Mike express build

[martin manning]

Maybe try to divide the amp and see if you can determine which side the problem is on. Disconnect the coupling cap from V2 and feed signal into the PI. You could use your generator and scope and examine the waveform, or connect the preamp output from another amp and play through it.

[Tillydog]

Maybe try to divide the amp and see if you can determine which side the problem is on. Disconnect the coupling cap from V2 and feed signal into the PI. You could use your generator and scope and examine the waveform, or connect the preamp output from another amp and play through it.

He did done do dat on Monday

I still think some scope pictures using a lower drive level might help

[Tillydog]

And NOTHING helped!

Were you listening, or just looking at the scope?

[martin manning]

Maybe try to divide the amp and see if you can determine which side the problem is on. Disconnect the coupling cap from V2 and feed signal into the PI. You could use your generator and scope and examine the waveform, or connect the preamp output from another amp and play through it.

He did done do dat on Monday

Monday's test put the signal in at V2, no?

[Tillydog]

Er... yes...

(Sorry!)

[geetarpicker]

Seems I read way back in the thread you were using an Airbrake into a resistive load. That said, how does the amp sound cranked straight into a speaker without any thing in between the amp and cab? Just wondering if your load box setup could be the culprit...

[Miket]

Hey guys, thanks for all the replies.

To answer your questions, no I haven't tried the amp without v2. That will be on the agenda for tonight.

Tilly - The signal level in the final scope pic was in the same range as my les Paul, the scope needs calibration (I only got it recently) so it's amplitude readings are off. When I said nothing helped regarding the last things I tried, that was listening tests, not scoping.

Regarding the attenuator, I'm using an air brake, and the swirl/phaser effect is more noticeable with it. But still there with no attenuation. (I think it's more noticeable with the attenuator because it's in a frequency range that's bypassed by the cap in the air brake)

I have been reading Merlins preamp book, regarding blocking and crossover distortion in the phase inverter and power amp stages. I'm thinking this might be the cause, or a factor now... It will probably be an avenue ill explore next. I might make the grid stoppers on the power amp a much larger value, see if that helps. Merlin brushes over using high value grid stoppers on the power tubes in his book, he mentions that you can use values as high as 1M, as long as the long as you don't surpass the tubes max pull down resistance rating (I think I worded that correctly, don't have the book in front of me.)
Would it be safe to run 1m on the express?

Thanks again guys

[Colossal]

Did you ground that second speaker jack?

I have been reading Merlins preamp book, regarding blocking and crossover distortion in the phase inverter and power amp stages. I'm thinking this might be the cause, or a factor now... It will probably be an avenue ill explore next. I might make the grid stoppers on the power amp a much larger value, see if that helps. Merlin brushes over using high value grid stoppers on the power tubes in his book, he mentions that you can use values as high as 1M, as long as the long as you don't surpass the tubes max pull down resistance rating (I think I worded that correctly, don't have the book in front of me.)
Would it be safe to run 1m on the express?

I can't tell from the photos but are all of your fly back diodes oriented correctly to ground?

[martin manning]

How about take the flyback diodes out? They have been known to cause trouble...

[Firestorm]

I have been reading Merlins preamp book, regarding blocking and crossover distortion in the phase inverter and power amp stages. I'm thinking this might be the cause, or a factor now... It will probably be an avenue ill explore next. I might make the grid stoppers on the power amp a much larger value, see if that helps. Merlin brushes over using high value grid stoppers on the power tubes in his book, he mentions that you can use values as high as 1M, as long as the long as you don't surpass the tubes max pull down resistance rating (I think I worded that correctly, don't have the book in front of me.)
Would it be safe to run 1m on the express?

Merlin was speaking of preamp tubes (it's worded a bit imprecisely). You can't use very large grid stops on output tubes or they will die. The grid return resistors are already over the published maximum grid circuit resistance values.

[Miket]

Regarding the attenuator, I'm using an air brake, and the swirl/phaser effect is more noticeable with it. But still there with no attenuation. (I think it's more noticeable with the attenuator because it's in a frequency range that's bypassed by the cap in the air brake)

I understood glens question and answered it.

Interestingly tonight I learned that the voltage spikes that can be seen in the sine wave, are an effect of the air brake. I ran the amp into a dummy load, and the voltage spikes went away (the amp still sounded bad however) so we can conclude that the spikes are not related to the amps problem or problems.

I ran signal directly into the phase inverter tonight, with c9 removed. And it was exactly the same. So the prob is definitely phase inverter and or output stage, or bias circuit I guess.
Regarding the bias circuit, I can't get below -33v with it, r29 is 150k, should I lower its value further?

Cheers

[Miket]

I never fitted the diodes on the output tubes, read too many bad reports.

And that second jack has just been removed for the time being

[KellyBass]

I think you just might be on the right track with the crossover distortion....but, from where? You're build is very clean.

I see that you measured and confirmed every resistor that went into the amp...did you check all of your caps as well? Just curious...

Darn near every swoosh issue I've heard had something to do with the PI. Why (exactly), I don't know...Anybody?

[JazzGuitarGimp]

I think you just might be on the right track with the crossover distortion

Low bias would be my first suspect if you're dealing with crossover distortion.

Miket, when you say "I can't get below -33V", do you mean to say that you need to go more negative, or are you looking at the number, -33V as an absolute number, and therefore mean you want to go higher (less negative)? If you are trying to get the bias voltage more positive (closer to 0V), then lowering the value of R29 will get you there. And if the amp is currently under biased to the point where you are in Class B territory, it may be exhibiting crossover distortion.

[Miket]

Hey guys. Had a bit of success with the amp last night. (bias was ok once I changed resistor back to 220k in the bias circuit).
I worked on the phase inverter, decided to change a few things at once, seeing as I was in the area.
I lowered c13 and 14 to 100nf, lowered r16 and 17 to 100k, and increased r18 and 19 to 100k.

This drastically lowered the amount that the output tubes are being overdriven. But in doing so seems to have removed about 98% of the swirl. And the distortion isn't a square wave at 9 o'clock position, like it use to be.

Now I need to go back and isolate further, to try and get it closer to being an express.

I did test all the caps that went into the circuit. But only for capacitance, I didn't have any other mens to test them at the time. Perhaps the coupling ones I removed were at fault...

Ah well, getting there either way.
Thanks again for the help guys!