Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jcm-jmp on June 20, 2011, 11:15:04 am

Title: Bogner Ubershall
Post by: jcm-jmp on June 20, 2011, 11:15:04 am

hey guys,

I have a bogner ubershall that is jumping in volume level on both channels. It also has a pretty loud 60 hz hum.
and the mids control on both channels seems to cause some un musical distortion when the mid controls are turned up on either channel.

time for a cap job?

Title: Re: Bogner Ubershall
Post by: phsyconoodler on June 20, 2011, 12:11:29 pm

Not likely.Time to find out what the problem really is.Most likely bad grounds or a bad pot,broken or cold solder joint.

Title: Re: Bogner Ubershall
Post by: jcm-jmp on June 20, 2011, 02:47:40 pm

lol phsyconoodler

yeah they hold thier schematics close to home. I have banged my head against the wall on this thing.

I will reflow all joints on all boards tonight.

all pots seem to be good.

the 4 large through board filter caps on the main board test ok now, but were at one point discharching through the metal top of the cap can (found out the hard way ouch!)
i will re attach all grounds while doing the reflow of all the joints on the boards.

I just cant seem to get away from thinking its a cap some where.
there is a non-musical distortion that happens when the mid pots of either channel are turned up to a well ballanced mid level as well.
Its not real audible, but is enough to say something isnt right here.

I also dont get any plate voltage on a couple of plates on the 12ax7's. i will post which tubes it is tonight. i think it is v2 and v4 not sure though.

Thanks

Robert

Title: Re: Bogner Ubershall
Post by: darryl on June 20, 2011, 09:13:59 pm

Quote from: jcm-jmp on June 20, 2011, 02:47:40 pm

I also dont get any plate voltage on a couple of plates on the 12ax7's. i will post which tubes it is tonight. i think it is v2 and v4 not sure though.

Some Bogner amps have unused valve sections. These are probably for factory-installed custom modifications, but they also make the circuit that much harder to analyse ( and then copy ).

Title: Re: Bogner Ubershall
Post by: jcm-jmp on June 21, 2011, 08:15:04 am

Ok I went through the amp last night and found some bad connections on the ribbon cable from the main baord to the efx board. I repaired these and it seemed to rid the amp of the disatortions asociated with the mid pots, but the volume level jummping up and down is still present not as bad though.

the tubes all have plate voltages now (channel switching kills plate voltages)

the volume jummping does not seem to happen on the clean channel anymore either but its still present on the dirty channel.