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Post subject: 6L6 WGC (TAD) bias question
Posted: Sat Jan 11, 2014 7:56 pm
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Aspiring Musician
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I know this is a Fender forum, but.................

I need suggestions on biasing a reissue Ampeg Superjet SL12T, (Hey, it's in a Fender chassis) SS rectifier, which is supposed to be close to 50w. The schematic recommends biasing @ 38v, way too low. My plate voltage is now 396v's, (which is close to the schem's 400v) and the bias now 45v. That's 17.8 watts, which is still too low, I think anyway. These tubes are supposed to be rated @ 30w. I'd like to crank it up but I'm wondering if I'd be better off swapping out some resistors, rect mod, etc., to get the plate voltage up first. I'm using an Amp-Head dual bias tester for the measurements. I've upgraded the stock OT and am considering upgrading the PT anyway.

These amps are kind of "lost in space" as far as info on mods.

Thanks for any input.

UN


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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sat Jan 11, 2014 10:10 pm
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Surely you mean 45 mV, not 45 V. :?: The accepted proper range is 15 watts to 21 watts for a 30 watt tube.

Bias it up to 21 watts if you feel the need, but IMO nearly 18 watts is plenty. No need to mess with the Plate voltage, just bias it as is.

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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sat Jan 11, 2014 10:48 pm
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Agree with shimmilou. Setting the bias hotter will cause the plate voltage to drop more. Right now you are at about 57% max plate dissipation (plate current is about 43 mA per the Weber bias calculator which puts the idle tube dissipation at 17 watts, not 17.8). You may be able to crank it a little hotter.

Leave the PT and power supply alone (unless parts need to be replaced). Raising the plate voltage will affect all the voltages which may change the tone. I see nothing wrong with the way things are now.

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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sun Jan 12, 2014 7:48 am
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Aspiring Musician
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Thanks for the advice guys. Yes I did mean mv's. I guess I'm just not satisfied with the tone. I realize that probably has more to do with design, so changing some coup-caps may help a little. I've done speaker swaps and tube swaps with some improvement but it's still very middy, with only a single tone control to work with. This design is different anyway, 1/2 of V1 is for the tremolo, the other 1/2 pre, V2 is the PI, then the power tubes, so I'm limited to a 12AX7 in V1 otherwise the tremolo is screwed up. I should probably get a balanced PI tube.

Even with my somewhat limited knowledge I still enjoy these challenges.

Thanks again.


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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sun Jan 12, 2014 10:10 am
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Have you tried a 12AT7, or a 12AU7 in the PI spot? This amp might also be a good candidate for some 6V6 output tubes, biased properly of course. You will get less output power with 6V6, but the tone might be more to your liking. The Plate voltage will rise when using 6V6, so you might be limited to those tubes that can handle voltage in excess or 430 V, such as JJ 6V6, GT 6V6 S (S version only) or EH 6V6. The manual mentions the possibility of EL34 also, but I would check the pin connections on the tube socket before trying EL34.

http://www.ampeg.com/pdf/SJ12T.pdf

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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sun Jan 12, 2014 11:06 am
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" I've upgraded the stock OT and am considering upgrading the PT anyway. "

Just outta curiosity, what make and model output tranny did you go to? This change can have a big influence over the final tone.


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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sun Jan 12, 2014 11:58 am
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Aspiring Musician
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I went with a Classic Tone 40-18010, which is 4.2k on the primary and a 50w, straight 8 ohm. It does sound better - twice the physical size too.

I like the idea of 6V6's. I do have a couple new matched JJ 6V6S tubes here. Can I just rebias by calculating and re-checking with these tubes? No other mod's for now? The schematic doesn't show what the spec's are on the power tranny and it isn't branded with any helpful hints as to the maker. SLM probably had them made or designed by someone else and I'm not set up to check PT spec's, haven't got to that knowledge point yet.

Thanks again.


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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sun Jan 12, 2014 12:13 pm
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Yes, plug in the 6V6, bias and go, no other mods necessary. :)

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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sun Jan 12, 2014 12:19 pm
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Yes, you can use the JJ 6V6S and re-bias to 6V6GT standards. The 6V6S can handle plate voltages of 450-475VDC (IME), as long as you keep the idle current, such that the dissipation doesn't exceed factory recommendations.

I'd bias the tubes to about 60-65% of max (bias to ~8.5 watts per tube), in your amp, and see how the thing sounds.

http://www.jj-electronic.com/pdf/6V6.pdf


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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sun Jan 12, 2014 12:46 pm
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Aspiring Musician
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Thanks for the help, I really do appreciate it. I really don't want to sink too much money into this amp if I can help it. That's why I didn't buy a MM ot tranny. I'd rather put money towards a blackface build. Well, it's potato and sausage soup and then tube changing time.


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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sun Jan 12, 2014 1:01 pm
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Nothing wrong with sausage and potato soup, upnorth.

Those of us with Irish, Polish, and/or German roots love that stuff!

Bon Appetit!

:wink:

Arjay

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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sun Jan 12, 2014 1:42 pm
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Aspiring Musician
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Well I rebiased to about 8 1/2 watts with the 6V6's. I'll try it in a little while.

Plate - 402v @ 21 - 22mv

The bias reading was low, about 4 when I started. The plate voltage didn't go up much. 402v with the 6V6's and 396v with the 6L6's. Should it have gone up more than that?


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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sun Jan 12, 2014 2:24 pm
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You did it correctly. I expected more of a Plate voltage increase, but your meter knows better.

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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sun Jan 12, 2014 2:48 pm
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Aspiring Musician
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Joined: Sun Oct 27, 2013 7:11 am
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I got a chance to play it for a while with the 6V6's and I can't really hear much difference. I didn't get the volume up past 4, but even the volume level doesn't seem that different. Maybe this amp is just doomed to the world of "thin and cold". I'll try a AT7 in the PI socket and see. I can hear the potential through the rough but overwhelming mids are blocking the warmth. (tried different guitars as well) Maybe I need to do some schematic comparing of other amps.

I'm glad I did the tube swapping and re-bias with the 6V6's though. A good thing to know and I'm sure it works good on some other amps.

Thanks


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Post subject: Re: 6L6 WGC (TAD) bias question
Posted: Sun Jan 12, 2014 3:11 pm
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Try going up on the idle bias, a little. Say to 9-10 watts. JJ 6V6S tend to need a bit hotter bias to warm up the midrange. Either that, or do you have another pair of 6V6GT's. Like maybe some old RCA's?

The response below "5" will prolly be about the same as a 6L6GC. Above "5" you should get OD sooner with earlier onset of compression.

Good luck!


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