Need amp tech advice plz!

mayfly

Senior member
Messages
8,309
=CB= said:
Patrick from Davis said:
Sorry, forgot to mention that the schematic I put up there has nothing to do with the amp that started the post.  Just an example of the 68k resistor in question...
Patrick

Well considering how 12AX7's dont have screen grids... no wonder they took it out!

That 68k resistor is usually called a grid resistor or grid-input resistor or isolation resistor.  It serves two purposes really, or maybe three. What it does is provide a constant relationship between the grid input, pickup (or mic) and the grid load.  This is important in many designs in order to maintain bias on the input stage, depending on the pickup used.  In practice it isolates the grid itself from the pickup.  The second thing it does, is isolate one input from another, such that the instruments controls don't interact (too much).  You will still get some interaction if say... you have one guitar played with max treble cut and the other not.  In that regard, its a mixing resistor as well.  The third thing it does, maybe... is provide some protection in the rare case of grid shorting to plate.  This "has" been known to happen, and of course, when it does... you get B+ on the pickup for a brief moment right before it blows the windings..... a capacitor input would be great protection, but there are other problems with that... however the resistor input does offer some degree of imperfect protection under such conditions.

But its no screen grid resistor.

Another reason for this resistor is that it interacts with the capacitance with the grid of the first input stage to create a low pass filter.  The knee of this filter is in the RF range, which should prevent you from picking up the local police radio.  It also aids in the stability of this stage by moving the pole so as to not create a positive feedback loop and thus oscillation in this stage, again up in the RF range.

Having said all that, I'm glad that the amp is working for you.  BTW - how does it sound?
 

Patrick from Davis

Senior member
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2,197
CB, thanks again for the info.  I only have one input on my lil homegrown so I never had to ponder the tone knobs from two instruments not playing nice with each other.  It does make sense that you could get signal sharing going to ground in that manner without a little circuit help.  As Mayfly put it, I have the resistor there so that my amp doesn't catch radio signals.  I caught a bad case of "Coldplay" one Saturday afternoon with my amp input, and it convinced me to add the resistor.  The guys that deal with that model/project looked at it as a gain cut for protection against radio.  So by adding it, it was a speed bump that stopped the small radio signal.  If that makes any sense.  It also make sense after reading your notes that it could be a sacrificial lamb in the pathway of B+ from a shorted tube.  Burn up the resistor not the pickup.

Tfarny, nice to hear that you figured out what was plaguing your amp.  The low wattage amps are a blast to play with and surprisingly loud.  My first foray into the idea was the 0.5 watt Zvex amp.  According to Zack, Malcolm Young has two, one at his house in Scotland, and one at the house in Australia.  I decided at that point to look into smaller amps, and I have been impressed by the cool number of options and sounds that are available.  I then started mucking around in the build your own scene from there.  I do understand the feeling you had when you got nothing out of it.  That happens and then it is time to start the learning process.  Like most things, you learn a more by making mistakes.  Unlike most things, tube amps can really kill you just by touching the wrong spot.  But, if you have the thing working and you know to take care, then get the wiring diagram and start reading up on the mods that you can do.  You will start to get a hand hold on a lot of what is said and then you can dig up info on your own when troubleshooting/modding.  If you are going to get some more Tubes for it, I always go with the Tube Depot because of how easy they make it to get what you need.  Have fun with the amp
Patrick

 

Superlizard

Senior member
Messages
2,514
"sovtek 12ax7wb"

Some of the worst-sounding preamp tubes I've ever heard.  They impart a very harsh, sterile and fuzzy top-end.  Not known for longevity, either.
 

-CB-

Senior member
Messages
5,427
Must have gotten some bad ones... as they tend to tame a top end in almost every amp I've tried them in.... especially a very brittle sounding Super Reverb that I have here
 

GoDrex

Senior member
Messages
3,619
my BH has a JJ power tube and a Electro-Harmonix preamp tube. Sounds good to me. Lots of clean head room and really loud. It's got a great clean tone, IMO. My strat sounds especially good through it - though the LP sounds great too. Also sounds great with an overdrive.
 

tfarny

Senior member
Messages
4,481
All interesting stuff. I'm buying three preamp tubes to play with: EH 12AX, Tung-Sol 12AX7, and JAN-Phillips 5751. I'm really curious to see what the 5751 does in this amp, it's supposed to have less gain and more clarity. Maybe I'll try the sovtek 12ax7wb again someday, but since I got a bum one, I thought I would give other companies a try.
 

-CB-

Senior member
Messages
5,427
FWIW, I've never been impressed with the Philips tubes, esp the JAN ones (military dispose-all crap that they are).

Some of the best tubes I've ever seen, bar none, are the EI Serbian 12AX7's.  But, they were noisey if you didn't hand pick one.  But talk about smooooooooooth breakup and not all farty or raspy when they did.  Just perfection.

The 12AX7WB has been, in the past, a good performer on higher gain circuits that get raspy, or on overly bright amps (like that Super Reverb I have).

I've also had good results with the 12AX7 sprial from EH.  Not overly great in tone, but quiet as can be, and good as a front end tube where any noise is going to get amplified in in later stages (hint: put noisy tubes at the arse end of the signal chain).

=CB=

 
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