Please try to be exact in your descriptions. I assume you mean 140V instead of 310-320V on the screens or screen grids. That voltage on the control grid, which is normally meant by saying 'grid' would lead to immediate destruction of your tubes.
I do not know if this is the only fault of the amp, but only half of the screen voltage present compared to the schematic is definitely wrong. Bear in mind that your meter loads the voltage you're measuring, if the meter has a very low internal resistance R_i, the readings are not correct. Though this should not be the case in the screen circuit, as this has to deliver some current in an output stage.
Greetings,
Andreas
OK, sorry, I am getting 140V on pin 4 of the output tubes (screen).
The ZD5, ZD6, ZD7 are no where to be found. I cannot find them.
Limono, I think I know what happened to your jacks. Mine were very loose when I first checked the amp. I tightened it from up top via the nut. While tightening the nut, I ended up turning the jack, which then touched the frame inside underneath the pcb, effectively grounding the cathode, and not allowing the break for the plug, and current to be read. I found and corrected the short and it and no longer touches the frame. Now I can break the connection when I insert the plug...
The ZD5, ZD6, ZD7 are no where to be found. I cannot find them.
Limono, I think I know what happened to your jacks. Mine were very loose when I first checked the amp. I tightened it from up top via the nut. While tightening the nut, I ended up turning the jack, which then touched the frame inside underneath the pcb, effectively grounding the cathode, and not allowing the break for the plug, and current to be read. I found and corrected the short and it and no longer touches the frame. Now I can break the connection when I insert the plug...
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Start from Pin 5 of the series regulator 6550 (V17). You should find the 1.8k grid resistor and the diodes after that.
When found, measure the voltage on top of the zener string, before the 1.8k resistor.
Greetings,
Andreas
... difficult to analyze zeners just by looking at them...
How do they measure? (voltage after each one, beginning at ground potential)
Greetings,
Andreas
EDIT: You edited your post, so I have to do the same, otherwise my question above makes no sense. Replace the diodes if they measure like that.
How do they measure? (voltage after each one, beginning at ground potential)
Greetings,
Andreas
EDIT: You edited your post, so I have to do the same, otherwise my question above makes no sense. Replace the diodes if they measure like that.
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Those zeners brake easily , also check thermistor . I tightened up the jacks and they still were shot. And I could not get reading ,unless I was bending the probe jack , and no nothing was touching the frame. If your phono bias sockets are Ok you should be abble to measure more than 1-3mA by now
I replaced the three Zeners and ....it works!
I can now tune up the bias to 50ma...thing is I don't remember which one of my tubes were matching, so now I have to try every single combo to get find them and get the sides even. I can set the bias to 50ma, but the DC trim can only get me within 5-7 ma to each other tube (I am only working on one side at a time)...
While I was in the amp and while waiting for teh zeners, I replaced the power indicator lamp to a cool blue LED, and painted the cage and bottom cover...she looks brand new...
Now that the bias can be set correctly, this thing really puts out some heat...I knew something was not right when I first got t as it was running way too cool...
pics later...
I can now tune up the bias to 50ma...thing is I don't remember which one of my tubes were matching, so now I have to try every single combo to get find them and get the sides even. I can set the bias to 50ma, but the DC trim can only get me within 5-7 ma to each other tube (I am only working on one side at a time)...
While I was in the amp and while waiting for teh zeners, I replaced the power indicator lamp to a cool blue LED, and painted the cage and bottom cover...she looks brand new...
Now that the bias can be set correctly, this thing really puts out some heat...I knew something was not right when I first got t as it was running way too cool...
pics later...
Attached are some pics of the amp...playing now and listening to some John Coltrane - Giant Steps...
I don't yet know if she will stay in the setup (the Krell KSA-50 Clone is tough to beat)...but she is up, biased up at 50ma and sounding sweet!!
Last pic is of the three bad Zeners and the Thermistor that crumbled away as soon as I touched it...
I don't yet know if she will stay in the setup (the Krell KSA-50 Clone is tough to beat)...but she is up, biased up at 50ma and sounding sweet!!
Last pic is of the three bad Zeners and the Thermistor that crumbled away as soon as I touched it...
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Limono, the pre is a 6SN7 Aikido I built, in a ARC style chassis (chassis off fleabay), and another amp with two Icepower ASP1000 modules powering my experimental Magnepan Tympani bass panels, also in an ARC Chassis. So the D-75 kinda looks like it belongs with the pre and bass amp...
The D-75 runs a pair of Martin Logan CLS IIa panels with bias/stepup tranny I built. I am quite happy with the results - if your ever down by Naperville, stop on by for a listen...
Yeah, I still have the bulb socket, so the LED can always be reversed...
Hey, did you ever do the D-75 mod to reduce the plate voltage (extend the life of the output tubes) per the ARC Database site for the D-75? I started last night and bailed at the last minute as I couldn't determine if I have a newer PCB, making it unnecessary...
The D-75 runs a pair of Martin Logan CLS IIa panels with bias/stepup tranny I built. I am quite happy with the results - if your ever down by Naperville, stop on by for a listen...
Yeah, I still have the bulb socket, so the LED can always be reversed...
Hey, did you ever do the D-75 mod to reduce the plate voltage (extend the life of the output tubes) per the ARC Database site for the D-75? I started last night and bailed at the last minute as I couldn't determine if I have a newer PCB, making it unnecessary...
No, I didn't do anything to D-75 except to replace what absolutely necessary. I've got D-51 to take care off and I put D-075 on flee -buy. Both of them are too weak to power Proac Studio 3 E.B.S on ATC drivers which I hauled home from Craigslist. Mids and highs are quite decent but the midbass, bass is rubbery and not very defined. I just moved to Des Plaines , left my horns behind and play around with what gets in my way (i.e vintage treasures
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In my experience, Tubes and Bass don't mix well...try a high current SS amp to see if the bass firms up. I really like these Icepowers for bass. They are not bad at HF, but best on bass...
I have a pair ICEPower 1000w@4 ohm Monoblocks for sale...your welcome to try them out... these same modules are running my bass panels...
Edit - my setup so far (a constant work in progress)
I have a pair ICEPower 1000w@4 ohm Monoblocks for sale...your welcome to try them out... these same modules are running my bass panels...
Edit - my setup so far (a constant work in progress)
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Problem with Proac is that it crosses at 500Hz and thats well into midband to mess with SS, tube biamp . I'd need capable SS power amp (like Bedini 100/100 I heard couple years ago) My complaint is mainly that the midbass and little bass (specs are 50hz-23kz -+ 2dB) those 10" ATC have lack color and tone.( I'm not sure they are capable to have any color and tone at all )Anyway , the setup is ARC Sp6 A and D51 all vintage parts so can't expect too much. (For your D-75 I'd add to PSU rail 2x 1000uF /350-400V , just strap it to PCB board on each side of original PSU caps row.)
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