Correct Bassman 5A6 circuit?

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Correct Bassman 5A6 circuit?

Postby OrangeJBL on Sat Jul 15, 2017 9:37 am

Hi there,
I am new to the forum and hope to be at the right place for the question I have.

First, however, a quick introduction: I live in Southern Bavaria, am an avid amateur guitarist and besides the music itself also have a pronounced interest in the technical side of the electric guitar and the associated amplification. My professional background is audio engineering, instrumentation and intellectual property. I have been playing Fender amps since 1974 but enjoy various other amps very much, as well.

Now, on to my question:
I am trying to find the schematic for the original late 1951 or early 1952 TV-front Bassman. Supposedly, it has the designation 5A6 .... and it can be found all over the internet. Except that obviously, the circuits published on the internet which I found (e.g. http://vintagefenderamprepair.com/wp-co ... ematic.pdf ) are not those of a Bassman since they sport 3 inputs and two volume controls (rather than the two inputs and single volume control the Bassman had). Also, this alleged 5A6 circuit shows two speakers (rather than one) which are moreover connected to two separate windings of the output transformer (or maybe it means to indicate TWO output transformers).

Therefore I conclude that this schematic is NOT that of the original Bassman Amp, and my question is whether any body here can point me to the correct circuit?

Many thanks in advance and a great weekend to all!

P.S.: The circuit found on the internet under the 5B6 designation very much seems to be a correct Bassman schematic corresponding to all other available info. I connect the 5B6 to the wide-panel version available in 1953 which apparently has the same structural design as the TV-front amp. There probably were not big alterations between the 5A6 and the 5B6 but it would be nice to have a schematic of the first Bassman.
OrangeJBL
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Re: Correct Bassman 5A6 circuit?

Postby Dean Jr. on Sat Jul 15, 2017 4:45 pm

the first BAssman amp....very late '51 or early '52...had a full coverage back panel with two ports near the middle of the panel. The chassis was on the bottom. There two controls....volume and tone??...and two inputs. The tube compliment was a 6SC7 for the preamp, a 6SL7 paraphase phase inverter, 2 x 6L6 power tubes, and a 5U4 rectifier....single 15" speaker.
That schematic you mention is the second version of the Bassman and came out in wide panel cosmetics.
IF you have one of those first versions of the BAssman, the schematic might be hard to find....and will be someone's hand-drawn schematic taken from the circuit of an actual amp, I would think. Does your amp use 6SC7's for the preamp? IF so, then the schematic for the 5A5 Pro might be the closest you will find.

https://images.search.yahoo.com/yhs/search?p=5A5+pro+scheamtic&fr=yhs-mozilla-003&hspart=mozilla&hsimp=yhs-003&imgurl=http%3A%2F%2Felektrotanya.com%2FPREVIEWS%2F63463243%2F23432455%2Ffender%2Ffender_pro_5a5.pdf_1.png#id=2&iurl=http%3A%2F%2Felektrotanya.com%2FPREVIEWS%2F63463243%2F23432455%2Ffender%2Ffender_pro_5a5.pdf_1.png&action=click

ONE could compare this schematic to the circuit and ascertain what differences...if any....there might be.
IF you are looking to build such an amp, that is another ballgame....but the 5A5 schematic and layout might be the closest thing to it. Fwiw, there are no voltages given in these old Fender schematics until the 1955 circuits...the 'E' versions.
IF you do have one of these early Bassman amps, some pictures would be of interest. :cheers:
Dean Jr.
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Re: Correct Bassman 5A6 circuit?

Postby OrangeJBL on Tue Jul 18, 2017 6:14 pm

Dear Dean,
thank you so much for your reply and support! Just to avoid misunderstandings: I do not own an old Bassman but I am researching the subject of the evolution of the Bassman (in fact up to the development of the first Marshall amps) for a talk I am giving at a meeting.

