Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mresistor on February 22, 2016, 09:23:35 am
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Looking at the Marshall JCM800 Lead 50W 1987 preamp I see that the first preamp stage has a 330uf bypass cap and 820 ohm cathode resistor. Has anyone every tried this in a Fender Champ/VChamp on the first preamp stage? What did it sound like? Is it higher gain? It's an awfully large bypass cap. I think that value was selected to allow more low freq. to pass. I would also like to know if in doing this, the plate voltage would have to be increased or decreased. I"m still looking but I haven't found what that voltage is in the JCM800 for that PS node.
http://el34world.com/charts/Schematics/files/marshall/Marshall_jcm800_lead_50w_1987.pdf (http://el34world.com/charts/Schematics/files/marshall/Marshall_jcm800_lead_50w_1987.pdf)
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It's an awfully large bypass cap
Yup, I believe a lot of folks don't even stick with 22uF in a champ, I believe you'll be pretty muddy at 330uF
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If you don't already have it, this would be a perfect opportunity to download and check out Merlin's load line plotter.
3/4 of the way down this page and the first link under DOWNLOADS
http://www.valvewizard.co.uk/links.html (http://www.valvewizard.co.uk/links.html)
It gives a great visual representation of how changing parameters affects conditions, power, and frequency response of the common triodes we use regularly. You'll need Excel to use it, or there's an Open Office version.
There is a field there to enter the supply voltage so that will show you what would happen to the curve if you raised/lowered the plate voltage.
To specifically answer your question, it depends which Champ circuit you're addressing...
The 5F1 has a 1.5K rk only with no cap, so not only are you halving the resistor value, you're adding a lot of gain just by adding the cap.
The 5F3 has the same 1.5K but with an already large 25uF cap. 25uF already provides full gain throughout the frequency range of the standard tuned guitar and could be referred to "as fully bypassed"...the 330uF is basically overkill, and probably wouldn't be very noticeable unless you were using a bass or drastically dropped tuned guitar.
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Here's a couple screen shots that might help for anyone who's interested
I just realized that I never changed the supply voltage field...so these are accounting for a 300VDC supply
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Thx SG I was looking at Merlins preamp info and the load lines while posting. Just wondered if anyone ever tried it and what it sounded like. Forget to say I was referring to an AA764 Blackface Champ/VC. I think I will try it just to play around. Interesting that JM used that big of a cap in 1987. I was thinking it may have been a misprint. Like maybe it was 33uf instead. 330uf is pretty good physical size too.
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more
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Thx SG I was looking at Merlins preamp info and the load lines while posting...
I'll leave these up for anyone who might be interested...helped me a lot when I was first trying get a grip
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I was thinking it may have been a misprint. Like maybe it was 33uf instead. 330uf is pretty good physical size too.
I think that's a carry-over from the 5F6A Bassman circuit. Sunn uses a huge cap on the preamp tube also. It may have something to do with reducing hum in the first stage due to heater/cathode leakage.
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If you don't already have it, this would be a perfect opportunity to download and check out Merlin's load line plotter.
3/4 of the way down this page and the first link under DOWNLOADS
http://www.valvewizard.co.uk/links.html (http://www.valvewizard.co.uk/links.html)
How do you open this download/program? I have windows 7.
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If you don't already have it, this would be a perfect opportunity to download and check out Merlin's load line plotter.
3/4 of the way down this page and the first link under DOWNLOADS
http://www.valvewizard.co.uk/links.html (http://www.valvewizard.co.uk/links.html)
How do you open this download/program? I have windows 7.
you need either Excel or Open Office
Excel is part of the Microsoft Office collection
Open Office is a FREE download, here:
https://www.openoffice.org/download/index.html (https://www.openoffice.org/download/index.html)
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*Attn: MERLIN*
Maybe Merlin will update that file one of these days? It goes back to a '97 version or similar of Office and the newer Excel doesn't like working with such older versions as there are conflicts due to this. Hmm, maybe he already has I don't know it?
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I haven't opened this particular file yet.
But OpenOffice is FREE! Runs fine on Win-doze, whatever version.
I try not to give Mr. Gates any more money than I absolutely have to. :BangHead:
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Looking at the Marshall JCM800 Lead 50W 1987 preamp I see that the first preamp stage has a 330uf bypass cap and 820 ohm cathode resistor. Has anyone every tried this in a Fender Champ/VChamp on the first preamp stage? What did it sound like? Is it higher gain? It's an awfully large bypass cap. ...
Your cap/resistor combo will be -3dB when Xc = R ("Xc" meaning capacitive reactance).
The formula to calculate Xc usually assume you know what specific frequency you wish to evalute, but we want to know frequency for a known Xc. So we'll swap those two values.
Frequency = 1/(2*Pi*Xc*C)
"C" is in Farads, so 330uF = 0.00033F, and we calculate -3dB at ~0.59Hz. As Silvergun's plot shows, gain is not "increased" it's just not reduced by local feedback across the cathode resistor even down to sub-sonic frequencies.
The 330uF is a carry-over, as Sluckey said, from the 5F6-A Bassman's 250uF, and rounded up to a value in the modern value scheme (0.01, 0.022, 0.033, 0.47, 0.68, etc).
I've had a theory for a number of years that because this was a bass amp (living in 60Hz and 120Hz), Fender tried every trick to make sure hum was nil in the input stage (except d.c. heaters, because that's just silly for an instrument amp in the 50's). A 12AY7 input tube was used (advertised for "low hum & noise in input stages of high gain audio amps"), and my theory is the super-huge bypass cap was to nix hum in the event an individual tube had heater-to-cathode leakage.
I know a modern boutique amp manufacturer contacted me a number of years ago with a hum they couldn't kill in a series of tweed-style amps. If they powered the heater with d.c. (from a battery), the hum was gone. If the powered the heater from a.c. and used a 25uF bypass cap, the hum was there. If they increased the bypass cap to 220uF on the input tube, the hum went away. Tube-swapping didn't seem to cure the hum. Turns out they had a whole batch of new-production tubes with heater-to-cathode leakage and the big bypass cap eliminated the coupling of that heater hum into the cathode.
Using the Xc formula again, 25uF looks like ~106Ω at 60Hz, while 250uF looks like 10.6Hz. A 330uF cap looks like ~8Ω at 60Hz.
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Thanks for all the replies. Appreciated.