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Some BF Fender questions......

12/14/1997 8:38 PM
Charles	Some BF Fender questions...... Seasons Greetings all! Maybe I just wonder about things too much, but let me throw some "stay awake at night" type questions to y'all. The other day, I was servicing a BF Bassman head. It was the one with the twin 220k feedback resistors on the power tubes, an AB165. Looking at the chassis, it was punched for a rectifier tube, and two additional power tubes. The openings were obviously done by Fender since the head was dead stock, and they put neat little covers over each opening. I've seen this sort of thing before, but got to wondering.... The Bassman chassis is a small one. What else did Fender make that used either a rectifier tube, or four power tubes on that chassis? Certainly there was no product that used a rectifier tube AND all four power tubes. Which brings me to "why?". Also, this particular head is at least late '66, but has the bias LEVEL control as stock (not the balance control). Again, if the circuit was AB165 (and all else says it is), and the head is a late '66...."why?" Does anyone think that they were just really "losing it" during the transition years?

12/14/1997 8:38 PM

Charles

Some BF Fender questions......

Seasons Greetings all!

Maybe I just wonder about things too much, but let me throw some "stay awake at night" type questions to y'all.

The other day, I was servicing a BF Bassman head. It was the one with the twin 220k feedback resistors on the power tubes, an AB165. Looking at the chassis, it was punched for a rectifier tube, and two additional power tubes. The openings were obviously done by Fender since the head was dead stock, and they put neat little covers over each opening. I've seen this sort of thing before, but got to wondering.... The Bassman chassis is a small one. What else did Fender make that used either a rectifier tube, or four power tubes on that chassis? Certainly there was no product that used a rectifier tube AND all four power tubes. Which brings me to "why?". Also, this particular head is at least late '66, but has the bias LEVEL control as stock (not the balance control). Again, if the circuit was AB165 (and all else says it is), and the head is a late '66...."why?" Does anyone think that they were just really "losing it" during the transition years?

12/15/1997 1:56 AM
Steve A.	Charles: This is just a wild guess, but perhaps the chassis was a CBS idea to reduce production costs: instead of having a separate chassis for each model, why not have just a few chassises in different widths to accommodate several models, with holes already punched in them to accommodate different combinations of tubes. Along those lines, perhaps some of the multi-use chassises would even have extra pot holes in them which would have been covered by the black face plate. If this is true, why don't we see more chassises like this one? Maybe CBS decided that they could save even more money by screwing up the traditional Fender sound, so they would sell fewer amps and could lay off workers at Fender... (I worked for Pacific Stereo in the 70's which was bought out by CBS, and we think they did stupid things to lose money just to be able to write off excess profits from other divisions.) Incidentally, I was just looking at the AB165 schematic last night wondering how to adapt the "deep" switch circuit to a traditional Fender guitar amp, and wondering which value deep cap would work best with a .047uf bass cap and .022uf midrange cap. (The AB165 uses 0.1uf caps for bass, midrange and the "deep" circuit.) Any ideas of what values I ought to try first? Steve Ahola

12/15/1997 1:56 AM

Steve A.

Charles:

This is just a wild guess, but perhaps the chassis was a CBS idea to reduce production costs: instead of having a separate chassis for each model, why not have just a few chassises in different widths to accommodate several models, with holes already punched in them to accommodate different combinations of tubes. Along those lines, perhaps some of the multi-use chassises would even have extra pot holes in them which would have been covered by the black face plate. If this is true, why don't we see more chassises like this one? Maybe CBS decided that they could save even more money by screwing up the traditional Fender sound, so they would sell fewer amps and could lay off workers at Fender... (I worked for Pacific Stereo in the 70's which was bought out by CBS, and we think they did stupid things to lose money just to be able to write off excess profits from other divisions.)

Incidentally, I was just looking at the AB165 schematic last night wondering how to adapt the "deep" switch circuit to a traditional Fender guitar amp, and wondering which value deep cap would work best with a .047uf bass cap and .022uf midrange cap. (The AB165 uses 0.1uf caps for bass, midrange and the "deep" circuit.) Any ideas of what values I ought to try first?

Steve Ahola

12/15/1997 4:17 PM
Charles	>> perhaps the chassis was a CBS idea to reduce production costs: instead of having a separate chassis for each model, why not have just a few chassises in different widths to accommodate several models, with holes already punched in them to accommodate different combinations of tubes.<< Steve, that was my initial thought. Then I actually looked at what was available during that general time period, and also looked at the sizes of the chassis'. There simply weren't any other chassis' I can find of the very short Bassman size that used the extra tubes. In fact, I can't find any other chassis of that size being used on any other product at all. So, that idea seemed less plausable after that. >>extra pot holes in them which would have been covered by the black face plate.<< I've seen that. In fact I have a Champ that's with only ONE extra hole. Very weird. And again... why? What product used four knobs on that chassis? None. My personal speculation on that one was that they were thinking about bringing out a 4-knob reverb unit in silverface >> Incidentally, I was just looking at the AB165 schematic last night wondering how to adapt the "deep" switch circuit to a traditional Fender guitar amp, and wondering which value deep cap would work best with a .047uf bass cap and .022uf midrange cap. (The AB165 uses 0.1uf caps for bass, midrange and the "deep" circuit.) Any ideas of what values I ought to try first? << I'm not too hot on the way deep switch works. It's only sort of "so-so" for bass use and really doesn't do much at all for helping the tone of the guitar. I always thought that a nice DPDT switch, switching in different values for the treble cap and middle cap would be interesting. I've never done it tho. The Bassman uses a 850pf, .1, .1 combination. Maybe switching that with the normal Fender values would be nice. Just a thought. Also, you gotta put some resistors in there to help reduce the "pop" when switching. I think 470k would be ok, but Fender used 1meg on later models to help keep the switching noise down. Charles

