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Old 18th September 2013, 03:08 PM   #1
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Default Single Ended 6L6GC Load

Hi, so I' messing with a 12W "super champ" type design and have been experimenting with different loads/speaker impedances and came up with something I think sounds good but does not quite look right on paper. The 6L6 is seeing around 360V on the plate, from the B+ I'm dropping down with a 1.5K resistor and then have another 470 Ohm resistor on G2. It's cathode biased with a 470 ohm power resistor and 50uF bypass cap. My OT has 5K and 8K impedances with 4/8/16 ohm taps. I figured 4K would be close to what I needed so I wired the 6L6GC up to the 8K primary and then connected the 16 ohm tap to an 8" speaker, which makes an equivalent 4K (right?). The interesting thing is that it sounds so much better with an 8 ohm dummy load in parallel to the speaker or another 8 ohm speaker parlleled, which makes the system equivalent to a 2K load on the 6L6GC...is that too low? I've been referencing an old GE datasheet which is mostly fixed bias scenarios but no single tubes have anything near as low as 2K. Just want to make sure I'm not reducing life or over stressing the transformers.

Also, the physical layout of the amp is the classic fender champ type with controls on top and tubes hanging upside down on bottom. After playing for a while (~2 hours) on high volumes (>5) the chassis gets pretty warm from the tube/transformer heat...is this common with amplifiers that hang tubes upside down? It's not like burning to the touch, just noticeably warmer after being played. I can't imagine a Tweed Bassman not getting super hot if that's the case. Thanks.
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Old 18th September 2013, 05:36 PM   #2
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The "operating conditions" you see in a tube data sheet assume that you want clean "HiFi" type sound. We aren't exactly operating a Champ as a class A HiFi amp when a pedal board is plugged in and set on "Full Metal Racket". It is true that the output tube will draw more peak current when you operate it into a low load impedance, but unless your transformers are undersized already it is doubtful that anything is going to blow up.

I made about a dozen "turbo Champs" about 15 years ago that used a KT88 and a Hammond 125CSE OPT which has 5 taps on the secondary for impedance and 450 volts of B+. I had a switch to allow adjusting the secondary tap for best tone. None of them ever blew up.

Heat rises. Ever felt the chassis of a Bandmaster after a 4 hour outdoor gig in the hot Florida sun.......Don't. The heat eventually kills the electrolytic caps and the old carbon resistors, not much else.
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Old 18th September 2013, 05:47 PM   #3
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Awesome, good to know. Man, a champ front end into a KT88 probably sounded pretty killer. Yeah, the OT is rated for 15 watts so figured at the end of the day if it's staying under that I should be set. Just wanted to a quick sanity check.

Yeah, if 12 watts is getting that hot I couldn't imagine what ~50 watts (or even 100 with the Twin) would do to a chassis' temperature. Just surprised that allowing a chassis to get that hot has stayed as an "acceptable" design. Did Fender ever do anything to try and reduce the heat? I was thinking about installing a fan or 2 but just like the upside down tubes, "if it ain't broke..."
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Old 18th September 2013, 06:17 PM   #4
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Fans and tubes aren't friend: if the fan cools unequally the glass can be broken. Use this rig as is.
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Old 18th September 2013, 07:25 PM   #5
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Cool, makes sense. Thanks.
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Old 18th September 2013, 08:15 PM   #6
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Quote:
Yeah, the OT is rated for 15 watts
A SE OPT will have a DC current rating. That's the one to watch out for. When you go over this spec, the OPT starts to saturate causing the current to go up, which increases the saturation, causing more tube current, causing...... Mild saturation causes an audible distortion which is quite undesirable in a HiFi system and somewhere in between acceptable to annoying in a guitar amp. The bass gets a raspy sound to it. Here the physical weight of the OPT is the major player. The original Fender Champ OPT was tiny. Most replacements are somewhat bigger. The Hammond 125CSE that I used in most of my Turbo Champs is 1.3 pounds and rated for 60 mA and only 8 watts. I ran up to 80 mA without any problems. I got about 15 watts out of a KT88, and maybe 10 with an EL34. The tubes or OPT didn't complain.

The power transformer needs to be capable of all the current your amp draws. I ran my output tubes anywhere from 50 to 80 mA, and the 12AX7 eats about 5 mA max. Just to be safe I would use a power trasnformer rated for at least 100 mA. The Allied 6K7VG that I used is good for 150+ mA.

Quote:
Man, a champ front end into a KT88 probably sounded pretty killer.
It had a fat sound that some players liked. I preferred the EL34 but I like the cranked to 11 sound. The EL34 can sit right on the edge between clean and nasty where a tweek of the guitar's volume knob will push it over the edge. Some also preferred the cheap Chinese 6L6GC. I built one for one of my daughters friends that used two 6X9 car speakers and a KT88. Too clean for my taste, and sounded awful with my Strat, but it did sound good for rythym with his ES335.
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Old 18th September 2013, 08:38 PM   #7
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So the PT is a Champ Xfmer rated at 70mA...think that's cutting it a little close? The company says it's capable of 15 watts. On the setup right now the 6L6 is using right around 55mA at idle (26 Volts / 470 Ohms). The PT gets warm but not crazy. The OT is an "upgraded" Champ Transformer rated at 70mA DC so sounds like that should be OK.

Thanks for the help.
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Old 18th September 2013, 09:04 PM   #8
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Quote:
So the PT is a Champ Xfmer rated at 70mA...think that's cutting it a little close?.....The PT gets warm but not crazy.
Probably OK. The 6L6GC eats about twice the heater (6.3 volt) power that the 6V6 did. Don't know if your PT is up to that, but either way, I wouldn't pop a KT88 in there.

I got a Fender Champ for Christmas when I was a kid in the 60's. I thought it sounded cool with a 6L6GC in it, but it eventually fried the PT.
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Old 28th September 2013, 03:40 PM   #9
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I am having a clearance issue with my upgraded OPT. Any suggestions? I have a 10 inch Weber with an offset baffle and there is space on the right side (looking into from rear) not to far from the 12AX7 but I was wondering if there would be any electromagnetic interference or hum issues. Also I would have to route the wires across the chassis and enter the factory grommet for connection. Was wondering if I did mount it there what position North-South, East-West would be best? Thanks
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Old 29th September 2013, 07:54 PM   #10
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You could seriously consider a second chassis with just the power supply (power trans, rectifier, and first filter caps) at the bottom of the chassis.

Generally, mount the power transformer core oriented 90 degrees from the output transformer core orientation (or vice-versa).
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