100w amp distorting way too early

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100w amp distorting way too early

Postby ioginy » Sun Feb 24, 2013 9:55 am

Good morning folks. I have run into a bit of a problem with an amp I am building. It is intended to be a bass amp, so ideally distortion should be as low as possible. I sadly don't have a schematic drawn up yet, but I will try to have one up this evening. Here are the basics though.

I am using Weber Transformers. Both designed for 80 watts. I am also using the output section of the 6A80 weber amp.
https://taweber.powweb.com/store/6a80_schem.jpg
Right off the bat the distortion was rather farty, so I changed out the .1 PI coupling caps to .02 with great results. Sadly it distorts very early.

my topology is as follows

1/2 12ax7 >>> volume >>> 1/2 12ax7 >>> baxendall tone stack >>> 6AV6 >>> 12at7 PI >>> 6l6 quad

I will also measure the voltages and post them as soon as I can.

Sorry for the vague description. I'm just running out the door but I will try to have everything useful up by this evening.

Thanks.
Cory.
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Re: 100w amp distorting way too early

Postby ioginy » Sun Feb 24, 2013 6:31 pm

Alrighty, here is the schematic with all the voltages. If there are any other measurements that might help, please let me know and I will get them ASAP. I didn't draw in the tone stack because I didn't think it had much to do with the distortion. I tried bypassing it and nothing changed other than the signal kicked up a notch. I can draw a copy of it though if anyone feels it may help.

Image
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Re: 100w amp distorting way too early

Postby Geek » Sun Feb 24, 2013 7:13 pm

ioginy wrote:I am using Weber Transformers.


I'd say that's 80% of your problem right there.

Guitar transformers are designed to saturate and pass as low as only 80 Hz. Bass goes to 41 and if you use a low-B string, 30Hz.

When it comes to bass, you need to think more HiFi than guitar for the power section.... solid power supply and rockin' output transformer.

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Re: 100w amp distorting way too early

Postby dhuebert » Fri Mar 01, 2013 2:27 pm

I had this problem with my BFA when I first built it. The problem turned out to be not enough headroom in the phase invertor. The signal was already distorted by the time it got to the output stage.

Don
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Re: 100w amp distorting way too early

Postby Shannon Parks » Sat Mar 02, 2013 6:08 am

As Don is saying, it's time to disconnect the negative feedback loop and use an oscope to check an input sine wave after every stage to find the culprit.

Shannon
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Re: 100w amp distorting way too early

Postby ioginy » Sat Mar 02, 2013 8:16 pm

Hifi for bass, that is good to know. Even with guitar is saturates way too early though. I'll try scoping it and see what I can find.

OUt of curiosity though, why disconnect the NFB loop?

I have an old telequipment S 51 A scope that seems to work fairly well, however I am very inexperienced with scoping. Any tips would be appreciated.

Thanks.
Cory.
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Re: 100w amp distorting way too early

Postby Shannon Parks » Sun Mar 03, 2013 7:52 am

The NFB will be feeding back distortion, so you'll see the distortion everywhere and it obscures the problem. With it disconnected you can just look after each stage to find the culprit. Mind your voltage rating on your probes and scope - try to look at the signal right after the AC coupling cap. Your gain will go way up, so just use a small 1kHz sine wave - maybe 10 to 50 millivolts. You can also use your volume pot to adjust it lower. If everything looks great, then keep increasing your input until one stage tanks.

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Re: 100w amp distorting way too early

Postby ioginy » Tue Mar 05, 2013 9:44 am

Aha, that makes sense. Thank you.

I tried scoping it and found that 1Khz @ 34 millivolts showed perfect waves after ever stage. When I turned it up to 52 millivolts (I am using my iphone as a tone generator, great app, but limited in it's output control) I got perfect waves up until after the 6AV6 where it started to clip. It is strange though, I tried removing that stage when I first noticed the distortion, but all it didn't reduce the distortion at all, it just didn't drive the PI as much so the whole thing was quieter.... too quiet. I could turn it all the way up and it didn't hurt my ears. Compared to my fender twin, this amp at full was the twin at maybe 3. Thus the 6AV6 to give a little bit of boost.
With the output of a guitar pickup being between 100 millivolts and 1 volt, I'm a little lost as to what I can do from here.
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Re: 100w amp distorting way too early

Postby Gingertube » Tue Mar 05, 2013 5:47 pm

You have a problem in the Phase Inverter - like Don says above.

