Bassman Chassis 1959 - What is under the hood ?

[Raja_Kentut]

bepone you are right, not a big deal to test all versions…
But what is a Sozo ?

[Roe]

The schematic you have is AFAIK the only one available for 5F6-A. Photos of vintage amps show that in production the tone stack used a 100k slope resistor (in place of 56k), and 100n bass cap (in place of 20n), and that the PI tail resistor was 6k8 (not 10k).

Is this right for the evolution of the fender tonestack:

1) 250pF, 0.02µF, 0.02µF
2) 250pF, 0.02µF 0.1µF
3) 100k slope resistor replaces 56k
4) 250pF, 0.05µF, 0.1µF

[Raja_Kentut]

I have summarized the tonestack versions mentioned here. If you know other versions used by Fender, please post them and I will add them

Kown Fender tonestack versions, used by Fender

BassmanToneStacks.jpg

[bepone]

bepone you are right, not a big deal to test all versions…
But what is a Sozo ?

sozo is polyester capacitor , good for tone stack

[martin manning]

Is this right for the evolution of the fender tonestack:
1) 250pF, 0.02µF, 0.02µF
2) 250pF, 0.02µF 0.1µF
3) 100k slope resistor replaces 56k
4) 250pF, 0.05µF, 0.1µF

An evolution of the values certainly makes more sense than random combinations.
1) are as seen in 5F6 and 5F6-A schematics,
2) + 3) are seen in fielded examples of the '59, and in the Reissue,
4) is AB763.
I tend to favor the values selected by Fender for the Reissue as being the definitive ones for the Tweed Bassman.

Do you know of any reference describing these steps?

[Roe]

Is this right for the evolution of the fender tonestack:
1) 250pF, 0.02µF, 0.02µF
2) 250pF, 0.02µF 0.1µF
3) 100k slope resistor replaces 56k
4) 250pF, 0.05µF, 0.1µF

An evolution of the values certainly makes more sense than random combinations.
1) are as seen in 5F6 and 5F6-A schematics,
2) + 3) are seen in fielded examples of the '59, and in the Reissue,
4) is AB763.
I tend to favor the values selected by Fender for the Reissue as being the definitive ones for the Tweed Bassman.

Do you know of any reference describing these steps?

The info I have seen is very fragmented, so this is an attempt to piece together information

[Raja_Kentut]

the build has started… I eliminated the Doghouse and place the capacitors where it makes the most sense electrically.
The rectifier tube is replaced by diodes, and the smoothing inductor is omitted.

IMG_1006.jpeg

IMG_0991.jpeg

[bepone]

I see that in series capacitor connections (47+47uF here) there is no resistors in paralell? They must be... A like for using UCC caps.

Making direct point to point and very short connections will not lead to original sound despite connecting all by schematic and have proper voltages...layout is making the sound, musical parameters which are not in the electrical schematic, like sustain, harshness, overdrive quality.

By cancelling the rectifier tube, all volages will be higher 20-30V, is this calculated in amp functionality? IMO 5881 tubes are then out of use here..

Also cheap 0.25-0.6W mf resistors is better to avoid in tube amp circuits, specially in UB+ line..

[Raja_Kentut]

Hello Bepone,
thank you for your views. I don't agree with you on all points, but I think we agree that building tube amplifiers is a kind of art.
This is my interpretation of the Bassman, close to the original, but changes that are acceptable to me and keep the basic character.
I've been building tube amps since my hair wasn't white and now I know what I'm doing and how and if changes are noticeable.

You live in beautiful Croatia, we should meet up in the summer and check out each other's builds, we can certainly inspire each other!
It would be great if that was the start of a "European tube amp builder festival" (HaHa, I'm just dreaming, but…. )

[bepone]

Hello Bepone,
thank you for your views. I don't agree with you on all points, but I think we agree that building tube amplifiers is a kind of art.
This is my interpretation of the Bassman, close to the original, but changes that are acceptable to me and keep the basic character.
I've been building tube amps since my hair wasn't white and now I know what I'm doing and how and if changes are noticeable.

You live in beautiful Croatia, we should meet up in the summer and check out each other's builds, we can certainly inspire each other!
It would be great if that was the start of a "European tube amp builder festival" (HaHa, I'm just dreaming, but…. )

ofcourse we can do, with some wine and beer will be a lot of discussion
there was in Croatia Triode festival every year, HiFI stuff and some guitar , but after 2021 there was no any. Maybe this year will be again, there are rumors, somewhere in Dalmatia..

[Raja_Kentut]

ofcourse we can do, with some wine and beer will be a lot of discussion
there was in Croatia Triode festival every year, HiFI stuff and some guitar , but after 2021 there was no any. Maybe this year will be again, there are rumors, somewhere in Dalmatia..

…please let me know when it happens! I‘ll bring the beer

[Raja_Kentut]

the wiring is done.
Circuit is more or less original, the changes can be seen in the final schematics below.
No hum at all, only some hiss when the Volume is fully on.
The main change is the replacement of the rectifier tube. To adjust the voltages behind the diode rectifier, a 120 Ohm resistor is in series with each diode. The Filter inductor was replaced by a 220 Ohm resistor.
Only one capacitor was omitted, it‘s the one between pin 1 and pin 6 of the phase inverter. What value is 47 MMF ?
The red disc at the right is a 14V VDR that protects the amplifier against open load condition.
The green yellow wire at the left will be taken out to connect PE in the final case.

IMG_1016.jpeg

IMG_1014.jpeg

[sluckey]

Only one capacitor was omitted, it‘s the one between pin 1 and pin 6 of the phase inverter. What value is 47 MMF ?

47MMF is 47µµF (micro-micro farads). That's old talk. Appropriate at the time of that drawing. Some of us old techs called it Micky Mike. Younger folks just say 47pF (picofarads).

[xtian]

This is a very attractive p-t-p build! Nice job!

[Reeltarded]

What an amazing build!