JMP Master Volume 6V6 20W Tone Controls - Odd

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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by pdf64 »

Littlewyan wrote: Thu Oct 26, 2017 9:06 pm...So it says to measure the centre tap voltage and then plate voltages, but I'm sure I'm right in saying that you need a pretty precise meter for this?
Sorry, I should have read Rob's procedure better myself :oops:
I don't know why he say to do it like that.
He writes in step 2 You can also measure the voltage drop directly by putting your meter probes on the plate pin and center tap but this is much more dangerous but I can't see why he thinks it's more dangerous.
I suggest to directly measure the voltage across the resistances of the OT half primaries; use probe clips if concerned about regular probe tips slipping off the terminal and shorting across terminals.

The risk of probe tips slipping is greatly reduced if they're kept sharp; I use a whetstone.

Sharp probe tips are very beneficial in making quick accurate measurements with minimal risk of the probe slipping.

You charts look fine; the slight treble hump may be due to the low setting of the pre gain and the parasitic 'bright' cap across the control (that's why hifi amps don't have high value volume controls).

Your tests are good but it may be better to also use a volume control (eg 500k) on the output of your sig gen, as it's quick to just sweep the control to change the level and source impedance, and it may better replicate the intended use, eg perhaps the 'all controls at 10' set up would show a problem if the source was 5mV @ 10k impedance and there was an inductive load.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

I definitely need better probes! Mine aren't that sharp and can slip easily, so during lunch today I'm going to look for a new set of probes and some with clips. And I wasn't sure about measuring the voltage drop across the primaries, I think I'd rather use that method. Surely it's much safer than the current method?!

I did every now and then get a lot of interference when I ran the signal really low but it was intermittent. I think it was a dodgy connection between my scope probe and the amp input. I have to connect a short patch lead into the input, then a croc clip cable from that to my scope probe. I wanted to keep the chassis in the head cab for shielding whilst I tested the amp.

I did also quickly measure the screen current and sag on the JTM50. The HT goes from 470V to 420V when it's running max clean wattage (56W) and then down to roughly 380/390V full whack. The screen current on max clean is 15mA but going up to full blast brings it up to 44mA.

On my 6V6 Amp, the HT only sags from 390V down to 360V full whack, so there is pretty much no sag at all in that amp. The screen current is about 9mA on clean I think and 18mA full whack. So I might put in 2.2K grid screen resistors to give it a bit more screen sag. Maybe put a resistor in with the choke to add a bit of sag there.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

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Littlewyan wrote: Fri Oct 27, 2017 10:06 am I definitely need better probes! Mine aren't that sharp and can slip easily, so during lunch today I'm going to look for a new set of probes and some with clips. And I wasn't sure about measuring the voltage drop across the primaries, I think I'd rather use that method. Surely it's much safer than the current method?!
If your probe tips aren't sharp, then sharpen them. Use a fine file the 1st time. The leads will fail eventually anyway, so may as well have them work properly while they can.
Yes, hanging a current meter off the HT is needlessly hazardous. OK if both ends are clipped in place beforehand, but ... clips sometimes become detached and then the free end is effectively a piece of wire dangling off the HT; and the temptation having measured one plate is to check the other, and the natural temptation is to move and re-attach the clip live, rather than properly de-energising things.
Yes I'm sure folks here have made current shunt measurements a zillion times without issue, but you only need to get unlucky / distracted once and you may not get a 2nd chance.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

Ill try and get the file out at the weekend to get them sharpened. And I wouldn't want to stick my hand into an amp live to move a clip on lead so there is no worry there.

I read one of your posts regarding screen grid resistors from a topic back in 2013. It was regarding something that I actually came across this morning in Meralin's Pre Amp book (section on Pentodes). If a higher value screen grid resistor is used then there is more local NFB on the screen grid which actually gives you more headroom but more compression at the same time.

So I'm wondering, do I want extra headroom? I intend to increase the NFB to try and flatten out the frequency response a bit more to narrow the tone gap between the 6V6s and EL34s (that's the plan anyway) so that will give me more headroom anyway. I wouldn't mind some extra compression though. Maybe it's best to have a resistor in with the choke to make the pre amp voltages sag more under load as well.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by pdf64 »

Headroom is one of those words that musicians use to mean various, sometimes contradictory, things.
Need to define technically what you mean by it, what goal you want to achieve.
Whatever, as individual screen grid resistor value is increased, both max power and gain (especially gain at high signal levels) will reduce.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

Headroom to me is the range of clean tone you get when turning down the guitar volume control. With the 82K feedback resistor I didn’t have much of a clean tone at all when I turned the guitar volume down.

