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How would this effect my tube amp?

3/5/2006 6:20 PM
How would this effect my tube amp?
How would this effect my tube amp?  
My 1970 YBA-1 Bass Master head had been worked on by somebody before I purchased it and there's a few things inside that I've noticed that aren't quite right.  
The thing that has me the most concerned is that there's a 10K/1W resistor that's missing.........  
Here's a schematic for reference:  
On my amp, R29(10k/1W) is missing. And the wire that's connected at the junction of R9(100k) and R10(100k) is connected between R28(4,7k/1W) and R29(10k/1W)! Which is also connected to the R22(82k) and R23(100k) junction.  
What is this doing to my amp anyway!?  
BTW, I will fix it, I won't leave it like this.
3/5/2006 8:20 PM

Oh, they just rearranged the B+ for the tubes a little, won't hurt it. You can put it back, or not.  
R8,9 are plate loads for the first preamp tube, so instead of going to the right end of R29 - there atop C15 - they eliminated R29 and moved the wire over to the other end of the resistor.  
So what it does is eliminate the voltage drop through the 10k R29. This means the preamp tubes will have an extra oh maybe 50 volts or so of B+ to play with.  
About the only potential downside would be that the preamp tubes share a B+ node with the phase splitter. Conceivably there could be a decoupling issue at some point, but unless the amp is motorboating, I doubt you'll see it.
3/6/2006 2:38 AM

Thanks for taking the time to respond to this question Enzo.  
The amp's probably been operating like this for many years, I was just wondering if anything was being slowly damaged due to this missing resistor.  
Thanks again!
3/11/2006 5:39 PM
John Culp

It's not uncommon to find production devices that differ in detail from the published schematic. They made running changes in what they put in them for various reasons. Or someone may have modified it later. When you're working on something old and previously owned, you don't always know for sure.

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