Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: plexi50 on August 07, 2010, 06:14:36 pm
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I have a THD Andy Marshall #55 made UniValve single ended KT88 amp. I have no B+ at the attenuator. I have B+ throughout the rest of the amp. The power tube plate high VDC is there but there is no bias grid voltage. I am assuming this is because the bias is running through the attenuator. Dont quote me on this but i have not had enough time to go through the amp yet. Just a quick check of the grid shows no VDC. I have checked the bias resistor/cap and they are good. The amber light on the panel does not function either as it is tied to the attenuator
(http://i356.photobucket.com/albums/oo5/plexijtm45/ChassisCage.jpg)
(http://i356.photobucket.com/albums/oo5/plexijtm45/InsideChassis-2.jpg)
(http://i356.photobucket.com/albums/oo5/plexijtm45/KT88.jpg)
(http://i356.photobucket.com/albums/oo5/plexijtm45/PS-2.jpg)
(http://i356.photobucket.com/albums/oo5/plexijtm45/Attenuator-1.jpg)
(http://i356.photobucket.com/albums/oo5/plexijtm45/Board2.jpg)
(http://i356.photobucket.com/albums/oo5/plexijtm45/Board-1.jpg)
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> no bias grid voltage
It is Cathode-Bias, isn't it? The grid should be at zero, the cathode should be a few dozen volts Positive, V/R tells you the cathode current.
What is the larger problem? Does the amp work? Do tubes explode?
BTW: melting tubes is not new (of course). My dad was just telling stories of buying steepletop radios at the salvation army when they were cheap, mid-1940s. He had one very deluxe rig with push-pull 2A3 (and he remembered "2A3" after all these years). Power up, plate went bright red and sagged into a lump.
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Hi,
THD manual says that the attenuator is a built-in Hot Plate Attenuator, so in my mind, it doesn't need a B+.
The manual also says that the amber light is part of the noise reduction system and it is powered by the guitar signal. Did you try it with guitar?
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The KT88 tube is dead. I tried both 6L6GC & EL34 tubes. Amp sounds good and amber light fluxuates with guitar signal for a minute. Then either power tube will begin to redplate. Plate is 360VDC with no load
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From what i am reading on the net the redpalting issue is a common and supposedly acceptable in the Univalve amp
It's not common or acceptable to me though. I am basically sitting here and watching a tube cook itself
My resistor is 310 ohm instead of the 300 ohm called for on the schematic / I wouldnt think a small drift like this would make the power tube noticably redplate
UPDATE: The LOV is awesome and there is no redplating at this setting
The HIV setting is where the redplating takes place and fluxuates as you play
The attenuator sounds like crap but functions. In conclusion all i had was a bad KT88 power tube
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Did you calculate the dissipation? How much does the voltage change from HI to LO settings? What is the voltage at the cathode?
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335VDC High and 257 VDC Low. Roughly 78 volts difference. I am getting .004 voltage pin 8. This is one of the first things i checked when i removed the bottom plate. When i said earlier that i had no grid bias voltage what i ment was there is no voltage on the resistor/cap combo to pin 8 the cathode. Then i discovered the KT88 to be dead and put in a TAD 6L6GC tube. Still no voltage on pin 8 that would calculate to the plate voltage of the power tube (335VDC). Does not look right to me. I have built single ended amps before and got the cathode voltage with no problem.
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I would check the cathode resistor and the cap and maybe try it without the cap. Secondly I would check that ground wire from those cathode components, looking at the pictures it looks like it is not going anywhere.
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Lifted cap from resistor and ground: Appears to be a bad cap. It wont bleed down on my analog Micronta
If i remember i dont need the cap but it can set the bass response (Tone to a degree) Dont know if thats the case here just yet
I put an alligator clip on the neg cap and as soon as i connected it to the ground side the voltage would dissapear
I only have a 330uf/16V cap and a 250uf/35V cap on hand. Have to play it and hear it first. Thanks for the tip. There is a White/Green wire that goes from the top terminal strip Resistor & -Cap to chassis ground
LV VDC & HV VDC Below: I am a stickler for pic's.
(http://i356.photobucket.com/albums/oo5/plexijtm45/BadCap.jpg)
(http://i356.photobucket.com/albums/oo5/plexijtm45/LVCathodeVDC.jpg)
(http://i356.photobucket.com/albums/oo5/plexijtm45/HVCathodeVDC.jpg)
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If i remember i dont need the cap but it can set the bass response (Tone to a degree) Dont know if thats the case here just yet
According to Merlin, if you want to maximise output power and input sensitivity, then you need that cap:
http://www.freewebs.com/valvewizard1/se.html (http://www.freewebs.com/valvewizard1/se.html)
That 35V is pretty close to that 27.38V. I would probably use at least a 50V part here, something like 100uF should be enough.
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I have a 100uf/100V cap i will try. Thanks VMS!