VHT D-50 Mod Journey

[bwest]

"You are wagging and about to throw kitchen sinks at tail-chasing" - aha but that's the fun of it!

I don't deny I'm not on the journey to make an ODS - just looking for something that sounds good to me and to document the process.

Speaker wise I have a vintage Jensen alnico 12 in an open back walnut cab and I also made a large closed back Warehouse "veteran 30" cab also. Using the jensen most these days and like it.

I'll take some pics and maybe throw a mic on my cab before doing more mods. Full disclosure I struggle to get great mic'd tones and that was one reason I went load box + IRs - just easier and sounded better and more repeatable.

I haven't put the plans online but I designed and built the load box - basically the equivalent circuit of a guitar speaker built off of thiele small parameters and with a 1K log pot in parallel. The 1K is huge compared to the 8ohm reactive load so it doesn't change the impedance profile, but it is still small compared to the input impedance of most preamps, so it works fine.

You also mentioned there is no interstage divider between 1a and 1b now - the clean gain serves as the divider. I AM now changing the response of the gain stages a bit by removing the load from 1a as you mentioned (I claimed its gain was max'd out, but actually the ac load will be <<220K because of the large tone stack load).

Anyhow - will try to post more next weekend when have time / access the amp! I may even do some FR plots using room eq wizard to show what the tone stack is doing.

[Reeltarded]

You do sound like you are on a mission. I like it.

[bwest]

Ok here are a bunch of pictures of the amp / guts post Mod1 (e.g. not stock wiring). I'll try to go back later on a few and add some labels, but wanted to get the pics up sooner vs later:

[bwest]

Ok 10 pic minimum - here are some more gut shots of the VHT D50:

[bwest]

Ok here's a labeled gut shot pic of Mod1 and what I did - did labels fast as making dinner so please follow up with any questions:

https://ampgarage.com/forum/download/fi ... w&id=64382

[bwest]

I've attached the LTspice schematic file for the stock VHT D50 schematic here. Note that I didn't really bother to model the power supply, but I did get the voltages roughly right at all the nodes.

On my copy the voltages are (all DC power on no signal):
460V on output transformer - note that this is the 4x 100uF stack - note that this stack is PRE standby switch, so when standby is off this is still ON. Worth noting if you like to use standby to work on your amp. Also on a positive note this giant stack of caps automatically bleeds to ground when turned off as each bank of 2x caps is paralled with a large 220K resistor. However because of the standby switch everything beyond it will not bleed to ground when off. Practically I find this not to be a big issue, but there's usually 40v left on the nodes after this when power is off - I'm going to install a 2meg resistor across them to cut this out

460V on screen resistors (pre-resistor)
445V for LTP phase inverter
378V for V2
375V for V1

[bwest]

For non-LTspice users here are pictures of the stock amp, both preamp and power amp. Note that the tone stack is wrong - I need to measure the real values and put them in. Also note that of course I don't know the values of the transformers, choke, etc. Where I had to guess because I couldn't measure a component or read it I put "guess". Also I don't know how to make a pot so I just put two resistors and hand change the values - that's why it looks like it does

Preamp
https://ampgarage.com/forum/download/fi ... w&id=64385

Power amp:
https://ampgarage.com/forum/download/fi ... w&id=64384

[bepone]

you have put ODS in LT spice? nice
i hope that you will get something, from many reasons. this is pro technical thread, but if you are thinking only about frequency response (it is ok to check the knee, and where f. starts to drop on HF) and not about material used.. maybe you can't get too far in "dumble" teritory ?
also thinking about Dumble and Marshall to be like the same amp philosophy is a little offside IMO

[dgrainger]

Are most of the resistors in that metal oxide?

[krd51@ptd.net]

I was having an issue with my VHT D50 when the OD was switched on. I loved the clean sound but the OD was way too bassy.
Bass notes on my Strat would just jump out and take over the mix.
VHT is using two 330pf snubber caps on V2. Original Dumble specs in the OD special #183 use 25pf caps here. So I changed out
the two 330pf caps for two 25pf caps and what a world of difference. It brightened up the OD right away. I read that some
use two 50pf caps for this mod. But I'm sticking with the orig. spec 25pf caps.
There is a Schematic that was posted on a previous post regarding the VHT50 vs. Dumble ODS 183 by NESKOR
https://ampgarage.com/forum/download/fi ... &mode=view

[bwest]

