25 watt valve pa bass cut question

[9 volts]

Hey there this is my first post. I'm in the process of restoring a valve pa and then altering it for guitar. I'm trying to determine the value of a blown resistor. It is part of a circuit that connects to a bass cut switch at the back of the head. From the second stage cathode of a 12ax7 the circuit goes through a 470k resistor then when the switch is connected in parallel through the blown resistor and a .047 uf capacitor. This then connects to a 20uf cap and then to the heater (pin2) of the two 6l6 valves. I'm replacing the 20uf electro but I'm wondering what the resistor value may be and two if it would be possible to replace the switch with a pot. Any insight to the circuit valued. Thanks

[Enzo]

And the brand and model of PA is... ?

[9 volts]

It's an Australian brand called awa.

[9 volts]

The model is pa1003z. I can't find a schematic anywhere

[Steve Conner]

Is the resistor burnt so bad that you can't read the value? What physical size and shape is it? Can you post a picture of the burnt item and surrounding guts? I think it's most likely to be a power supply dropper resistor that was taken out by an electrolytic cap going bad.

[9 volts]

picture

the first stripe on the resistor is blue.

[Steve Conner]

Have you got a bigger version of that picture? I can't really see much of anything.

[9 volts]

Sorry about that. This one should be a better size.

[Steve Conner]

Hmm, OK, it looks like the bass cut switch is part of the negative feedback around the output stage, kind of like a presence control, and that resistor is part of the bass cut thing. Still no idea what value it is though, sorry but maybe someone else will.

It looks like a Williamson type design though, which is more of a hi-fi amp. You're probably going to want to rewire the whole thing to a Marshall or Fender circuit, or whatever, so I wouldn't worry too much.

[9 volts]

Thanks. Since this part of the circuit is connected to a switch I've left it off for now. I've replaced the electro caps and a few plate resistors and we are beginning to rock.
I have a 68 k resistor on the input. It's plenty loud enough for now. I've noticed that when I have an over drive on, if I tap the metal telecaster knobs I get earthing buzz. Do I have an earthing problem?
Also if I was going to overdrive the second stage could I replace the 220k resistor that feeds into its grid with a 1 meg pot? (I'm new to this, I've made pedals for a while, restored some valve radios and I'm trying to get an angle on valve amps etc.) Thanks again

[Arthur B.]

It looks like a Williamson type design though, which is more of a hi-fi amp. You're probably going to want to rewire the whole thing to a Marshall or Fender circuit, or whatever, so I wouldn't worry too much.

I thought the Marshall Major as well as some Gibson amps wrere based on the Williamson circuit?

[9 volts]

I've fixed the first problem (bad connection on the input. Can someone point me in the right direction for the second one (how to overdrive the second stage) Much appreciated. Thanks

[Steve Conner]

Did you try it with a Tube Screamer or similar drive pedal? That will overdrive the second stage sure enough

If you want the answer that I think you wanted, then you probably need to add an extra gain stage. It usually takes about three tube gain stages to get things really grinding, when you consider losses in tone stacks and such like. If you want to wail like Van Halen, you probably need four stages.

If it's a multi-channel amp, you may be able to steal a triode from one of the other channels. We can't really help with that unless you trace the schematic and post it.

[9 volts]

Thanks, I've built quite a few overdrives and fuzz's and luv them all. I've experimented with the different ways to saturate a transistor and was trying to apply it to a valve. I'm going to trace the schematic. I suppose I was thinking about a pedal I built which was a three stage jfet based on the marshall 18. Three jfets and a subtle overdrive. My amp has two 12ax7 tubes that feed into two 6l6 tubes. There is a volume inbetween the first side of the first 12ax7 and the second side. I thought maybe adding a higher gain in there may have broken up the sound a little (not like van halen, just a simple blues thing). At the moment it has a very clean sound which is cool also, I'm just trying to get a feel for this stuff. Thanks again.
ps Hey one of my favourite guitarists the late great Robert Quine used a Tube screamer.

[9 volts]

awa valve pa schematic

This has been a steep learning curve for me. After some reading I set about drawing up a schematic for my 25 watt valve pa. I documented it without the added phono/radio input.( these give me 4 extra holes for pots and a place for a standby). I like the sound of the amp as it is but want to enhance it. I figure I'll add a fender style treble, middle and bass tone stack and a post volume pot. (maybe a brite capacitor also). As for placement, from what I've read the best place for the tonestack would be on the pink line between the second and third 12ax7 stage and the best place for the post volume would be either after the tone pot or a dual pot on the green lines between the fourth 12ax7 and the 7025 grid. Any advice on this is greatly appreciated. (eg is this going to create too much gain?)
Also if anyone has advice on the blown resistor value I'd love to hear, I know the first line was blue so it's a 6 something. I figured a 600k but more likely a 6k.
Finally I have an extra space for another valve (eg 12ax7) which I may put in later down the line (good idea?) I've drawn the schematic to the best of my abilty, I think it's pretty much error free
(*third version seems to be getting closer!). Thanks again