Question about need for biasing a '65 Princeton

[Tom Huff]

Hey Guys, This may seem ridiculous but I want to be absolutely sure about this.

I bought a matched pair of NOS Marconi 6V6G power tubes for my '65 Princeton. I want to think I can just put them in and go but I know that some Princetons, mainly the later ones I've seen have a bias trim pot. I don't see one in mine. Does that mean mine is fixed bias and I should be OK?

Here's a gut shot for your confirmation.

Thanks for your answers! I'd really like to try out these new tubes today or tomorrow!!!!!!

 

[cronnin]

Re: Question about need for biasing a '65 Princeton

Try here, lots of usefull stuff for Princeton.

http://www.tdpri.com/forum/amp-central-station/192130-71-princeton-reverb-amp-maintanence.html

 

[glassman]

Re: Question about need for biasing a '65 Princeton

You should be ok.

I would definitely consider monitoring the bias current initially even though you can't adjust it. I'd hook up the probes and observe it for 10 minutes or so to assure yourself that everything is stable...and that the tubes themselves are ok and in a range that is proper for the amp.

 

[Curly]

Re: Question about need for biasing a '65 Princeton

yeah, as long as the power tubes are within normal specs, you should be ok, but as mentioned, you should also verify that.

when power tubes aren't normal specs, then you have to adjust the bias... and since Princetons don't have a bias pot, that means changing out the value of the bias resistors

FWIW, in general, NOS tubes should be more consistent than current production

 

[Tom Huff]

Re: Question about need for biasing a '65 Princeton

Thanks. If it comes to changing bias resistors I'll take it to a tech. I have a set of probes so I'll run it with them for an hour or so to make sure all is well.

Thanks again guys.

 

[kmcguitars]

Re: Question about need for biasing a '65 Princeton

Yup, A Princeton is a Fixed/Bias Nonadjustable amp stock. Here is a direct link to the Hoffman sites "library of information" Biascircuits I see a link w/t some pics of the mod was posted above, But this one takes you to the page. I did this mod to my 79' PR, And it works like a charm if you like to tweak your Bias. The best thing about it (as w/t any mod to a vintage amp) Save your original parts, And it's completely
irreversible, Pretty easy to do Depending on your soldering Skills? And only cost a few bucks in parts.:cool2:
BTW, The bias cap on your bias board next to the 27k bias resistor looks to be the only non-original component in your amp?

 

[zzmoore]

Re: Question about need for biasing a '65 Princeton

Any amp that has a "fixed bias" needs to have its bias checked when power tubes are changed. It is not too hard to add a bias pot, but........ That is one God Damn clean looking example of a BF Princeton. I am not sure I would consider adding a bias pot and the other necessities. If the bias is too hot or cold, just change the resistor.
Next item....what were you going to do if the amp did not sound/feel right after the new tubes were installed??? Do not trade your tone and playability for the convenience of not biasing your amp.
The bias cap was changed (that is good) and so was the diode. Unless some anal, OCD, tech stuffed the shells of your old by-pass caps, your amp is do for a cap job. That would also include the can cap in the power supply.
Nice Amp!!!
How many wires are on the power cord???

 

[Gearjoneser]

Re: Question about need for biasing a '65 Princeton

I'm just jealous of your amp. I want that SOB!!! haha

Curly's right, but I wouldn't worry much about it, just play it. With the initial design, they probably found the median bias point for 6V6's. As long as the plates in your tubes don't display any orange glow on them, when running, you're fine.

As for new tubes, I've found that JJ's bias up just fine in fixed bias amps. Some others run hot or cold.

 

[Tom Huff]

Re: Question about need for biasing a '65 Princeton

Any amp that has a "fixed bias" needs to have its bias checked when power tubes are changed. It is not too hard to add a bias pot, but........ That is one God Damn clean looking example of a BF Princeton. I am not sure I would consider adding a bias pot and the other necessities. If the bias is too hot or cold, just change the resistor.
Next item....what were you going to do if the amp did not sound/feel right after the new tubes were installed??? Do not trade your tone and playability for the convenience of not biasing your amp.
The bias cap was changed (that is good) and so was the diode. Unless some anal, OCD, tech stuffed the shells of your old by-pass caps, your amp is do for a cap job. That would also include the can cap in the power supply.
Nice Amp!!!
How many wires are on the power cord???

Wow. I didn't expect the amp lust from you guys but I understand why because this is my little keeper amp.

The old caps are the old caps... Will they affect anything other than noise levels because the amp sounds just perfect imo. There is almost no noise when I'm not playing and my tech friend had it on some kind of scope and said it was the cleanest amp he'd ever seen. I think he said there is literally no pre-amp breakup. Does that sound right? It's been about 10 years since he had it and I don't remember if I'm saying what he said correctly. But he was impressed.

He also put a 2 prong cord on it about 10 years ago. He said it was for safety reasons but got my approval.

At this point, although I can easily bias an AB amp with pots I'm a bit uncertain about biasing by changing resisters so I'll probably call him and see if he'd like to do it. Hell, he'd probably get a kick out of seeing the Marconis anyway.