Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: moonbird on July 21, 2014, 01:05:59 pm
-
Hi there -
I am looking at buying one of Doug's Princeton Reverb boards for a project but I have a couple of questions:
1. What is the difference between the Princeton Reverb and the AB763 single channel topology *aside* from the different PI? I can't quite figure out if the vibrato is different or not (I *am* aware that Doug's AB763 uses bias wiggling vibrato vs the traditional Fender design)?
2. I would like to try a paraphase PI in this amp - would the AB763 be a better choice since it would have some extra triodes available? Could/should I still use the Hammond board or should I just start fresh (LOL). I like the "safety net" of the proven design I guess.
Thanks.
-
I like the "safety net" of the proven design I guess.
I agree....
Anytime you can follow one of these templates that has been supplied here it is a good idea, and wise practice...
Not sure if you have seen this yet, but if I was gonna build one of these I would tape this over my bench and not look elsewhere....
Clear, clean layouts like this are priceless:
http://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf (http://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf)
...and if I wanted to try a paraphase inverter, I would just make the adjustments on his board....you shouldn't need any more tube sockets,,,the paraphase will use the same triode that the cathodyne would
...and read this: http://www.valvewizard.co.uk/paraphase.html (http://www.valvewizard.co.uk/paraphase.html)
Could/should I still use the Hammond board
I'm assuming you mean Hoffman board ....?
-
SG --
Thanks much for the encouragement. I really like how the paraphase sounds - particularly for a blues amp which is what I am trying to build here (Supro in Fender clothing). Thanks much for all the great advice too! I had forgotten about Merlin (very dumb). This is my first go at "designing" something (more like leap frogging really) so all the "nets" are very comforting (LOL).
-
I can't quite figure out if the vibrato is different or not (I *am* aware that Doug's AB763 uses bias wiggling vibrato vs the traditional Fender design)?
The tremolo circuits of both amps are functionally the same. But...
The PR uses a single triode tremolo circuit. The oscillator directly modulates the bias for the power tubes. Hoffman's AB763 uses two triodes for the tremolo circuit. One triode is the oscillator. The oscillator connects to a cathode follower and the CF modulates the bias for the output tubes.
-
Here's a quick little cut and paste mockup of what it might take to get you close to adding the paraphrase to that layout...and Doug's board
Notice the balance pot added as a grid leak for the paraphrase input...
This was done very quickly,,,so let sluckey have a look and make sure I'm not steering you in the wrong direction.
Again,,,,this is a totally unconfirmed first draft (I didn't pay any attention to values), just trying to see the path with you :icon_biggrin:
-
The oscillator connects to a cathode follower and the CF modulates the bias for the output tubes.
Sluckey - Thanks very much - that really helps me and it really makes sense. Would it be true to say the cathode follower makes the vibrato circuit more efficient because it is lower impedence? Could a mosfet follower be inserted to do the same thing in the Princeton design? Wow thanks.
-
SG -
VERY COOL dude!! I gotta study this tonight when I have some time. Thanks much. :worthy1:
-
SG -
VERY COOL dude!! I gotta study this tonight when I have some time. Thanks much. :worthy1:
No problem moon,
Hopefully it'll get confirmed or denied......I'm just the artist,,,not the engineer :wink:
-
SG -
With ya there brother!! Except I ain't really much of an "artist" either. More like a "hyperactive hack" (Hx2) I am afraid :laugh: .
-
I took another quick look and added a coupling cap between our balance pot and the bias circuit....
:dontknow:
-
More grist ....
http://fenderguru.com/amps/princeton-reverb (http://fenderguru.com/amps/princeton-reverb)
I need to look at everything to try to decide which version of the Princeton Doug's board implements ...
Oh shucks -- I gotta read a bunch of stuff about tube amps .... boo hoo :l2:
-
All -
Could mention that I have a Onetics "Blues" OT slated for this amp. It is much bigger than the standard PR OT. So I will want to error on the high freq side with the caps I think.
Man that stuff Merlin hints at (dissimilar paraphase tube) certainly gets my head going. Best to stay stock to start probably and get it running first. But I will definitely want to install that trim pot for the power tube grid leak on the paraphase tube side. Gotta goof around with that.