I was having an issue with my VHT D50 when the OD was switched on. I loved the clean sound but the OD was way too bassy.
Bass notes on my Strat would just jump out and take over the mix.
VHT is using two 330pf snubber caps on V2. Original Dumble specs in the OD special #183 use 25pf caps here. So I changed out
the two 330pf caps for two 25pf caps and what a world of difference. It brightened up the OD right away. I read that some
use two 50pf caps for this mod. But I'm sticking with the orig. spec 25pf caps.
There is a Schematic that was posted on a previous post regarding the VHT50 vs. Dumble ODS 183 by NESKOR
https://ampgarage.com/forum/download/fi ... &mode=view

Wow thank you so much for posting that schematic! Clearly I had missed it before. I'll update my LTspice tone stack values to reflect these values and look for other issues.

Your experience with the 330pf snubbers is interesting - I'll pull up spice and look at the frequency response differences, but at least according to spice (which is dependent on the tube model, so could be wrong) this should have only added a bit of highs back. That said I'm not discounting your lived experiences at all - these are really reactive circuits and it is hard to capture everything including feel etc in a simple model

[bwest]

Quick follow-up, which I guess I will call "mod 1.1":
- I swapped the preamp tubes out. Stock tubes are unbranded Chinese tubes. I honestly didn't expect much difference in the clean but figured I might hear a difference in V2, but I put in what I had (Tung Sol in V2, Sovtek LPS in V1 and V3). Actually did sound better - I kind of couldn't believe it. When I flip the switches on the back I hear a bit of ringing, so one or more of these is slightly microphonic (I will tap test later to figure out which) but generally the amp felt brighter and more "alive" especially in the clean channel. Definitely a positive change
- I deleted the ~150K resistor in series with the level control for the OD section to boost the available volume from the OD section. This worked, but it loads down the V2b triode much more (I need to measure the tone stack resistance, but the level pot is 100K, so that's the max resistance possible now). I would say it worsened the OD character slightly as it is probably now a pretty steep load line - will play with decreasing V2b load as well
- I increased the coupling cap size on the output of the OD section V2b from 5n to 50n. That is because lows were being rolled off with the 5n going into the ~200K of resistance of the resistor + (100K pot || tone stack), and dropping the 150K resistor would have rolled off more lows. IMO since I'm running the power section clean I only want to roll of low frequencies BEFORE the OD section, not after, and since I"m not planning to distort the power section I'm not worried about blocking distortion there, so 50n is fine - I'll model it - may even go bigger

[bwest]

Here is what I'm talking about re: the 5n coupling cap right after the OD section stock:
https://ampgarage.com/forum/download/fi ... w&id=64438

The red trace is right before the cap, the green trace is right after. As you can see about ~4db of bass is being dropped at 80hz (low e guitar). So I replaced this with a 50n

[bwest]

@krd51 here is the quick analysis of 330pf vs 25pf snubber caps. Basically what I did was the copy the whole OD section so the loading, input setup, etc was all exactly the same.

Anyhow the difference is that the 25pf adds about 1db more high content at 4K and a bit more beyond:
https://ampgarage.com/forum/download/fi ... w&id=64439


To be clear, I'm not trying to imply what you experienced didn't happen - just sharing what the model would imply would have happened from a frequency response standpoint

[bwest]

Here's another one I thought you guys might find interesting - the effect of the V1B feedback

With feedback:
https://ampgarage.com/forum/download/fi ... w&id=64440

Without feedback:
https://ampgarage.com/forum/download/fi ... w&id=64441


Ok so lets look at some values:
~50hz, 33.6db with, 36.5db without - boost of 3ish
~300 hz, 32db with, 35db without - boost of 3ish
2k 33.6 with, 35 without - boost of 1.4ish

So it is very broad feedback but more feedback in the bass - why? I think it has to do with the 500pF cap in parallel to the 220K resistor grid stopper. If I remember right basically a voltage divider is formed here from the 40meg resistor to the grid stopper to the bottom of the volume pot. So the smaller the feedback resistor related to the bottom of the volume pot + grid stopper, the more feedback. If I'm right that means:
- When you turn up the volume you get more feedback on V1B (it will fight the knob a bit and increase headroom, which helps explain why it feels like I have trouble distorting V1B - headroom increases as I turn up clean gain)
- Higher frequencies see less resistance, so get slightly less feedback, because they bypass the 220K

Anyhow I think that is right but please correct me if I'm wrong here