Really strange EL34 PP Power Amp....

[Dutch66]

Hi,

after (re)building a few SE (hifi) amps, I just finished building my first all tube
guitar amp. It has a JCM800-like pre-amp, 2 x 12AX7 followed by the EQ with some tweeks for frequency shifting with the mid controle.
Then a switchable fx-loop is inserted based on a single 12AU7, and this all works fine, actually I am really amazed about the result considering this is my first (serious) build.

Now the problem starts with the 2 x EL34 PP poweramp, (see schematic), for some reason its way to loud...... I know it sound strange as everything seems to work rather smoothly.
All voltage values are in line, no overheating, I did multiple checks on the lay-out and consequently the wiring, but once again, everything looks fine.

http://www.uploadarchief.net/files/download/pa3.jpg

http://www.uploadarchief.net/files/download/rt801-1.jpg


However, turning the master up say about 5 % makes my 4 x 12 inch goes
extremely loud, I mean in an unhealthy way.

I lowered the R value from the feedback circuit from 100K to 56K to 33K, in a desperate attempt to reduce the power.

I must be overlooking something here.

Any help, suggestions or feedback is more then welcome

Thanks in advance !

[imaradiostar]

Assuming voltages are normal this thing should make a pretty clean 50 watts and a good bit more than that distorted. Have you compared it to anything else in a similar power range? 50 watts is painfully loud to my ears! I use a THD hotplate with any amp over about 5 watts if I'm trying to get musically interesting power tube distortion going.

You say it's loud but not in a good way- have you tried running a sine wave into the phase inverter input and scoping it or even just listening to it to see if it's adding harmonics or crossover distortion? What about it is bad?

You could always get rid of a 12ax7 and an EL34 and make it much lower power- I built a plexi circuit this way with about 10 watts output for my little brother. It's a tiny amp on a tiny chassis- 11" by 5" and it sounds fabulous and thick into a 4x12.

[Steve Conner]

I agree, 50 watts into a 4x12 is going to be really loud. It was designed to be heard over a rock drummer, before PA systems were invented. When I've played master volume amps at home, I always find I can't turn up the master more than a crack. The higher settings are for gigs only, and even then, sound guys try to stop me turning it up all the way.

If you want to keep the high power capability for use at gigs and jam sessions, then you might want to look at an attenuator or power scaling to tame it for home use. If you're happy with the sound of the preamp without power tube distortion, maybe you could put an attenuator between the preamp and the MV, like the "-20dB" button that some old Japanese stereos had for late-night listening.

If you don't need the volume, then a smaller power stage is probably the way to go. (imaradiostar, what OT did you use for that build with one EL34?)

[MWJB]

270K grid load resistors are a bit on the big side for EL34, 220K are the biggest I'd use, or even 100K.

Your 82K & 100K PI plate resistors are the wrong way round, the 82K should be on the A triode of the PI (fed by tone stack). Did you start off with 22K as your tail reistor, or was this a measure to restore balance?

Why are the EL34 pin 2 grounded? You should have a twisted pair, one leg of the heaters feeding EL34s pin2, the other leg ffeding pin 7?

It's normal to tie pins 1 & 8 together, then to ground via the 1R, rather than have the 1R resistor between 1 & 8 (don't know how much difference this makes?).

That chassis looks way too small to build this amp with a trouble free layout, do you have any pics of the circuit?

"All voltages are fine" OK, but what are they (inc. EL34 plate current)?

Are you sure that your problem lies in the power amp? You seem to have only one channel, what did you do with the spare input triode? Please post preamp schem.

[Robert M. Martinelli]

I agree with imaradiostar and Steve, to the point that I even doubt you have a problem....I had the chance to play and field-test a 50 W 2x12 JCM800 in the early '80s and I remember it to be capable of deafening sond levels. It's not only the power itself, but the dynamics, the harmonic content....50 W out of a tube amp can be terribly loud...

If you measure the plate currents and voltages injecting a sine-wave of the appropriate amplitude at the power-amp's input, then, and only then, you' ll be able to state the actual power fed to the OT; ( the actual power fed to the speakers will be somewhat lower, depending on the losses in the OT ).

Relying on your ear to state the power output makes for inaccurate statements, "loudness" is a questionable concept as it depends on many factors. ( A 25 W amp would be only -3dB away, and 3 dB are considered the slightest sound level variation the human ear can perceive ).

If the amp's output power is too much for your purposes, how about a "Triode operation" switch ?

Hope this helps

Best regards

Bob

[MWJB]

"I agree with imaradiostar and Steve, to the point that I even doubt you have a problem...." You could be right Bob, but given the number of 12A? stages, the tiny chassis, the questionmark over what the 12A? stages do, the fact that not a single voltage or current is listed & finally the fact that Dutch66 states "extremely loud, I mean in an unhealthy way" (I'm assuming that this means that it doesn't sound right, rather than is just loud like 50W amp) - all mean that it's worth having a deeper look before writing it off as a typical example of type.

[Robert M. Martinelli]

Hi MWJB,
I agree with you, that was the reason why I advised Dutch66 to inject a sine-wave and take some measurements to state the actual output power.
Best regards
Bob

[MattT]

Some good suggestions already. Another thing I noticed that hasn't been mentioned is that you used a 22k tail resistor but still have a 470R cathode resistor for the PI. The PI idle current is set by the cathode resistor but all that current has to go through the tail resistor to ground and that current * tail resistance yields a voltage drop across the tail. So when you adjust the tail very often you should be adjusting the cathode resistor too. That's why you often see 470R with 10k, 820R with 22k, and 1.2k with 47k (in Voxes). Yes...some amps did use 470R/22k but the cathode resistor affects gain just like it does in 'regular' gain stage; i.e. lowering the cathode resistor increases gain (to a point).

So you could increase the 470R to 820R and reduce the PI gain a little bit and it might also increase the PI headroom a bit too.

I agree that first thing I would do is put signal into the amp and measure the voltage swing stage-by-stage. Be sure to note all your settings (vol/bass/mid/treble/master).

[imaradiostar]

If you don't need the volume, then a smaller power stage is probably the way to go. (imaradiostar, what OT did you use for that build with one EL34?)

I used a Hammond 125ese. It's too large and I'd say it's overkill for a guitar amp. If I had it to do over I'd use the OT20SE from Musicalpowersupplies.com.

It might be fun and interesting to build the amp with a single ended output section driving a load and use the 50 watt output section to re-amp the smaller power amp.

jamie