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#1
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MK III biaset zoomz to 26v during warmup
I'm rebuilding a pair of MK III's. Just now working on the first.
My plan was to replace all tubes AND: -1 Replace selenium stack w/ diode & bias caps/resistors with a kit from Uncle Neddy. Test voltages. -2 Replace quad cap with triple cap + 1 cap with another kit from Ned. Test voltages. -3 Replace driver board with stuffed board from Classic Valves. Test voltages. Enjoy some Dave Brubeck on my rebuild 60 honkin' watt amps. Of course, I never took baseline voltage measurements (for I am RAT Boy... and I knows it all). I replaced the bias circuit (being very careful to get it all wired back correctly). Using original tubes, I powered it up and checked the biaset... 1v, 1.9v 2v, 4v, 26v!!! PULL THE PLUG! I powered it up again and began to check the voltages and compare them to those listed in manual... 5AR4 voltages are OK on pins 4 & 6 but pins 2 & 8 are sky high. PULL THE PLUG!!! Replaced the Quad filter with Ned's (triple + 1) kit... same issue. Is my PT fried? What should resistances at PT leads be? Could the choke be causing this? Ideas? |
#2
Posted to rec.audio.tubes
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MK III biaset zoomz to 26v during warmup
- - : R A T B o y : - - wrote: Is my PT fried? What should resistances at PT leads be? Could the choke be causing this? Ideas? Hi RATs! RATboy, you are playing with powerful forces. Please do the basic homework before you kill yourself, or worse, ruin a good amp. An email to this herd of wretched posers is not they way to learn basic electronics. Sorry. Read some books. Build some circuits. Ignorance is bliss. Bliss is for idiots. Happy Ears! Al |
#3
Posted to rec.audio.tubes
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MK III biaset zoomz to 26v during warmup
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#5
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MK III biaset zoomz to 26v during warmup
Jon Yaeger wrote: There is no conduction with a positive bias . . . Eh ? Graham |
#6
Posted to rec.audio.tubes
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MK III biaset zoomz to 26v during warmup
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#7
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MK III biaset zoomz to 26v during warmup
in article , François Yves Le Gal
at wrote on 10/1/06 7:02 PM: On Sun, 01 Oct 2006 17:22:18 -0400, Jon Yaeger wrote: The recommended bias voltage for this configuration is -70 volts, +/- 25%. When you say 26 volts do you mean POSITIVE 26 volts? I guess that there's 26 V on the "biaset" connector on the front panel, which should give a 1.56 V reading (same as an unloaded D cell) when bias is properly set. I was talking about the grid of the output tubes. Guess I should have clarified that. |
#8
Posted to rec.audio.tubes
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MK III biaset zoomz to 26v during warmup
Jon Yaeger wrote: in article , at wrote on 10/1/06 5:02 PM: - - : R A T B o y : - - wrote: Is my PT fried? What should resistances at PT leads be? Could the choke be causing this? Ideas? Hi RATs! RATboy, you are playing with powerful forces. Please do the basic homework before you kill yourself, or worse, ruin a good amp. An email to this herd of wretched posers is not they way to learn basic electronics. Sorry. Read some books. Build some circuits. Ignorance is bliss. Bliss is for idiots. Happy Ears! Al Al, Some assistance here would be better than poetic derision . . . . Jon Hi Jon, OK. I was caught up in the romance of the moment. - Ratboy, Back up a few yards and start fresh 1. Did the amp work before you tried to improve it? Happy Ears! Al |
#9
Posted to rec.audio.tubes
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MK III biaset zoomz to 26v during warmup
Jon Yaeger wrote: in article , Eeyore at wrote on 10/1/06 5:54 PM: Jon Yaeger wrote: There is no conduction with a positive bias . . . Eh ? Graham If you bias a grid positive with respect to the cathode, it will not conduct. So how is it that many high-power RF amps operate in this way ? Graham |
#10
Posted to rec.audio.tubes
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MK III biaset zoomz to 26v during warmup
in article , Eeyore at
