Laney GH100TI preamp

[Sensei Tim]

i had a request to build one of these preamp sims.

I drew up the high gain input.  low gain should be pretty straight forward to add.

my layout borrows heavily from the dr. boogie layout.

a few things that i'm not sure about:

1. power filtering:  i left the filtering the same as the dr. boogie with a 100u filter cap and a two 100R resistors (isolation?).  i assume this will be sufficient.  can someone confirm?

2. V2b in the schematic has 2x100k bias resistor. i assume that a 100k trimmer is going to be sufficient for jfets?

3. i am not entirely sure how to add the presence and resonance controls in the circuit (lower left of the schematic)  Should I include C16 (100n) also?  

4. I'm not exactly sure how the resonance circuit works?  it's just adding in a 100k resistor in parallel to R39 right before it hits the output?  

5. In the schematic it shows the presence and resonance portions in parallel with the output transformer.  Not exactly sure what's going on  with this?  

[Sensei Tim]

100u cap was on the wrong line
switch shouldn't have shorted the capacitors when off

[glasspak73]

Is V4 tube in the schematic some type of a booster for the preamp? Can V4 also be converted to Jfet and used at the end of the simulator circuit? or would this make any difference on how this will sound?

[rocket88]

i know this is one of the custom ones for iommi, but i thought he used a supergroup mostly and a 100BL  before that. i wonder how much different they are.

[Sensei Tim]

i don't know what v4 does... it's after the effects loop but before the power tubes...

wouldn't make sense to have more gain stages after the effects loop.

[Ciaran Haslett]

Its the phase inverter required for the push pull power section in the amp.  Unless you design a push pull output for a pedal it's not worth simulating.

[Travis]

V4 is the phase inverter

[Sensei Tim]

can anyone explain how the resonance switch works?

[Ciaran Haslett]

It's placing another 100K resistor in parallel with the 100K in the negative feedback path.  Without doing the math, this will boost (or more accurately....not attenuate) a specific frequency range.

[Sensei Tim]

Ciaran,

ok.  so is there an obvious way to add this to a pedal, or will this only work with an amp due to the interaction between the power amp section?

[Ciaran Haslett]

Each amplifying stage inverts phase by 180 degrees.  I suppose you could feed some signal from a later JFET back to the gate of an earlier one thats out of phase.  I haven't toyed much with this stuff so I'm just speculating.  Sounds interesting though.

[Travis]

The type of PI in this amp is called a “long tailed pair”. The long tailed pair or LTP produces gain and can contribute clipping of its own as well as driving the power tubes harder

Imagine if you have a lot of gain in the first preamp stages, all of that gain hits the front end of the PI.

I would have to say that generally in an amp where you are paying particular attention to the clipping characteristics, the PI is a critical element beyond just allowing push pull operation.

With that said it would be weird to implement it in a stompbox preamp. It would probably make more sense to approximate it with a single gain stage.

To further complicate things, most people will use a preamp stompbox into the front end of their amp, where the extra gain stage in the pedal may be overkill. I guess the key as always is to use the ol ears

[Sensei Tim]

that being said, i assume Laneys are based on marshalls.

looking at the jcm800 layout:



and then looking at the 2204 schematic



i would say the the jcm800 layout here incorporates the phase inverter tubes... very interesting

[rocket88]

correct the JCM800 layout does. side note, i just looked at the supergroup schematic and it's damn near identical to the sunn model t, which is also similar to the JCM800, dual rectifier, etc. which goes all the way back to the fender bassman. with that in mind, one could change component values to have any of these preamps converted from tubes to FETs. i'm not so sure that having the phase inverter is really necessary, as mentioned before. the mini model t doesn't have it converted and its perfect for the application, so i may just leave it off.

[traktop]

 I´ve got a post phase inverter vol pot installed on my hiwatt dr103, so I actually could hear what´s going on in there without the output tubes saturating at all.
 The original master volume goes right after the tone stack, then to a buffer and then hits the PI.
 Even being a pretty "clean"amp, you get some crunch from dimming both preamp channels without the PI afecting that saturation. The moment you open the master volume tap enough, you can clearly hear how the PI saturates, (a lot), and how that rounds the tone preparing it for the final blast.
 Your idea of swapping parts on the jcm layout to match a sort of a single preamp channel model t, (for example... ), got me thinking that could be a good try.
  Anyway, maybe just tunning a simple gain stage, as Travis said, would do something similar.


 

[Travis]

Didn’t Ciaran mod the Model T FET adaptation for more gain? Could be making up for not having the clipping and gain of the PI stage

[rocket88]

Hey Travis he didn't, but someone who built it In the comments changes the source cap and resistors to get more gain out of it.

[Ciaran Haslett]

Yeah wasn't me.  It already had more than enough for my tastes.  I've toyed around with AC bypass caps with other marshall type get conversion.  There comes a point where the high and low freq content really starts to get in the way.  So you can only go so far with out adding some extra filtering.

Take a look at the ROG Thunderchief schematic.  They put a HPF on the "phase inverter" and LPF on the "power amp"....probably to overcome what I said above.

[Sensei Tim]

when the boss is away...

I *think* this is correct.  I used the JCM800 layout from Mark as a starting point.

NOTE: i am not sure if the trimmer on Q4 should be 100k or 200k-ish based on the schematic

[traktop]

Travis wrote

Didn’t Ciaran mod the Model T FET adaptation for more gain? Could be making up for not having the clipping and gain of the PI stage

I change both 470k resistors before Q3 to 100k, so both preamp channels hit that fet harder.
That way you can get far more gain, (plenty enough to make some heavy riffs), but it only worked right with 9v. Over 9v, the thing started squealing when preamp controls passed 1 o´clock.
You can hear it here:
https://soundcloudx.com/thrashtop/model-t-les-paul