I am also considering a triode/pentode tube for V1. I can "steal" a config from some other web designs to get started. I just love a pentode in the preamp for a bluesy amp.
To partially counteract the paraphase "grubbiness" (Merlin's term), maybe a PP reverb pan driver would be fun to put in this thing ... or maybe tubenit's 6BM8 reverb - which would free up a triode for the vibrato perhaps. Garsh my little brain is just "squirming like a toad" ...
-
I am also considering a triode/pentode tube for V1. I can "steal" a config from some other web designs to get started. I just love a pentode in the preamp for a bluesy amp.
FWIW (not much) I don't like a pentode strapped to make a triode. When I did it, it just sounded thin. Also I think Merlin is right when he says that to get the full benefit of a pentode, it's nice to have a gain stage in front of it to get the full flavor. THAT said, there's lots of guys like 'em at the input stage, so whaddaIknow? :icon_biggrin:
-
Would it be true to say the cathode follower makes the vibrato circuit more efficient because it is lower impedence? Could a mosfet follower be inserted to do the same thing in the Princeton design?
You could say it's more efficient. Doug's trem circuit is copied from the 6G16 Vibroverb circuit which uses 6L6s. 6l6 bias is a little harder to wiggle than a 6V6 circuit, so the CF buffer helps prevent oscillator drop out due to loading. Also, the PR was the cheapest reverb amp Fender built. The trem circuit was probably one of the cost cutter ideas.
Doug's PR project uses the PR AA1164 circuit.
-
Hi John -
Thanks much for the input. My plan was to put the triode first and then the pentode. Still have many things to think about so I need lots of help. Thanks again.
-
Sluckey - Thanks for the confirm. I got a chance to look at it last night. Does SG's layout look to be on the right track? I need to start working on drawing all this. Thanks again.
-
Hey moon,
A layout is only as good as the schematic it reflects,,,and can be difficult for anyone to confirm without a schematic to compare it to...
I took a couple minutes over lunch to sketch this up quick...
I think it matches what I drew as a proposed layout for the paraphase substitution.... I tried :icon_biggrin:
And here's a little more fun:
http://www.ampbooks.com/home/amplifier-calculators/paraphase/ (http://www.ampbooks.com/home/amplifier-calculators/paraphase/)
-
I need to look at everything to try to decide which version of the Princeton Doug's board implements ...
How many are there?
This is the one I probably used
http://el34world.com/charts/Schematics/files/fender/Fender_PRINCETON_REVERB_AA1164.pdf (http://el34world.com/charts/Schematics/files/fender/Fender_PRINCETON_REVERB_AA1164.pdf)
-
Doug -
Thanks for your time. When I first looked at the list of schematics called Princeton - it looked like there are many (my bad there); but from what I could find and read, once you look more closely I think the AA1164 is the only one it could really be if you are looking pre-CBS. There is a CBS version (couple really) but they are only minimally different from what I read. There is a GREAT piece on them at the ampguru link above if it strikes yer fancy. Again -- thanks kindly for the great support!!
-
I just finished a new layout with the schematic and a BOM for the Princeton reverb
Hit refresh on your browser after clicking on the links or you will not see the latest version
Layout and schematic
http://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf (http://el34world.com/Hoffman/files/Hoffman_PrincetonReverb.pdf)
BOM
http://el34world.com/Hoffman/files/Hoffman_PrincetonReverb_BOM.pdf (http://el34world.com/Hoffman/files/Hoffman_PrincetonReverb_BOM.pdf)
-
:worthy1: My next build in the future. I have a question about the rectifier, can you use a solid state instead? I am building the Revibe at the moment and it uses the soild state instead of the vacuum tube
-
yes,
Use a plug in or just hard wire diodes
-
Thanks Doug
-
Doug
Any chance in the future that the BOM for this can be converted into a ordering platform for us who love this site :icon_biggrin: ? I know you dont do kits but say that spread sheet had a ordering page link so you can either click to want or not want each item then submit order? I know it would save me from making mistakes
-
Princeton reverb already has a public parts list here.
http://hoffmanamps.com/MyStore/catalog/PartsListIndex.htm (http://hoffmanamps.com/MyStore/catalog/PartsListIndex.htm)
-
:worthy1: Once again