Here is the thinking:

B+ is C = 417V
Cathodes are sitting at 98V
So you effectively have cathode to B+ of 315V.
For maximum clean headroom, you want the anodes sitting at 1/2 to 2/3 of this voltage differential above the cathodes , that is between about 98 + 157 = 255V and to 98 + 210V = 318V, say between 255V and 320V.
They are currently at 224V and 222V which means you have only 120V from anode to cathode (Vak) AND you are using a 12AT7 which has much more grid current than an 12AX7, particularly at low Vak. Negative going output swing capability will be MUCH less than positive going and that high grid current region will be introducing extra distortion and not of the desirable type.
Increase that 470 Ohm PI bias resistor. That will drop the cathode voltage a bit and increase the anode voltages .
Try say 680 Ohms - you want the anode voltages in that 255 to 320 V range (or thinking another way, perhaps a better way, you want 157 to 210V between the cathode and anodes, ideally aim for about 180V).
That should give you a fair bit more clean headroom.

What this is doing is equalizing the swing capability in both directions, The PI cathode voltage won't vary a lot because as one triode conducts more the other conducts less. Positive going swing is limited by hitting B+ when current in that side of the PI goes to 0 and there is no voltage drop across the anode load resistor, negative going swing is limited by the cathode voltage AND the saturation voltage of the triode (that is the voltage across the tube when the signal on the grid swings positive far enough that the effective grid to cathode voltage (Vg1k) is 0V and the tube is conducting max current. That is where that operating point (of anodes at 1/2 to 2/3 cathode to B+) recommendation comes from. The 1/2 is basically neglecting tube saturation voltage and so is absolute minimum, the 2/3 is about max. without overly affecting the positive swing capability.

The oscilloscope checks mentioned above would probably show you this problem quite well.

Hope this makes sense to you.

Cheers,
Ian
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Re: 100w amp distorting way too early

Postby ioginy » Sun Apr 07, 2013 10:45 am

I tried adjusting the PI cathode, and found that I had to increase it up to 940 to get a 180v between the cathode and anodes. The distortion has become smoother, but has not reduced in any way. I'm going to try and scope the preceding gain stages and see if increasing the cathode resistor further up the line will help. I seem to remember the distortion showing up on the scope after the tone stacks pick up stage. I'll see if that cleans things up and if it does, maybe do away with the 6AV6 if i'm not getting enough push and put in a dual triode. We'll see.
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Re: 100w amp distorting way too early

Postby COOL C » Thu Jun 20, 2013 11:55 pm

HI..I have built a few guitar amps over the years and I always used that Fender type phase splitter.It has a lot of gain for simple circuit.Never had distortion problems.There is great web site called The valve wizard.Google should get you there.Maybe you have looked at it,if not it is a very informing site.Regards COOL C.
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Re: 100w amp distorting way too early

Postby nigelwright7557 » Mon Jun 24, 2013 2:20 pm

I have just built a 80 watt valve amp.
There is a dual gain stage, into a tone stack, another gain stage into phase splitter then output stage.

I found I had to go through it carefully from the front end looking at all the voltages to get it right.
I had to adjust the resistors to get some of the voltages spot on.
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Re: 100w amp distorting way too early

Postby soundmasterg » Sat Jun 29, 2013 11:53 pm

You may try a 12AU7 for the first stage...or a 12AY7 if you have one. You can also adjust the cathode resistor up in value in the second and third stages so those stages don't distort. You may try using a cathode cap on the first stage or bias it with an LED as going with no cathode cap on all stages can sound kind of constipated. I know the cathode cap adds more gain, but if you then adjust the voltages and/or the cathode resistor value of the second and third stages you can get them to accept a larger input signal so they won't distort. Larger voltage and larger values of cathode resistors will allow for a larger input signal without distortion. Check at the grid and at the output of each stage at each level setting on your input so you can see where it first starts to distort. You should be able to make it work fine without distortion with those transformers, though you may not get the bottom end you want with them perhaps.

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