Having Tube Amp Troubles

[Zben3129]

So I just finished up building the AX84 P1 amp, and it works but I'm having some troubles.

It sounds kinda quiet to me, about the same as my 1w firefly, when max volume, and the tonestack pots have a significant effect on the volume. The mid pot also causes blocking distortion when cranked.

More troubling, I'm either doing something wrong trying to measure the plate voltage on the el84 or something funky is going on. When I measure the B+ with no tubes it's about 325v, which is good, but when i try to measure the voltage with the tubes in, the meter doesn't give me any reading. If I try to measure it using the 10ADC unfused setting on the DMM, however, sparks fly where i touch the probe to the tube socket pin for the plate. I stupidly held the probe there for a second trying to read the voltage and the first resistor in the power supply started to smoke.

Any thoughts?

[iccaros]

how are you trying to read current? as the 10ADC is a current reading mode..
where are you reading the 325v?
do you have pictures..


to measure voltage, you should, using alligator clips, clip to the + side of the supply cap for each B+ step

do you have pictures of your tone stack, this is very easy to mess up as the schematic shows the order different than people put it on the amp.

[Zben3129]

I just found this old multimeter cuz my good one is.....apparently missing. Been away from DIY for a couple years now and just got back into it.....hadn't had the time or money. Anyways, i wasn't using a 10ADC setting, theres 2 places on the DMM where the + probe can plug in, one called 10ADC unfused and one thats something else. I am trying to read the voltage on the plate, with the common lead clipped to ground, and when the tubes are in, it reads like it does when the voltage exceeds the range selected. I'm on a 600v DC mode however, and the B+ without tubes is 325v, and the amp sounds pretty decent, so i can't understand why the DMM wont give a reading.

I didn't mix up the TMB tonestack, cuz I've made that mistake before so I always double check now. I can take pictures tomorrow, but the wires for the stack are mostly hidden above the pots by a lip of the chassis

[Zben3129]

Been playing through the amp for about an hour now, and it definitely can't be too big of an issue because the thing pretty much works......still having the same problems as before however  can't seem to fix it.

[iccaros]

for Volume, what is the speaker?
What is the audio transformer?
are you doing relative volume? as my VOX AC4TV, which a P1 is very close just drop the full tone stack, is only slightly louder than my firefly with a 12au7 and not any louder than with a ECC99.

I had volume issues in my last build, turned out I had miss wired my volume pots to the wrong taps.. Did not see it until I use a scope to see at what levels audio was at each stage. I first used a voltage meter set to AC voltage to get an ideal.
A bad ground will also cause this issue,
With out pictures or more readings this is the best I can do.. IF you are in the Seattle area you could bring it over and have a fresh set of eyes look at it..

[Zben3129]

I've been running it through a Celestion 30w speaker in a closed back cab. After playing it some more, and comparing it to my firefly, it is definitely not as loud as the 1w firefly. Also, none of the tone controls are really working correctly, and theres really no area of the amp where I can get a clean tone. The amp sounds very tinny and has pretty much no bass. Very harsh and trebly. I'm gonna see if I can dig out my old meter and get some voltage readings, I'm assuming for now that something screwy is going on with the meter I'm using.

[Zben3129]

Managed to take some voltages with the preamp tube in, with no power tube installed, and heres what I came up with:

http://www.ax84.com/p1/AX84_P1_101004.pdf   (pdf of schematic)

B+ - 322
B+1 - 321
B+2 - 317
B+3 - 285

V1 -12AX7
First Gain Stage:
Pin 6 - 176
Pin 7 - 0
Pin 8 - 1.7

Second Gain Stage:
Pin 1 - 93 <---???
Pin 2 - 0
Pin 3 - 0.8

V2 - EL84
Pin 2 - 29
Pin 3 - 0
Pin 7 - 320
Pin 9 - 316



Seems like my problem is in the second half of the 12ax7, which is connected to the tonestack. Plate voltage is way too low.

[PRR]

V1 -12AX7
Second Gain Stage:
Pin 1 - 93  {should be 140}
Pin 2 - 0  {should be 0}
Pin 3 - 0.8  {should be 1.15}
V2 - EL84
Pin 2 - 29 {should be dead-zero}

Tone-driver plate is low, power tube grid is high.... DC is sneaking through.

The EL84 pin 2 voltage changes with Master setting, right?

OK. Your tonestack IS mis-wired.

> The mid pot also causes blocking distortion when cranked.

This clue suggests that C9 is not blocking DC. (If C8 were at fault, the Bass knob would throw more DC sneakage.) Since new caps are rarely shorted (IF rated >300V), I would look for mis-wire or solder-splash.

[Zben3129]

V1 -12AX7
Second Gain Stage:
Pin 1 - 93  {should be 140}
Pin 2 - 0  {should be 0}
Pin 3 - 0.8  {should be 1.15}
V2 - EL84
Pin 2 - 29 {should be dead-zero}

Tone-driver plate is low, power tube grid is high.... DC is sneaking through.

The EL84 pin 2 voltage changes with Master setting, right?

OK. Your tonestack IS mis-wired.

> The mid pot also causes blocking distortion when cranked.

This clue suggests that C9 is not blocking DC. (If C8 were at fault, the Bass knob would throw more DC sneakage.) Since new caps are rarely shorted (IF rated >300V), I would look for mis-wire or solder-splash.

Just clamped the DMM onto pin 2 of the EL84 and started fiddling with the knobs and watching the voltage, and found some relationships.

First, the mid pot has the most noticable effect. While sweeping it, the voltage goes from 0v to 24v, seemingly linear-ish. If the mid pot is turned all the way down, the voltage on pin 2 of the EL84 is 0, no matter what I do to the other knobs.

So far, here is the setup:

MV - fully turned (as in, if playing guitar fully turned would be max volume)
T - Not turned at all
M - Fully turned
B - Not turned at all

From here, I left the mid pot alone and messed with the other pots -
  The treble pot had a slight effect on voltage, fully turning it raised the voltage into the 30s.
  The bass pot had a large effect, swinging the voltage from 24 all the way up into the 50s
  The master volume lowers the voltage as the pot is lowered, down to 0

[Zben3129]

Well, got that problem solved. Too late to play the amp and see if there's any more issues, gotta wait till tomorrow on that one.

After checking my wiring many times, sure I would find a wiring error or solder mess-up or wires touching, I couldn't find anything.

Turns out one of the caps was bad. Either the .022uf or the .01uf was letting voltage through, because I replaced both of them and now there's no more dc on the grid of the EL84.

Thanks for the help guys