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SEplex style build questions (fizz, fuzz, fart?)
SEplex style build questions (fizz, fuzz, fart?)
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Old 19th December 2017, 03:29 AM   #11
thoglette is offline thoglette  Australia
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Quote:
Originally Posted by PRR View Post
Hey, if we wanted predictable response we could use chips and transistors.
I've got a 12AX7/807 amp which oscillates (starts buzzing and drops gain, blue glow on 807 moves to high power location).

But only after playing power chords for a few minutes with everything set to 11. And I need to turn the amp off to stop it.
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Old 19th December 2017, 06:06 AM   #12
GeorgK is offline GeorgK  Austria
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Originally Posted by jarrodthebobo View Post
I'm a bit confused; how exactly is he master 'inside' the NFB loop? There isn't really anywhere else the master can be put in this circuit and remain effective, is there?
The FB resistor provides a certain amount of feedback, but the open loop gain of the stage varies with the master control.
But agreed, predictability is boring

Actually i am rather questioning the the need of NFB itself. the power section will probably be stable without.
And I wonder if the Feedback loop is trying to correct possible distortion of the last triode, which actually is desired. Otherwise you could also move the master directly after the tone stack.

Last edited by GeorgK; 19th December 2017 at 06:12 AM.
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Old 19th December 2017, 06:51 PM   #13
jarrodthebobo is offline jarrodthebobo
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Honestly I think the NFB loop is there to try and as closely match the original circuit this was based off of (1959 plexi, but with a single channel). The original amp has a phase inverter and negative feedback, which I'm sure is somewhat important to the sound of the amp.

With the master volume pot where it is, is the amount of negative feedback decreasing with increase in master volume? or is it increasing? I've noticed if you bring the master volume up, not only do you get a nice power tube saturation from the single el84, but it almost seems like some of the harshness goes away. I had just assumed that the distortion generated by the el84 was somewhat 'covering up' the distortion generated by the 12ax7s, hiding some of that harshness. Could it be due to varying NFB?
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Old 19th December 2017, 07:58 PM   #14
GeorgK is offline GeorgK  Austria
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Well, the Marshall does not have its master volume directly before the power tube(s) and therefore it is outside the loop. The last triode in you amp, as you stated, is where the PI used to be.

I think the behavior of the NFB loop with variable gain is rather complex. I know that there are lots of designs with a master on the input of the power tubes, but all that come to my mind at the moment are PP, and the usual long-tailed inverters behave different from your single triode when pushed hard.
Just thought it could be a good idea to see what happens without NFB. Or with the master pot before the input of the last triode. Or both.
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Old 20th December 2017, 01:53 AM   #15
jarrodthebobo is offline jarrodthebobo
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Alright Maybe I'll play around with that when I get a break from work! Will be interesting to see what happens, hahah
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Old 20th December 2017, 04:49 AM   #16
PRR is online now PRR  United States
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Quote:
Originally Posted by jarrodthebobo View Post
... how exactly is he master 'inside' the NFB loop? There isn't really anywhere else the master can be put in this circuit and remain effective, is there?...
It could be at the "boost switch" point.

That would not "strangle" the final section much, true. Designers have to balance various tradeoffs.

It's clearly inside the NFB loop, whether we like it or not. The amp sure would be "stable" without NFB; this is not one of those wimp transistor amps that doesn't know what to do (way too much gain) until NFB tells it. However when driving a lumpy loudspeaker there is a different sound between naked pentode and NFB pentode.

AND "some of the harshness goes away".

We can trim an amp (EQ and speaker and box) to be "good" either way. But combining gain, power, harshness, and damping all in one knob is either serendipity or consternation (depending on luck and beer).
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Old 21st December 2017, 05:07 AM   #17
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by audiowize View Post
A 100K grid stopper isn't acceptable for an EL84. A 100K grid leak resistor on an EL84 is OK. My guess is that you have a 100K grid leak and a 2K grid stopper.
I agree that typical grid stoppers are a kilo ohm or two.

However, I just took a look at an EL84 datasheet. As with all pentodes, input capacitance is very low - 10.8 pF in this case. No Miller effect to speak of thanks to the screen grid, so the quoted 10.8 pF won't change enormously with voltage gain and operating point.

Let's suppose we've built the circuit clumsily, and managed to add another 9.2 pF of stray capacitance from the socket, poorly laid out wiring, etc. So now we have 20 pF of input capacitance altogether.

So, if we had a 100k grid stopper, and 20 pF input capacitance, what would the 3 dB cutoff frequency be?

80 kHz, that's what. And that's about ten times higher than the highest frequency we need to worry about for a guitar amp!

