PPIMV biasing issue

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geoff 1965
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PPIMV biasing issue

Post by geoff 1965 »

hello guys,
i've come across a gremlin whilst biasing my lite2 cage amp,i have a dual pot PPIMV and with the MV on about 1/4 volume i'm getting a voltage drop of -11.6 volts on V3 and -10.4 volts on V4.
however when i increase the MV to past 3/4 volume the voltage drop on V3 increases from -11.6 to -16.4 volts but the voltage drop on V4 decreases from -10.4 to -7.9 volts!
is this normal for a push/pull amp?
my OT is a hammond 1750PA and the primary resistance is unequal with blue to CT 369 ohms and brown to CT 329 ohms.
the tubes are a matched pair of JJ's and test good,appreciate any help with this.
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geoff 1965
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Re: PPIMV biasing issue

Post by geoff 1965 »

Hello ampaholic's!
had to post this just to amuse the senior amp techs! a case of trusting something and paying for it! with some help from a couple of lads on PPWatt,com i found what was causing the increase/decrease biasing problem,bear in mind i had a pair of marshall JJ's which tested very strong with no leaks or shorts on my heathkit tube tester! at first i thought it was the cascaded preamp signal being too strong but it was doing the same in parallel so i proceeded to do the following;
add the 470K grid resistors-no difference!
swap the 10% taper dual pot for a standard taper-no difference!
change to 1meg dual pot with 1meg grids-no difference!
this is where i discovered dc volts on the el84 grids,as much as 1090mv on one tube! so proceeded to remove all the coupling caps and test them for leakage-no leakage-no difference!
i was at my wits end at this point thinking where the hell is this dc coming from and finally did what i should have done first and swapped the el84's for a pair of ruby's and now the bias is stable on both tubes with no dc voltage on the grids! after 2 days of testing,soldering and scratching my head!
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JMPGuitars
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Re: PPIMV biasing issue

Post by JMPGuitars »

Ouch, what a headache. I avoid JJ EL84s, I had a series of unreliable 84s (a lot of them), and never went back.
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geoff 1965
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Re: PPIMV biasing issue

Post by geoff 1965 »

yeah Josh,
i've got 2 sets of matched paired JJ's "marshall" stamped and they all are leaking dc to the grid's! don't know if this is a common problem or the DSL40 amp they came from had a fault which caused this?
which tubes have you found best? i like the look of the harma retro's that are supposed to be close to the old mullards but have no microphonic's,has anyone tried them?
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Re: PPIMV biasing issue

Post by JMPGuitars »

I mostly use reissue Mullard, reissue Tung Sol, or TAD. I still use JJ for preamp or other stuff sometimes, but never EL84.
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Re: PPIMV biasing issue

Post by Daviedawg »

Just catching up Geoff. I had my grandsons to stay for the week. That is a bummer. But you got there which is great.

I have a pair of JJs which were bought as my reserves when I built my 18 watt. So they have only had a test of a few hours. I ran Sovteks for the first few months while I fiddled with it. Then NOS odds and ends. But for at least 5 years I have been running a pair of Zaerix which have been great.

Dd
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geoff 1965
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Re: PPIMV biasing issue

Post by geoff 1965 »

5 years! that's good Dd,what dissipation are you running them?
mine are at 10 watts with a 180R cathode resistor,150R stock was a bit too close to the 12W max.
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Re: PPIMV biasing issue

Post by Daviedawg »

I am not sure I ever worked out the dissipation. But I do know that I tried hard to get the cathode voltage right on 12 volts. That extra wee bit from 11 volts which I had with cathode R of 150 ohms makes a big difference to the tone. It really lives when you play hard.

I am sitting trying to remember what my cathode resistor was when I finally settled on it. I think it is 130 ohms. But I would have to look. It was part of my messing around after it was up and running. The valves are not suffering it seems for being biased a bit hotter.

Dd
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geoff 1965
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Re: PPIMV biasing issue

Post by geoff 1965 »

interesting! yeah i tried a 160R and it did sound good but i had the JJ's in at that time which i now know had dc on the grids! so maybe i can try the 150 & 160 again with the good tubes in.all my testing & experimenting has been on the preamp with the parallel/cascade so i can fine tune the el84's now for the best tone.
with 180R and the good tubes i've got 12.3volts
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