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Author Topic: VHT Classic 18 Preamp Modification  (Read 605 times)

Tomsouthtrader

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VHT Classic 18 Preamp Modification
« on: September 21, 2020, 08:14:49 AM »
Hello There
I have been trying to modify my VHT Classic 18 (basically a cheep Chinese amp based loosely on a Marshall 18w circuit).
Essentially I am trying to completely bypass the awe full input gain stage that uses a TL072. And wire the guitar straight to the input of the first preamp tube..
I have found the schematic for the amp, and working from that I got the soldering iron out..
My aim was to completely remove the TL072 from the circuit and wire the input straight into the tube.
The schematic I have attached shows what I have done to the circuit.
The amp didn't blow up thankfully.
And it does make a sound... And The tone itself is not to bad (at least through the LO input).
However it now has a couple of issues  :-[

The input is now incredibly noisy. There is some sound even when the master is at minimum.
Also the treble control when turned all the way up makes make an awful noise (almost like some self oscillation). However I had never turned this all the way up before attempting this modification.

So I am asking for you knowlage and advice on how to fix my 'mod'  <3)

Link to the schematic and my 'MOD'..
https://drive.google.com/drive/folders/1Q-AdViC60HTd6p0KYzZEajgf1M5A8tYZ?usp=sharing

tonyharker

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Re: VHT Classic 18 Preamp Modification
« Reply #1 on: September 21, 2020, 08:22:42 AM »
You have forgotten that the TL072 is a dual op-amp.  What does the other half do?

Tomsouthtrader

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Re: VHT Classic 18 Preamp Modification
« Reply #2 on: September 21, 2020, 08:32:11 AM »
From what I can tell the circuit only utilizes one side of at as 3 pins are just left floating..
And I figured the power for it on the schematic is 'IC1-2'??

joecool85

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Re: VHT Classic 18 Preamp Modification
« Reply #3 on: September 21, 2020, 09:20:23 AM »
From what I can tell the circuit only utilizes one side of at as 3 pins are just left floating..
And I figured the power for it on the schematic is 'IC1-2'??

You should be looking for IC4 (or IC4a/b).  I'm only seeing IC4 at the beginning of the schematic, makes me wonder if there is a typo there.  How many IC's do you see on the PCB?  My guess is that IC4 (the one you are trying to disconnect) is still connected at one or more of the pins and causing an issue.

**edit**
To keep it simple, I suggest trying the mod this way (cut trace on red line, add jumper on green line):
« Last Edit: September 21, 2020, 09:29:10 AM by joecool85 »
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Tomsouthtrader

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Re: VHT Classic 18 Preamp Modification
« Reply #4 on: September 21, 2020, 11:39:17 AM »
Hey,

So there is only 3 IC's on the board, and having followed the traces and double checking the pads.. It does infact just have 3 pins floating and the power pins going to the + and - 15v rails respectively.

But Having now having tried it without the capacitor before the input of the 12AX7 it sounds great!

My only questions now are..
How do I properly add a volume pot to the new input?
And is a capacitor before the valve needed?

Also many thanks for your reply's!!! <3)

phatt

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Re: VHT Classic 18 Preamp Modification
« Reply #5 on: September 25, 2020, 03:38:12 AM »


My only questions now are..
How do I properly add a volume pot to the new input?
And is a capacitor before the valve needed?

Also many thanks for your reply's!!! <3)

Hey Tom,
If you want volume control at input then Return to original input BUT insert a 100k ~250k pot in front of C52.
This will open up much better control over how the Amp responds to your rig.

If thee amplifiers overall system tone is bass heavy then you can reduce the value of C52. If on the other hand it lacks bottom end then remove C52.
Also R76 is 100k You can always make it 220k or higher,, that will give the input higher sensitivity.
Don't go crazy as it might squeal like a pig if the input is too sensitive. I'd try 220k.

Also worth note is you have all 3 plate resistors at 220k (being R3, R6 & R30) which to me is a bit too much. I'd be changing 2 back to 100k which might help civilize the outcome.
If you want to get more power stage magic OD then try changing R52 from 2k2 up to 10k or even 20k for some softer power stage compression.
A lot of the magic of the old famous valve circuits was in the power supply voltage drops,, far more useful than buying fancy pants  brand name Valves 8|
Phil.