I took my SB200 off the shelf today and when I starting using amp the output wattage was low with a 50w drive I was only getting about 100w and I then put 200w drive into it I got only 250w out and one tube then the other started turning red. I replaced one tube 3 months ago with a china one and other seemed ok I was getting about 600w out then with a 100 watts in. I just donot have any luck with this amp for sure thought it was fixed. Meter shows 2200v but goes down to 1600v on first keydown of 200w drive but when you talk it raises back to 1800v any help will be great Thanks Jimbo165
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Heathkit SB200 tubes getting red
- Thread starter Jimbo165
- Start date
The first step to curing the problem is to stop driving the ever loving piss out of it. Sorry to sound harsh but that amp and those tubes were NEVER designed for that drive level.....not even for short periods. You probably have weak tubes, poor bias, or maybe bad filter capacitors if the voltage drops waaaay down on key up.
To me it sounds like the amp is not properly loaded and tuned. Tubes that turn red are not weak, that is until you keep them red long enough. When they turn red it's because they are drawing too much plate current and a weak tube could not do that. More important than the plate voltage is the current. How many ma. is the thing drawing under these conditions? Probably too much.
Check the idling current first by just grounding the relay terminal on the back and checking the front panel current meter. If that checks out OK and the plate current only rises too high as more drive is applied, your problem is in the output tuning section, otherwise known as the tank circuit. It's very likely it may simply be not adjusted correctly on the load and tune.
Following the factory tuning instructions would be best to be sure you're not over loading the amp but for a test you can just tune them both for maximum output. Don't just adjust them once, go over them both a few times to make sure you can't get more efficiency out of the circuit. Both controls interact with each other and is why they may need to be adjusted more than once if they are way out of tune. Be mindful of the tube temperature during this tuning and let them cool if they go red.
If that fails and you have the ability to try the amp on another band, it may reveal just this band is defective. In this case the most likely cause would be a bad contact on the band switch from over driving the amp. The most drive you should ever apply to this amp is about a 25 watt carrier for AM and 100 watts PEP for SSB. I'm not thinking you've burned up the input circuit yet since you can get the tubes to turn red when keyed. However, this assumes the bias idling current is correct.
Check the idling current first by just grounding the relay terminal on the back and checking the front panel current meter. If that checks out OK and the plate current only rises too high as more drive is applied, your problem is in the output tuning section, otherwise known as the tank circuit. It's very likely it may simply be not adjusted correctly on the load and tune.
Following the factory tuning instructions would be best to be sure you're not over loading the amp but for a test you can just tune them both for maximum output. Don't just adjust them once, go over them both a few times to make sure you can't get more efficiency out of the circuit. Both controls interact with each other and is why they may need to be adjusted more than once if they are way out of tune. Be mindful of the tube temperature during this tuning and let them cool if they go red.
If that fails and you have the ability to try the amp on another band, it may reveal just this band is defective. In this case the most likely cause would be a bad contact on the band switch from over driving the amp. The most drive you should ever apply to this amp is about a 25 watt carrier for AM and 100 watts PEP for SSB. I'm not thinking you've burned up the input circuit yet since you can get the tubes to turn red when keyed. However, this assumes the bias idling current is correct.
I hear you on the lack of sleep CK and it gets me too. Back when sweep tubes where $5 each and tube testers were priceless, I would test them by seeing how much current they could support flat out. If they didn't turn red, a new $5 tube went in. If anyone has some of those tubes that turn red, send them to me. I'll take all of them.
All kidding aside, just because a tube can turn the plate red does not mean it's a strong tube. Many of those Chinese tubes suffer from poor quality control affecting the grid inside the tube. Alignment can be off, grid wires can break off and short to the plate or the cathode. If the grid shorts to the plate, you get fireworks from meter shunt resistors, grid resistors, grid chokes and capacitors.
If the grid shorts to the cathode (filament) you could have the exact symptoms this Heathkit is demonstrating. This effectively removes the cathode bias by shunting it to ground through the grid. That will cause excessive plate current even before drive is applied and cherry out the plates on the tubes. This will also cause the output to be very low since the drive is seeing a much lower impedance and the tubes are operating very inefficient due to the extreme bias current.
All kidding aside, just because a tube can turn the plate red does not mean it's a strong tube. Many of those Chinese tubes suffer from poor quality control affecting the grid inside the tube. Alignment can be off, grid wires can break off and short to the plate or the cathode. If the grid shorts to the plate, you get fireworks from meter shunt resistors, grid resistors, grid chokes and capacitors.
