Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Planobilly on August 10, 2023, 08:40:51 pm

Title: Supro S-6424 start up voltages issues. Need recomendations
Post by: Planobilly on August 10, 2023, 08:40:51 pm

I started the amp with no major issues. The tremolo is not working and I am looking for the cause. I injected a 1000hz signal an there is gain.

I changed the 150 ohm cathode resistor to 330 ohms and reduced the current from 32mv to 21 mv. I ASSUME I need to increase the value of the cathode resistor to reduce current to around 17mv. Yes?

If these voltages stay the same when I find the issue with the tremolo, how should I proceed from there? 372VDC is pretty hot to run the EL84s. The amp will only be use here at home for recording at lower volume and I am not concerned about shorter tube life.


V1 Pin 1 204VDC
V1 Pin 6 151VDC

V2 Pin 1 134VDC
V2 Pin 6 178VDC

V3 Pin 1 310VDC
V3 Pin 6

V4 Pin 7 372VDC
V5 Pin 7 372VDC

Primary has two taps, one at 115 VAC and One at 125VAC
Currently connected to the 115 primary at 339VAC

A at 22uf Cap 378
B at 10uf Cap 368
C at 10uf Cap 287

Voltages when connected to 125 primary 307VAC with 150ohm cathode resistor

A 321VDC
B 307VDC
C 244VDC

V1 Pin 1 232VDC
V1 Pin 6 140VDC

V2 Pin 1 106VDC
V2 Pin 6 not recorded

V3 Pin 1 243VDC
V3 Pin 6 170VDC

V4 Pin 7 was around 320VDC but I forgot to record it.

Sugestions?

Thanks,

Billy

Title: Re: Supro S-6424 start up voltages issues. Need recomendations
Post by: Planobilly on August 10, 2023, 08:49:06 pm

Perhaps less work to read.

(https://i.imgur.com/dPbJnQ7.jpg)

Billy

Title: Re: Supro S-6424 start up voltages issues. Need recomendations
Post by: sluckey on August 10, 2023, 09:01:51 pm

Use the 125V primary tap.

Title: Re: Supro S-6424 start up voltages issues. Need recomendations
Post by: PRR on August 10, 2023, 09:25:57 pm

If your only compliaint is no-trem, why are you fretting about the power stage bias? Unlike some other amps, this design only makes tremolo in one preamp, not the power stage.

Is the tremolo oscillator wobbling as it should? (When un-strapped.)

Title: Re: Supro S-6424 start up voltages issues. Need recomendations
Post by: Planobilly on August 10, 2023, 11:15:36 pm

I mainally wanted to get you guys openion about the voltage issues.
The tremolo is not working. I will find and fix that issue.

I will revert back to the 125VAC primary which is what I thought would be best as Steve has indicated also. The mains here at my house are at 123VAC.

I would like to know what the value of the resistor that replaced the LED. I can not read the color code on the photo of the amp.

Thanks,

Billy

Title: Re: Supro S-6424 start up voltages issues. Need recomendations
Post by: sluckey on August 10, 2023, 11:53:04 pm

Maybe you can read the color code in the attached pic. If not, then download my original drawing. It includes the original schematic with the resistor value.

(http://sluckeyamps.com/supro/board.jpg)

Title: Re: Supro S-6424 start up voltages issues. Need recomendations
Post by: Planobilly on August 11, 2023, 07:46:28 am

Thanks, I see it on the original schematic.

Billy