300b get set go problems

2nd harmonics for the masses

300b get set go problems

Postby chopdr230 » Mon Sep 29, 2008 10:17 pm

I finished my amp and all the voltages looked good so i went for it and there is sound but not much of it . I have 90+ db speakers and have had a 6t9 single ended amp hooked up to the and it way out powered this amp . For some reason if i turn up the source volume up past half way it gets all distorted and sounds like it is being overdriven but the volume is really low ?????? I used 3 different sources ipod, computer , and turn table with jolida phono stage and bottlehead line stage . I dont know what i did wrong but this thing sounds like poop . I am using rh-60 outputs and there are 6 taps 8,16ohms, ul,ac ground , and ground is the ac ground the b+ ? Please help i put alot of time and money in this and i know that i must of done somthing wrong . Oh yeah if i leave the source on when i power up it gets real loud and sounds good for a few sec then goes quit and over driven sounding ? Thanks AL
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Postby WA4SWJ » Tue Sep 30, 2008 6:08 am

Hi Al,

The best thing you can do is post some good, relatively close up pictures of the underside of the amp so we can look at it and see what might be wrong. I've built a get*set*go and it sounds great, even with some relatively low efficiency speakers I have. I am using 6B4G's and not 300B's though. As you say, something must be wrong because it's a good amp design.

Regards,
Ed Long
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The 6sl7 is running really cool

Postby chopdr230 » Tue Sep 30, 2008 10:14 am

the heaters light up but this tube is cold to the touch and the rest are nice and warm it is new old stock sylvania 6sl7gt is this interchangeable with the 6sl7 and if so do they normally run real cool ? I tested it on my tube tester and all is well ?????? When i get home from work i will post a pic . Thanks AL
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Postby SDS-PAGE » Tue Sep 30, 2008 10:40 am

Have you taken any voltage readings yet? What are you getting on the 300Bs' plate? What are you getting at the cathode? It take a while for those 300Bs to heat up. Have you gone over your GSG board again, particularly the driver section? Take voltage reading on the 6sl7 too. It's hard for you to figure out what's going on w/o first finding out how your tubes are biased. I built two GSG 300B hybrids, and they could be driven pretty hard without noticeable distortion. You should get around 4.5 wpc w/ ~385VDC on the plate. -Min
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Postby chopdr230 » Tue Sep 30, 2008 12:35 pm

My B+ is at 435 volts . Isnt that Really High????
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Postby chopdr230 » Tue Sep 30, 2008 12:36 pm

My B+ is at 435 volts . Isnt that Really High????
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Postby SDS-PAGE » Tue Sep 30, 2008 1:04 pm

That's fine for 300Bs, but I could never get the B+ that high with my GSG. What about the cathode and 6sl7 voltages?
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I think its the 300b's

Postby chopdr230 » Tue Sep 30, 2008 1:06 pm

the right channel is louder than the left so i switched tubes side to side and the problem followed the 300's they are new electro harmonics did i kill em or could they have been bad from the factory ??? I checked my voltages again and it looks good to me ????I dont have a 4 pin tube tester so i cant see whats up with the tubes . I would hate to put another set in and burn them up ....thanks for the help AL
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Postby SDS-PAGE » Tue Sep 30, 2008 1:51 pm

EH 300Bs are somewhat wimpy. I once had a similar problem, where cathode voltage shot up to 150 vdc on EH 300Bs simultaneously. The problem is likely due to tha fact that they are not heated up long enough before the B+ is applied. Never had such a problem with JJ 300Bs. You can try installing a standby switch or get a H+ delay ciruit by Sophia Electric. BTW, are you using a SS rectifier to get such a high plate voltage?
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no ss

Postby chopdr230 » Tue Sep 30, 2008 2:54 pm

Its tube rectified . Why would one tube be worse than the other the problem follows the tube ?? Are the tubes bad now or would they work if i put a stand by switch in ? Thanks AL
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Postby chopdr230 » Tue Sep 30, 2008 3:41 pm

The stand by switch didnt do the trick . One of the tubes is done for the other sounds decent in both positions but still is very distorted with higher source volume . The b+ is 375 with tubes in place the other mesurement was without 300b installed . The place i got the tubes from is going to replace them i just hope i didnt take them out . Any one had bad luck like this with eh 300b tubes . Thanks AL
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Postby mesherm » Tue Sep 30, 2008 7:01 pm

I have a set of EH 300Bs in an amp I built and they kick booty. My GSG amp uses EL34s so a lot of the GSG PCB parts are not used.
I would double check the componenets in the cathode/filament supply circuitry. Also check for a missing ground connection.
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
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What ground connection

Postby chopdr230 » Tue Sep 30, 2008 7:28 pm

do you mean a pcb component not going to ground ? I didnt hook up the safety ground . I ordered some jj tubes they should be here soon i will go threw every thing one more time and if the voltages still look good i will try them . How do you post a pic on here i have a mac ? Thanks AL
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Postby mesherm » Wed Oct 01, 2008 1:43 am

From J3 and J4 terminals 4 and 5 should go to 300B pins 1 and 4. terminal 3 for J3 and J4 should be connected to 300B pin 3.
The lead from the output transformer marked 2.5K or whatever OT you are using should go to 300B pin 2. The other OT lead probably marked B+ should be connected naturally to B+.
Check the diodes D1 to D8 to make sure they are oriented the right way.
Also check capacitors c11 to c14 to make sure the polarity is right. Sometimes when you install components on the opposite side of the PCB to flush fit into the chassis you can accidently reverse them. Same with tube sockets and their numbers. I have made that mistake a few times myself.
Measure the voltage across and of the three R15 or R16 resistors to check the self bias voltage on the 300B. You should be getting around 65-70 volts.
Double check that R7 and R8 are both 270k (270,000) ohms.
300Bs would be happier if R9 and R10 were 200K but 475k shouldnt cause the problems you describe.
Check R18 to make sure it is 10K brown-black-orange.
I assume you are using 5 volt filament trannies for the 300Bs, make sure the center tap is not connected if you are going through the GSG PCB since the filament grounding is handled on the PCB.
You must be using a separate 6.3 volt filament supply for the 6AX5 and 6SL7. Have you made sure that the traces are cut on the PCB to separate them from your 5 volt 300B supply?



Also what part of the country are you located?
Mike's N-1 Rule: When looking for N number of components to finish a job, you have a 95% chance of only finding N-1 of them.
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Postby ecir38 » Wed Oct 01, 2008 6:28 am

Also, did you change R19 & R20 to 1.5 ohms, 5W? This and how meshern described the 300b being wired is the only thing that varied from the original schematic.
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