Author Topic: Tube circuit  (Read 2708 times)

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Offline ELS122Topic starter

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Tube circuit
« on: September 07, 2019, 12:13:26 pm »
can you explain why are there multiple capacitors on the cathode for the left side of the 6N2P tube
 

Offline T3sl4co1l

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Re: Tube circuit
« Reply #1 on: September 07, 2019, 12:27:15 pm »
Negative feedback, the 150p is compensation.

Tim
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Offline 001

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Re: Tube circuit
« Reply #2 on: September 07, 2019, 12:38:26 pm »
it is not more when some sort of fixed equaliser, isn`t it?
 
 

Offline ELS122Topic starter

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Re: Tube circuit
« Reply #3 on: September 07, 2019, 12:40:04 pm »
so can I just bypass it and put a 2k resistor to make it just a regular amp whit no tone control?
 

Offline T3sl4co1l

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Re: Tube circuit
« Reply #4 on: September 07, 2019, 02:30:40 pm »
Weird... they put the tone control, inside the feedback loop?  But most of the feedback is also bypassed away by the 15.0 cap.  So it really shouldn't do much of anything.  But then why connect it at all?

Maybe they just needed to use up more capacitors...

Anyway, cutting out the two extra(?) pots and the tone circuit, so the 6N2P left plate is cap-coupled to the right grid, is enough to do that.  Then probably cutting the 15u cap and adjusting feedback values, would get a pretty ordinary amp with flat response and modest overall gain.

Tim
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Offline ELS122Topic starter

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Re: Tube circuit
« Reply #5 on: September 07, 2019, 05:53:32 pm »
Russian logic sometimes can't be explained. for example, this radio has 2x8inch speakers and 1 2x4inch one but it has only 1 6p14p tube. I think they were really optimistic about the power output of this circuit!
 

Offline ELS122Topic starter

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Re: Tube circuit
« Reply #6 on: September 07, 2019, 06:06:41 pm »
so this should work, right?
 

Offline T3sl4co1l

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Re: Tube circuit
« Reply #7 on: September 07, 2019, 11:11:08 pm »
Short out the 150p, and there should be a, 470k or so from right grid to GND.  Otherwise yep, that's it. :)

Note that without feedback, it will have quite a bit of distortion at full output.  Gain may be pretty high as well.  Better perhaps for a guitar amp than general audio, but you'll figure things out as you use it I suppose.

Russian logic sometimes can't be explained. for example, this radio has 2x8inch speakers and 1 2x4inch one but it has only 1 6p14p tube. I think they were really optimistic about the power output of this circuit!

Yuo see Ivan, more speakers make moar sound, puny valve into make big!

Tim
« Last Edit: September 07, 2019, 11:13:20 pm by T3sl4co1l »
Seven Transistor Labs, LLC
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Offline floobydust

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Re: Tube circuit
« Reply #8 on: September 07, 2019, 11:43:22 pm »
There's two feedback loops - one for tone controls/EQ and another for the amplifier.
I would keep the original's overall loop feedback, from loudspeaker output to cathode of 6N2. That 1k, 150pF, 20k, 351R - or some network like it.

The tone control/EQ feedback is a second 150pF cap at terminal "16" which I would delete. Just as Tim says, 6N2 pin 1 to cap to pin 7 with 470k to GND.

Russian circuits are very strange to figure out.
What's with the tweeter return being the bypassed cathode of the output tube? Ahhh so you don't need a non-polarized electrolytic cap in the crossover network.
 

Offline ELS122Topic starter

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Re: Tube circuit
« Reply #9 on: September 08, 2019, 05:16:06 am »
yes this is meant to be a guitar amp, and also it distorts even now on full volume but the distortion is just terrible sounding. maybe need to replace the speakers because they're full of dirt and the old oil-paper caps that somehow are not bypassing large amounts of dc yet...
 

