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Discussion in 'The Workbench' started by NutBuster81, Oct 18, 2021.
Where are your resistors that should be on the jacks? There should be a 1 megohm grid leak resistor running from the junction of the jack hot and the 68K input resistor to ground. This is your audio ground reference and it's more important than you think. It sets the input stage gain and sets the input tube bias point.
To quote a poster from long ago, the function of the input grid resistors is:
They weren't visible in previous photos. Just checking to see that they were there.
Nothing seems to jump out as an obvious problem. Can you tell us what voltage you have at each pin of each tube?
I'll have to get those readings this weekend. I just got done reflow in every connection that looked iffy and no change.
I just don't get why both inputs 2 sounds great but not the inputs 1.
As you turn up volume 1 it doesn't hardly get any louder and starts to pop and crackle in either both high/low #1 inputs when you play notes just seems to me the volume 1 pot might be bad? Also when you dime it I can here a fast faint motorboating with a hissing sound, not loud but it definitely sounds like oscillation of some sort...
I presume that having replaced the OT that you know the primaries are correctly wired?
I'm pretty sure but it's been 4 years since Iast worked on it so can't say for certain. What are the symptoms of the primaries being wired wrong?
Looks well put together.
Only things I see are bigger cap on v1b (as should be 0.0022uF). I like 4nF or 100pF bright cap.
Sounds like a dodgy component or possibly misplaced wire but don’t see why just bright channel.
Jesus, I must be getting old...it wasn't the OT I changed it was the PT!
Thanks I'll look at that resistor. And I was trying out different bright caps and NFB setups thinking something was causing the super thin sound...but nothing worked
But you say normal channel is ok?
As has been stated above we need to start at basics. If you can post a global preamp board shot or two from above. Next a voltage chart of each valve pin (including heaters- I’ve seen issue where v1 was missing the heater on v1b before.
Then we must think faulty components only on bright channel (eg silver micas on pot or bypass on 470k). There was a bad batch a few years ago that caused much head scratching!
Check the green grid wire from the 68k resistors that is under the board and goes to pin 7 on v1. Check and double check those solder joints.
I just checked those solder joints and they appear fine, but I went ahead and resoldered them and put a connector in the Jacks and measured the 4 - 68k resistors and all 4 tested @ 70k
The motorboating is gone but still have a high pitch whine/hum only on input 1 high & low. Inputs 2 high & low still sounds good.
What wattage bias resistor do I need? I have a 33k in there now and my bias won't go below 40ma. I need a 27k or lower but can't remember how many watts.
Nvm on my last question, I found my stash of resistors. Got the bias where I feel comfortable with, but didn't help with my volume 1. As I was biasing it I noticed the screen's on my tester just go blank on volume 1. Volume 2 fluctuates abit but never blanks out.
Got any recommendations on somebody that I can send this to and take a look at this thing? There are no techs around here. I'm officially throwing in the towel.
But if the second channel is fine, there's only a very small part of the amp you need to dig into. It's basically one triode of an ECC83, 1 pot, and a few caps and resistors
Exactly! It was the damn pot! It test fine but I jumpered in a new one and it came to life. That's the last time I ever trust a multimeter that's for sure!