Bassman copy Mods

[wahmi]

hi, i gave up on changing the power section of my italian bassman copy. ( original thread https://ampgarage.com/forum/viewtopic.php?f=6&t=34622 )

Here a part of preamp schématic, an a draft layout (incomplete) of power section.

amplification-schema-sound-99336.png

power amp layout.png

I want to transform Bass channel (normal on bassman) to a TMB JTM45 channel:

what do you think about this + TMB 5F6 mod ?

amplification-sound-jtm-45-99336.png

I also think about a passive loop and a Trainwreck Type 3 Post Phase Inverter Master Volume.

power amp layout MV.png

I think about this but need your Help for pot value ( and all advices are welcome)

[Stevem]

One big thing you need to understand about effects loops used with time delay devices like Reverb, Echo, Chours and Flangers is this.
If your running any one or more of these devices when the amp is driven hard enough to clip the output stage then these effects devices sound like crap!

This is because some of the distortion is being applied to the effect your using and when you hear for example Reverb or Echo added to a guitar track recorded in a studio these effects are added last or at mix down with the end result being that the effect is not clipped/ distorted!

A useful effects loop also needs to be buffered!

In short I find effects loops useless but for strapping in added gain stages or more EQ control in between gain stages.

I also find them more useful when there in parallel with the signal layout of the amp then in the normal way of being in series.
Just my 2 cents worth.

[pdf64]

I agree with all that, except -

…
A useful effects loop also needs to be buffered!
…

The ‘send’ just needs a lowish impedance to avoid excessive treble loss. Unless cabling is very long / of high capacitance, a send impedance of up to about 22k works fine without compromising bandwidth.
The signal level will need reducing anyway before the send output, and so a send output which meets that requirement can be achieved by taking its feed directly from a potential divider, provided that its shunt resistance doesn’t exceed 22k, no output buffer required.
Consider the number dumblator type fx loops there are, which, for reasons beyond me, use a 250k pot between the buffer and send output. ie the output impedance of the pot to the send will be up to 62k.
Yet such loops are consider technically acceptable, and often tonally enhancing.
Hence the only active circuit required by a loop is often a recovery stage, ie to overcome the loss due to the potential divider.

[wahmi]

thanks for your advices, i know I know the quality of a passive loop for modulation effects.

But in my case it could not be worse than a front connection.

I intend to use it a lot to plug tube preamps in the power section.

Do you think my MV wiring is good?

[wahmi]

do you see any light dumble mod i can do on a channel ?

[ChopSauce]

Not sure what you're talking about, with respect to Dumble mod?

I'm definitely not a qualified person but from what I understand (the power section not being discussed) "a Dumble" is :

1 - an amazing tone stack section with a full load of options ;

2 - an overdrive section, which you might implement by sacrificing one channel ; &

3 - a phase inverter which I do not know anything about and which might be a subject for mods.

l would tend to advise you against 2 because I think you will be loosing much of the interest of a bassman kind of amp but maybe - or not - there's something to do with 3 ?

1. is left to your courage!

Note: I could read here that implementing a Dumble kind of circuit using P2P wiring is a recipe for disaster.

[wahmi]

I was thinking particularly of the overdrive section.
My goal is not to be 100% dumble faithful but to have a "inspired" crunch channel. a bit like a pedal but with a full tonestack.

I agree for P2P but my amp is not ( i put a picture). I think i will etch some pcb section to do that.

[ChopSauce]

I think Merlin Blencowe reported one study of the Dumble tone stack in both rock & jazz positions, which might help you to approach reasonably well one of these variations with something more classical/simple.

Not sure whether that was here or on his website...

[pompeiisneaks]

I've just finished building a dumble modded bassman and it really has a great sounding drive channel, somewhat like a marshal like sound I'd say... you can easily mod one channel that way if you've got the spare tubes... Here's the schematic and layout... he left the normal channel alone and modded the other channel:

https://ampgarage.com/forum/viewtopic.php?f=4&t=33721

The build thread is here:

https://ampgarage.com/forum/viewtopic.php?f=4&t=33901

You could possibly just mirror that drive channel to mod yours?

~Phil

[wahmi]

thx, yes i already follow your thread but i don't understand some of part.

I will take time

[wahmi]

i was working on your schematic and... it reminded me something .... Here is the diagram of the blackheart bh15 (which I own) and that I wanted to check. Looks like it would be a more suitable base for a dumble mod.

[ChopSauce]

I'm not sure the BH15 power transformer will support one more preamp tube - as is required by adding an overdrive section...

That would be helpfull that you finish tracing the circuit and post it so you & we exactly know where you start from.

I'd be tempted to gut it and rebuild an entire amp keeping just the power section & the phase inverter as is.