wrote on 10/1/06 9:30 PM: Jon Yaeger wrote: in article , Eeyore at wrote on 10/1/06 5:54 PM: Jon Yaeger wrote: There is no conduction with a positive bias . . . Eh ? Graham If you bias a grid positive with respect to the cathode, it will not conduct. So how is it that many high-power RF amps operate in this way ? Graham Graham. My sincere apology. I thought we were talking about a Mark III audio amplifier. Just for my edification, how many AUDIO frequency amps operate like your subject RF amp? I've not seen a push-pull output stage or a SE for that matter, with the plate at B+ and a grounded cathode that conducts when the grid goes positive. Please enlighten me. I'm ready to give up my hobby once you show me the light. Jon |
#11
Posted to rec.audio.tubes
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MK III biaset zoomz to 26v during warmup
Jon Yaeger wrote: in article , Eeyore at wrote on 10/1/06 5:54 PM: Jon Yaeger wrote: There is no conduction with a positive bias . . . Eh ? Graham If you bias a grid positive with respect to the cathode, it will not conduct. On the contrary Jon, If a grid is biased positively and cathodes are ar 0V, then the tube will be turned on fully and the there will also be grid current. Maybe the cathodes are not grounded properly and the 11.2 ohm current sensing resistor is open because getting 26V across the 11.2 ohms is a very large amount of current. But if the 11.2 ohm is open, the voltage at the cathodes will measure very high without the tubes conducting any current. What is the applied NEGATIVE applied bias voltage at the grids? If the bias supply diode has been reveresed to produce a +ve supply instead of a -ve supply the cathode current will be maximal saturation current. If the current of 2 amps is maintained longer than 20 seconds in the two output tubes the PT and OPT could be seriously damaged because these ancient bits of old audio junk don't have automatic over current detection that turns off the amp if the cathode voltage exceeds the 1.56V that should appear across the 11.2 ohms by 50%. Having only ONE bias supply for 2 output tubes is a very poor way to design any amp and it should be altered so there is a bias set pot for EACH tube. When installing revised bias set circuits only 3 watt wire wound pots should be used. Patrick Turner. |
#12
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MK III biaset zoomz to 26v during warmup
Jon Yaeger wrote: I was talking about the grid of the output tubes. Guess I should have clarified that. Hi RATs! I withdraw my apology. Asking the ignorant sluts on this NG to teach you basic electronics is suicide, So, RATboy, did the amp work before you decided to improve it? Happy Ears! Al |
#13
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MK III biaset zoomz to 26v during warmup
Patrick, You are absolutely right. So are you, Al. Don't know what I was smokin' or thinking. Apologies to all. Jon |
#14
Posted to rec.audio.tubes
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MK III biaset zoomz to 26v during warmup
Jon Yaeger wrote: in article , Eeyore wrote Jon Yaeger wrote: in article , Eeyore wrote Jon Yaeger wrote: There is no conduction with a positive bias . . . Eh ? Graham If you bias a grid positive with respect to the cathode, it will not conduct. So how is it that many high-power RF amps operate in this way ? Graham Graham. My sincere apology. I thought we were talking about a Mark III audio amplifier. Just for my edification, how many AUDIO frequency amps operate like your subject RF amp? None of them do so intentionally AFAIK ! I've not seen a push-pull output stage or a SE for that matter, with the plate at B+ and a grounded cathode that conducts when the grid goes positive. It gets pretty non-linear AIUI but the RF guys aren't especially bothered about that. Please enlighten me. I'm ready to give up my hobby once you show me the light. About positive grid bias ? I only know of it in passing really. I wouldn't want *anyone* to give up their hobby btw. Graham |
#15
Posted to rec.audio.tubes
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MK III biaset zoomz to 26v during warmup
Patrick Turner wrote: Having only ONE bias supply for 2 output tubes is a very poor way to design any amp and it should be altered so there is a bias set pot for EACH tube. That's much better pratice. When installing revised bias set circuits only 3 watt wire wound pots should be used. We discussed this at least a year ago funnily enough. I'd actually use a cermet pot out of preference, they're easily as reliable and also have a finer setting accuracy. Also it's advisable to wire the wiper of the bias pot to the the most negative part of the -ve bias circuitry via large value R in the rare event that the wiper should ever go open circuit. Graham |
#16
Posted to rec.audio.tubes
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MK III biaset zoomz to 26v during warmup
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#17
Posted to rec.audio.tubes
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MK III biaset zoomz to 26v during warmup
Eeyore wrote: Patrick Turner wrote: Having only ONE bias supply for 2 output tubes is a very poor way to design any amp and it should be altered so there is a bias set pot for EACH tube. That's much better pratice. When installing revised bias set circuits only 3 watt wire wound pots should be used. We discussed this at least a year ago funnily enough. I'd actually use a cermet pot out of preference, they're easily as reliable and also have a finer setting accuracy. Also it's advisable to wire the wiper of the bias pot to the the most negative part of the -ve bias circuitry via large value R in the rare event that the wiper should ever go open circuit. Graham Cermets are OK but a wire wound of 10k with a range of -30V to -50V is usually quite OK for tube biasing. The range of voltage adjustment should never allow the tube to be biased fully on, or biased right off. Owners of tube amps routinely make terrible mistakes with bias pots and owners are responsible for much of the smoke clouds seen in loungerooms from time to time. All old amps like the crap from Dynaco should be fitted with 10ohm R to each output tube cathode and biased with separate bias pots and have active protection measures against excessive cathode currents. Fixed bias isn't self regulating!!!! As tubes age and approach the end of life the applied grid voltage fails to control the anode current which suddenly can begin to rise and send a tube into the thermal run-away condition and the tube may sit there red hot until it dies maybe after an hour of sitting there with glowing red hot anode and 250mA instead of 40mA. There is no cathode resistance to offset the effect. Even with a cathode resistance, the same thing eventually happens, but at least the Rk can be rated to fuse open if the Ik rises enough, thus saving the OPT primary. Excessive Ia may cause the OPT primary to go open or get really hot, causing insulation breakdown of the enamel wire giving shorted turns, but not cause enough current to flow to make a fuse anywhere blow. Hence maybe a large cloud of smoke and large repair bill. Old amps need active protection more than new amps. Who would drive an old Mustang and use it without a working water temperature meter and oil pressure guage? Patrick Turner. |
#18
Posted to rec.audio.tubes
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MK III biaset zoomz to 26v during warmup
I had wandering bias on my Dynaco ST70, which I fought for some time.
It turned out to be the rectifier tube socket. Try swapping out the sockets, if you haven't done so already. BTW, all the cleaning in the world didn't fix my problem. Only a new socket did. FWIW. Bob H. - - : R A T B o y : - - wrote: I'm rebuilding a pair of MK III's. Just now working on the first. My plan was to replace all tubes AND: -1 Replace selenium stack w/ diode & bias caps/resistors with a kit from Uncle Neddy. Test voltages. -2 Replace quad cap with triple cap + 1 cap with another kit from Ned. Test voltages. -3 Replace driver board with stuffed board from Classic Valves. Test voltages. Enjoy some Dave Brubeck on my rebuild 60 honkin' watt amps. Of course, I never took baseline voltage measurements (for I am RAT Boy... and I knows it all). I replaced the bias circuit (being very careful to get it all wired back correctly). Using original tubes, I powered it up and checked the biaset... 1v, 1.9v 2v, 4v, 26v!!! PULL THE PLUG! I powered it up again and began to check the voltages and compare them to those listed in manual... 5AR4 voltages are OK on pins 4 & 6 but pins 2 & 8 are sky high. PULL THE PLUG!!! Replaced the Quad filter with Ned's (triple + 1) kit... same issue. Is my PT fried? What should resistances at PT leads be? Could the choke be causing this? Ideas? |
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