I've recently acquired one of the above mentioned amps on the cheap.. and have a set of MM trannies and choke laying around and wouldn't mind tarting the amp up a bit.
So I've purchased the usual array of components for the signal path and hell bent on adding a choke albeit not really required.. Sadly my ability far outweighs my talent when it comes to the finer points of reading schematics. I've built a few amps over the years but we're talking a good ten years since my last so I'm a tad rusty.
Any help in pointing me in the right direction would be awesome!!
first, make sure you're comfortable discharging caps at 450 volts or more.
Second, discharge those caps safely before you get in the guts. There be dragons here. (you did say you've built amps before, but just being cautious.)
Now, it looks to me like the best spot on that schematic is where that 1k resistor is. It's usually one or the other, (choke or resistor) i.e. LC or RC network, but that schematic might need the 1k dropper there. (i.e. between C71 and R120)
It should be between 10 and 20 Henries per the old schematics, rated at 100mA.
I'd think you might want to try somehow putting it after C71 and then into R120, as the choke will likely only be in the hundreds of ohms not thousand. you 'could' just try swapping that 1k with the choke, but it may have too high of voltages on the B+ rail after.
Thanks so much for the reply. The R120 was my best guess also, plus I've seen a post around the traps from Lyle Caldwell who did a similar mod.. but you can never be too careful!! Thought this is the best place to ask and I've been a long time lurker, figured now's as good a time as any!!
The choke is rated at 20H 100ma, so we're in business there and no worries regarding the voltages, I know what I'm doing there at least.
When you say a dropper resistor, I assume you mean a resistor in-line between the C71 and R120? Just double checking I understand correctly..
R120 is a dropper resistor, it's inline with the B+ which means it will lower voltages a small amount across it. If you just remove it and replace it with the choke, the choke will have 'some' resistance but not 1k most likely. (I think you can measure it with a DMM) If you add the choke inline before it, then you keep the general voltages similar, but slightly lower, instead of replacing and raising them a bit.
Roe wrote: ↑Wed Apr 03, 2019 1:30 pm
480R is about right for an ac30
Normally yes but the schematic linked shows 1k dropper, not sure if it will matter, but it's worth considering.
You could put it in w/o the resistor, measure B+ at the anodes of the EL84's and make sure it doesn't exceed limits. Also raising the voltage raises the bias a bit, I believe and Vox tends to bias them pretty hot, like over 100% if I recall and for cathode biased tubes it's best to keep them under max diss But then again, some people claim that's what gives the vox i'ts sound are the hard ridden el84's