So there appears to really be nothing wrong with using 100 kilo ohm grid stoppers with EL84 valves, as long as you make sure the total grid-cathode resistance doesn't exceed the data sheet maximum of 300k (in the Mullard datasheet I'm looking at). You could use a 100k grid stopper and a 180k grid bias resistor, and the total of 280k would be under that 300k limit.

Incidentally, Merlin Blencowe makes the point in his preamp book that using much larger-than-usual grid stoppers for output valves can solve many unpleasant sonic problems (particularly those related to blocking distortion and/or excessive bias shift under overdrive), and yet this is rarely done in commercial designs, probably because everyone is still copying ancient circuits designed decades ago.

-Gnobuddy
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Old 21st December 2017, 06:37 AM   #18
jarrodthebobo is offline jarrodthebobo
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I was able to finally poke around a bit inside the amp today; my new OT came in so I hopped on in to put the Classic tone 40-18031 15w tranny. I made sure to mark off the original tranny's wires and where the lead to and installed the new output tranny.

Eureka! It works!... but Only on the 'clean' channel? The preamp section I had been mucking around with no longer produced any sound. Changing tubes didn't help this. Decided that well, maybe something is wrong with the tranny; so I put the original back in. Now there is NO output at all! How does that make any sense?

There is hum coming from the speaker, which can be changed slightly by messing around with the master volume knob. Poking the plates and such on the 12ax7s produce clicks when using my multimeter; and I can't find any circuit dis-ruptions that are obvious to the eye. Neither tranny is 'open' either.

I've rolled tubes through every slot but to no avail; I honestly have no idea what happened to my amp (after it was finally starting to sound so good too... and I built a new cabinet for it... ugh).

Do you guys have any idea as to what could have happened? I spent litterally the ENTIRE DAY trying to figure out what went wrong, but I have nothing to show for my labors.

Welp... lets go down the check list I guess....

DC on the B+s all checked out; around 340 with some variations here and there down the chain. Getting around 390v on the 6v6, so that seems normal.

Flipping to AC and trying to trace the sound of my guitar through the circuit (i dont have a scope unfortunately), I was able to clearly see the guitar getting amplified up to where the 470k and small cap are paralleled to the input of v1b. The reading there showed about 1.4Vs, and I assume thats normal as the reading on the unamplifed signal was in the mvs. Past that its kinda hard to follow as there is some HV that sneaks in on the plates. I do not appear to be getting any signal after the caps going into the tone circuit however (none of the caps show ANY AC swing no matter how hard I strum my guitar).

Is it possible that I somehow popped all the caps in the preamp? The clean channel, (or the separate 12ax7 which is completly isoloated from the rest of the 'marshall' circuit) has this same issue; no AC swing after the tone caps. However, there does appear to be a very minute swing when the probe is placed on the input to the 6v6 and the guitar is strummed hard; couple of MVs of swing.

seeing 340v DC or so on the common and 7.5k secondaries of the output tranny; not sure if thats normal. No DC present on the primaries; which is good.

What exactly is going on?!!? I really hope I don't have to replace every cap in this circuit... could it be a diode issue? Or a fuze? I'd assume not a fuze as all the tubes light up and their is 'some' audio in the form of pops and hum so.... jee what the hell is going on!
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Old 21st December 2017, 07:08 AM   #19
GeorgK is offline GeorgK  Austria
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Quote:
Originally Posted by jarrodthebobo View Post
seeing 340v DC or so on the common and 7.5k secondaries of the output tranny; not sure if thats normal. No DC present on the primaries; which is good.
To me, if I did not own a scope, this would be the moment to get one

The transformer winding you refer to as "secondary" is on the side on the tube, right? Usually the "speaker winding" is called that way.

Anyway... What are the DC values on the Power tube? You did not mention cathode, grid and screen voltage. Exactly the same voltage on the plate and the HV connection of the transformer would be strange too.

I do not think you have a defective cap. At the moment I would rather blame some contact problem in the output stage or anything similar. Maybe the cathode resistor is blown?

Last edited by GeorgK; 21st December 2017 at 07:15 AM.
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Old 21st December 2017, 07:17 AM   #20
jarrodthebobo is offline jarrodthebobo
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But if it was just a contact problem, why would the clean channel have initially worked when the other channel hadn't?

I'll play around with the amp some more in the morning and take down all the voltages I can... hahah, I'm so frustrated right now.

And yes, you're correct; I meant the winding on the side of the tube for the tranny, haha.

And I know I need an oscilloscope; I've been wanting one for a while but just keep putting it off honestly.
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