12/15/1997 4:17 PM

Charles

>> perhaps the chassis was a CBS idea to reduce production costs: instead of having a separate chassis for each model, why not have just a few chassises in different widths to accommodate several models, with holes already punched in them to accommodate different combinations of tubes.<<

Steve, that was my initial thought. Then I actually looked at what was available during that general time period, and also looked at the sizes of the chassis'. There simply weren't any other chassis' I can find of the very short Bassman size that used the extra tubes. In fact, I can't find any other chassis of that size being used on any other product at all. So, that idea seemed less plausable after that.

>>extra pot holes in them which would have been
covered by the black face plate.<<

I've seen that. In fact I have a Champ that's with only ONE extra hole. Very weird. And again... why? What product used four knobs on that chassis? None. My personal speculation on that one was that they were thinking about bringing out a 4-knob reverb unit in silverface

>> Incidentally, I was just looking at the AB165 schematic last night wondering how to adapt the "deep" switch circuit to a traditional Fender guitar amp, and wondering which value deep cap would work best with a .047uf bass cap and .022uf midrange cap. (The AB165 uses 0.1uf caps for bass, midrange and the "deep" circuit.) Any ideas of what values I ought to try first? <<

I'm not too hot on the way deep switch works. It's only sort of "so-so" for bass use and really doesn't do much at all for helping the tone of the guitar. I always thought that a nice DPDT switch, switching in different values for the treble cap and middle cap would be interesting. I've never done it tho. The Bassman uses a 850pf, .1, .1 combination. Maybe switching that with the normal Fender values would be nice. Just a thought.

Also, you gotta put some resistors in there to help reduce the "pop" when switching. I think 470k would be ok, but Fender used 1meg on later models to help keep the switching noise down.

Charles

12/16/1997 11:19 AM
John Greene	I'll bet the answer is much simpler. There could have been a model in the works that was canned just prior to production and they used up the inventory. Since these amps were all basically hand assembled, it's not too difficult to do inline changes to burn inventory. If it fits, use it. I worked on an early 67 bassman head with the 200Ks and it didn't have any extra holes. That bassman had the bias 'balance' adjustment which I converted back to a level adjustment. It also had a really annoying AC hum. I tried removing the 220K feedback resistors on the power tubes and the hum went away. The amp sounded good so I left them off. The other problem was a popping and fizzing. I managed to track that down to one of the 220K summing resistors feeding the 12AX7 before the phase splitter. I swapped them out and no more popping and fizzing. The feedback resistors on the output tubes really had me puzzled. I can only guess that the reason was to keep the gain of the output tubes down to prevent damage from the mucho-mondo power requirements of bass freqs. Anyone have any other theories on this? --johng

12/16/1997 11:19 AM

John Greene

I'll bet the answer is much simpler. There could have been a model in the works that was canned just prior to production and they used up the inventory. Since these amps were all basically hand assembled, it's not too difficult to do inline changes to burn inventory. If it fits, use it.

I worked on an early 67 bassman head with the 200Ks and it didn't have any extra holes.

That bassman had the bias 'balance' adjustment which I converted back to a level adjustment. It also had a really annoying AC hum. I tried removing the 220K feedback resistors on the power tubes and the hum went away. The amp sounded good so I left them off. The other problem was a popping and fizzing. I managed to track that down to one of the 220K summing resistors feeding the 12AX7 before the phase splitter. I swapped them out and no more popping and fizzing.

The feedback resistors on the output tubes really had me puzzled. I can only guess that the reason was to keep the gain of the output tubes down to prevent damage from the mucho-mondo power requirements of bass freqs. Anyone have any other theories on this?

--johng

12/16/1997 11:51 AM
J Fletcher	Well I worked on one of those Bassman amps a few years back that had a weird harmonic accompanying every guitar note played.After "arresting all the usual suspects"with no success,I removed the 220k feedback resistors and the harmonic disappeared.I've seen BF Concerts and Bandmasters with extra socket holes punched for 4 output tubes.I figured they were using the same chassis for the Showman.

12/16/1997 5:54 PM
Charles	See... same application of "huh?", just a different amp. The Concert and Bandmaster (and Super Reverb and Pro Reverb) all had the same size chassis (22-1/2"). The Showman had a longer chassis (close to 25").

12/17/1997 8:13 AM
J Fletcher	Concert and Bandmaster had the same size chassis,but the Super Rev is slightly longer I think,and the Pro Rev is the same as the Twin Rev which is slightly longer again.Talking about BF amps here.Don't think a BF Showman is 25" long,although the white one might be.

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