Offline ELS122Topic starter

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Re: Tube circuit
« Reply #10 on: September 08, 2019, 05:46:09 am »
I've studied a bit of negative feedback loops and came up whit this circuit. also added 'presence' control
 

Offline ELS122Topic starter

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Re: Tube circuit
« Reply #11 on: September 08, 2019, 01:02:09 pm »
Is this a capacitor?? In the schematic this is labelled as a cap
 

Offline 001

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Re: Tube circuit
« Reply #12 on: September 08, 2019, 01:06:21 pm »
Russian logic sometimes can't be explained. for example, this radio has 2x8inch speakers and 1 2x4inch one but it has only 1 6p14p tube. I think they were really optimistic about the power output of this circuit!

Oh boy
It is not for power reason  :-DD
In my childhood it was a sort of "surrounding" for "premium" audio
 

Offline ELS122Topic starter

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Re: Tube circuit
« Reply #13 on: September 08, 2019, 01:27:15 pm »
Short out the 150p, and there should be a, 470k or so from right grid to GND.  Otherwise yep, that's it. :)

Note that without feedback, it will have quite a bit of distortion at full output.  Gain may be pretty high as well.  Better perhaps for a guitar amp than general audio, but you'll figure things out as you use it I suppose.

Russian logic sometimes can't be explained. for example, this radio has 2x8inch speakers and 1 2x4inch one but it has only 1 6p14p tube. I think they were really optimistic about the power output of this circuit!



Yuo see Ivan, more speakers make moar sound, puny valve into make big!

Tim

Ummm. I think my mind wasn’t working right when I made that comment, now it’s obvious why there is loads of speakers...
« Last Edit: September 08, 2019, 01:28:54 pm by ELS122 »
 

Offline floobydust

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Re: Tube circuit
« Reply #14 on: September 08, 2019, 06:50:01 pm »
Is this a capacitor?? In the schematic this is labelled as a cap

Yes, looks like 150pF +/-10% tubular capacitor

You can also add a potentiometer between the two 6N2 stages, if you want to overdrive the input tube for some crunch. That would look like a 250-500k pot where you have the 300k at pin 7, wiper goes to pin 7. Most amplifiers bypass the 2k cathode resistor on the input tube to keep gain high.

Older amps used several high efficiency, low power loudspeakers as an alternative to the cost of a higher power amplifier, to get the same SPL. If you play a gig in a big room, you will see it works just as well.
 

Offline ELS122Topic starter

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Re: Tube circuit
« Reply #15 on: September 08, 2019, 07:48:40 pm »
I just connected the 2 tube sides together put a 270k resistor to ground and it worked beautifully until I turn it up to when it really distorts, then it is ear piercingly loud and this also happened before you just had to crank it up more before! what could be the cause or the fix to this?
 

Offline ELS122Topic starter

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Re: Tube circuit
« Reply #16 on: September 08, 2019, 07:54:54 pm »
also, the amplifier tubes are standard ones (not vibration resistant) I have some 6p14p-k's that I'll try swapping in tomorrow!
 

Offline ELS122Topic starter

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Re: Tube circuit
« Reply #17 on: September 09, 2019, 11:51:57 am »
I changed the power tube and now it’s a lot better but not completely gone, so maybe I need to put the speakers in a different enclosure!
 

Offline floobydust

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Re: Tube circuit
« Reply #18 on: September 10, 2019, 03:17:57 am »
The amp might need less feedback or the presence control is a bit too much.

Moving the volume control to after the first tube would upset the feedback loop here.
You'd have to change how it's done - make the feedback come into the second tube's cathode and flip the output transformer's secondary wires, to keep it the correct phase.

This is commonly how it's done on Fender, Vox, Marshall etc. where the guitar goes right into the first tube- instead of after a volume control, because you already have it in the guitar. So you can overdrive the first tube but keep the amp's volume low.
 

Offline ELS122Topic starter

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Re: Tube circuit
« Reply #19 on: September 10, 2019, 01:49:00 pm »
so the feedback loop needs to be in the same stage as the volume control?

also, I haven't modified any feedback loop, I just connected pins 14 and 13 together whit a 270k resistor to ground
« Last Edit: September 10, 2019, 01:50:38 pm by ELS122 »
 

Offline ELS122Topic starter

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Re: Tube circuit
« Reply #20 on: September 10, 2019, 04:40:12 pm »
I'll try this.
 


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