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Post subject: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Wed Oct 05, 2011 7:34 am
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Hi
I am new to all valve amps and so I have been poking around inside my 2010 'The Twin' to try and learn a bit about more about bias levels etc etc. I am very aware of the voltages that are present even after the amp is switched off and am taking precautions to 'poke' safely :). I have worked with telecomms equipment all my life, which is why I feel I should be making the effort to understand my guitar and equipment more.
I have recently changed my O/P tubes to 6L6GC winged C s . I biased these at 70mV for each pair using the test points provided on the rear of the chassis, after taking advice from you guys, based on the information on my service sheet that states +475VDC as the supplied B+
When I actually measure directly onto the valve base socket on pin 3 using a Fluke DVM, I am getting +499VDC. The test point TP36 and TP37( which puts you directly on the track to each pair of pin 3s) on the service sheet 57014000 states that this should be +465VDC.
Set up is the same as for measuring/setting bias, ie all gain, vol, reverb, trem, pres controls on 1
From what I have read, this voltage would seem to be right on the maximum for this type of tube. I havent got the original values for when the Fender supplied GT 6L6GC tubes were in place. When the new winged Cs were fitted they had a bias level of about 55mA compared to the originals which were set at 70mA.
Can anyone assure me that all is well, and that the tubes arnt gonna die in 3 months :O
FYI, Fender state that the bias should be set at 60mV, and I have adjusted it back to this value for the time being. The winged Cs are the originals, as supplied and tested by Watford Valves of England.
I hope I have given you all the relevant info :)
Edit : no I havent, this amp is in England and so the wall supply is a nominal 240VAC, although the chassis has been tagged with a 230VAC designation. But it is meant for the UK market.
Your thoughts will be much appreciated
Mel


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Post subject: Re: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Wed Oct 05, 2011 8:22 am
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The Plate voltage may be high because of the higher input voltage. I believe that there is another tap on the power transformer to change to 240 volt input (instead of 230), that should lower the Plate voltage also.

60 mV, or even 70 mV at either bias test point, is too low. You set the bias dependent on Plate voltage and tube Plate wattage rating, and each test point is for two output tubes. You want the idle to be between 50% and 70% of the tubes rating, and for a 30 watt tube this would be 15 watts to 21 watts.

At roughly 499 Plate volts, 70 mV at either test point would be 15 watts idle per tube, while 95 mV would be 21 watts per tube. At roughly 475 Plate volts, 72 mV for 15 watts and 98 mV for 21 watts. 15 watts for the most clean headroom, 21 watts for earlier breakup. I usually set mine at around 55% to 60%.

I don't know if the "Winged C" can handle that high of a Plate voltage or not (ask the tube supplier), but you should definitely try to lower that voltage with the power transformer taps (if available), as the other amp voltages will be a little high also. :idea:

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Post subject: Re: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Wed Oct 05, 2011 9:17 am
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Thanks for the input shimmilou.
I have checked the service sheet and the only option that includes the UK on the primary wiring is shown as "230V and 240V for Uk and Aust Units"
If you have access to the Fender Schematic 57014000 rev F you will see it is the top option for the primary power supply.
Well..............., you know whats coming don't you?? :)
It appears that the version I bought was intended for Europe only @ 230V.
I have rearranged the wiring from the power transformer so that it matches the correct set up for the UK. Its only one wire really, the wht/bk moves to P208 as nc and the bk is now in its place on the power switch.
I have measured the plate voltage on pin 3 of each tube and....
its now 477V. Thats close enough for me to the quoted B+ supply of 470V.

When I decided I wanted a Fender The Twin, I had difficulty locating one in the UK, in fact it seemed like they were no longer being made as I could only find one supplier that had one in stock. This was the last one they had. The tag inside states that it was manufactured in October last year, but maybe it should have gone to Europe and not to the UK dealer??
I will set the bias @ 70mV as I need clean headroom and not early breakup :)
Many thanks Shimmilou, oh, I measured at the wall outlet and we have 244V at the mo but that will prob drop a bit later this evening
I wonder what it will sound like now its running at the correct i/p voltage??
See u later :)
mel


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Post subject: Re: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Wed Oct 05, 2011 9:43 am
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Good job! It seems that we assumed on the other thread that the Plate voltage was correct. With this experience, I would say that you are no longer new to tube amps. 8)

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Post subject: Re: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Wed Oct 05, 2011 9:59 am
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Hey thanks for the help shimmilou. All is well.
I'm glad I am naturally inquisitive by nature, I dont know how long it would have held up had I not 'poked' around inside :)
Cheers for now :D
mel


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Post subject: Re: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Wed Oct 05, 2011 10:31 am
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Good job rewiring that primary for the correct input voltage, Mel.

