valve amp pa question

[9 volts]

Hey there, Im trying to change a 25 watt valve pa to suit guitar. The first stage has a 1.8k resistor from cathode to ground and a 2.2 m resistor from ground to grid.When I connect my jack to grid and ground it's really loud. Looking at the 18 watt schematic, should I change the 1.8k to something like a 820r and the 2.2m to 1m. Will this reduce the volume of the first stage?
Many thanks......
ps proceeding with care

[Sir H C]

I would keep it as is.  You don't say what tube it is there, but I assume a 12AX7.  Usually the 820 ohm is low for most applications (fender jumps the two cathodes and uses an 820 ohm resistor, but here the resistor is really 1.6k per side).

To reduce volume, cut the capacitor on the cathode to ground, if one is there.  If not, to lower the gain, increase the cathode resistor value, up to about 10k max.

[9 volts]

Its a 12ax7. Thanks there is no cap so maybe I'll try just lifting the 1.8 up to 10k. The original mike connection was through a screened isolation transformer which has been disconnected so I'm just adding a 1/4 inch jack for guitar. Guitar input to grid, ground to cathode resistor...yes? I've been doing alot of searching on line but limited info. Thanks you guys are Gems!

[Ronsonic]

Why change that to 10K? The 1.8K is probably fine, unless you want to get rid of that nasty gain. Actually it mostly depends on the plate resistor, what value is that? If it's the usual 100-220K then you'll be wanting a cathode R in the 820 - 2.7K range. 10K is for amps that should never distort.

You'll probably want to add 10-68K in series between jack tip and grid. The 2.2M to ground is fine.

If you want to reduce the volume wouldn't you just use the knobs on the amp and/or your guitar?

Ron

[9 volts]

The plate resistor is 220k. I tried adding a 68k resistor. It reduced the sound level too much and seems to has introduced some hiss also. But this is good news as the volume dropped. I'll try adding a 10 or 22k instead. The reason I have to do so is that it was so loud the sound was coming through with the volume at zero. at 'one' it  was really loud. (i want to overdrive this a little so, I'm leaving the 1.8k in there for the  time being). What are the advantages/disadvantages of adding a resistor to the input/grid as opposed to altering the cathode resistor? Thanks again!

[Sir H C]

The grid will load your guitar and keep the highs a bit better (lower system impedance requires more parasitic cap to kill highs).  The second resistor (when no cathode cap is used) sets your gain in conjunction with the plate resistor.

Since the grid takes on current the current in the plate and cathode resistors is the same.  Assume the tube is perfectly huge gain, the the gain is the plate resistor/cathode resistor.  Increase cathode or decrease plate resistance and you decrease gain.

All this goes out the window with a cathode bypass cap which in effect shorts that resistor out setting the gain to whatever the tube can do.

Why not in this case just put the cathode to ground?  Biasing.  That cathode resistor sets everything happy.

[Ronsonic]

If there's too much sound coming through with the volume at zero, then that's a different issue. Usually that's a bad ground and an additional source of noise. If adding a resistor in series from input jack to grid adds noise, then I'd consider that an issue with how it was added / shielding/ grounding all like that. Might have to do a bit more reassembly to test it fairly.

Have you done the routine maintenance on this thing; clean pots, resolder skitchey joints, contact clean and resecure pot-chassis grounds and all like that? You may need to give yourself a cleaner slate to work from.

Ron

[9 volts]

The hiss increases with volume. All connections look OK. After reading RG's article on trouble shooting valve amps I've decided to replace the plate resistor and the decoupling capacitor which goes to the first valve (8uf), as the hiss increases with volume and is present when nothing is pluged in. I'll leave the 68k resistor in there until I get this worked out as it keeps the input signal quiet. I've used shielded wire for the input. Keep you posted on the outcome. Thanks again

[Ronsonic]

Is this noise apparent with the input shorted? Do you have a shorting jack for the input?

If there's that much noise with the input shorted then you do have a problem.

Ron

[9 volts]

The jack I used has an integral switch which contacts the signal prong when the plug is removed, same hiss when no instrument is connected. I remember that the mike input hissed when the volume was up before I put it in. I tried another 12ax7 which had the same result. There is also a phono jack that comes in at the second stage which doesn't have the hiss so I'm pretty sure the problem is the first stage.

[9 volts]

Well. so far I've replaced the caps and experimented with a resistor on the input wire of 33k and 68k. I'm going to put in a 47k next as they both sound good but the 33k brings in a little more hiss (more gain). Any reasons for using or not using a metal as opposed to carbon resistor here. (less noisier?). Also I replaced the two 8uf caps that go to the two 12ax7's. Someone mentioned I could boost these to 22uf. I understand this reduces hum.(not a problem at the moment) Are there any other reasons for doing this?
Finally I reduced the cap that goes to the volume from the first stage to .01 from .047 (I'm reading an article from Geo about converting a pa to guitar amp which encouraged this). After the volume there is a 220k resistor that leads into the next stage of the same 12ax7. If I add a 500k resistor to this in serial world it at gain to that second stage? Thanks again

[9 volts]

Hey I just found this great link
http://www.diystompboxes.com/pedals/tubedummy.html//
which is going to answer alot of my questions and give a greater understanding of what going on. I did my input wire again today with a 56k, bigger coaxial wire and heat shrink wrap and things are looking and sounding good. I'm going to do some more research and reading before tackling the next step of more mods.

[9 volts]

Well. Things have been happening wih the amp project...all good so far, I've been looking into cathode capacitors as a means to 'roll off'. I found a great site with a table that explains which caps to use with which resistor value. The table uses a 100k  to plate resistor. Should I change my 220k plate resistor down to 100k and add a cathode cap?  Seems like thats what most of the guitar amp schematics I look at do this.
At present I'm still using 2.2m grid to ground and 1.8k cathode.Thanks