It is currently Mon Mar 16, 2020 3:35 pm

All times are UTC - 7 hours



Post new topic Reply to topic  [ 59 posts ]  Go to page Previous  1, 2, 3, 4  Next
Go to page Previous  1, 2, 3, 4  Next
Author Message
Post subject: Re: 1968 Bassman
Posted: Sun Feb 09, 2014 4:05 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Balance pot circuit of the AB165 converted to bias pot of the AA864. Loading the phase inverter anodes with 120k/100k-ohm resistors.


Image


Top
Profile
Fender Play Winter Sale 2020
Post subject: Re: 1968 Bassman
Posted: Sun Feb 09, 2014 5:15 pm
Offline
Professional Musician
Professional Musician

Joined: Fri Jun 26, 2009 6:53 pm
Posts: 2252
Location: Harvard IL
Very nice !!!! It's coming into focus!!! Art

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


Top
Profile
Post subject: Re: 1968 Bassman
Posted: Sun Feb 09, 2014 5:36 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
aclempoppi wrote:
Very nice !!!! It's coming into focus!!! Art


+1

I never got used to that complicated mess in the AB165 chassis.

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: 1968 Bassman
Posted: Sun Feb 09, 2014 6:51 pm
Offline
Professional Musician
Professional Musician

Joined: Tue Sep 11, 2012 2:14 pm
Posts: 2561
Retroverbial wrote:
aclempoppi wrote:
Very nice !!!! It's coming into focus!!! Art


+1

I never got used to that complicated mess in the AB165 chassis.

Arjay



Isn't that Bias Balanced circuit also used in the SF Princeton Reverb.. ?? Sure looks like it.

Is it worth it to revert to the old adjustable Bias?

cheers!

_________________
Image

'11 FSR Am. Vtg. Ltd. Ed. CAR '57 Stratocaster (SN# LE02639)
'14 American Deluxe Ash Stratocaster
'12 Telebration Empress Telecaster
'99 Deluxe Nashville Telecaster
'12 FSR Telecaster HH
'10 Heritage H-535
'99 Martin DC-1E
'13 Lanikai Tenor Ukulele


Top
Profile
Post subject: Re: 1968 Bassman
Posted: Sun Feb 09, 2014 7:21 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Lightnin MN wrote:
Isn't that Bias Balanced circuit also used in the SF Princeton Reverb.. ?? Sure looks like it.

Is it worth it to revert to the old adjustable Bias?

cheers!


The PR never had a balance pot, since the vibrato/tremolo circuit wags the idle bias of the output tubes. I believe that the DR always had a bias pot. The silverface 6L6GC amps seemed to have gone to the balance pot.

The balance pot really doesn't effect the overall bias of the output tubes. Only one side of the push-pull pair. To balance the idle current flow between the two push-pull pair(s). In order to bias both pairs of tubes, you need to juggle the set resistors, in the fixed bias supply.

Changing these balance pot amps to bias pot is not a big thing. Remove a few resistors hanging on the pot. Put in a pair of good tolerance 220k-ohm resistors. Maybe move a wire or two on the circuit and pot.

Changing a Bassman AB165 with its oddball global negative feedback, altered phase inverter stage, added local feedback on V3 and the output tubes is a BIG hassle. Believe me. Almost easier to buy a new Bassman AA864 tagboard and populate it with correct components in correct positions. Then, swap out the boards.

I'm still at the PI section. Worse thing is pulling lines under the top board. And reconfiguring the mess. Advice: test amp on speakers you don't care about. :lol: Photos to follow.


Top
Profile
Post subject: Re: 1968 Bassman
Posted: Sun Feb 09, 2014 8:14 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Bias circuit and phase inverter sections reverted (hopefully, correctly :D ) from AB165 to AA864. I replaced the 47k-ohm/1mfd GNFB network with a 1k-ohm resistor. Rewired the GFNB point to the bottom half of the 12AT7 long-tailed PI. Note pretty better change over the stock AB165.


Original AB165 layout:


Image




Partial conversion to AA864:


Image


Top
Profile
Post subject: Re: 1968 Bassman
Posted: Sun Feb 09, 2014 8:20 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Partially following Tone Lizard's mod recommendations:


Image


Top
Profile
Post subject: Re: 1968 Bassman
Posted: Sun Feb 09, 2014 10:07 pm
Offline
Aspiring Musician
Aspiring Musician
User avatar

Joined: Sun Apr 14, 2013 4:41 pm
Posts: 764
Location: Sydney. Australia
BMW2002Ti, its very interesting watching your skilful conversion documented with photos.

Could you please explain (in simple words for us who are not very technical) what are the advantages of converting the circuit from AB165 to AA864?!Do you get different tonal spectrum, response, headroom, or anything else?!

Many thanks. :D


Top
Profile
Post subject: Re: 1968 Bassman
Posted: Sun Feb 09, 2014 10:28 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
From Tone Lizard Site.



