Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: six el six on August 26, 2011, 10:41:22 am

Title: Boogie 50 Caliber repair
Post by: six el six on August 26, 2011, 10:41:22 am

This amp is passing a little signal. One 6l6 is glowing VERY brightly. It had problems before I replaced the filters. Plate voltage on the power tubes is 50vdc or so below what it should be according to schemo. All diodes check ok w/analog and digital meters. I'm posting the schemo and my voltages. If anybody has the time to look this over I very, very much appreciate it. Thanks.

http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11624 (http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11624)

Title: Re: Boogie 50 Caliber repair
Post by: phsyconoodler on August 26, 2011, 11:01:54 am

Bias supply or screen resistor comes to mind here.If one tube is sucking a lot of current the plate voltage will be drawn down like that.
  Get the bias supply up to par and check those screen resistors.
Your voltages show positive voltages on pin 5 of the 6L6's.Your issue lies there.

Title: Re: Boogie 50 Caliber repair
Post by: stratele52 on August 26, 2011, 03:08:38 pm

SIX el SIX your tube is red platin = danger to destroy it in few minutes.
You have a bias problem OR your tube is short

remove the 2 6L6  and mesure bias negative voltage on each 6L6 socket pin 5 you must read something like - 30 DC volts ( minus 30 Volts DC ).

Power ON the amp without those tubes to  test

Title: Re: Boogie 50 Caliber repair
Post by: Shrapnel on August 27, 2011, 01:42:04 am

Quote from: stratele52 on August 26, 2011, 03:08:38 pm

remove the 2 6L6  and mesure bias negative voltage on each 6L6 socket pin 5 you must read something like - 30 DC volts ( minus 30 Volts DC ).

Power ON the amp without those tubes to  test

According to the schematic, it looks like that bias voltage should be -47VDC, not -30VDC, If it is at -30VDC, it might be too little to keep the tubes from red-plating.

Check pin 5 to ground for this voltage with the 6L6 tubes removed.

Title: Re: Boogie 50 Caliber repair
Post by: stratele52 on August 27, 2011, 05:01:05 am

My answer ( -30 volts bias ) is only useful to find why your 6L6 red platin not to do a proper Bias. The important is to know if some bias voltage is there or not.
If not there , tube probably short.

By the way the schematic -47 volts is ONLY  good for reference AND if you use the same brand name  AND batch of tubes when amp was desingn.
Today the tubes could be not the same for use -47 volts

Title: Re: Boogie 50 Caliber repair
Post by: EL34 on August 27, 2011, 05:07:12 pm

Looked at your voltage chart
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11624 (http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11624)

pin 5 on the power tubes is totally out of whack????

Title: Re: Boogie 50 Caliber repair
Post by: stratele52 on August 28, 2011, 04:39:48 am

You have no bias voltage on pin 5 ?????

Your tube are in real danger to be destroy. Check your bias circuit. Do that without the 6L6 tubes in the amp.

Title: Re: Boogie 50 Caliber repair
Post by: EL34 on August 28, 2011, 06:27:31 am

BTW, I added a few new features recently to the valve recording program.

You can now go back and edit your voltage s
You can also request that files be deleted if they are not needed.

The edit button is under the notes field
http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11624 (http://www.el34world.com/charts/valve/ValveData.php?e=view&f=11624)

Title: Re: Boogie 50 Caliber repair
Post by: six el six on August 28, 2011, 01:17:11 pm

UPDATE:

The main offender was a bad coupling cap going from one of the plates on the phase inverter tube to one of the power tubes. It was completely shot i.e. tested bad with a digital cap meter and an old school eye tube checker. I think I could have caught this by checking for DC voltage on both sides of the coupling caps. I also resoldered the 220uF/63V electrolytic in the bias supply just in case I didn't get a good solder joint the first time. Then I  double checked all the leads I'd disconnected/connected on the circuit board AND I double checked all the solder joints on components I tested out of circuit (the diodes).

I used Illinois caps for the filters. They sound fine. All voltages now read just a little over what the schematic reads + wall AC voltage difference.

It's funny. I loved these Boogies when I was young back in the 80's. Things change.

Thanks for the help from everyone.

Title: Re: Boogie 50 Caliber repair
Post by: sluckey on August 28, 2011, 01:26:16 pm

That could account for no bias voltage on one 6L6 but what about the other? What voltage do you measure on pin 5 of EACH 6L6? If it ain't -40vdc to -50vdc, you probably ain't done yet.

Title: Re: Boogie 50 Caliber repair
Post by: stratele52 on August 28, 2011, 01:36:36 pm

SIX el SIX,your capacitor in phase inverter plate was short ( you write shot ) ? If it was short circuit it could send some B+ voltage to bias circuit and stop the minus 40 volts.

That probably why your tube gloww red

Title: Re: Boogie 50 Caliber repair
Post by: six el six on August 28, 2011, 03:24:56 pm

I just posted the new voltages. Bias voltage is at -44 on both power tubes now. Thanks again.

Title: Re: Boogie 50 Caliber repair
Post by: stratele52 on August 28, 2011, 03:32:39 pm

Good , so the amp will work now ?

Title: Re: Boogie 50 Caliber repair
Post by: six el six on August 29, 2011, 03:33:59 am

It works. I thought Boogies were great back in the eighties when tonal anemia was de rigueur.