And a good call to Lou for inquiring about it (I always thought that the UK used the same wall voltage as the rest of the continent).

The 499VDC at the plates of those winged C's was definitely too high for their "comfort level". You shouldn't have any problems with 477.

RAWK ON!

8)

Arjay

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Post subject: Re: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Wed Oct 05, 2011 12:25 pm
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Hi Arjay,
well I always thought that Europe was pretty much the same nominal voltage as us in the UK.
When we go on vacation we take hair dryers etc with us and just use an adapter for the wall socket.
But, there ya go, 10 volts of primary voltage does make a difference. I wonder now whether the original GT/Fender tubes will sound better. They have only had about 20 hrs of playing on them. They are marked as winged C s but the glass has been constructed differently to the original winged C s I have just installed.
I have had this amp apart so manytimes in the last month the screws are looking like real veterans lol :)
I used to be like this years ago with hifi eqmt, technophiles I think we are called. More interested in the eqmt than the actual sound. Sad eh.
But, having played and twiddled with various bias settings just now, I can confirm there is a sweet spot for whatever your ideal tone is. I have arrived at about 65mA for a clean tone that just starts to break up when I dig in hard at the levels I play in my band.
This forum is brilliant, you guys are so willing to share your knowledge and experience.
Its a valuable resource and who ever your gods are, he,she,they will recognise your efforts to help others. :) Arjay and Lou take note.
Time for tea, as we say in England
Cheers,
mel
www.dawntreadermusic.co.uk


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Post subject: Re: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Wed Oct 05, 2011 4:36 pm
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dawntreader wrote:
well I always thought that Europe was pretty much the same nominal voltage as us in the UK.


I think UK voltage is "left-handed" as opposed to "right-handed" like the rest of Europe also. :lol:

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Post subject: Re: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Wed Oct 05, 2011 6:33 pm
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bluesky636 wrote:
I think UK voltage is "left-handed" as opposed to "right-handed" like the rest of Europe also. :lol:


WEISENHEIMER!

:mrgreen:

Arjay

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Post subject: Re: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Wed Oct 05, 2011 6:40 pm
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Retroverbial wrote:
bluesky636 wrote:
I think UK voltage is "left-handed" as opposed to "right-handed" like the rest of Europe also. :lol:


WEISENHEIMER!

:mrgreen:

Arjay


Only part. Mostly Polack. :lol:

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Post subject: Re: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Wed Oct 05, 2011 6:49 pm
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Mebbe we're cousins.

:mrgreen:

Arjay

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Post subject: Re: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Wed Oct 05, 2011 9:35 pm
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Retroverbial wrote:
Mebbe we're cousins.

:mrgreen:

Arjay


That is a frightening thought. :lol: :lol: :lol:

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Post subject: Re: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Wed Oct 05, 2011 11:00 pm
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Only if the thought of a vodka-soused horseback-mounted saber-wielding cossack scares the hell out of you (that's the other half of my maternal geneology).

:mrgreen:

Arjay

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Post subject: Re: Plate voltage of 6L6GC winged C in 'The Twin'
Posted: Thu Oct 06, 2011 2:23 am
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dawntreader wrote

Quote:
Edit : no I havent, this amp is in England and so the wall supply is a nominal 240VAC, although the chassis has been tagged with a 230VAC designation. But it is meant for the UK market.
Your thoughts will be much appreciated
Mel




On a related theme i had a surprise with a 5E3 build i completed 2 or 3 months back. When the B+ was measured it came out high, on measuring the mains supply (here in the UK) it was 247 v ! :shock:
I doubled checked and it was correct. I thought the voltage had been reduced in recent years obviously not.
I mentioned this to tube depot where the kit came from and Rob Hull suggested this as a possible fix.

http://www.geofex.com/article_folders/v ... ntvolt.htm

I fitted a 9 volt version and now the supply is a more reasonable 238 v

Dont want to highjack the subject but a recent problem where i thought the speaker was toast and i asked about speaker changes turned out to be knackered valves, preamps microphonic and power valves sounding poor.
Fitted old spec Mullard/Philips preamps with the Mullards in at present has really quietened the amp down with respect to low level hum/hiss. They say old valves were better made?
Al


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