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 Post subject: 6L6G plate voltages
PostPosted: Dec Fri 14, 2018 4:06 am 
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Location: Haddon Heights NJ
I'm rebuilding a no brand tube amp that has a pair of 6L6 push pull. It was no power transformer. I was looking at some Fender schematics to get an idea of the type of transformer that i will need and something caught my attention. In most models that uses the 6l6 the plate voltage is pretty high (http://ampwares.com/schematics/bandmaster_ab763.pdf) over 400V. Looking at the specs for that tube in a push pull configuration is showing a maximum voltage of 360V, the typical operation shows voltages of around 270V. I'm reading something wrong on the specs? What are the consequences of driving the tube at a much higher voltage that the recommended one.

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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Fri 14, 2018 4:27 am 
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Location: Albuquerque, NM 87123
You should use 6L6GC tubes or you might have a problem.


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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Fri 14, 2018 4:55 am 
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dan3460 wrote:
I'm rebuilding a no brand tube amp that has a pair of 6L6 push pull. It was no power transformer. I was looking at some Fender schematics to get an idea of the type of transformer that i will need and something caught my attention. In most models that uses the 6l6 the plate voltage is pretty high (http://ampwares.com/schematics/bandmaster_ab763.pdf) over 400V. Looking at the specs for that tube in a push pull configuration is showing a maximum voltage of 360V, the typical operation shows voltages of around 270V. I'm reading something wrong on the specs? What are the consequences of driving the tube at a much higher voltage that the recommended one.
First thing is, you said 6L6 but that schematic uses a 6L6GC. They have different ratings, as do the 6L6G and 6L6GB. What tube is the circuit designed for?

I don't know where you're getting "max" plate from but the RCA 6L6 specs max plate/screen at 400V/300V when running Class AB1 and the 6L6GC specs 500V/450V (if using UL the screen rating is upped to 500V). There are numerous 'typicals' for Class AB1 ranging from 360V/270V to 450V/400V.


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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Fri 14, 2018 5:24 am 
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Here's another example of a more robust version of the 6L6, the 6L6WGB or 5881.

https://www.radiomuseum.org/tubes/tube_6l6wgb.html

No doubt, some designs pushed output tubes beyond their ratings, and it was a good idea to have spares handy. Back then it didn't cost an arm and a leg for a pair of tubes.

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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Sat 15, 2018 4:06 am 
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Joined: Nov Fri 15, 2013 10:17 pm
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Location: Haddon Heights NJ
Thanks for the advice. I didn't know that there are different types of 6L6's. I thought that only the envelope changed depending on the letters behind the 6L6. I need to reverse engineer the circuit, I only have a shaky handwritten indication that the amp used a 6L6.

Thanks again,

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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Sat 15, 2018 11:18 am 
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dan3460 wrote:
I thought that only the envelope changed depending on the letters behind the 6L6.


That's usually the case, but the 6L6 is an exception to that.

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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Sat 15, 2018 1:41 pm 
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TPAairman wrote:
dan3460 wrote:
I thought that only the envelope changed depending on the letters behind the 6L6.


That's usually the case, but the 6L6 is an exception to that.


Always learn something new!! Thanks.

Careful with the turbine, they tend to be suckers :D

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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Sat 15, 2018 4:18 pm 
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Location: Fayette County, Pa
Look at your output transformer too. (Size and impedances) If you are reverse engineering a Fender amp, keep in mind that the lower power ones used 6V6s in the output. These generally put out around 15 watts and ran at a lower plate voltage. (Around 300 volts) i have seen cases where people have switched to 6L6s thinking they would get more power..... (Usually that just overloaded the power transformer and took out the amp)


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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Sat 15, 2018 4:37 pm 
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Joined: Nov Fri 15, 2013 10:17 pm
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Location: Haddon Heights NJ
Ok. understood. I don't have the amp with me so i can't take pictures. Could not find any markings on the amp, my best guess is it is a fender clone made in the early 60's (just a guess by some of the caps). You may be right, the owner remembers his father taking the power transformer for rewiring and it was never installed back.

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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Sun 16, 2018 7:18 am 
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The Fender Deluxe 5E3 is a wonderful amp, uses 6V6's, and is plenty loud. Check the schematic. It might match what you've got there.

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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Sun 16, 2018 2:13 pm 
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Joined: Nov Fri 15, 2013 10:17 pm
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I'll get the amp after the holidays, will reverse engineer then.
Thanks all

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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Sun 16, 2018 4:09 pm 
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The 6L6GC is the uprated version of the 6L6GB, with button-stem construction and a much thicker plate, allowing it to dissipate 30 watts; the previous versions were only rated at 19 watts. Although the GC version will replace the older ones, those cannot be used where the design calls for the 6L6GC.

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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Mon 17, 2018 2:41 am 
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Tim Tress wrote:
The 6L6GC is the uprated version of the 6L6GB, with button-stem construction and a much thicker plate, allowing it to dissipate 30 watts; the previous versions were only rated at 19 watts. Although the GC version will replace the older ones, those cannot be used where the design calls for the 6L6GC.
All true except the ratings are misleading as they changed rating systems from Design Center to Design Maximum. Design Maximum is about 15% higher for plate Watt (10% for plate volt) than the same Design Center rating so the difference in plate Watts is not quite so dramatic as it seems at first blush.

Design Center subsumes line voltage variation into the ratings whereas with Design Maximum you have to take that into consideration yourself.


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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Mon 17, 2018 10:44 pm 
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So, with today's higher line voltages, it would be important not to exceed the maximum ratings.

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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Mon 17, 2018 11:13 pm 
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Tim Tress wrote:
So, with today's higher line voltages, it would be important not to exceed the maximum ratings.
Yes and no. It was always the case that with Design Maximum you have to take into account line voltage tolerances (rise and fall) yourself.

Contrary to the popular belief, power company voltage at the pole is the same now as then. What has 'changed' is the amount of loss in home wiring. Mainly it's gotten better so the losses are less as well. The other change is where the 'spec' is defined. The old "115" / "117" nomenclature was AT THE PLUG (or the device in the case of heavy duty appliance motors) and assumed a particular line loss. Now days it's spec'd at the pole.

When designing equipment you should take into account the maximum and minimum line voltage (including worst case wiring losses), and have the device operate properly over that range, not just what you 'think' it will 'usually' be. Generally speaking, the "utilization voltage" ranges from 104 VAC to 127 VAC (often rounded up to 130VAC). So "back in the day" they had to design for 127 VAC (as well as 104 VAC) even though the device said "117 VAC" on the rear sticker.


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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Fri 28, 2018 6:43 pm 
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Location: Gainesville, Florida
the power transformer is missing :shock:


Last edited by tubeAMP on Dec Fri 28, 2018 11:15 pm, edited 1 time in total.

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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Fri 28, 2018 9:11 pm 
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Joined: Sep Tue 15, 2015 1:16 am
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Location: 18424 PA
Are you going to use it as a guitar amp? If yes, then running max voltage is what you want. Most of the problems come from running the dissipation too high, not the plate voltage.


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 Post subject: Re: 6L6G plate voltages
PostPosted: Dec Fri 28, 2018 10:28 pm 
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Joined: Nov Fri 15, 2013 10:17 pm
Posts: 2002
Location: Haddon Heights NJ
n3uvt wrote:
Are you going to use it as a guitar amp? If yes, then running max voltage is what you want. Most of the problems come from running the dissipation too high, not the plate voltage.

It is a guitar amp, it is a no brand (at least i couldn't find it anywhere). I still don't have the amp so i cannot look at the actual circuit yet.

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