Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ernie_jr on January 09, 2011, 01:57:31 pm

Title: 5f6a help
Post by: ernie_jr on January 09, 2011, 01:57:31 pm: I just finished my 5f6a rebuild, took my hoffman board and reassembled it into a new and better chassis. Fired it up, no smoke, etc.
It works, but it is very distorted. tried different O/P tubes and different preamp tubes, no change. Before the rebuild, it worked fine.
Any ideas where to begin looking?
Thanks for your help,
Ernie
Title: Re: 5f6a help
Post by: The_Gaz on January 09, 2011, 03:50:38 pm: Maybe the a wrong value in the phase inverter? I think most would suggest taking you schematic and a highlighter, and tracing through the circuit, checking component values as you go alone. The 5F6A is relatively simple - shouldn't take too long.
Title: Re: 5f6a help
Post by: The_Gaz on January 09, 2011, 03:54:00 pm: This also might be a good excuse to build one of Doug's circuit tracers to aurally find where the signal begins to distort, or and excuse pick up a cheap scope for the same purpose. The method's the same: inject a sine wave into the amp's input, and follow the AC signal from in input jack to output, and see (or hear) where the signal becomes distorted. Good luck.
Title: Re: 5f6a help
Post by: ernie_jr on January 09, 2011, 05:00:09 pm: all tubes are correct and circuit has been traced twice, again this is a board removed from a working and and put into a new chassis, same tubes, same everything.
Thanks,
Ernie
Title: Re: 5f6a help
Post by: ernie_jr on January 09, 2011, 05:06:05 pm: also, want to say the amp is dead quiet at idle, no hum at all
Ernie
Title: Re: 5f6a help
Post by: sluckey on January 09, 2011, 05:08:09 pm: Quote
this is a board removed from a working and and put into a new chassis, same tubes, same everything.
If all the components came from a working amp there must be a wiring error somewhere. Keep looking. It'll turn up.
Title: Re: 5f6a help
Post by: LooseChange on January 09, 2011, 06:01:35 pm: Check your voltages!
Title: Re: 5f6a help
Post by: The_Gaz on January 09, 2011, 07:25:56 pm: Oops, I guess I misunderstood. Sorry. That's good advice about checking yr voltages from LooseChange.
Title: Re: 5f6a help
Post by: Geezer on January 09, 2011, 07:52:54 pm: Quote from: LooseChange on January 09, 2011, 06:01:35 pm
Check your voltages!

And record them & post them here> http://www.el34world.com/Forum/index.php?topic=10653.msg97821#msg97821

Or just list them on this thread........
Title: Re: 5f6a help
Post by: ernie_jr on January 17, 2011, 10:41:41 am: http://www.el34world.com/charts/valve/ValveData.php?e=view&f=10360

see if this works for my volatages.
thanks,
ernie

note: this is using a 5u4gb recto and 807 power tubes, same as i had before
Title: Re: 5f6a help
Post by: ernie_jr on January 17, 2011, 10:49:10 am: also of note, the distortion gets a bit worse after the amp has been on for a while
thanks,
ernie
Title: Re: 5f6a help
Post by: sluckey on January 17, 2011, 10:56:59 am: Your voltages look normal to me, at least there are no red flags. I still suspect a wiring error.
Title: Re: 5f6a help
Post by: ernie_jr on January 19, 2011, 04:15:02 pm: OK i am lost. all wiring is correct, parts are all correct. board used to work in old chassis, using it for bass, very loud, no hum or hissing,
just when I play, it is very distorted, almost like playing thru a fuzz box.
thanks,
ernie
Title: Re: 5f6a help
Post by: plexi50 on January 19, 2011, 04:47:16 pm: When it gets to where you can not find the problem it's time to take out the DVM and check any and all wires and circuit connections for continuity to ruel out any wiring errors or broken connections

Also recheck all the resistor values just for fun. They can open up internally when the amp gets hot after a while
Title: Re: 5f6a help
Post by: DummyLoad on January 19, 2011, 10:50:51 pm: divide and conquer...
use a test lead - attach to the grid of the PI and inject a tone. work your back back.

use your PC for a tone source if you have to - isolate with a .1-1uF 600V non-polarized coupling cap.

would you consider posting some hi-res pics? sometimes another person looking for the error can spot the error simply b/c they have not been fatigued by searching for extended periods.
Title: Re: 5f6a help
Post by: phsyconoodler on January 20, 2011, 10:06:38 am: If it was wired right it would be working.
Those are Doug Hoffmans words and they are true.
You have made a mistake and you need to either show your work up close or correct it.There are a few areas to check if voltages are correct.Output transformer wiring.Screen resistors,input jacks,grounds,cold solder joints,output jacks,grid stopper resistors.It may be oscillating because of lead dress.Swapping an entire board into another chassis is a big job and fraught with potential problems.
Solder has a way of getting into stuff like the interior of pots and can bridge connections under boards and on tube sockets.
Divide and conquer.One way is to separate the amp into small sections with masking tape and make 200% sure that the one small area is right before moving to the next section.
Title: Re: 5f6a help
Post by: bluesbear on January 20, 2011, 03:50:23 pm: I had the same symptom a few months ago when I rewired an old Bassman head to be a 5F6a. It was such a little thing that I couldn't see it till I actually started tracing where the voltage was coming from. Read this post from start to finish because it sounds exactly like your problem:

http://www.el34world.com/Forum/index.php?topic=9805.0

I wouldn't be surprised if you did the something like I did when you moved the board to the new chassis.
Hope this helps.
Good luck!
Dave