Newbie first amp distortion troubles

[shzmm]

hello all,
this is my first post to this forum and my first tube amp build from scratch.

basically i would appreciate it highly if someone would mind looking over the schematic (sorry that first cathode resistor on the 12ax7 should be 2.7K) and see if there any problems. i "designed" it by taking the majority of an older ax84 p1 schematic and adding the 18watt marshall tone control.
having now built it, it sounds ok until it hits break up which is quite early due to the triode wiring. the problem is a fizzy distortion-- when the input signal is strong there is an ugly fuzz that dies well before the main signal. i would say the frequency is in the lower mids.
so far i have:
changed speaker
changed power tube
changed bias on the preamp and power tubes
brought the heater voltage down from a 7.6 to 5.8 with inverted diodes
increased the resistance into the el84 control grid up 100k
all to no avail.
the only thing i can think of which i haven't tried is reducing the value of the coupling capacitors further. but then why would i be getting too much bass?
is this what blocking distortion sounds like?
i'll try to record a bit of the signal tonight if that helps.
thanks
jeff

[shzmm]

here's the updated schematic and an mp3 of the ugly sound
thanks again

[Fred2202]

Try putting a 220K resistor on the second triode grid to control blocking distortion. It may roll off your highs a tiny bit. You could also try biasing the second triode a little warmer by changing the cathode resistor to 820 or even 680 ohms. That sometimes gets rid of the fizzy stuff and gives a smoother transition into clipping.

[Bruce / Mission Amps]

here's the updated schematic and an mp3 of the ugly sound
thanks again

I wonder if you have your .00047uF coupling cap and the .022uF tone cap reversed.
Also, your drawing has the FWB rectifier wired as a negative power supply... the cathodes of the final two diodes go to the filter caps and the anodes should be grounded, yours is backwards.
I'd use a larger 22K or maybe higher dropping resistor for your preamp tube too.
Supplying +300v to that first stage really gasses up the preamp gain and a little EL84 only needs about 10v-12v of signal to drive it past clipping.
And the 1M grid load resistor on the EL84 could be reduced down to 180K-220K with some benefit in tone

[shzmm]

thank you both for your replies.
unfortunately i am out of town for the next couple days so i can´t change anything right away. however perhaps i could ask a few questions to get myself prepared.
having had a day away and some time to think about it, i think the problem could well be between the two triode stages. i think i´ll try a blocking resistor first, but i think dropping the voltage to the preamp as suggested by bruce is a good idea. i don´t think i´ll be changing to another output tube anytime soon, if ever.
i will keep everyone updated when i get back into it
thanks for the help

[Fred2202]

High preamp voltages can cause harsh sound and fizzy distortion. That 22K dropping resistor that Bruce mentioned should help. I'd shoot for 220 - 250 volts at the B+ rail for the preamp tube. It should take some of that nasty edge off of the sound.

[shzmm]

hello again
back home now and have fiddled around a bit with the values of the cathode resistors and the dropping resistor to the preamp tubes B+ voltages. i also put a 220k resistor after the gain control on the grid to the second stage of the 12ax7
unfortunately, i am still getting a fizzy sound. the distortion changes certainly with change in cathode resistor but there is nothing yet approaching a good sound.
at the moment, i have two questions... when setting the bias on the preamp tube i was shooting for something more around 1.5V yet i read .9V. is this a product of the fact that specs are not exact representations or am i missing something in my calculations?
otherwise, is it possible that something else is causing the bad sound-- for instance the output transformer? i am using old transformers from a tape machine which used el84s as the output tubes.
thanks for any help. included is the new and (corrected!) schematic

[Fred2202]

I would concentrate on the second triode as there is no distortion going on in the first triode unless you drive it with a boost pedal. For smoother distortion you want a voltage reading on the plate to be 50 to 60% of the voltage at the B+ rail. Right now you have about 210 volts at the plate which is about 77% of the 270 volts at the B+ rail. I would lower the cathode resistor and/or raise the plate resistor until you get about 145 - 165 volts at the plate of the triode and see how that sounds. You only have one tube for the preamp so you are not going to really get a really smooth distortion but it can still sound good and crunchy.

[Fred2202]

I also wanted to mention...As Bruce said below you may be driving the snot out of the El84...lowering the grid resistor from one meg down to 470K or 220K may also help with the fizziness.

[shzmm]

thanks alot for sticking with me on this, it's turning out to be much more difficult than i thought starting out!
first i changed the cathode resistor on the second triode. however when i bring that up or down it affects the B+ voltage. so then i put in a 220k resistor from B+. that brought the plate voltage to 190V-- but my B+ has increased to 285V.
my bias voltage is now at .7V with a 1.8k.
if i continue increasing the value of the plate resistor don't i risk losing current?

maybe it is time to start over again with this. i need to understand why the values i am getting are so radically different from what i expected. when i look at the values from the first schematic i posted things seem to be much more in order. somehow bringing down the B+ for the preamp is creating some strange problems.

(i forgot to add that 220k grid resistor in the last schematic, that did help)

thanks again

[MWJB]

5.8VAC for the heaters is too low, go back up to <6.2VAC (6.3 to 6.9VAC is typical in guitar amps).

Personally I'd simplify things, lose the 1K power supply dropping resistor feeding the OT & rebias the EL84 (220R).

Why have you separated the volume & tone controls, why not have both situated between the 12AX7 triodes?

Rewire the power supply with a traditional screen node & filter cap.

What's the layout like, any pics?

[shzmm]

thanks for the reply mwjb you brought up some good points.
the thing with the heater voltage is that i had around 7.6VAC before so i added
a pair of 1n5404 diodes per output which brought it down to the 5.8VAC. i thought it would be a bit higher but i figured it would at least do less harm.

the reasoning behind moving the tone control was due to the origin of the schematic-- i got it from the ax84 website. my thought was that it would allow me to have tone control post distortion, but at the moment i'm not picky.

same with the triode wiring, it was on the website and i figured that i wouldn't need an extra 2 watts as this is for at home use and was interested in the difference between triode and pentode distortion.

i'll be getting a chance to work on it again later so i will try to post some pics, i'm sure there's something there to critique!
i appreciate your response.

[shzmm]

hi fred
so i just did what you suggested, and voila! big improvement... i ended up using 330k plate resistors and didn't play with the cathode resistors much.
the thing i need to figure out for myself now is why the larger resistors increase the B+. i now have around 280V B+ and 150/145V at the plate.
so around 55-60%. regardless the sound is much much better.
i am including pictures and i know it isn't the greatest job of wiring and layout-- running the input, controls and power outside of the chassis!-- but, surprisingly there is no hum and very little amp noise. go figure.
thanks so much for your help it was a great learning experience.

[Fred2202]

I am glad it helped. The 330K plate resistors are a little on the high side but I have seen higher in some schematics..up to 470K. I think the reason the B+ went up (somebody please correct me if I am wrong) is the tube is drawing less current with the bigger plate resistor. If you lower the value of your cathode resistor the tube draws more current and the plate voltage will also drop. In my limited experience too high of a B+ to the preamp tube (especially the first tube) will give preamp distortion with a nasty edge to it. Once you get up around 300 volts or higher...you get a very HiFi like clean sound but the tube clips in a fizzy nasty way when overdriven.