huge plate voltage drop on cathode biased amp

[Heavenamps]

I've built an amp with a power section that is quite similar to an ac30. It's cathode biased with four el84's but when I connect the bias circuit (a 56ohm power resistor, in parallel with a 225uF cap) I am seeing a huge voltage drop on the plates, from about 350V down to about 130V. The voltage on the cathode is around 3.5V. I've checked and rechecked my wiring against my schematic and can't find anything wrong with it. Attached is the schematic from which I based my design. Any ideas as to what might cause this large of a drop?

[sluckey]

Is the cap installed backwards? Remove the cap. Any better?

[Heavenamps]

I don't think the cap is backwards. The negative side is grounded.

[sluckey]

Remove the cap. Any better?

[Heavenamps]

I Removed the cap with the same result.

[Heavenamps]

So I tried it with the bias circuit connected to the cathode but the screen resistors disconnected. While screen resistors are disconnected I my 350V but when I reconnect them the same drop occurs and I'm left with around 120V again

[tubenit]

Double and triple check the values on the screen resistors and are they on the correct pin.  And check the value of the dropping resistors in the B+ rail.

If neither of those reveals the issue.  Measure the 6.3v and see if that changes when the screen resistors are connected?

Have you tried it with two EL84's and a larger cathode resistor to see what the voltages are?

With respect, Tubenit

[sluckey]

With the amp turned off measure the resistance from pin 3 8 of the EL84s to ground. What have you?

[tubenit]

Also measure pin3 to ground please and let us know.

[sluckey]

Also measure pin3 to ground please and let us know.

Crap! That's what I meant to say.

[Heavenamps]

The values of the screen resistors are all 100 ohms going to pin 9 of each tube. I thought it might be something in the dropping resistors but I've tested those and they're all fine (as far as I can tell). With only 2 power tubes the plate voltage goes to about 193V. The resistance from pin 3 to ground is 55.8 ohms which is my cathode bias resistance.

[sluckey]

Could you post some hi-rez pics of this amp? Are the power tubes red plating?

The resistance from pin 3 to ground is 55.8 ohms which is my cathode bias resistance.

You actually measured that with one meter lead connected directly to pin 3 and the other meter lead connected to chassis?

I keep harping about the cathode pin 3 because there's a confusing conflict with the voltages you posted in the original message. The plate voltage is dropping extremely low which indicates that a very high current is flowing through the tubes. BUT, you only have 3.5v on the cathode and that indicates there is very little current flowing through the tubes. This would make sense ***IF*** the cathode resistor was very small. This is the easy part to troubleshoot. But if the cathode resistance really is 56Ω, we need to look elsewhere.

Start by measuring the voltage of pin 2 for all four output tubes. They should all be zero volts. Next, measure the resistance of pin 2 (meter lead directly on the socket pin) of all tubes to chassis. Should be about 222K for all four tubes.

Now check the resistance of the OT primary. Each side to center tap will likely be a couple hundred ohms and should be about the same. A very high reading is not good and could cause the symptoms you describe.

If the cathode and grid measurements and OT resistance are correct we may need to look at the power supply or a bad connection between the power supply and the tube plates.

Do these tests and report back. Then we'll dive deeper if we need to.

[tubeswell]

What PT are you using?

[PRR]

Are you testing with a Lamp Limiter? With a small bulb?

If not that...... There's around 3K of extra resistance in the B+ loop. Throwing about 16 Watts of heat, so you should feel the heat rising past your nose.

[Heavenamps]

I discovered the problem! So I had the B+ voltage connected to the wrong pin on the rectifier tube. But an even greater thing causing the issue was that, l was using a home made current limiting device to protect the circuit, which was eating up all my current. Like a bone head I didn't check the AC voltage coming into the amp from the wall. I didn't notice it until one of you suggested I check the heater voltages. When I did, they were very low. I checked the AC voltage coming in and it measured 60VAC! I unplugged it from the current limiter, made sure my fuse was still good, plugged it into the wall directly, and bingo! Everything is measuring perfectly! Thanks to all of you for helping me out!