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Post subject: BF SR problem(s)
Posted: Fri Apr 13, 2012 8:25 pm
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Got this BFSR (AA 763) with this odd problem. First a schemo/electrical Q... Why did Fender go to the inbalanced PI voltage on the anode halves on the phase inverter in the AB763 versus "balanced" on the AA763 (82/100 K-hom pairing versus 100/100 k-hom)?

http://www.webphix.com/schematic%20heav ... _schem.pdf

Thanks.

Next the amp's issue. NORMAL channel plays way louder (even overdrives before "5" than the VIBRATO channel. All input and EFX stage tubes were rotated then changed out. Still same issue.

Then, the most odd of all problems... Any guitar plugged into Instrument 1 is consdierably louder than same guitar in INSTR 2. AN BOTH CHANNELS! With same overwhelming volume from NORMAL and very low on VIBRATO. Efven tried a 12AY7 in NORMAL and an super hot Tungsram ECC83 in VIBRATO. Just get way more OD on NORMAL and more hiss on VIBRATO.

I've been told to go though all pico-farad cap, esp the 10pf/330K-ohm network connecting the VIBRATO to the grid of 2nd half of V1.


http://www.webphix.com/schematic%20heav ... _schem.pdf

All PS lytics, bypass lytics, grid-to-ground resistors on the 6L6GC replaces (one measured 272 k-ohm, then other 312 k-ohm.... so much for buying matched 6L6GC's). In there now are Dales at 220.2 and 220.4 k-homs. Also changed anode feeding resistors are new Dales at 100 & 100.1 (original were 115 and 128k-ohms).

The darn amp NOW measures 41.2mA on one RCA BP and 40.8 on the other. Never got even within 5mA before swapping resistor networks. FWIW.

Sure made the output section SUPER QUIET. No 12AX7 or 12AX7, just the GZ34 and 6L6GC and you can't hear even whiie noise through the quad of 10-inchers.

In fact no noise, at all, until those two input tubes are put into play (V1 and V2). All input jckes have been removed, new pots, new resistors, and jacks. Nada.

Thanks, for any suggestions. May need to break open my friend's singal generator and scope. Gotta grind out three graces in a row. More new on Monday, I most.

Have a great weekend! :D


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Post subject: Re: BF SR problem(s)
Posted: Sat Apr 14, 2012 12:14 am
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Beemer, you got to be jivin' about the 82-100ohm setup. You're funny! Art

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Post subject: Re: BF SR problem(s)
Posted: Sat Apr 14, 2012 12:33 am
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The only reason I can think of is, Leo wanted a deliberate mismatch in transconductance between the phase inverter halves to create some assymetry at the output stage to artificially saturate the output tranny prematurely. The resultant odd-order harmonics would be enhanced and compressed, contributing to the amp's voice.

Arjay

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Post subject: Re: BF SR problem(s)
Posted: Sat Apr 14, 2012 7:29 am
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And it hasn't been modded?

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Post subject: Re: BF SR problem(s)
Posted: Sat Apr 14, 2012 9:13 am
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I wondered that too tyronne, any modifications?

It looks like the 82K/100K vs the 100K/100K resistors on the PI Plates have to do with whether or not there are grid stoppers on the output tubes. No grid stoppers on output tubes means that the 100K/100K are used for PI Plates. Note that all of the other ab circuits have grid stoppers on the output tubes while the aa circuits do not (Bassman exception :? ).

As far as the difference between input 1 or 2, input 2 is attenuated (68K) compared to input 1 (34K) so input 1 will be louder.

I would have to have a look at the circuit as to why the normal channel is louder, should be louder on the vibrato channel.

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Post subject: Re: BF SR problem(s)
Posted: Sat Apr 14, 2012 10:06 pm
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FWIW,
My recently aquired silverface bandmaster reverb had a similiar problem where the normal channel was clearly louder than the vibrato channel. I inadvertantly resolved the problem by replacing the 4.7k and 1k resistors in the power supply filter section. The main reason I replaced those resistors was because of an unrelated issue. The 4.7k resistor had been previously replaced with a 10k and the 1k resistor was replaced with a 2.2k, I still don't know why they did that. Anyways, all I did was install the correct value resistors as per the schematic and this made all the difference in the world. I haven't experienced the volume difference on the two channels since doing this.
HTH,
Abe.

