Express build 1st startup, B+ voltages too high?

shipwreck · Post by **shipwreck** » Mon Jun 12, 2017 11:14 am

Completed an Express with parts from rj guitars, and fired it up with tubes last night. The power tubes started redplating after around a minute, so had to turn it off quickly. Today I measured all the B+ voltages without tubes, here's what I got (allow for small discrepancies that could be the fault of my cheap DMM):

B+ 416V
B+1 414V
B+2 416V
B+3 410V
B+4 409V
B+5 407V

Plate voltages:

V1 pin 1/6 416V
V2 pin 1 404V
V3 pin 1 405V
V3 pin 6 403V

AC voltage in: 237V

How much can I expect the B+ and plate voltages to drop under load (with tubes installed)? Are my voltages without tubes reasonable, or too high?

Are the power tubes red plating due to parasitic oscillations? What measures can I take to identify, locate and remedy the problem?

didit · Post by **didit** » Mon Jun 12, 2017 11:51 am

shipwreck wrote: ↑Mon Jun 12, 2017 11:14 am How much can I expect the B+ and plate voltages to drop under load (with tubes installed)? Are my voltages without tubes reasonable, or too high?

The right zone. Secondary is 270-300V. Without tubes an unloaded capacitive input, so multiply by 1.414 (sqrt(2)).

Are the power tubes red plating due to parasitic oscillations? What measures can I take to identify, locate and remedy the problem?

Could be. Deserved reputation that Express is naturally on the edge of stability. Some details on the build? Pictures would almost certainly help. You should be able to install just power tubes and bias up without red-plating. You neglected to include your bias circuit voltages.

Best .. Ian

shipwreck · Post by **shipwreck** » Mon Jun 12, 2017 12:56 pm

Some details on the build? Pictures would almost certainly help. You should be able to install just power tubes and bias up without red-plating. You neglected to include your bias circuit voltages.

Best .. Ian[/quote]

Thanks for the comments, I will put some pictures up shortly. I am fairly new to amp building, and I am not sure where to measure the bias circuit voltages. I have bias test points installed, but I am waiting for some 1 ohm bias sense resistors before I can use these. Could you explain where I should measure the bias?

didit · Post by **didit** » Mon Jun 12, 2017 1:03 pm

shipwreck wrote: ↑Mon Jun 12, 2017 12:56 pm Could you explain where I should measure the bias?

Referencing the TAG schematic, junction of C22, R30 & R31; and if you cannot adjust to -33V, the voltages on other side of both R30 & R31.

shipwreck · Post by **shipwreck** » Mon Jun 12, 2017 2:13 pm

I get 0.0 voltage reading from the junction you mentioned to ground.

I get red-plating starting up with just the power tubes.

shipwreck · Post by **shipwreck** » Mon Jun 12, 2017 2:28 pm

On the other sid of R30 I get -9 VDC, on the other side of R31 0V.

joeboo88 · Post by **joeboo88** » Mon Jun 12, 2017 3:45 pm

Hi Shipwreck: what are those 2 resistors under the yellow alligator clip? they kinda look like 1 ohms? which should be on pin 1 and 8 - to ground on the el34's and a connection to the bias test ports
I just looked at the layout on the DIY forum, and didn't see any resistors where you have them ( under the alligator clip).

I hope this helps

shipwreck · Post by **shipwreck** » Mon Jun 12, 2017 4:43 pm

joeboo88 wrote:Hi Shipwreck: what are those 2 resistors under the yellow alligator clip? they kinda look like 1 ohms? which should be on pin 1 and 8 - to ground on the el34's and a connection to the bias test ports
I just looked at the layout on the DIY forum, and didn't see any resistors where you have them ( under the alligator clip).

I hope this helps

Those are R23 and R24, located on the power supply board instead of on the can caps. Drain resistors, I think?

YouTube · Post by **xtian** » Mon Jun 12, 2017 5:05 pm

Please remove your power tubes. Power up and measure DC at pins 5 on the power tube sockets. That's where your negative bias voltage should be. Adjust the bias pot on the rear panel for maximum negative DC. You should be able to get -30 or -40ish volts.

shipwreck · Post by **shipwreck** » Mon Jun 12, 2017 5:22 pm

xtian wrote:Please remove your power tubes. Power up and measure DC at pins 5 on the power tube sockets. That's where your negative bias voltage should be. Adjust the bias pot on the rear panel for maximum negative DC. You should be able to get -30 or -40ish volts.

Hello Xtian, I went to ampgarage to get more eyes. Good to see you here as well. I get 0 volts on pin 5 of either tube, something is obviously wrong with my bias circuit, but what?

shipwreck · Post by **shipwreck** » Mon Jun 12, 2017 5:35 pm

I get -9 V DC after D11. Before R29 and the rectifier diodes I have 598-600V AC, right at my meter's limit.

Measured R29 to 220k, as called for by the schematic. And R31 to 15k.

Could the bias capacitors be faulty, or the wrong value? They are marked 10mf 100V.

didit · Post by **didit** » Mon Jun 12, 2017 5:42 pm

Cannot see the bias pot. The circuit grounds off that. Perhaps that's missing? Definitely keep the power tubes on the bench until you have correct negative bias voltage.

Updated: I see it now, on chassis back panel; and appears correctly wired. Bolt and post perhaps your capacitor ground? Wired to the pot or that common ground is recommended.

sluckey · Post by **sluckey** » Mon Jun 12, 2017 5:45 pm

Resolder R29 and D11 from the component side of the board. If that doesn't fix it, replace R29 and/or D11.

didit · Post by **didit** » Mon Jun 12, 2017 6:06 pm

Check for continuity to ground from the PCB trace wiring positive leads from two 10mF electrolytics. My guess is that it's floating.

shipwreck · Post by **shipwreck** » Mon Jun 12, 2017 6:38 pm

didit wrote:Check for continuity to ground from the PCB trace wiring positive leads from two 10mF electrolytics. My guess is that it's floating.

I have continuity to ground from the + side of those capacitors.

Reflowed all solder joints in the bias circuit. Still -9V at the junction of C21 and R30, and 0V at the junction of C22 and R30. I measured R30 to 15k, so maybe it is C22 that is faulty? Is -9V after D11 an expected measurement?

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