JMP Master Volume 6V6 20W Tone Controls - Odd

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Littlewyan
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JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

When playing my JMP Master Volume amp at practice last night (2204/2203 preamp on a 20W poweramp) I was struggling to get a nice distortion sound and on top of that a nice clean sound turned down.

I had my settings like this:
Preamp Gain - 7
Master Volume - 8
Treble - 6
Middle - 7
Bass - 5 (Sometimes 8)
Presence - 5

The sound was really spiky, harsh, sounded like there was a load of hash in the distortion and the clean sound (guitar volume turned down) had a big mid hump that made strumming sound really bold and stand out.

Anyway at the end of the night (had to find out then didn't I!) I was messing about trying Solodallas settings for the Let There Be Rock tone and it sounded great. When experimenting I found that if I turned the Middle up past 5 the tone would get spikey and the bad tone would come out. Also with the master on 8 it seems the pre amp gain was best set at 6.

So what I'm getting at is how different the 2204/2203 tone controls work compared to the 1987/1959 amps. Anybody else find this? Maybe it's the JJ 6V6s?

I suspected that the primaries might be the wrong way round giving me positive feedback but the amp wouldn't clean up if that were the case.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by pdf64 »

Bear in mind that capacitance is connecting everything to everything else, so there will be various feedback loops, positive and negative, some more significant than others, and a lot of positive feedback is needed for actual oscillation to break out (lots of threads about trem oscillators not working).
Best to check the amp output with a scope and inductive load.
To verify the polarity of the power amp feedback loop, open the loop and see if the gain goes up or down.
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Littlewyan
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

Checked the NFB and it's correct. I actually thought it wasn't at first, made it incorrect, scratched my head for an hour, researched exactly how a LTPI works and now it all makes sense. So thank you for in a way prompting me to do that :).

I think it's just the tone of the amp and the 6V6s I'm using. I do recall these JJ 6V6s had a very scooped tone compared to EL34s in my TW Express.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by pdf64 »

Yes, different power tubes can react rather differently to being overdriven by an already overdriven processed signal.
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Littlewyan
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

Yes, I was actually going to try and bring these in closer to EL34s by simulating the screen voltage drop with a larger screen grid resistor. Beam Tetrodes draw a lot less screen current than Pentodes. So I was going to get my JTM50 on the bench to measure the screen current draw at full tilt.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by pdf64 »

If it's put on a switch, it can be surprising how big a value resistor can be put in series with the choke without there being much difference, even with the amp cranking out signal. With a 6L6 p-p amp it ended up being ~2k2 before the switch seemed to do anything.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by mojotom »

You can mod the PI values to better match the 6V6. 22k instead of 10k for exemple.

A 100pF Fizz cap might help too (across PI output plates).
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Littlewyan
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

Yeah the 6V6s only draw 18mA when the amp is going full blast so it will probably be a 2.2K screen grid resistor at minimum. I won't exactly replicate the voltage drop of an EL34 as obviously the plate current draw is less on the 6V6 as well so ill probably work out the plate current/screen current ratio and work from that. I think pentode screens are meant to draw 20% of the plate current and beam tetrodes draw 10% but we'll see how it works in real life ;)

I thought the standard Marshall PI values were good for 6V6s? I might take the grid leak resistors down a bit as they are on 220K and really should be lower. Won't the 22K resistor give me less drive to the output valves?

100pF fizz cap might not be a bad shout. I noticed on the Marshall DSL15 that they use a 470pF Fizz Cap, whereas on the DSL40 (EL34s) they use 100pF. Maybe they ran into the same problem?

I'm also going to double check the bias by using the shunt method. Putting my meter across the primary winding. I absolutely HATE using this method because of the dangers of it but I want to make sure my 1Ohm Cathode Resistors aren't off.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by pdf64 »

Littlewyan wrote: Thu Oct 26, 2017 7:19 am...I thought the standard Marshall PI values were good for 6V6s? I might take the grid leak resistors down a bit as they are on 220K and really should be lower. Won't the 22K resistor give me less drive to the output valves?
I think that the basic dc operating points of EL34 and 6V6 are broadly similar, so the bias voltage and hence the drive signal magnitudes should likewise follow suit.
Smaller grid leaks on the power tubes / large tail resistor value on the LTP will tend to reduce the gain / max signal level from the LTP.
Littlewyan wrote: Thu Oct 26, 2017 7:19 am...100pF fizz cap might not be a bad shout. I noticed on the Marshall DSL15 that they use a 470pF Fizz Cap, whereas on the DSL40 (EL34s) they use 100pF. Maybe they ran into the same problem?
Have you checked for oscillation yet?
Littlewyan wrote: Thu Oct 26, 2017 7:19 am...I'm also going to double check the bias by using the shunt method. Putting my meter across the primary winding. I absolutely HATE using this method because of the dangers of it but I want to make sure my 1Ohm Cathode Resistors aren't off.
No need to the dreaded current shunt method which (due to the parallel path) will tend to under read the actual current anyway.
Just measure the voltage across OT primary CT to each plate, then (after de-energising and isolating) measure the resistances between those points and divide the voltage by the resistance https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm
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Littlewyan
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

Yeah the only reason for the smaller grid leak resistors was that officially the limit is 100K and I'm using 220K. In theory 220K is safe as long as I don't bias over 70% but I just like to give myself a little headroom. Might change them to 150K.

Haven't yet checked oscillation, I ran out of time the other night. I will get onto that either tonight or at the weekend. Shall I just test with a clean sine wave in and ensure it's clean coming out?

I've tried that method before but it didn't work for me as the meter I was using only gave me a rounded up figure. So I wouldn't get 453.2V I would just get 453V. I think the meter I've got now is the same so might have to use the current shunt method.
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Littlewyan
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

Actually saying that I was checking the tone controls the other night so I deliberately distorted the amp, put on the spectrum analyzer and was watching which frequencies changed with the controls. I don't recall seeing anything unusual come up but I was only checking up to 6Khz. One thing I did find was the output wattage increases until I get to about 8/9 on the volume, after that it decreases slightly by 1 or 2 watts. I think this can be normal though as my TW Express does this as well. I am just measuring this using a multimeter though.
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by pdf64 »

Littlewyan wrote: Thu Oct 26, 2017 11:35 am...Haven't yet checked oscillation, I ran out of time the other night. I will get onto that either tonight or at the weekend. Shall I just test with a clean sine wave in and ensure it's clean coming out?

I've tried that method before but it didn't work for me as the meter I was using only gave me a rounded up figure. So I wouldn't get 453.2V I would just get 453V...
Yes, check the output waveform over the full range of input signal levels and output power levels, and at various control settings, using a sine input.

If you're getting 453V between the OT CT and power tube plate at idle then something is very amiss :lol:
Have a good read the method again.
I'd expect 3 or 4Vdc across ~150 ohms of each half primary.
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Littlewyan
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

Oh no sorry that was an example :lol:

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

Post by Littlewyan »

I had a bit of time tonight so quickly hooked the amp up to a load box and put a scope on the output. I don't think I found any oscillation.

However with these settings:

Preamp 2
Master 6
All Tone Controls 5

I got a little hump on the spectrum analyzer around 40Khz. However the waveform looked fine?
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Littlewyan
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Re: JMP Master Volume 6V6 20W Tone Controls - Odd

Post by Littlewyan »

This is with every control on 10. These were all done with a 1Khz sine wave hitting the front end with 100mV.
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