Fixed Bias Amp Question

[j mascis]

Hey guys.

I want to change the 6v6 tubes in my fixed bias amp ('62 princeton).

What does this entail?

If I just put new tubes in, I take it that they won't be dialed in and might run hot/cold depending how the former tubes were biased. Is that correct?
What did people do in the 60s when these amps first came out? Just swap in new tubes and hope for the best, or did they take it to a tech each time they changed tubes?

If I bring it to a tech, what exactly does a tech do to optimize the tubes on these? (i.e. since there is no trim pot what does he do to get the voltage ideal?). And how can I do it at home?

My idea was to have a tech install a trim pot, but that might ruin the value of the amp.

Thanks.

[Ursa Minor]

Are you changing the current tubes because they don't sound good? Or just tying different ones?

I use a Weber Bias Rite to to get the mA reading from each 6v6. The plate voltage you'll have to get from inside the amp while it's running. VERY DANGEROUS. My 6g2 is somewhere in the 300vdc range and my PR is over 400vdc. Someone could install external points to mature this with a voltmeter....but that's kinda over the top in my opinion.

On a 62 Princeton you'd need to change a resistor to dial in how hot or cold your tubes are. I installed a trim pot in my PR to do this and it makes it very easy to dial them in. My 6g2 is just the stock resistor since the pairs of tubes I have are all within the same range and I don't think they need to be fuzt'd with.

Not sure if folks took their amps in to be biased back in the day. But I'm wagering to bet most folks simply didn't have that kind of info and were probably fine with plug and play.

[j mascis]

Not sure if folks took their amps in to be biased back in the day. But I'm wagering to bet most folks simply didn't have that kind of info and were probably fine with plug and play.

That's what I was thinking, so tubes probably burned out quicker, and maybe they weren't getting ideal tone.

My current tubes are okay, but they are old Sylanvias, and I came into some RCAs that I wanted to try out just to see the difference. I probably won't change them until the Sylvanias burn out so as not to waste tubes. But, when I do change them out, can I just plug the RCAs in and hope for the best, or do I need to bring them to a tech each time? That can get really expensive.

I read up on the mod changing the resistor and installing a trim pot. I could have a tech do that, but it might kill the value of the amp. I don't plan to sell it, so I wouldn't care, but if I came into an emergency where I had to sell it I might get killed on price for the mod.

Would you say my options are to bring it to a tech each time I change the tubes, or to mod it? It seems trying to find the plate voltage at home is too dangerous since I've never worked in that much detail on amps. I'm still unclear on what the tech does when I bring it to him since there is no trim pot. Does he actually change out a resistor each time to match the new set of tubes? I.e. how does he change plate voltage without any bias/trim pot?

Sorry if these are stupid questions. I bought the amp based on the sound and didn't really pay attention to the fixed bias issue.

[HNB]

I am guessing back in the day you would just grab a new tube and pop it in. I doubt a lot of musicians took apart their amp and uses a meter on it to adjust the guts of the amp.

I would go the trim pot way. You could always have it reversed and tbh, I am not sure why a buyer would look at that mod and say you ruined the amp since it makes it easier to run better. It would be like throwing a fit over putting a correct three prong grounded plug and disconnecting that huge ground capacitor. You aren't looking to change the amp, just make servicing it easier.

[Ursa Minor]

The bias pot mod is pretty much non invasive since it can go in the ground switch (irrelevant after a three prong cord is installed.) It really just adds range to the current bias resistor. I can't say it will kill the value if makes it more serviceable. Especially in this day and age where folks want to try tubes out and tweak. It can be reversed pretty easily too.

The old Sylvanias may have been original and probably still sound great. I love Sylvanias from this era and earlier. The RCAs are awesome too, of course. I'd say try them out. Just swap them out and see if you like them. If something seems odd turn off the amp and swap them back. Chances are they'll sound pretty great without the need to worry about the tech. Tubes are always going to be a little mismatched (or a lot sometimes!) and that's okay. These amps were made for that. I've always felt that having perfectly matched tubes was bullshit. There's no point, the sky doesn't just open up and give you holy grail tonez.

If you do take it to a tech, ask if you can watch and maybe get a tech that's willing to take you through what their doing so you can understand what goes into it and what you're paying for. But there's no need to take it in every time you need to change tubes.

Any pics? I love brown Princetons! There hasn't been nearly enough chatter about them on here lately.

[j mascis]

Thanks guys. I'll try to get a photo up soon. It's not a museum quality piece, but it's a nice amp. The brown Princetons are the best of all worlds, imo, with the only drawbacks being this biasing issue and the lack of reverb (the fantastic tremolo makes up for it somewhat). I like the old Sylvanias for sure but got a few RCAs at a great price and was curious to try them.

Is it accurate to summarize that these are the issues with dropping tubes in without taking it to a tech:

1. They might run hot and burn out quicker.
2. They might run cold and tax some other part of the amp. (this would be the biggest concern, if it's valid).
3. Headroom might change and it breaks up earlier or later.
4. Maybe some minor tonal changes? Though, those could originate from using different tubes.

If I'm totally informed I'll make a better decision. My main concern would be damaging the vital parts of the amp.

ps. Regarding mismatched tubes, anyone know how that works? Does the hotter tube burn out quicker and eventually comes down to match the other tube?

[muchxs]

What did people do in the 60s when these amps first came out? Just swap in new tubes and hope for the best, or did they take it to a tech each time they changed tubes?

... so tubes probably burned out quicker, and maybe they weren't getting ideal tone.+

Having been there... we just replaced the bad tubes. That's why Fender came up with the '70s bias balance circuit in lieu of actual adjustable bias. Stick mismatched tubes in there, fudge the balance... good to go!

Comparing apples to oranges... I have a batch (more than a dozen) original Tung-Sol 5881s manufactured in the late 1950s. They're astonishingly consistent. They bias within 1 millamp across the entire batch.

Hassle with "NOS" these days is we don't know where they've been.

If I bring it to a tech, what exactly does a tech do to optimize the tubes on these? (i.e. since there is no trim pot what does he do to get the voltage ideal?). And how can I do it at home?

I designed a custom bias supply board that uses a small Marshall style pot. It's simple... unsolder the three wires that go to the supply. Remove the two screws that hold it in place. Save the old board for posterity. Drop the new board in where the old one came out.

My idea was to have a tech install a trim pot, but that might ruin the value of the amp.

Your amp's real value is as a good tool to make music. The way it "ruins" the value of the amp is if you perceive the amp to be de- valued after necessary service. The silly "collector" market is anti- music IMO. The reality of the situation is parts wear out and need to be replaced. The further reality is a vintage amp with matched tubes properly biased yields better tone and reliability.

[MechaBulletBill]

The silly "collector" market is anti- music IMO.

That's an excellent way to describe it. Thank you for that!

[NoiseNoiseNoise]

The silly "collector" market is anti- music IMO.

That's an excellent way to describe it. Thank you for that!

+1!!

[46346]

Having been there... we just replaced the bad tubes. That's why Fender came up with the '70s bias balance circuit in lieu of actual adjustable bias. Stick mismatched tubes in there, fudge the balance... good to go!

Comparing apples to oranges... I have a batch (more than a dozen) original Tung-Sol 5881s manufactured in the late 1950s. They're astonishingly consistent. They bias within 1 millamp across the entire batch.

indeed, biasing was less of an issue back then as the classic American tube makers were in their prime (50's and 60's), and quality was consistent. RCA, Tung-Sol, GE, even Sylvania and Westinghouse were nailing it. most of the time, if you stuck to the same brand, you didn't have to re-bias.