For my money, the 'best' Bassman head would have to be the AA864. This would give the least amount of 'work' involved in order to have a very good guitar amplifier. The 'worst' candidate? I'll vote for the AA371. The most common seems to be the AB165 (they were issued for almost twenty years), and these are not great for guitar players, as we'll see in a minute. The '6G6' series tends to come in a lovely 'Blonde' vinyl, and are highly collectible.

These are also the only Bassman 'heads' to feature a 'Presence' control. You can think about all of these opinions on various Bassman circuits this way; many classic Bassman derivatives and today's boutique amplifiers all follow the 5F6 circuitry, whilst none I can name follow the AB165 circuit verbatim.

---------------------------

Circuit model. Tube Compliment. Brief description.

6G6
(1961-1962) 4 x 7025 2 x 5881.

Odd circuits aplenty. DC coupled first gain stages, cathode-driven 'Bass' control in 'Bass' channel. 'Treble' control is after 3rd gain stage. Tapped 'Treble' control in 'Normal' channel.


6G6A
(1962-1964) As above.

Linear-taper 'Volume' control in 'Bass' channel. Also, has more inverse feedback.



AA270
(1968-1983) 3 x 7025, 1 x 12AT7 2 x 6L6GC.

Odd 3rd gain stage common to both 'channels', with local feedback. Very odd inverse feedback circuit. Not a great guitar circuit. Increased output power, post 1976.



AA371
(1968-1983) As above.

Odd 3rd gain stage common to both channels, with more local feedback than AA270. The 'King' of screwball inverse feedback circuits; awful for guitar. Increased output power, post 1976.



AA864
(1964-1967) As above.

Very close to 'classic' circuit. 'Bass' channel has extra gain stage. High amount of inverse feedback.


AB165
(1964-1983) As above.

Odd 3rd gain stage common to both channels, with local feedback. Inverse feedback circuit boosts bass frequencies; not great for guitar. Local feedback at output stage. Increased output power, post 1976.



AC568
(1968-1983) As above.

See AA270.


Top
Profile
Post subject: Re: 1968 Bassman
Posted: Sun Feb 09, 2014 10:30 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
BMW2002Ti wrote:
Partially following Tone Lizard's mod recommendations:


Image


That looks good, Steve. Of course, you've already upgraded the rectifier's diode stack and I'm assuming the death cap has been ditched and a grounded power cable installed.

Rawk on!

8)

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: 1968 Bassman
Posted: Sun Feb 09, 2014 11:43 pm
Offline
Aspiring Musician
Aspiring Musician
User avatar

Joined: Sun Apr 14, 2013 4:41 pm
Posts: 764
Location: Sydney. Australia
BMW2002Ti,

thank you for the great info ... :D


Top
Profile
Post subject: Re: 1968 Bassman
Posted: Mon Feb 10, 2014 2:24 am
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Replaced 0.1mfd coupling cap to PI top section with a 500 pico-farad. As per AA864 circuit. You can see the silver mica cap, next to the two 33mfd/160VDC cathode bypass cap in first photo. GNFB loop now feeds bottom half of phase inverter.

Primary output tranny leads reversed on the 6L6GC's.




Image





470k-ohm local feedback resistor removed from grid of V3.



Image


Top
Profile
Post subject: Re: 1968 Bassman
Posted: Mon Feb 10, 2014 6:38 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
All this yaddy-yaddy and photos... no tones. :lol:

Ran outta time. Will try to measure V's and test amp later this weekend. Thank goodness for the three-day weekend!


Top
Profile
Post subject: Re: 1968 Bassman
Posted: Sat Feb 15, 2014 7:06 pm
Offline
Rock Star
Rock Star
User avatar

Joined: Sat Jul 03, 2010 2:18 pm
Posts: 6544
Will measure and balance the "tail" of the long-tailed phase inverter's resistors. Not sure why, but many articles stress the 1meg, 1meg, 470, 22k-ohm tail resistors of the Bassman. Much more so than any other long-tailed PI driven Fender amp. Even though the setup is basically the same with all amps.

Image


Top
Profile
Post subject: Re: 1968 Bassman
Posted: Sat Feb 15, 2014 8:52 pm
Offline
Rock Icon
Rock Icon
User avatar

Joined: Wed Mar 31, 2010 12:48 am
Posts: 26417
Location: Tombstone Territory
BMW2002Ti wrote:
Will measure and balance the "tail" of the long-tailed phase inverter's resistors. Not sure why, but many articles stress the 1meg, 1meg, 470, 22k-ohm tail resistors of the Bassman. Much more so than any other long-tailed PI driven Fender amp. Even though the setup is basically the same with all amps.

Image


It could be that symmetry is much more important in the PI stage than with later Bassman revisions due to the traditional idle-bias circuit combined with the altered negative feedback network.

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
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 59 posts ]  Go to page Previous  1, 2, 3, 4  Next
Go to page Previous  1, 2, 3, 4  Next

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: