Speaker question ?

[Roly]

Good to hear.

Tonestacks are a rich and wide open area for the amp tinkerer.  Most of them have an insertion loss of around 20dB so they are pretty interchangeable, the most common limiting factor being the number of holes in the front panel (heh heh).

I changed the grid resistor from 18K to 1.4K that was the lowest value I had. I also added another resistor in parallel to the 560K plate resistor 510K, it should be roughly half now.

I hope you mean "cathode" resistor down to 1k4.

As original the Mic input stage uses "contact" bias and will be overdriven by a guitar signal, so it needs to be modified to use more conventional cathode bias.

You will find a table of typical values for use with a 12AX7 here.

The two most common in guitar amps are;

    100k anode load and a bypassed 1k5 in the cathode for a gain of 52, and
    220k anode load and a bypassed 2k2 in the cathode for a gain of 59.

While these are for a nominal 300V supply they should be okay down to at least 200V.  The main tweek is the cathode resistor value to get the resting anode voltage somewhere around 1/2 to 2/3rds of the stage HT supply.

Normally the cathode resistor would be bypassed by a low voltage 25uF cap for full bandwidth gain, but as you are finding it a bit bassy a tweek is to reduce the value of this cap.  In some amps it is reduced very considerably to make a "lead" channel, perhaps as low as 0.1uF, but feel free to experiment to taste.  The cathode voltage is normally only a couple of volts.

The grid resistor should be reduce to between 1Meg and 2.7Meg (see below).


Assuming you are currently connecting your guitar to the "Phono" input; this has a 1meg pot which will define the input impedance up to a setting of 7 or 8/10 and this should be satisfactory for just about any guitar.


I have done some experimenting with amp input impedances (resistances) and found that one meg should be the lowest you should go with a passive guitar (one without a battery inside), going lower tends to make the pickup dull, and that increasing the input resistance to 2 or 3 megs can give and improvement by unloading the pickups and making them more "lively".  You could try inserting a resistor, say half to one meg, between the Phone input and the volume pot, i.e. in series with the input, not across it.  This will reduce the signal level by half and present the guitar with a high input resistance.  You will find a detailed discussion here.  This does not apply to guitars with active pickups, those powered by an internal 9V battery which are already unloaded by an internal preamp/buffer.

HTH

[SpareRibs]

Hello,
      Yes Roly I did indeed mean the cathode resistor. I used the jack on the mike input but I bypassed the thing that looks like a tube socket. I wired a 68K resistor, a .01 cap  both in line directly into #2 on the 12AX7. Coming out the #1, the (Fender Tweed Deluxe) tone stack into #1 on the 6AU6. I used the existing potentiometers. I also left the 330K resistor on the tone control wiper I think it may have remove that to let the signal pass into the 6AU6 with less resistance.
       I got all of that done, it still sounds a bit tubby. Tomorrow I am going to try different values of cathode bypass capacitors, currently there is a 25uf - 25v. I am going to try 10, 5, and 1uf. I think I will start at 1 and work up, as I already know 25 is to much.
       It is going well. I think most of you guys could have done this in an afternoon, however I have gone
this far without any clouds of smoke

[Roly]

Since you have moved away from the original circuit may I suggest that you at least draw and post the bit that you are modifying so we can better follow what you are doing?  TIA

[SpareRibs]

Hello,
      The Fender Tweed Deluxe tone and volume circuit is in (Converting An Existing Amp Chassis For Guitar Use by Niels"Petco"Nielson) Pg.7. I put it between the first half of the 12AX7 pin #1 and pin #1 on the 6AU6, also I properly grounded it.
      I could not find a way to bring both drawings together. I was wondering why you would want it posted, but now I can see there are no two alike.
      I would also like to ask could that treble circuit be eliminated ? It would only constitute cutting one wire at pin #5 on the 6AU6. It seems like it is out of the circuit and only makes obnoxious noise.
      I hope this helps. Thank you for your continued intrest.
     

[Roly]

Typical triodes
http://www.ozvalveamps.org/rcatriodes.htm

...more...
http://www.ozvalveamps.org/ampstages.htm#inputstage

Typical triode guitar input stage



Fenderish tonestack and gain recovery post amp



Both the pot and the cap in the top cut has to be suspect, and it's not what we want anyway.  The idea is to "tune" or tweek the bandpass and controls to be more guitar amp-like.  Fresh hardware.

A typical limitation at this point with small PA's is that there is only one hole for a tone control, so the Big Muff single knob is worth a look (In Tone Stack Calc).

