Heavy modded Bassman/JTM45

JHS · Post by **JHS** » 23 Jul 2007, 14:32

Just a suggestion for modding your Fender or Marshal amp.
Have fun with it. I hope this link will work.

https://img407.imageshack.us/my.php?ima ... prebk4.gif

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JHS

Post by **modman** » 10 Aug 2007, 01:42

Hi JHS,

I edited your post, as these small preview urls don't seem to work on this forum. I'm checking this out.

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Could you explain a bit more about what this mod does exactly? How did you come up with this?

soulsonic · Post by **soulsonic** » 10 Aug 2007, 04:55

It looks more like a total rebuild than a mod. It adds two tubes and makes it switchable between a Fender clean channel and another channel which can be switched to be either like a stock bassman channel or some kind of hot-rodded channel.

Have you built these mods?

JHS · Post by **JHS** » 13 Aug 2007, 20:31

The base was the Arrendondo-mod from the eighties.

Arrendondo has made a lot of things with the Marshalls yielding in more or less useable sounds (IMHO there are a lot of useless features in his mod).

This mod is way simpler, and delivers the best sounds from the Arrendondo tuned Marshalls and the original sound of the Amp when the added tube is bypassed.

The new Laney 1-channel tubeamps deliver similar sounds, the boost stage is copied from Arrendondo. A JTM45/Bassman with a good tube-treble booster in front of it sounds nearly identical.

JHS

analogguru · Post by **analogguru** » 13 Aug 2007, 20:45

V4a without a pulldown directly at the grid appears to me a little bit "brutal".

250µF at the cathode of V1b appears to me a little bit "oversized".

analogguru

JHS · Post by **JHS** » 13 Aug 2007, 23:42

There is a common 1M pulldown at the input.

The Clean BF-preamp (V5a and 5b) and V4b can be dropped.

250uF is std. at this stage, for that deep lush bass but it can be reduced 'til 4,7uF to tighten the lows, but I prefer to throttle down the Bass-pot (around 0-2) to tighten the bass.

The classic Blues-setting on those JTM 45 amps is Bass= 0 and Treble=0 and a RM TB in front of it.

V4a is a treble booster for the classic JTM-preamp, less noise, more dynamics and more organic sounding than a Rangemaster TB and a stock JTM45/Bassman.

This mod can be uses with any Plexi, 2203/4, old Stramp and Laney AOR amps and others, it's universal.

JHS

analogguru · Post by **analogguru** » 13 Aug 2007, 23:58

There is a common 1M pulldown at the input.

Yes I have seen that, but what happens, when the switch - which sudddenly occurs - has contact problems ?

250uF is std. at this stage, for that deep lush bass but it can be reduced 'til 4,7uF to tighten the lows, but I prefer to throttle down the Bass-pot (around 0-2) to tighten the bass.

Hmmm....we have a plate resistor of 100k and a cathode resistor of 1k5. The lower (-3db) corner frequency with a 22µF bypass cap is 9,3 Hz, with a 250µ approx. 0,9 Hz...... the lowest frequency of a guitar approx 80 Hz, the lowest audible frequency 20 Hz...... how does the 250µ produce, effect or contribute to a "deeper lush bass" than a 22µF

analogguru

JHS · Post by **JHS** » 14 Aug 2007, 09:24

22uF is enough when you calculate this stage but you can hear a difference in sound with a 250uF.

The frequency-curve for electrolytic is not linear at low frequencies, they act. more or less, due to their losses, like a bass cut at low frequencies.
That's the reason while they are 10x oversized (compensate this effect a bit). They also loose capacity over the years when they dry out (about 30-50%), another reason why they are oversized.

I will think about the pulldown resistor again. Furthermore I recomment to use opto-couplers (the ones Soldano uses) instead of the switches (no relais, not fast enough).

JHS

Goop_buster · Post by **Goop_buster** » 14 Aug 2007, 11:24

JHS wrote:22uF is enough when you calculate this stage but you can hear a difference in sound with a 250uF.

JHS

Confirmed

That 250uF in the old bassman contributes to the character IRL even though 22uF should be enough logically.

Although I think there is a shared cathode-"thing"+ 820 ohms in the original.

If that 250uF is good or bad is another issue

soulsonic · Post by **soulsonic** » 14 Aug 2007, 22:22

My experience with using K bypass caps that large is it usually leads to blocking distortion and a strange "smearing" of the frequency vs. time response.