Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on November 01, 2015, 03:12:33 pm

Title: Musicman HD-130 Reverb / Bias Problem
Post by: plexi50 on November 01, 2015, 03:12:33 pm: I have (2) EL34 power tubes linked together as a pair off one of the coupling caps that want to red plate.

Yet a bias voltage of -42 is present on all tubes and does not fluxuate.

I can not really make out for certain the bias trimmer voltage number on the schematic.

My max voltage at point (Y) is -43vdc

The left pair of power tubes is not effected. This amp has a 12AX7 PI driver
Title: Re: Musicman HD-130 Reverb / Bias Problem
Post by: plexi50 on November 01, 2015, 03:37:22 pm: I just backed down the bias trimmer pot and i have -31.35vdc.
A nice blue glow and no apparent red plating at -31.35

I could swear i am seeing -45 volts at point (Y) on the schematic
There was a faint bias hum with -43 on the grid of the EL34 tubes

Do i even have an issue now?

Hi power -31.35
Low Power -21.30

Sorry for any confusion. I'm Confused today.
Title: Re: Musicman HD-130 Reverb / Bias Problem
Post by: DummyLoad on November 01, 2015, 03:39:50 pm: check c46 and c47

--pete
Title: Re: Musicman HD-130 Reverb / Bias Problem
Post by: DummyLoad on November 01, 2015, 03:41:51 pm: Quote from: DummyLoad on November 01, 2015, 03:39:50 pm
check c46 and c47

--pete

do you see -46V at point C and +46V at point A on recto board?

--pete
Title: Re: Musicman HD-130 Reverb / Bias Problem
Post by: plexi50 on November 01, 2015, 04:00:49 pm: Quote from: DummyLoad on November 01, 2015, 03:41:51 pm
Quote from: DummyLoad on November 01, 2015, 03:39:50 pm
check c46 and c47

--pete

do you see -46V at point C and +46V at point A on recto board?

I have -44.5V on both point C and +44.5V at point A on recto board

It started to slowly red plate @ -31.35. I raised the voltage until the red plate went away @ -37VDC

C47 & C46 are not saturated or look bad. I suppose changing them wouldn't hurt either. They are 37 years old.

--pete
Title: Re: Musicman HD-130 Reverb / Bias Problem
Post by: plexi50 on November 01, 2015, 04:50:55 pm: I put new caps in C46 & C47. I still have -44.5. No more red plating after raising grid voltage to -37.5

But why was only that one tube trying to red plate. They are brand new tubes. Not that, that means a lot these days.
Title: Re: Musicman HD-130 Reverb / Bias Problem
Post by: Willabe on November 01, 2015, 05:38:42 pm: Looks like the output tube sockets have a few different components on them?
Title: Re: Musicman HD-130 Reverb / Bias Problem
Post by: plexi50 on November 01, 2015, 06:32:56 pm: Musicman amps are built a little differently. They have 1kv ceramic caps and a few diodes on the tube sockets. 700VDC plate voltage.
I have had it on for over 2 hours and no red plating has taken place. Sounds good as well. They are good amps.
Title: Re: Musicman HD-130 Reverb / Bias Problem
Post by: DummyLoad on November 01, 2015, 06:45:20 pm: Quote from: plexi50 on November 01, 2015, 04:50:55 pm
I put new caps in C46 & C47. I still have -44.5. No more red plating after raising grid voltage to -37.5

But why was only that one tube trying to red plate. They are brand new tubes. Not that, that means a lot these days.

that's probably it. hate that scheme to get +/-16V. notice the PCB burn around the zeners. it's not like Vregs weren't available in those days... :w2:

--pete
Title: Re: Musicman HD-130 Reverb / Bias Problem
Post by: plexi50 on November 01, 2015, 07:20:03 pm: Thanks for the help!
Title: Re: Musicman HD-130 Reverb / Bias Problem
Post by: DummyLoad on November 02, 2015, 12:04:23 am: YW, though you had it covered already. :icon_biggrin:

--pete
Title: Re: Musicman HD-130 Reverb / Bias Problem
Post by: plexi50 on November 02, 2015, 05:46:11 am: The new old stimulus for amps/ :laugh: