Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ledvedder on August 24, 2022, 08:41:07 am

Title: Voltage drop with power tubes installed
Post by: Ledvedder on August 24, 2022, 08:41:07 am

Hi guys,
I'm troubleshooting a 1987/2204 that I've built. I used a Modulus plexi kit, but I'm building it to 2204 spec. For the power section, I've used Ceriatone's 2204 wiring layout. I'm getting a significant voltage drop when power tubes are installed. I've recorded the readings on all el34 socket pins. The top values are without power tubes, with preamp tubes. The bottom values are with both power and preamp tubes installed. I'm getting roughly the same readings as pins 2 on pins 7.

Any ideas where I should start?

Title: Re: Voltage drop with power tubes installed
Post by: Cirrus on August 24, 2022, 09:56:12 am

Do the readings change if you pull the phase inverter? Just wondering if there's any oscillation that might cause the readings to be off...

Title: Re: Voltage drop with power tubes installed
Post by: pdf64 on August 24, 2022, 12:45:40 pm

Light bulb limiter in circuit?

Title: Re: Voltage drop with power tubes installed
Post by: Ledvedder on August 24, 2022, 01:08:52 pm

I'm guessing the light bulb limiter was causing issues with my voltages. Now I'm able to get power tubes biased to 32 and 36mV respectively. Plate voltage is then 332v.

Now I'm getting a super annoying hum and a faint high pitched squeal through the speakers that I'll have to figure out.

Title: Re: Voltage drop with power tubes installed
Post by: pdf64 on August 24, 2022, 05:24:43 pm

The bulb wattage you had in was fine for ‘no valves’ testing. But higher power is needed for the next stages, testing it with valves, and then under load.
Once the build and valves have then proved themselves good, it’s safe to continue testing / troubleshooting without light bulb limiter.

Title: Re: Voltage drop with power tubes installed
Post by: Ledvedder on August 24, 2022, 10:37:34 pm

Yep, I got all voltages up to acceptable values after I removed the bulb limiter. I didn't realize it would cause that much of a drop. I'm getting around 432v on the plates. Now I'm troubleshooting other issues with the preamp. I'm getting a hum that increases with the MV, but no guitar signal.

Title: Re: Voltage drop with power tubes installed
Post by: tubeswell on August 25, 2022, 04:42:43 am

Take VDC readings on all HT supply nodes, plates, and cathodes of all tubes, and report back.


And do a pop test on all grids and plates

Title: Re: Voltage drop with power tubes installed
Post by: sluckey on August 25, 2022, 05:18:10 am

Quote from: Ledvedder on August 24, 2022, 08:41:07 am

I used a Modulus plexi kit, but I'm building it to 2204 spec. For the power section, I've used Ceriatone's 2204 wiring layout.

What could possibly go wrong with that plan???   :icon_biggrin:

I suspect a wiring error.

Title: Re: Voltage drop with power tubes installed
Post by: Ledvedder on August 25, 2022, 10:53:36 am

I posted wrong information.