If the grid shorts to the cathode (filament) you could have the exact symptoms this Heathkit is demonstrating. This effectively removes the cathode bias by shunting it to ground through the grid. That will cause excessive plate current even before drive is applied and cherry out the plates on the tubes. This will also cause the output to be very low since the drive is seeing a much lower impedance and the tubes are operating very inefficient due to the extreme bias current.
Thats ok C.K. your excused just dont let it happen again or you my friend will get a timeout!!! LMA
!!
Red plating = bad.
Proper bias = very important.
Do you have access to a good tube tester? I would not want to guess on whether a tube is sound. A bad tube can cause numerous other components in the circuit to fail. Not a chance I'd want to take.
Proper bias = very important.
Do you have access to a good tube tester? I would not want to guess on whether a tube is sound. A bad tube can cause numerous other components in the circuit to fail. Not a chance I'd want to take.
Thats ok C.K. your excused just dont let it happen again or you my friend will get a timeout!!! LMA
Hell I've had so many timeouts lately I thought I was on vacation.
First and foremost be carefull in that amp!
Was the amp working then started giving this issue?
The info that Shockwave gave is the correct way. Get the manual. Make sure you are driving it properly.
More times when I get these amps in I find that they have been well over driven. Folks come in and tell me their 200 no longer puts out 1500 watts.
I just tell them that is a good thing because it will not put out over 600 watts PEP ssb and about 150 to 160 compined key on AM. Just remember the carrier must be reduced to 1/4 output power for proper AM linearity. If you run the carrier up too high this also can cause issue in bias and turn the tubes red.
Was the amp working then started giving this issue?
The info that Shockwave gave is the correct way. Get the manual. Make sure you are driving it properly.
More times when I get these amps in I find that they have been well over driven. Folks come in and tell me their 200 no longer puts out 1500 watts.
I just tell them that is a good thing because it will not put out over 600 watts PEP ssb and about 150 to 160 compined key on AM. Just remember the carrier must be reduced to 1/4 output power for proper AM linearity. If you run the carrier up too high this also can cause issue in bias and turn the tubes red.
I can't help but wonder if this is the same amp as before...driven till it begged for mercy....
All the Best
Gary
http://www.worldwidedx.com/amplifiers/153408-heathkit-sb201a.htmlary
All the Best
Gary
http://www.worldwidedx.com/amplifiers/153408-heathkit-sb201a.htmlary
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SB200 tubes getting red
All of a sudden its working did not do anything except turn it up and down a few times but still does not put out as much as we believe it should. Radio Tech and Wavrider thanks part of the problem was tuning and deadkey. I also found out it does make a difference on what you use to drive amp I tried using Siltronix 1011C and amp puts out about 250w and tubes started to turn red but I readjusted tune and load and it worked ok. SB200 worked the best and tubes did not color at all when using my 4pill amp driving it. 200w on low or 300w on high and SB200 really seem to like the 300w drive the best. I do not think this is my kind of amp it just not put out enough watts for what you have to do get out any power. I will keep it a little longer but first chance I get to get my money back its gone. Thanks for your help. Jimbo
All of a sudden its working did not do anything except turn it up and down a few times but still does not put out as much as we believe it should. Radio Tech and Wavrider thanks part of the problem was tuning and deadkey. I also found out it does make a difference on what you use to drive amp I tried using Siltronix 1011C and amp puts out about 250w and tubes started to turn red but I readjusted tune and load and it worked ok. SB200 worked the best and tubes did not color at all when using my 4pill amp driving it. 200w on low or 300w on high and SB200 really seem to like the 300w drive the best. I do not think this is my kind of amp it just not put out enough watts for what you have to do get out any power. I will keep it a little longer but first chance I get to get my money back its gone. Thanks for your help. Jimbo
70-80W PEP is optimal drive for that amp. 200-300W PEP input is asking for trouble.
I ran 15W PEP into my SB200 when I had it (Wavrider has it now) and it put out 200-210W into my Bird meter. Ran great. No need to kill it with all that input.
~Cheers~
I ran 15W PEP into my SB200 when I had it (Wavrider has it now) and it put out 200-210W into my Bird meter. Ran great. No need to kill it with all that input.
~Cheers~