Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J Rindt on December 15, 2016, 02:54:56 pm
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My Neighbors band, guitar player, has one of these.
FWIW, from time to time i like to mention.....I Do Not Play Guitar. I am a washed up drummer that fell into "amp repair".
Anyway.....the head came to me with weird problems. Both channels were Loud/Noisy when either channel was at Zero...cleaned up when the Pre/Post knobs got to 3-4, then got Noisy again toward max rotation. So for about 50% of the middle of the pot rotation, the amp sounded "fine".
Chop-Sticking revealed the Loop Jacks were not always closing (common, right.?)
So i cleaned and exercised the jacks...so far so good.
This is, or can be a High Gain Head, so it can be pretty "noisy". How much is normal is beyond my experience.
I started to scope just the clean channel. I put 1 kHz at 200 mV into the amp. It goes into and out of V1a cleanly, and gets amplified cleanly.
Right after V1a, headed toward V6a, it hits C51 and R19. At that point (in round numbers) the signal is clean and 9.5V P to P.
After it crosses C51/R19 it is much dirtier. Still a sine wave, but with lots of little symmetrical, saw-tooth noise on the wave, and the amplitude drops to 150mV.
Is that "normal" for this amp.?
This is before it hits the Pre/Clean Pot and with the Crunch Button off. I am assuming the Buttons and relays are as the schematic shows. There is a lot of stuff (to my eye) in that Crunch Circuit, and i do not fully understand how it functions.
Sorry for the long post. I hope you get the gist of my question.
Thank You
http://el34world.com/charts/Schematics/files/peavey/5150-2.pdf (http://el34world.com/charts/Schematics/files/peavey/5150-2.pdf)
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After it crosses C51/R19 it is much dirtier. Still a sine wave, but with lots of little symmetrical, saw-tooth noise on the wave, and the amplitude drops to 150mV.
Is that "normal" for this amp.?
swap out c51 n see. in the pre circuit you can unplug the "next stage" tube until it's needed. this sorta knida helps if you have interactions 'tween stages. once you clean up V1, install and move to V2 sorta progress.
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That certainly makes sense.
This is a Peavey and is PCB. This might be a major task to get at the bottom of this board, but i will give it a shot.
Like i say, all these parts get to be more than i can figure out, but.....is C51 bypassing higher frequencies around R19, based on where that pot VR1 is set.?
Thank You
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Oh Man...talk a bout chasing my tail.
I have a 70 Dollar Audo Generator that is about the size of a calculator.
That was my problem. Fortunately or Unfortunately...it got progressively worse. A slight touch of the amplitude knob set off some real bad distortion, that is how i first noticed it. It was intermittent at first, but now it is unusable.
Time for something better.
Thanks Again
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That was my problem
I had a HV probe send me in circles for awhile! When my 35yr old function Gen took a dump I found a software one that has worked well for me over the past 3 yrs, think I paid $30