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FDP Forum / Fender Amps: Vintage (before 1985) / '60 Fender Bassman: 6L6GCs to 5881s. Need biasing advice

theotherkind

Kentucky, USA

Jul 13th, 2016 05:53 PM   Edit   Profile   Print Topic   Search Topic

I'd love to try some of the new Tung-Sol 5881s in my '60 Bassman. It's set up for 6L6GC's and there's no bias pot. I have an Amp Maniac voltage regulator which can easily take the wall voltage down to 110 volts or even less.
There's no amp tech near me and I'd really prefer not to mess around with putting a bias pot in it. I have no idea how to bias an amp by substituting different resistor values, and I don't know where to begin with the lowered wall voltage. How should I bias this amp? Advice? Thanks!

SoK66
Contributing Member
*******

USA

We had the hit but Van got the money
Jul 14th, 2016 08:35 AM   Edit   Profile   Print Topic   Search Topic

If you don't want to add a bias pot then you have to adjust the bias by changing the bias resistor, which is the 56k one between the two 8uf 150v caps. Not sure how you check bia, either with a probe or the 1ohm resistor method on the power tubes, but it's not too difficult. If the amp really is set up for the 6L6GC, which are 30 watt tubes, I'd surmise the bias resistor has been reduced in value from the 56k. If it hasn't been changed then the amp was not really adjusted for them. The Tung-Sol 5881s are 23 watt tubes and bias at a lower current rating. If the 56k is still in there, I'd just put the 5881s in there and se how they bias up.

Wall voltage spec on a 5F6-A is 117vac. Set your buck transformer (Amp Maniac) to whatever setting gets you closest to 117vac. Then check the bias.

Cool amp!

Doc Sarvis
Contributing Member
**********
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USA/Salt Lake City

Tuned Strings and Tight Lines
Jul 14th, 2016 05:13 PM   Edit   Profile   Print Topic   Search Topic

I generally install bias pots on all my amps but in a pinch different tubes will show different values within a certain range so it's possible to buy several pairs and see which one gets you close. I've done this a few times for friends on tweed and brownface amps because I have a good tube collection to draw from.

BbendFender
Contributing Member
*********

American Patriot

I'm on guard these days.
Jul 14th, 2016 08:38 PM   Edit   Profile   Print Topic   Search Topic

Would this work on the Bassman? No alteration to the amp.

Uncle Doug and bias adjust pot.

SoK66
Contributing Member
*******

USA

We had the hit but Van got the money
Jul 15th, 2016 04:13 PM   Edit   Profile   Print Topic   Search Topic

Same basic idea. I use little trim pots I get from Doug Hoffman's site. The pic shows what I did on my former '59 Bassman RI.

Bassman bias trim pot

theotherkind

Kentucky, USA

Jul 19th, 2016 09:11 AM   Edit   Profile   Print Topic   Search Topic

That looks like a tolerable alteration, SoK66. So you just soldered the pot to the two resistors and that lower bias cap?
Did you get the 10kl pot, or the 50k?
Edit: and by the way, the 56k resistor hasn't been changed out.

(This message was last edited by theotherkind at 11:37 AM, Jul 19th, 2016)

SoK66
Contributing Member
*******

USA

We had the hit but Van got the money
Jul 19th, 2016 07:13 PM   Edit   Profile   Print Topic   Search Topic

Yes. I use Doug's 50k pots. On that amp it's a 50k pot & 10k resistor.

If the 56k resistor hasn't been changed and the 15K equalization resistor across the caps hasn't been messed with then nothing was done to the amp. Someone just changed out the tubes. In that case I'd just put those 5881s in there and see how it biases up.

theotherkind

Kentucky, USA

Jul 19th, 2016 08:02 PM   Edit   Profile   Print Topic   Search Topic

Even if nothing was changed and I wanted to put in a bias pot, what would you recommend I do?

SoK66
Contributing Member
*******

USA

We had the hit but Van got the money
Jul 21st, 2016 12:10 PM   Edit   Profile   Print Topic   Search Topic

I'd carefully unsolder the 56k and store it away, then install 50k trim pot & 10k resistor per the photo link.

theotherkind

Kentucky, USA

Jul 29th, 2016 08:54 AM   Edit   Profile   Print Topic   Search Topic

Thanks again, SoK66!

super mario
Contributing Member
*********

USA

tone to the bone no other way
Jul 29th, 2016 09:27 AM   Edit   Profile   Print Topic   Search Topic

I did a bias pot adjustment mod to my Pignose G60VR. This amp was built and sold by Pignose in the early 2000's as a 6L6 amp. It was always super bright almost brittle in tone as the bias was fixed and on the cold side. One thing that they said it could do is handle 5881's which intrigued me. After researching and a lot of questions, I opted to mod it to have an adjustable bias. It was the best mod as I could now dial in either 5881's or 6L6's to their sweet spot. I run 5881's along with 5751 preamp tubes to give it an early breakup. "It" has a wonderful tone that gets dirt without pedal assist a decent volumes. Good luck on your amp mod.


FDP Forum / Fender Amps: Vintage (before 1985) / '60 Fender Bassman: 6L6GCs to 5881s. Need biasing advice




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