I have done some more sleuthing around and have arrived at the following conclusions:
- the circuit labelled 5A6 or "Old Bassman" which is found the internet (e.g. here: http://vintagefenderamprepair.com/wp-co ... ematic.pdf ) and in literature (e.g. in Aspen Pittman's Tube amp book and Michael Doyle's "History of Marshall" from 2013) cannot be that of the TC-front Bassman for the reasons I have mentioned in my first post.
- rather, this circuit could be that of a Fender Dual Professional amp: it does have the corresponding input configuration and - maybe even more importantly - it has two separate output transformers (mounted to the two speaker frames each).

I think your approach to tie in the 5A5 Pro amp is really a very good one. The similarities between it and the 5A6B-Bassman schematic (which I find reasonably trustworthy) are very strong, and it seems reasonable that Fender would not change too many details. Also, Teagle & Sprung write in their 1995 book "Fender Amps - the first 50 years" that the first P-Basses were delivered with TV-front Pro amps because the Bassman was not yet available. That would make it likely that indeed the first Bassman was a single channel Pro with a different back panel.

Again, my full appreciation for your input - the discussion with you has helped me a lot!

Tale care!

P.S.: I have read the book on Marshall amps I mention above only today and I am a big shocked that they did not see that they have the schematic of an entirely different amp. What's even weirder: Alexander Dumble contributes (in this book) a little analysis of the circuit shown and relates it to the "Old Bassman". This analysis now of course has very dubious relevance to the actual Bassman amps - and I find it almost a bit sad that someone as well known as AD does not pay a bit more attention.
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Re: Correct Bassman 5A6 circuit?

Postby Dean Jr. on Tue Jul 18, 2017 6:56 pm

good luck with the project, Orange.
I don't have that Teagle/sprung book that was published in 2013. I have the Doyle book that was published in 1993...."The History of Marshall'. The larger Fender amps before 1955 are very similar to each other. In 1955/1956, all of those larger amps took imho three different paths. There is the family of the Super/Bandmaster/ Pro amps. Then the 5E7 Twin sort of sits by itself...but has close ties to those first three amps. In 1956 the 5F6A BAssman and the 5F8A Twin came out....they are in essence identical but for the difference in the output sections. These are the basis for those first Marshalls. The Marshall amp has little to do with any other Fender, imho.

Again...good luck with the project. Some of that early Fender history can be murky, I suppose. I have minimal experience with them. I have resurrected a '49 Deluxe, and I have a '51 Deluxe in for repairs right now. All of the rest of the tweeds I have owned or worked on were from 1952 and later with correct schematics available. The earliest Bassman I have worked on was a November, 1954 4 x 10 5D6. About the only tweed from the time frame of 1952-1960 that I have not owned or worked on would be a 5F8A HI power Twin.
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Re: Correct Bassman 5A6 circuit?

Postby OrangeJBL on Wed Jul 19, 2017 5:10 am

Thanks again! I can see that you certainly have a lot of experience with Tweeds - if I ever need hands-on-info on those I will certainly try to get in touch.

Best regards and wishes!
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Re: Correct Bassman 5A6 circuit?

Postby Naked Clarke on Thu Jul 20, 2017 4:42 pm

Dean Jr. wrote:In 1956 the 5F6A BAssman and the 5F8A Twin came out....The earliest Bassman I have worked on was a November, 1954 4 x 10 5D6. About the only tweed from the time frame of 1952-1960 that I have not owned or worked on would be a 5F8A HI power Twin.


The 5D6A Bassman was produced through about September 1955 when the 5E6 circuit was introduced. The 5F6 circuit came out in June 1957. The 5F6A was not produced until around January 1958.
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Re: Correct Bassman 5A6 circuit?

Postby Dean Jr. on Thu Jul 20, 2017 5:23 pm

Naked, Clark, thanks for that... I get 'hung up' on the schematic drawing dates....F being 1956. When I get the actual amp in-hand, then I know when THAT one was built. The only 5F6A's I have owned or worked on were a '58 and two '59's....so that fits with your info.
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