[SpareRibs]

Hello,
      I am going to incorporate your second drawing into the schematic and post it back later today.The PA has three holes in the front, another one could be utilized if I dumped the power on light. I don't know a lot about uploading and downloading things, but I will struggle through it .
      I didn't think the Fender tone stack would be sufficient as a Fender Deluxe is kind of a crude amp to start with. I actually wanted a tone stack with mid range but I didn't know where to look. However I will worry about that later. The first thing is to iron the bugs out of this.
      Thank you once again for your time and effort.

[SpareRibs]

Hello,
      I hope this works. (GREAT), are we on the same page ? I think I put it in the right position. I had never thought of putting the tone stack in front of the 12AX7, Duh !!!! Maybe that's why they call it a preamp.
      I made one mistake, on the cathode resistor of the 12AX7 I marked it 1.5K, but I know it is 1500 as per the schematic. Thank you for the diagram. I am going to start on it right away.

[Roly]

1k5 == 1500 ohms!


Input -> 12AX7a-> (Gain)-> tonestack-> 12AX7b-> Master Vol-> PI-> OP->

[Roly]

The attached circuit is unfinished, potentially full of value errors because the original was so lo-def; hasn't run yet in LTSpice because the OPT is still unknown - we need to know the inductance and resistance of each winding section.

It also already departs from the original because I didn't see a lot of point in redrafting it when we have a rough copy and a sample of the beast, so it's what I'd be thinking of doing (remembering that the PI may still have a couple of "issues").

A bunch of voltage measurements, supplies, anodes and cathodes, warm idle and normal programme drive, would be a great help.


Also...
http://www.r-type.org/pdfs/6au6-1.pdf

[SpareRibs]

Hello Roly,
      I have spent a lot of time pondering over the diagram you posted. I do not know how to check all of the voltage measurements of supplies, warm idle, or normal program drive. I intend to incorporate all of the changes to the left of R6.
      I have one question at this point.  Coming off the plate of the 6AU6, (on the original schematic) there is a resistor immediately to the right, a 560K that ties the 6AU6 plate to the plate of the second half of the 12AX7. Is that connection to be eliminated ? As on the drawing you posted it is not there. I want to be sure before I start tearing things out. I will have both schematics and make notations as I go along. So in the
worst case scenario I will know how things were before I started
      Thank you for the effort you expended in creating the drawing. Also in case no one else told you yet,
                                                            Happy New Year !!!!!

       

[Roly]

Appended cct 0v2 annotated showing:

Red - main HT feed to output stage
Yellow - screen HT feed to OP stage
Orange - HT to PI and preamp
Green - signal flow

Master vol-> 6AU6 -v----------> 6V6a
                            '-> 12AX7-> 6V6b

The output of the 6AU6 is attenuated by R4 and R5 and then inverted by the 12AX7 stage, the drive balance set by the attenuator trim, R6.


Can you go over this cct for values errors, and tell me what you actually found in the chassis, TIA.

[SpareRibs]

Hello,
      Yes Roly, I will post those readings as soon as I finish cobbling it together. I should have it done sometime
tomorrow. I will also give you the readings of the 6AU6.
       Should the guitar be plugged in, or will that matter ?
                                                                                                                           

[Roly]

Normally initial standard test conditions are with the input shorted.  In most guitar amps, but not all, the input jack socket already takes care of this, otherwise it's a clip lead or shorted plug.

For signal tests something like a cassette player or MP3/WAV player will serve as a satisfactory signal generator, particularly if you have pre recorded a number of test tones and sweeps in something like Audacity.

Tone tests with a guitar follow when you have the mechanics sorted, then you get down to fine tuning/tweeking.

First we apply power for a smoke test.

Then we look at the DC conditions of each stage, starting with the output pair, and working back to the input.

HT1,2,3, every anode and cathode voltage from output to input.


Then we take Berlin.

[SpareRibs]

Hello,
      OK, I'm on it will post results later today.

[SpareRibs]

Hello,
      I spent most of the day scratching my head and soldering. I am almost done. I have to make another hole in the front panel for the gain control. That is going to regulate the value of the 25uf bypass cap correct ? I
removed the pot for the treble control (original schematic) was stacked on top of an on and off switch. I just
removed it since that treble circuit was going to go anyway, and replaced the rotary on and off switch with a toggle switch
      Now I have the on and off toggle, bass, treble, and master volume. That makes it cleaner and the pots are the proper value. When I get the other hole for the gain everything will be arranged in better order.
      OK as soon as I complete all of this tomorrow I will do the readings. I thought I was going to get it all done today (Ha Ha Ha) !!!!
       Thank you for all your help so far. I could have never imagined doing all of this on my own. I didn't know
where to start or what to do.