How add bias control pot to 1973 Fender super 6 reverb?

[Hammerstone]

I have a Fender Super 6 Reverb amp (silverface)- the tone of the amp sucked when bought it in 1989, so i had it hotrodded - the guy put in a preamp gain knob in back and another know that seems to behave as an attenuator...not sure what all he did. anywah i was happy with the sound back then being a young metal head and all. But now i want the fat bluesy sound and have figured out how to use the attenuator to 0verdrive the power tubes - a-hah!! ok, so i checked my tube bias and it is WAY cold. and there is no bias adjust pot..... I read an article on the internet about how to add a bias adjust pot for these amps (they are the same as the fender twin reverbs), and it said use a 100Kohm, 1/2 watt pot- but when i went to get one, the pots i saw listed as amp bias pots all seemed to be 10Kohm. so i want to make sure of what i am putting in my amp so i dont blow it up!! Can anyone give me some input on this so i can get it right?

[steve25]

Well first things first. This is a loud 100 watt RMS amp, and with the bias set better or not you are not gonna get a good blues sound out of it unlees you are able to plat it on a volume setting of 7 or 8 with those 6 -10 drivers.
I would think about stepping down to a 30 watt or so amp with 2-12s at most.

[Hammerstone]

I know what you are saying, and this amp has an attenuator built-in, so i can dial the channel volume and the master volume both to 10, overdriving the power tubes, and keep the volume coming out the speakers to whatever level i want- and it sounds pretty good. Now that i have checked the bias (using a bias tester where you plug the tester into the tube socket and then the tube into the other side of the tester and then it tells you plate voltage -about 425V- and the mA for that tube (21 to 25 mA) and then you look at a Table to see is that cold, medium or hot bias for that type of tube) . Well checking the table, this bias was off the chart in the cold direction, and i would really like to see what it would sound like with the bias set on hot. but these later silverfaces do not have bias adjust pots in them. So I want to add one and see what happens....

[Kap'n]

If the amp has been modded there's no telling how to change the bias supply...that is, if it already hasn't been changed.

Do you have any documentation? A schematic?

Any tech worth his salt who did major reconstructive mods would have improved the bias system.

[Hammerstone]

Unfortunately, I do not have anything representing the mods this guy made- I even tried contacting the place of business (Friendly River Music in Portland, ME) where i had it done, but the owner has closed the shop and now only operates as a small mail order business, and he has no idea even who the tech might have been that made the mod. Apparently, back then a lot of the metal guys in CA were sending Marshalls over to him to get hot rodded, so i asked him if he wanted to do it to mine and so he did- i loved the mean sounding distortion it gained, but he never really tried to tell me what he did, and i was a dumb punk and didnt try to find out. Looking inside there, it just looks like he rerouted a bunch of stuff in there with wiring all hooked up to the two new pots he installed in the back panel. Since the tube bias tester is showing they are runnning cold, wouldnt this be all that matters? take a look at this reference for the mod i am wanting to implement

http://www.diyguitarist.com/GuitarAmps/TR-BiasMod.htm

my question is whether the 100k pot would be the right resistance, as I am not finding one offered, and wondering if this is a typo on the writer's part. The tube bias pots i have seen advertized on some websites are 10K.

[Kap'n]

Can you take some photos of the inside of your amp?

[Hammerstone]

Sure will - it may take me a day or 2 to get to it. how do you put photos in these messages?

[Kap'n]

You need to host them somewhere first, then link them.

[steve25]

A 100 k pot sounds really out of wack for anything to do with a bias circiut in that amp no matter how you might mod it. As I look at the schematic of a AB763 layout of a bandmaster their is less than 26k of total resistance in the bais circuit( ac thru dc portion) so like I said 100k is way out of wack unless the bias tap on the amps PT has crapped out, and the bias voltage is now coming off of the B+ string.

[Hammerstone]

Yeah, i was thinking that, considering that the resistor i would be replacing is only 15K, how much higher would the resistance need to be to get the voltage up to what it needs to be. Since its a Pot and i dont have to turn it all the way up, i guess it would be ok, just would have a lot of unnecessary top end - the burning question to me is should it be a linear pot or an audio pot? SInce the Bandmaster is only 26K, that makes me think i could cut down to a lower resistance- maybe 40K or thereabouts, just to have some wiggle room (remember, this is a pot, you can set it where you want it). Makes me laugh, thinking about some people when they find out i play electric guitar they say- man, they are SOOO loud- i tell em, hey the volume knob goes down too. So does anyone know about the linear versus audio issue?

[steve25]

If that amp is any where near stock, than find that 2.2k resistor that comes off the juction of the the bias diode and the 70mfd 100 volt cap. Remove that resistor and place in serise a 10k pot wired up center to etheir end, and Iam gonna guess that with that pot set at about 4k you will be ableto get those outputs up to 32 to 35 ma where they should be.

[steve25]

You might want to check first that you have that pot set near 2.2k when you first fire the amp back up so you do not red plate and burn up the output tubes!

[Hammerstone]

10-4! I appreciate the input