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Post subject: Re: BF SR problem(s)
Posted: Wed Apr 18, 2012 6:26 am
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RJ...did the amps get shipped with the 12AX7 in the V6 pos. instead of the 12AU7?
that's interesting.


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Post subject: Re: BF SR problem(s)
Posted: Wed Apr 18, 2012 6:58 am
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It's my understanding that the correct tube complement was installed at the factory. However, whenever the amp was brought in for servicing the first thing a tech would do is to change the 12AU7 to a 12AX7 to match what the schematic indicated was correct. From there, whatever issue the amp happened to suffer from would cascade, relegating it to the status of a big, obnoxious "bench hog". Apparently FMI sent out several TSB's to all dealers to clarify errors in the drawings but since so few stores had ever actually seen a 400PS, let alone sold one, they went into the trash instead of being kept on file for use by the shops' repair staff.

Arjay

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Post subject: Re: BF SR problem(s)
Posted: Wed Apr 18, 2012 12:54 pm
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Man! thanks for the trivia! :D


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Post subject: Re: BF SR problem(s)
Posted: Wed Apr 18, 2012 6:22 pm
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Beemer...did you get that BFSR fixed up? Hope so, cause those amps are some of the greatest sounding BF amps there are..IMHO. How about an update?

T2

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Post subject: Re: BF SR problem(s)
Posted: Wed Apr 18, 2012 7:55 pm
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Sorry. Had to work the last three days (12-hour graves). Will get to the amp this weekend. Really an odd problem. Seems to be in the input section. Maybe, between the jack and grid. Will post on results later.

And got that 1961 Ampeg Jet in, too. Going to be a busy weekend!


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Post subject: Re: BF SR problem(s)
Posted: Thu Apr 19, 2012 12:03 am
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Beemer, I'm sure you'll get both of those amps strairght up! Although, the Ampeg might be a wild ride!
Ajay, I was thinkin' that the Musicmaster, with the X-former inverter, might have been aimed at Jazz players or recording applications. Art

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Post subject: Re: BF SR problem(s)
Posted: Thu Apr 19, 2012 12:37 am
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It was an uninspiring entry-level bass amp for students with only the most rudimentary of controls (volume and tone), a generic 12-inch speaker (usually a CTS alnico), and three tubes (6AQ5 power tubes early on, changed to 6V6GTA for the final production editions).

Arjay

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Post subject: Re: BF SR problem(s)
Posted: Thu Apr 19, 2012 1:24 am
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Dunno if that V6 is a phase inverter tube. 12AU7 can handle a bit more current than a 12AX7. FWIW.

A lot of Ampegs have 12AU7 as PI's. The BEST 12AU7 (which is a lousy sounding tube, in general) is the CBS-Hytron 5814A, 7016, or 7016. These singer make those amps SING. A V22 with this tube just lights those 7027A on fire.


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Post subject: Re: BF SR problem(s)
Posted: Thu Apr 19, 2012 10:53 am
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The 400PS used a 6L6GC (!) as a driver stage for the phase-inversion transformer. The 12AU7 was used as a "pre-driver" driver for the 6L6. Here's the schematic......

http://ampwares.com/schematics/400ps.pdf

Note the erroneously-labeled V6.

Arjay

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Post subject: Re: BF SR problem(s)
Posted: Thu Apr 19, 2012 4:42 pm
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Arjay, Thanks! Wow! What a monster! 6L6GC coupled to high voltage output stage. Where in the world would you find those commercial grade output tubes? I assume that's what they are with that kind of designation (10309).

I'd like to see the PT on that amp.


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Post subject: Re: BF SR problem(s)
Posted: Thu Apr 19, 2012 5:02 pm
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Those PS 400's were monsters...I had a bass player in Florida who played one, the head alone had its own stand and it needed it! It would take two to carry that behemoth! It was loud too. He even had the folded horn cabinet with an 18 inch speaker if I remember right. I had to run my Concert II dimed to keep up with him at outdoor gigs and still had to get him to turn it down! I want to say it had six 6L6's in the output...Arjay would that be correct? My memory on some details is not the greatest anymore!

FWIW

Oh and Beemer, good luck with your weekend...sounds like you had a long week and a fun weekend coming up.

T2

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Post subject: Re: BF SR problem(s)
Posted: Thu Apr 19, 2012 6:40 pm
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GE 6550's, exclusively -- made to Ed Jahn's demanding specs. The plate voltages (circa 700VDC IIRC) would melt lesser bottles.

Arjay

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Post subject: Re: BF SR problem(s)
Posted: Thu Apr 19, 2012 7:43 pm
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Arjay & Strat,

Another amp tech told me he worked on a couple of these. And one came with six selected TungSol 6550's! Imagine the cost to fix that thing today. Either a sextet of GE or TS is going to set you back a good grand or two.

Plus, he said the thing ran transmitter range voltages. Like close to 1KV. So all the parts in the output stages needed to be special mil-spec epoxy-coated like commercial AM transmitters. And when the thing would go unstable, it would eat tubes faster than you could shut the thing down.

And I thought Ampeg SVT's were crazy... :lol:


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Post subject: Re: BF SR problem(s)
Posted: Fri Apr 20, 2012 5:24 am
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Guys, dig this GREAT site. Esp those absolutely MELTED 6550's!!! What an increadible beast of an amp.

http://www.timeelect.com/400-PS-IDX.htm


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Post subject: Re: BF SR problem(s)
Posted: Fri Apr 20, 2012 11:00 am
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You'd need a steel-frame bench to work on that sucka!! :lol:

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