Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on November 01, 2013, 01:17:15 am
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Hi guys, thought I punish myself after having a couple of good builds under my belt.
The power amp side is working fine and got 350v on the 6V6s and so far sounds fine.
With some juggling I got close to the preamp voltages as per the old schems, and again the preamp sounds fine.
The STINGER(fuzz) is not working great. The node voltages as per the schem has it at 255v at the moment it 261v and wired as per the schen I would think that it would have a fuzz like effect,WRONG, with the preamp volume at 0 the stinger volume can be turned up and is fine at 1-2 but anything after that is distorted and dam ugly. The Colour (tone) just adds a sound like a swarm of bees and a lot of squealing.
My thinking to how this circuit should perform would be that t input signal is amplified to the max and just added back into the circuit, pretty simple one would think :dontknow:
If anyone could expand on the workings of this circuit on whether this will actually work, OR could this be another case of it really didn't work but they still went a head and sold it as is??????
Looking through the other schems and there is another take with a different tube (6JW8)and it only uses the pentode half and wired in after the first half of the 12AX7, so at the moment I an playing around with this circuit to see what happens. Thanks
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Hi guys, This one didn't work either :cussing: all I am getting is a blocking distortion type sound when hitting the strings :think1:
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WTH? triode has very sick bias.
original stinger used just the pentode - triode used for trem. at least it was in the rebel.
http://www.el34world.com/charts/Schematics/files/garnet/garnet_lb100ft_rebel_deluxe.pdf (http://www.el34world.com/charts/Schematics/files/garnet/garnet_lb100ft_rebel_deluxe.pdf)
TOO much gain - suggest ditching the 6AN8 triode gain stage and use the bias values shown in the schematic. for guitar amp, grid leak bias with pentodes sucks at best.
--pete
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Thanks for the reply DL, I didn't think that much could go wrong when wiring as per the schem/s but either TUBES its sound S**T.
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Thanks DL,I looked through all the schems and no matter what I tried I couldn't get them to work for various reasons but I was able to get this to work with the 6JW8 as per the rebel deluxe. I even tried it with the triode as a CF and again it was way too distorted.
I getting the FUZZ that is similar sound that a pair of LEDs produce and has the BUZZ that comes with it, tried a stack of things to smooth out the buzziness but nothing works :think1: :help:
This has left me with a triode, so the tremolo has been added. Works but very weak so tried this http://www.el34world.com/Forum/index.php?topic=15087.0 (http://www.el34world.com/Forum/index.php?topic=15087.0) helped a little but very hard to hear.
Added a PPIMV as the two volumes were miles apart when flicking between the two.
There is still more tweaking that can be done and its turning out to be a good build.Thanks
O yeah, BTO........ BLUES TONE OVERBRIVE :icon_biggrin:
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tremolo has been added. Works but very weak so tried this http://www.el34world.com/Forum/index.php?topic=15087.0 (http://www.el34world.com/Forum/index.php?topic=15087.0) helped a little but very hard to hear.
Your circuit is not quite right. Point X should not connect directly to point BIAS. Doing so just dumps the trem signal straight to ground thru your 100µF bias filter cap. The correct way to wire it would be to first remove the "X" and "BIAS" connections from your MV. Now connect point X to the left side of a 250K intensity pot and connect point BIAS to the right side of the intensity pot. Finally, connect the wiper to the MV where you originally had the BIAS connected. Refer to the Princeton Reverb schematic.
And also change that .01µF cap on the plate of V1B to a .1µF. Low frequency trem signal likes big coupling caps.
These changes will get your trem working. You may need to tweak some components to your own taste. And remember, there is a trade off between strong trem and 'proper' bias adjustment with this type of circuit.
EDIT... added pic
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Thanks Sluckey, Tremolo now coming through LOUD and STRONG.
I see what you mean about the trade off. With the bias set at 23mA, it spikes up to about 50mA and I get a suspicious smell from the 6V6s. :dontknow:
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Sluckey, Found another helpful bit. There is a 68K resistor across the wiper and the bias lug. This trims the spike to about 27mAs at very quick intervals. So i'm thinking these sharp spikes won't do any harm :dontknow:
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... This trims the spike to about 27mAs at very quick intervals. So i'm thinking these sharp spikes won't do any harm :dontknow:
A class A output stage might swing from its idle current to 2x idle current (say 23mA to 46mA) with every input signal cycle.
A class AB output stage might swing from its idle current to 3-4x idle current (say 23mA to 69-92mA) with every input signal cycle.
The average current is what the tube feels (your meter may have detected that average or it might be measuring peak current, hard to tell from your wording of measuring 50mA). You can trim the no-trem idle current down in most cases to compensate for the higher average current with the trem on.
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Timbo,
I love the cool innovative ideas that you come up with.
Out of curiousity, why did you go fixed bias instead of cathode biased? I think with PPIMV and tremolo, I would've gone cathode biased on the amp which might have been simpler. Not saying that's better, just simply how I'd approach it.
There are some good cathode biased tremolos out there:
http://www.el34world.com/charts/Schematics/files/Vox/vox_cambridge_reverb.pdf (http://www.el34world.com/charts/Schematics/files/Vox/vox_cambridge_reverb.pdf)
http://www.el34world.com/charts/Schematics/files/epiphone/EPIPHONE_EA-35T.pdf (http://www.el34world.com/charts/Schematics/files/epiphone/EPIPHONE_EA-35T.pdf)
http://www.el34world.com/charts/Schematics/files/gibson/GA-17RVT_SCOUT.pdf (http://www.el34world.com/charts/Schematics/files/gibson/GA-17RVT_SCOUT.pdf)
With respect, Tubenit
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Put a 1N4007 (or similar) diode across the outer lugs of the intensity pot, cathode to the bias side. Does that help smooth out the current spikes? What other effects if any do you notice with the diode?
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HBP,When the bias is set and measuring across the 1r resistor(idle set at 23ma) that measurement then spikes up to 50mAs. This was causing the power tubes to red plate although I am only guessing that they were red plating cause the groove tubes are black and can't see whats happening inside, just a smell :icon_biggrin:
Tubenit, I'm still searching for that "ultimate" amp and with every build there is something that stands out that I like. I found that in the original silver jubilee I had a cathode/fixed bias switch and this gave me a good comparison between the two and I have to say that the fixed bias sounded much better. To me its away to have the power tubes working on "the edge" where as cathode bias sounded like the tubes were being held back off "the edge" but that may be different between amps :dontknow:
Adding the tremolo to this fixed bias amp only came about cause finding that the pentode half has more than enough GAIN by itself that it left the triode out in the cold, there was plenty of info from garnet to use one of theirs but there always has to be a problem but without these problems this would be NO fun :laugh:
I'm a bit of a DUMBO when it comes what works and what doesn't work :l2:
Now sluckey, With the 68k between the middle lug and the Bias side lug as said this tames the spike to about 28mAs( again the bias is set at 23mAs with the trem disconnected) and the trem is smooth and lush, sounding great :icon_biggrin:
With the 68K removed and the 1N4007 across the outer lugs and the Cathode at the bias end the bias would fluctuate between 18-21mAs and the Speed was the same but the intensity went from 0 to a wobbly.
With the 68k reconnected this brought it all back to having a good range across the speed dial and the intensity has a good range as well :worthy1: with the bias staying between 18-21mAs. This is GOOD... NO :l2:
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With those few little problems fixed this one is just about done :wav: The head cab is built ready for tolex and a faceplate to be made. The fuzz circuit is about the only thing that has a minor buzz on the end of a struck note, it sounds similar to that of diode clipping but other than that works well.
I never liked tremolo/vibrato but after have this I'm liking it a lot :icon_biggrin:
Thanks to those for the help. :icon_biggrin: