It is currently Mon Mar 16, 2020 8:30 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 8 posts ] 
Author Message
Post subject: Princeton Reverb (B1270) reverted back to AB764
Posted: Mon Apr 09, 2012 12:42 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Redid a costumer's PR from what was supposed to be a "stock" B1270 amp to AB764. The guts of amp shows more than a moderate amount of modifications. Again, those crazy Torres-labeled SBE 718P coupling caps. Which are really pretty good caps & hard to find these days. But, did not sound good at all in the setup or mods done to this amp. Very much in-your-face tone with none of the sweetness or fine detail PR are noted for. Note: cap can was not the correct one (a 20/20/20 450 VDC three stage can, instead for four). No attempt to add another 20mfd cap. This threw the voltages off, esp on the critical input and consertina PI stages.

http://www.kbapps.com/audio/schematics/ ... b1270.html

Anyhow, after some swapping of caps (to Mallory-150 and Russian K40Y-9 PIO). Redoing the tone stack. Modding the PS rail to GZ34 standards (I used that 5V4GA labeled mil spec Philips 5AR4). Nice used Weber alnico. Amp is sounding back to 1964 standards.

BTW.... this amp REALLY likes a well layed out PI section. I even used matched grid-to-ground Mills WW resistors (hand measured 220.4 and 220.6 K-ohms). And those mil spec Russian PIO's (0.1 mfd/600VDC) and not the 0.15mfd/600VDC OD 718P that were in the amp. The amp's response to transients and bass took off. Like I added a 15-inch speaker to the cab. Very clean and tight bass.

Finally, used some high testing Ken-Rad VT-107A (1942 era) 6V6G/GT. Very nice tubes, for those looking for good used tubes. Biased them to about 10.5 watts per tube. Darn things hardly even warm up, even after an hour of use above "5."

About 4 hours of work. Well worth the effort. As primo AB764 or even good AB1164 are getting harder to find and more $$$ these days.

Before. (Fairly messy layout and use of that wax-laden breadboard):

Image

After. I also replaced the cap can with an FP 20/20/20/20 at 500VDC can, which is now avaiable at AES. Nearly double the size of the 450VDC one. Much more electrolytic fluid. Should have a much longer life.

Image

Front grille changed out. 5AR4 visible at left. Great Ken-Rad tubes

Image

Image


Last edited by BMW2002Ti on Mon Apr 09, 2012 2:53 am, edited 2 times in total.

Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: Princeton Reverb (B1270) reverted back to AB764
Posted: Mon Apr 09, 2012 1:01 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
The chassis does not match the cabinet (note the tube chart within).

Any issues with the larger 20/20/20/20/500V cap can as far as it fitting the standard PR mounting accommodations?

Nice work, Steve!

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: Princeton Reverb (B1270) reverted back to AB764
Posted: Mon Apr 09, 2012 2:16 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Arjay,

Thanks, for the compliment!

No the 500VDC cap can fits fine. You can see how tall it is in the lower photo (this photo was taken after all the internal mods were done). It looks like the can is close to the one speaker's screw, but is actually a good 1/2-inch or so away.

The owner changed out the cab to fit a 12-inch speaker. I think he added the tube chart. Which I told him this ends up fine... to remind him that the amp is setup for a GZ34. Just don't forget that 12AT7!

The 5U4GB run a tiny bit lower B+ and quite a bit lower bias current through those 6V6GT's. I'd say about 16-18mA per tube. Versus 22-24mA with the GZ34, at idle. The plate voltages on the output tubes are fairly close, at idle. 412 versus 415 VDC. The 5U4GB is an NOS GE. I've found that the several multi-tap Fender amps I've worked on, run a slightly lower B+ than the amps setup for 120VAC, only. Is that your experience?

Most PR I've worked on usually run the B+ in the 420-430VDC range. But, I remember a couple of BMR's (TFL 5005D) also had lower B+ than I expected.

For this amp & 6V6GT tubes. I dropped the 27k-ohm (C- to ground) resistor in the bias supply to 34k-ohms. And use two 33mfd/160VDC F&T 'lytic parallel, in place of the stock 50mfd/70VDC cap. Kept the 100k-ohm dropping resistor and changed the diode to a UF4004.

So far so good. Very sweet tone with both the Telecaster and Gibson LPS.

Boy, that "pull" option on the volume control REALLY makes this amp sound crazy! Like a super-duper presence boost. Not sure if that circuit was modded after the original sale, but it has to be one of the lousiest options I've heard on any Fender amp.


Top
Profile
Post subject: Re: Princeton Reverb (B1270) reverted back to AB764
Posted: Mon Apr 09, 2012 2:50 am
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
The only "export" Fender I've ever worked on was a 100W MV Twin, about 25 or 30 years ago. I think the schem specified 445VDC for the B+ with the standby switch in "play" mode but I measured only about 425VDC at pin 3 of all four jugs. The amp otherwise sounded fine and seemed to play okay. I was never certain as to the reason for the low plate voltage but I suppose it could be the nature of those multi-tap PT's.

Arjay

_________________
"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


Top
Profile
Post subject: Re: Princeton Reverb (B1270) reverted back to AB764
Posted: Mon Apr 09, 2012 6:37 am
Offline
Rock Star
Rock Star
User avatar

Joined: Tue Mar 01, 2011 3:10 pm
Posts: 5646
Location: Gateway to the West
Steve, nice job there. How rare to find an Export Model with the "Return to Blackface" Cosmestics. Don't see those everyday.

T2

_________________
-----------------------------------------------------------
What time is it? It's Fender Time.


Top
Profile
Post subject: Re: Princeton Reverb (B1270) reverted back to AB764
Posted: Mon Apr 09, 2012 4:19 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Yeah, choice components and work, Beemer! That wax is a real drag. Usually scrub it off the areas I work on. Really like your use of the Mills, I'll have to try some. Where can I get them? What type of disc caps are those? You didn't mess with the .02's in the tone stack? Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Post subject: Re: Princeton Reverb (B1270) reverted back to AB764
Posted: Mon Apr 09, 2012 8:02 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Yeah, sounded ok with the 0.02 mfd coupling cap. I have a bunch of 0.022mfd/1KV Russian PIO coming this way. These are pretty good caps, for the money. Love these and 630VDC rated ones. Metal clad, tough little puppies.

Thanks! for all the comments. Nice to get these CBS-era amps back to 1960s' tone standards.


Top
Profile
Post subject: Re: Princeton Reverb (B1270) reverted back to AB764
Posted: Tue Apr 10, 2012 1:31 am
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Yeah man, the .02 caps are just a personal thing. Appreciate your opinion," sounded OK". In two channel amps, I usually keep one channel in the generic configuration. Again, great job! Art

_________________
None of Us are free, if One of Us is chained !


Top
Profile
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 

All times are UTC - 7 hours

Fender Play Winter Sale 2020

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum

Search for:
Jump to: