My First build completed problems (100w push-pull)

[Nephilyn]

Hi All. I have recently completed my first build a 100w push-pull type amp with 3 preamp gain stages a cathode follower,
phase spliter/inverter stage with a quad el34(Electo Harmonix) output stage fixed biased with each output tube biased at 32mA @ 450VDC(plate).
The amp design is a combination of the SEL preamp stage + moded 50w to 100w output stage all from AX84.com

These are my problems:
1. My preamp tubes(12AX7) won't bias more than 1.5mA even if I bring the cathode resistor down in size.
(I need them to bias around 4mA.)

2.The power output/volume of the amp is very low (25 watts aprox.) even at full volume.

Would the OT primary impedence affect power output? (my OT is custom built with a primary impedence of 3200 ohms).




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[M Fowler]

As long as the power supply used was adequate for 100w section and the voltages are good throughout the preamp, PI and power amp sections, the amp sould be flapping your pants in the wind with a good power chord.

So many things can go wrong when building such as the input jack not being grounded or output jacks not being grounded. Pots not being grounded.
Poor solder joints and poor lead dress. Inadequate voltages, etc.

Start from the output section and work your way through the amp with a probe doing the pop test if no equipment is being used. You should be able to hear where the problem is with a guitar or signal at the input with speaker attached.

Don't forget about poor tubes as a source of the problem as well.

Mark

[Phil_S]

I'm just an amateur here. The first thing I want to do is the math. At idle, you've got the tubes at .032A @ 450V. If you have 4 tubes that's only 57W as idle. I'm thinking it's on the cold side and this may be affecting all sorts of things. I think I'd want to get up to about 70W at idle and to do this you will need to move your bias voltage down (less negative).

You don't say what your bias voltage is. I'm thinking it should be somewhere in the mid 30's. If you colder than -38V (like -40) that would probably explain the low power.

As for the preamp tubes, I don't see getting 4mA out of a 12AX7. At 1.5mA you are already over the book spec of 1.2mA (one triode). How did you arrive at 4mA for a 12AX7? If you want this sort of current to flow through a preamp tube, perhaps you should be looking at a 12AT7?

[Tillydog]

1. My preamp tubes(12AX7) won't bias more than 1.5mA even if I bring the cathode resistor down in size.
(I need them to bias around 4mA.)

2.The power output/volume of the amp is very low (25 watts aprox.) even at full volume.

How big are your anode resistors?

100k anode resistors (& CF cathode resistor) are standard on the SEL.

4ma through 100k = 400V!

What is your B+ to the preamp? Presumably about 400V (which won't leave you with much headroom...!)

(I don't understand why you're trying to set the bias of the preamp valves like this, anyway... The SEL has pretty conventional cathode bias resistors: 2k7 / 1k2 / 820R ).

Tone-stack wiring is another area where it's easy to put a foot wrong...

You have got a coupling cap between the input to the PI & the MV control, haven't you?

0.02p

Andy

[John_P_WI]

450v on the plates with fixed bias and an OT w/ 3200 primary and a matched secondary to the load should give around 63 watts out.

[Nephilyn]

Thanks for the reply guys...

@ Phil_S My power tube bias current was calculated like this:

Pa = maximum plate dissipation of tube
Va = anode(plate) Voltage

Pa(max) = 25W (EL34) ; Va = 450V

I(bias) = 0.7*Pa(max)/Va = 0.7*25/450 = 38.9mA

My preamp tube bias was calculated like this:

Pa(max) = 1W (12AX7 Electro Harmonix) ; Va = 240V

I(bias) = 0.9*Pa(max)/Va = 0.9*1/240 = 3.75mA

[Tillydog]

Preamp bias is chosen for the desired operating point, not for power dissipation. If you've changed the cathode resistors from the SEL schematic then I think you should change them back to stock and see what problems you're left with.

another 0.02p

Andy

[Tone Lover]

Im 7 or 8 amps down the road and I still have the same problem somewhere I will always miss a ground, its a lot easier for me to find now that I do it on almost every one. but depending on where the ground or cold solder is on the ground it will do what you are discribing .

Im just giving my 2 cents because this is where I have had problems and what you are discribing has happened several times to me, and can be very frustrating .
Im now pretty good at trouble shooting an amp because of the struggles Ive had in the past. So every time you get frustrated just walk away for a bit it helps.
Thanks Bill
PS. these guys are great with help, get some voltages on your tubes that will help them also

[Phil_S]

You are showing some theoretical math?

Yes, 38.9mA is 70W for the power amp.

Forget about what you are doing with the preamp tube math. They are what they are and leave them alone. Look at the spec sheet and graphs for a 12AX7. 1.2mA +/- is what you'll get and it's not really important as long as you built it to the schematic. See what Tilly said.

I didn't consider your OT at 3.2K. I feel certain that John knows what he's saying about 63W. If you are getting 57.6W (32mA * 450 *4), that is only 10% less and well within tolerances for tube amp calculations.

Back to your original post. You say output is only 25W. That is not consistent with your data, see above. You will get more than 57W when you push the amp with real signal.

What is your real issue here? Not loud enough? Not as loud as expected? Not loud at all? You just really want the math to tell you an answer?

25W is loud enough to get over a heavy handed drummer with the right speaker. 57W is uncomfortably loud unless you are in a huge space or outside. 100W is not even 10% louder than 57W, so that couldn't be the issue, as you can measure the difference, but it is difficult actually hear the difference.

You still didn't give the grid voltage.

[ToneMerc]

You are showing some theoretical math?

Yes, 38.9mA is 70W for the power amp.

Forget about what you are doing with the preamp tube math. They are what they are and leave them alone. Look at the spec sheet and graphs for a 12AX7. 1.2mA +/- is what you'll get and it's not really important as long as you built it to the schematic. See what Tilly said.

I didn't consider your OT at 3.2K. I feel certain that John knows what he's saying about 63W. If you are getting 57.6W (32mA * 450 *4), that is only 10% less and well within tolerances for tube amp calculations.

Ditto, he's also leaving some watts on the table by choosing a 3.2K over 1.7-2K as well.

TM

[M Fowler]

Not enough info in the OP to make any assumptions I didn't think.

Need PT specs and voltage chart to begin with.

[jjman]

The inappropriate biasing of the preamp stage could make that stage clip before the output clips, preventing the output from reaching its full output potential.

Wattage is not a starting point for calculating appropriate bias of a 12ax7.

[Structo]

These guys all know more about this than I do but I agree about leaving the preamp alone unless there is an error there.

Most amps follow the 67:1 rule on the plate to cathode resistors.

Like 100K/ 1K5 or 150K/ or 2K2 220K/ 3K3

That puts the tube in the center of it's design.

What you can alter though is the dropping resistor that feeds the plate resistors.
Different amps have different voltage ranges which they operate on.

I'm a Dumble guy so I shoot for around 190-200v at the plates of the 12ax7.

[Phil_S]

Hi All. I have recently completed my first build a 100w push-pull type amp with 3 preamp gain stages a cathode follower,
phase spliter/inverter stage with a quad el34(Electo Harmonix) output stage fixed biased...

Just re-reading here...it seems no one noted what you accomplished. An amp like this is a fairly complex build. I don't believe anyone here would have suggested this as a first build. The fact that you got it to pass a signal is no small accomplishment. I think you deserve kudos for getting this far! Eventually, you'll get it running the way you'd like.