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Post subject: Re: BF SR problem(s)
Posted: Sat Apr 14, 2012 11:41 pm
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Beemer, You're testing us on the long tail phase inverter theory, right? Art

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Post subject: Re: BF SR problem(s)
Posted: Sun Apr 15, 2012 9:52 pm
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Sorry, haven't had a chance to look at the amp--- working graveyards, all weekend. Thanks, for all the input.

The last thing I did was pull both input jacks (NORMAL) out of the chassis. And disconnected the input lead going to V1's grid. And guess what? With all tubes 12AX7 inserted (including V1 and V2) & no other component removed--- the VIBRATO channel was back to full loudness.

Will need to check it out further, tomorrow. Get back to you all.


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Post subject: Re: BF SR problem(s)
Posted: Mon Apr 16, 2012 12:48 am
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So you're looking at the 1meg grid leak, or some sort of bleed over leak? Not sure I understand this problem. What was suspect about the jacks? Art

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Post subject: Re: BF SR problem(s)
Posted: Tue Apr 17, 2012 2:48 am
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Didn't mean to give the phase inverter circuit any slight. Any discussion of it's function, attributes, and shortcomings are great! OK, I'll throw in my 2cents; so there is the inherent 7% disparity between the inverted and non inverted signal. The compensation was called close enough to readily available resistors-82 and 100 ohm. It seems the balance wasn't worth trying to compensate for later on, and the 100ohm output resistors were accepted. The 2nd harmonics generated by this imbalance(must have) seemed to warm the progressively sterile sounding amp of the late'60-early-mid '70 era. My guess is that a design engineer had some input from a person with ears. The use of the AC coupled long tailed inverter must have been an economic and efficience oriented decision. The previous cathodyne and paraphrase designs are great, in their own rights( each have inherent benefits and drawbacks)!The perfect inverter was available, but transformers are expensive and heavy. Fender didn't have a problem using them in the 400 PS and a couple of other high-brow amps. I would appreciate any opinions! JMHO, Art

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Post subject: Re: BF SR problem(s)
Posted: Tue Apr 17, 2012 6:00 am
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You guys make my brain hurt ! :mrgreen:

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Post subject: Re: BF SR problem(s)
Posted: Tue Apr 17, 2012 6:11 am
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blues bondsman wrote:
You guys make my brain hurt ! :mrgreen:

:lol: :lol: Mine too!...I love it when these guys get serious. :D


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Post subject: Re: BF SR problem(s)
Posted: Tue Apr 17, 2012 11:50 am
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aclempoppi wrote:
The perfect inverter was available, but transformers are expensive and heavy. Fender didn't have a problem using them in the 400 PS and a couple of other high-brow amps.


"High-brow" is not exactly the phrase I'd use to describe Fender's Musicmaster Bass Amp. I think the retail back in the day was around $259 or so.

:mrgreen:

Arjay

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Post subject: Re: BF SR problem(s)
Posted: Tue Apr 17, 2012 6:29 pm
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Arjay, your point is well taken. Why do you think Fender decided to use the X-former in the lower powered Musicmaster Bass Amp? Art

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Post subject: Re: BF SR problem(s)
Posted: Tue Apr 17, 2012 10:47 pm
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Likely a proof-of-concept field test before using this type of phase inversion on a larger, more expensive amp such as the 400PS. The center-tap tranny technique works well but is more expensive in the long run than the more common balanced "long tail" dual-triode system. However, the 400PS was an exception to all of the rules when conceived. A pity that only the GE-made 6550's were up to the challenge posed by Ed Jahn's enigmatic design. FMI itself committed a major faux pas by misprinting the schematic with an incorrect tube listed in the V6 position -- a 12AU7 was specified but the draftsman labeled it as a 12AX7 (which would limit the amp to substantially less than its design output). As well, most service centers lacked the requisite types of precision test equipment and familiarity with the model to properly tune the output stage or perform repairs.

Arjay

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