After a lot of reading up I think I’ll leave the screen grids alone.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

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Littlewyan wrote: Sun Nov 05, 2017 12:42 amHeadroom to me is the range of clean tone you get when turning down the guitar volume control...
Isn't that the inverse of gain? In that as gain increases, overdrive will occur at a lower signal level and so the range of clean will decrease.
Littlewyan wrote: Sun Nov 05, 2017 12:42 am...With the 82K feedback resistor I didn’t have much of a clean tone at all when I turned the guitar volume down...
82k as opposed to ...100k?
I think that if system characteristics are very sensitive to small changes in the degree of NFB, it may indicate that its margin of stability is low.
It may be worth investigating that using test conditions as close as feasible to those under which the system performance seems affected, eg rig up a vol control and guitar cable between sig gen and amp, inductive load on amp.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

No sorry I didn’t explain very well. I never had a 100K fb resistor. I had 82k and I could get a clean tone but the range was very small, which is normal for these amps. So going to a 56K feedback resistor was good as it gave me slightly more range than before. Might put an even smaller value in to give me more range and tighten up the amp a bit more.

Also 6V6s have a higher anode resistance compared to EL34s so have lower damping. This I think is a big part of why 6v6s sound different. So having more NFB should help with this a bit.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by sluckey »

NFB... I noticed with my Plexi6V6 that the presence control had almost no effect (that I could hear) with the typical 100K feedback resistor. I experimented with many resistance values and finally ended up with a 22K.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

I read about you doing that. I calculated that my amp should have a bit more NFB than my TW Express, yet the presence control on the Express has more effect than my 20w Master Volume. I wonder if it’s the damping factor I mentioned in my last message. Maybe a 6v6 needs more NFB for the presence control to have an effect on tone? I’ve used the control with a scope on the amp and it definitely works but I can see on the spectrum analyser that it doesn’t make much difference.

I looked at the Marshall DSL15 (2x6V6) and DSL40 (2xEL34) amps and noticed the NFB was very different. The DSL15 had 22k FB resistor and the DSL40 had 100K. Same OT tap and very similar presence control. I did some calculations and the DSL15 has loads more NFB than the DSL40. Interesting.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by sluckey »

I didn't put much thought into the NFB circuit when I built the amp, but after reading this thread I kinda believe that the 6V6s were probably a big factor in the lack of presence control with that big 100K resistor. I didn't do any measurements. I just tuned it by ear. :mrgreen:
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

Maybe I’m putting a bit too much thought into it! Unfortunately I can’t tune by ear easily as I have to take the amp to a rehearsal room to crank it.

82k on the 8Ohm Tap is roughly the same NFB as 100k ok the 4Ohm tap on a 50w amp. I think 56K on 8ohm is the same as 100k on 4ohm on a 100w amp. Ill measure with my spectrum analyser to see what the presence control does before changing values.

What setup did you use for the screen grids? 1k resistors and a choke? Have you compared your’s to a 50w plexi?
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by sluckey »

My B+ rail is straightforward. There's a 14H choke between the plate (400v) and screen (397v) nodes. All 32µF caps. 470Ω screen resistors. All iron came from a Hammond organ AO-43 reverb amp. I don't have a 50W Plexi but I do have the November amp from ax84.com. It's a Plexi preamp into a fixed bias EL84 power amp. Both amps have a Marshall growl that I like but I prefer the EL84 amp for this sound. Here's my schematic...

http://sluckeyamps.com/6v6plexi/6v6plexi.pdf
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by pdf64 »

For negative feedback to take effect, it requires sufficient open loop gain to work with .
Usually with guitar amps the ratio between open loop and closed loop gain is ~6dB (check with a resistive load).
As gain within the loop is reduced, eg with a type 1-3 MV, the ratio will reduce, closing the loop will make less and less difference.
So as open loop is reduced, to attempt to maintain the loops efficacy, the feedback sample will need to be increased, to get back to the 6dB gain reduction when the loop is closed.
6V6 have rather lower gain than EL34, hence the 100k series feedback resistor could be reduced in order to keep the loop effective.
It's not really more negative feedback, rather alteration of the feedback sample so that it's commensurate with the open loop gain.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by sluckey »

That's a much better way to look at it. THX...
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