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Post subject: Modded 1970 Princeton with 6L6s... Dangerous?
Posted: Sun Jul 25, 2010 7:44 pm
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So I'm about to do an amp trade for a 1970 Princeton which has been extensively modified. Reverb has been added, as well as a mid control knob and standby switch. The original speaker was also replaced with a Copperhead. What caught my eye most though was the fact that amp runs on 6L6s rather than 6V6s. Someone replaced the stock transformer with an early 1970's Vibrolux transformer so it could handle the new tubes.

The mods have apparently been done gradually over an extended period of time and it seems the amp has held up just fine. It sounds killer, thats for sure; a little Princeton, a little Deluxe, and a bit Vibrolux in a very portable package. But I admit that I'm kinda frightened by the 6L6s in a Princeton... With the more capable PT, do I have anything to worry about?

Thanks,
-Dan


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Posted: Sun Jul 25, 2010 7:54 pm
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That depends.

Was the filter cap can upgraded to handle the additional current from the new power tranny?

Was a variable biasing circuit added?

Was the output tranny upgraded to handle the additional power?

For every one of these hotrod jobs done correctly, I've seen a dozen that are merely three-alarm conflagrations waiting to ignite.

I'd recommend more research on this amp before you hitch your wagon to it.

Best of luck, HTH

Arjay

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Posted: Sun Jul 25, 2010 11:11 pm
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Very interesting ! The Princeton actually has a higher filtering value situation, than the Princeton Reverb. I can only see where two additional filter caps maybe used, but maybe this has already been done. If the output trans hasn't been changed, then I'd just go with 6V6's and adjust the bias, a variable pot is nice, but not necessary. IMO the 6L6's are not a very good match for the original OT. Something like, NOS Tungsol 5881 tubes, would be a closer impedance match. Which rectifier tube is in the amp? You know, I don't see any real problems, especially if the amp is stable and has had plenty of hours on it, in this configuration. Art

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Posted: Sun Jul 25, 2010 11:52 pm
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aclempoppi wrote:
IMO the 6L6's are not a very good match for the original OT.


+1

The problem if using 6L6's with the original Princeton O/T is that the plate-to-plate load resistance of the tranny's secondary winding is designed for a pair of 6V6's at +/- 8KΩ, whereas the 6L6's need to see around 5kΩ (specs for the 5881 are similar so they're really not a viable option either). That's more of a mismatch than I would ever be comfortable with or confident in.

If the O/T were upgraded to that from a Tremolux or Vibrolux (or the equivalent from Heyboer or MM) I'd consider the conversion to be correctly engineered and executed.

My two cents -- FWIW

Arjay

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Posted: Mon Jul 26, 2010 12:10 am
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Yep, 6V6's if the OT is stock. Art

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Posted: Mon Jul 26, 2010 5:03 am
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You know,I've never seen the value of changing an amp like this one has been...especially when a larger amp can easily be found like SF DRs and even SF Vibrolux's that are the Princeton's bigger brothers.
It's almost like somebody had too much time on their hands,but then again that's how Mesa Boogies came into existence.


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Posted: Mon Jul 26, 2010 7:34 am
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Yeah, you know I love your typical Princeton and it really doesn't need to be changed... But I must say this thing sounds good!

I'm not sure about the OT, I'll have to find out. Of course, if I'm really not comfortable I can just bias it back to accept 6V6s I guess.

Thanks for all the input, you guys have really helped!
-Dan


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Posted: Mon Jul 26, 2010 8:23 am
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Rebelsoul wrote:
You know,I've never seen the value of changing an amp like this one has been...especially when a larger amp can easily be found like SF DRs and even SF Vibrolux's that are the Princeton's bigger brothers.
It's almost like somebody had too much time on their hands,but then again that's how Mesa Boogies came into existence.


Well, Fender guitars and amps seemed to have been designed with the hot-rodder in mind (since it's so easy to do for those with the desire AND the requisite skills).

Personally, I limit my amp activities to "re-arranging the furniture" (ie: converting a piggyback to a combo or vice versa) and upgrading to premium speakers.

I think the only electrical mods I've made to any amps in the last 20 years or so were to have a totally-reversible standby switch added to my original '65 Princeton Reverb and installing a multi-tap O/T to my DRRI.

Supro and I have a mutual friend who builds amps from the chassis up, using basic kits. This fellow often deviates from the schematic by adding custom features not found on the original prototype(s) but in this case, the gentleman is very capabile at what he does and the changes are engineered into the build from the outset. I consider this type of work to be the paradigm in "amp hot-rodding".

I hate to see amps, either vintage or contemporary, hacked up by some clueless weisenheimer who thought he knew more than Leo Fender.

Arjay

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