Marshall Super bass component anomalies

[Timbo]

According to the model number - 1992, this is a super bass 100. I don’t think, from the soldering, components and general condition that it’s been modified from new, however some of the components appear to be more in keeping with the 1959 super lead. Specifically the coupling capacitors from the first stage are 0.22 and 0.022, whereas on the SB schematic they’re both 0.022. Also, this one has individual cathode resistors on V1, the second one bypassed by a capacitor labelled 0.047/10/250, which doesn’t agree with either model schematic. Interestingly, but maybe not relevant, the screen printing in the back panel misses either lead or bass and just reads MkII Super 100 (it hasn’t been rubbed off, just never printed).
I haven’t worked on one of these before, so I don’t know if this kind of variation is to be expected or not. Can anyone provide words of wisdom?

[drewl]

Looks stock to me.
Employee mistake?
Part fell into the wrong bin?

Common Marshall weirdness I'd say.

[Jon Snell]

For the bass version, thew 320 or 330uF should be fitted otherwise it will be a little peaky to say the least!

[Timbo]

For the bass version, thew 320 or 330uF should be fitted otherwise it will be a little peaky to say the least!

This has an 820 ohm with no bypass cap on V1a and a 4.7k with 0.047uf bypass cap on V1b. What do you think they had in mind?!

[Chuck H]

This has an 820 ohm with no bypass cap on V1a and a 4.7k with 0.047uf bypass cap on V1b. What do you think they had in mind?!

I saw that on your board and I'll confirm that it's what is there. Further, it looks to be stock installation. Weird because there is no schematic that supports this circuit. There's a chance it's a modification even though it doesn't look that way. In any case... It's not chapter and verse for that circuit or any respected Marshall schematic. I wouldn't think twice about changing it to the stock SB circuit or the stock 1959 circuit. Whatever you choose.

[Jon Snell]

If one works out the extra gain when using a decoupling capacitor, in this case the extra gain doesn't start until HF or above 8kHZ. (F=1/2pi RC)Way beyond the loudspeakers' cut off frequency.
I would say either a mistake of a misguided attempt to brighten up a bass amp.

[Timbo]

Excellent. Thanks, those are exactly the replies I was hoping for.

[mozz]

I'd flip the board over and see if the solder joints are original. I'd at least listen to it anyway thru a proper cabinet. I don't know if they have any kind of serial number checker.

[tubby]

Might be a 1992N, see lower left corner in attached schemo. Does it have the James tonestack?

[drewl]

This has an 820 ohm with no bypass cap on V1a and a 4.7k with 0.047uf bypass cap on V1b. What do you think they had in mind?!

Ah but the Club and Country models did use a 4.7k/.047 combo, or a cap close and with the cap being foot switchable they were quite versatile and crunchy.
I believe they were built around the same time.

[Timbo]

well done sir! That appears to be exactly what I have. Can you explain the idea behind this design??

Might be a 1992N, see lower left corner in attached schemo. Does it have the James tonestack?

[tubby]

As I understand it, it is a somewhat rarer version of the Super Bass circuit. It is the only model that comes to my mind where Marshall doesn't use the classic TMB tonestack. Because the plate-fed James tone-stack has inherently a little less gain, it often gets (mistakenly IMHO) converted to standard 1992 specs. I have the very same amp from '79 and it is great for guitar!

Here's some more info and tones:

https://youtu.be/qUPNmkFtkkw?feature=shared

[Helmholtz]

Can you explain the idea behind this design??

Splitting the cathodes allows for differently voiced input stages.
The bright channel with the bypass cap boosts frequencies above 720Hz.
The normal channel has a flat frequency response down to zero Hz.

https://www.ampbooks.com/mobile/ampl...or/calculator/

[Timbo]

Tubby, thanks so much for this, I would never have found that otherwise.

As I understand it, it is a somewhat rarer version of the Super Bass circuit. It is the only model that comes to my mind where Marshall doesn't use the classic TMB tonestack. Because the plate-fed James tone-stack has inherently a little less gain, it often gets (mistakenly IMHO) converted to standard 1992 specs. I have the very same amp from '79 and it is great for guitar!

Here's some more info and tones:

https://youtu.be/qUPNmkFtkkw?feature=shared

[Chuck H]

The lower left in tubby's posted schematic (as we are instructed to observe) is the same split cathode values as the 1959 model. 2.7k/680n. Not 4.7k/47n. There's a big difference. But there is something to what tubby is saying I think because your amp, as I can see in the photos, is NOT using the typical TMB tone stack (I haven't tried to pidgeon evaluate it by tracing) and it's also clear that V2B is UNUSED. As in no cathode follower for feeding the tone stack. And an unused triode on board. If it is a james stack that would be another unusual feature. VERY un-Marshall like. I do think it's important to recognize that the good performance noted by tubby ISN'T represented by your circuit as it DOESN'T match the schematic he's talking about WRT the split cathode values. So the idea that it's a mistake to change the circuit is entirely up to you. As in "How does it sound?", "How do alternative circuits sound?"

This is a rare bird Marshall to be sure.