Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 28, 2024, 07:50:34 PM

Login with username, password and session length

Recent Posts

 

Peavey Bandit T-Dynamics Diode Clipping ?

Started by Monkey_Relish, June 27, 2017, 09:01:35 AM

Previous topic - Next topic

Monkey_Relish

Hi all, first time poster.

I recently picked up a Peavey Studio Pro 112 "Silver Stripe". Basically an Express without the T-Dynamics pot and a crappier speaker.
Now, as I can't leave anything alone, I emailed Peavey for the schematic so I could add the T-Dynamics pot.
The T-Dynamics circuit is the same as a Bandit, Silver Stripe and Red Stripe are mostly identical.
Adding the T-Dynamics pot is relatively easy.



Here's how I think the T-Dynamics section works.

This is the distortion section controlled by the T-Dynamics pot. U5A and U5B are the pre and post gain sections.
The T-Dynamics is a dual gang 10KA/10KC pot. Log 10k and reverse log 10k.
With the pot at 100%, the pre gain is at minimum, and the post gain is at maximum. This requires more signal into this stage before clipping occurs.
With the pot at 20%, the pre gain is at maximum, and the post gain is at minimum. This requires less signal into this stage before clipping occurs.
The output signal from the post gain stage is the same for any pot position.
The studio Pro has no T-Dynamics pot. It is the same circuit board with no pot, but set for 100% with a jumper

Now, the parallel op-amps U4A and U4B, I think that U4B passes the positive part of the waveform, and U4A the negative part. Not 100% sure here.

From a link ENZO pointed me towards on the PEAVEY forum :-

QuoteChapter 3 TransTube® by Hartley Peavey
Most people ASSUME that our TDynamics control is nothing more than "master volume"... WRONG!
The T-Dynamics variably engages a clipping circuit in the output stage that effectively lowers the power amp's headroom,
thus emulating a lower powered amp.
When the T-Dynamics control is turned down, it is not just acting as a volume control for the amp,
it actually decreases the available voltage swing just as a smaller/less powerful amplifier would do
while retaining all the vital TransTube overload characteristics that so effectively emulates a tube amp's sound and feel.

So, if the T-Dynamics section is effectively a variable clipping circuit, it could be modified to change the "Tube Breakup" sound this circuit is meant to emulate.



So, I circled what I think are the hard clipping diodes in red (CR32, 34, 44 & 46).
As for diode mods to change the distortion characteristics, these could be changed for Germanium diodes, two for each 1N4148.
Or, for assymetrical clipping, differing amount of diodes for each, say bridging CR46.

Or, the circuit could be changed to soft clipping, like a tube screamer. Removing the hard clipping diodes, and adding the diodes in blue.
I THINK they are shown the right way.

Am I reading this circuit right ?

Monkey Relish

teemuk

Long story short: I will not work the way you have drawn it.

Monkey_Relish

OK, there is a fair bit going on there. So, the "Hard Clipping" diodes I circled aren't there for clipping ? Or is this happening before the middle two op amps ?

I probably should just leave it alone and go play with pedal circuits LOL.

Mr Relish

teemuk

#3
They are for clipping.

But you don't get soft clipping the way you have drawn the circuit.

Look at the feedback loop; as is there's 100% negative feedback resulting into gain of unity. For both DC and AC. The amps are just unity gain "buffers" isolating their input and output circuits.

Now, the way you have drawn those "soft clipping" diodes there results into negative feedback being removed until diode forward voltage is reached. The amp has gain of nearly infinity (for both AC and DC) until Vf is exceeded, and then signal is abruptly clipped.

I think this is not your intention....

If you want that thing to work you need to create proper feedback loops for those stages. Google non-inverting amp for example.

Now another problem: This thing works with sliding DC offsets, so you don't want any DC gain. DC gain will disrupt the operation of the circuit.
Secondly, the gain levels of the entire power amp circuit are tweaked for a circuit where those amps are just unity gain buffers. Add voltage gain there and probably you start to get clipping distortion in unwanted places. Non-inverting amp with diodes in feedback loop also will not "clamp" to specific voltage threshold. And you want that clamping because that's only thing preventing overdrive of the following power amp stage.

My bet is that adding voltage gain there, and increasing clipping threshold - in any form - disrupts the circuit's operation. It's designed to work optimally as is. If you don't know how to tweak it - and you obviously don't - my advise is to do nothing for it because there's a huge chance you just turn things worse - or turn a fully functional amp non-functional. There's very little you will gain in practice from turning the clipping of one half wave a bit softer. As is, it already clips softer than most genuine tube power amps.

Also, trust me: One thing you do not want from that circuit is excessively asymmetric clipping of those "push-pull" pairs. It will not work properly.

Get back to this when you fully understand how that circuit actually operates.

Monkey_Relish

Thanks for the reply.
Yes, there's a whole lot going on there that I would need to understand first.
I will definitely leave it alone, No point messing up what is a surprisingly good amp.

Mr Relish