VHT D-50 Mod Journey

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bwest
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VHT D-50 Mod Journey

Post by bwest »

As a bit of context, I've done amp mods/builds before, but I've never much documented the process. Here I figured with a bit of planning ahead, I might be able to make a thread which contributes to the DIY community knowledge base. So over the next few weeks/months as I have time I'll post updates, pics, sound clips etc. I've got a few parts written already which I'll drop in below. Please enjoy but remember I'm new to the forum and an amateur tinkerer (and reasonably poor guitarist) so take that into account when reading the below :-).

Part 1 - For as exciting as an affordable “D” style ample clone is, at least to me, I’m surprised there is so little content on the web about the VHT D-50. In fact when I was looking for an amp to mod to behave like a big clean D amp or the John Mayer Two Rock, I was very close to buying an old Fender Silverface bandmaster or bassman or an old Traynor and doing a ton of restoration work, only to then do the mod work, mostly because I thought that was my best option for a high quality non-PCB mod platform with 2x 6L6 tubes, at a decent price, with decent starting hardware/chassis/cabinet.

So imagine my surprise then when I came across the D-50 / D-Fifty. Ok so here’s a really handsome looking (to me) head, in a compact enough size (thank goodness), with all the wiring done on an eyelet board, and oh yeah there’s a footswitchable overdrive channel and preamp boost? For <$1000 new? Not a kit, but a full blown built amp with tubes?

First a note on why I care about non-PCB wiring. No, it is not because I think PCB are the devil or will make my guitar sound like plastic or something. The simple reason is they are easy to modify and are physically robust. I recently used a Ceriatone kit as a donor to start a highly modified Marshall project of my own design. And I really liked the kit quality and Nik over there was great to deal with. The thing was that I wanted to paint outside the lines, and that meant cutting some traces and running some extra wires. Also I spent a lot of time holding the board up to the light remembering what connected to what. Also on a prior mod project (plexifying the Marshall Class 5) I soldered one pad enough times trying components that the pad lifted, which I can assure you led to no end of headaches and troubleshooting until I identified the board as the problem. Anyhow with turrets, eyelet, or P2P what you see is what you get – unless your soldering is atrocious or two wires are crossed, if it looks right it will work. So to be clear, if you are building a thing as intended and you aren’t swapping parts a lot (e.g. you are a normal builder), a PCB is great and makes your life easier – for me I wanted no PCB hence originally looking at the vintage amps.

A bit about my amp lineup – I have the highly modded / custom Marshall 1959-inspired head which I designed/built, which puts out ~25 watts clean with El34s and probably ~40 watts distorted, but it also does many other sounds in the Marshall family because of the mods. I have the Marshall thing covered. Then I have an old Hammond organ reverb amp (AO-44) which was modded into a ~13 watt EL84 guitar amp by someone else, and then I modded it further to fix the flubby distortion and cathodyne inverter distortion issues – sounds great now – got a tweed/Vox thing pretty nailed now too. But I didn’t have a clean Fendery amp, and “D” amps have their own halo, so I wanted to try to achieve that clean “D” sound. The extra OD channel on the D-50 was just pure gravy as far as I was concerned.

D-50 first playing impressions. I mean of course I had to plug it in first right? I have a strat that has been in the process of being built for ~4 years (kids…) so for now most of my playing is via a neck-thru PRS-style guitar with a Bare Knuckle rebel yell in the bridge (think Seymour Duncan JB) and in the neck a Dimarzio unpotted PAF pickup – pretty sure it is the PAF 59. Both sound great. On the clean (non-OD engaged) side, I did really like the clean sound but it was very clean. Super clean. Big tight low end. I liked the rock position best, by far (it also has meaningfully more output) and the mid switch “up” (more mids), so it stays locked there. Interestingly the bright switch (3 options, none in the middle and 2 different caps) didn’t do nearly as much as it does in my Marshall style amps – will have to look at the circuit to see why. Even playing through humbuckers, with the clean preamp turned all the way up, the mid switch up, and pretty noonish tone settings, I could not get the clean preamp to distort one bit. No I didn’t put a boost in front, yes of course I’m sure you can make it distort if you do that. Worth noting also this amp is dead quiet – no real hum or hiss. Clicking on the OD and…to be honest I just didn’t like it at all. It has a very “saxaphoney” sound to me / almost like a fuzz. It sounded to me like there was just too much bass in the circuit (for my taste). Rolling off the guitar helped a bit, as did fiddling with the OD trim and OD drive controls, but regardless the tone isn’t what I’m looking for. That said if I had a very low output guitar and it was very bright (like say a vintage tele) this might work great, so your mileage may vary. I will say though the distorted tone isn’t like what you get with a Marshall or a Tweed, so it did check the box of being “different”.

Turning off OD and cranking the power section with an attenuator – it very abruptly distorts and I didn’t love the distorted tone – guessing there is a lot of feedback in there.
bwest
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Re: VHT D-50 Mod Journey

Post by bwest »

Part 2 – time to crack open the amp and see what’s going on in there. Immediately I had an issue where the amp wouldn’t come out of the case… what could be the problem? Turns out the handle is held in place by t-nuts (good) and there is an interference with them and the case sliding out. Kind of obvious in retrospect but anyhow unscrew your handle first just to make sure none of your wiring gets snagged.

I should mention before getting to deep in here that I emailed VHT to make sure they would be ok with me sharing my schematics, mods, pics, etc and they were very supportive and also expressed interest in the project. Thanks VHT for being so supportive of the DIY community!

First impression of the wiring – pretty impressed. Eyelet board as advertised. Only PCBs are small ones for the footswitch relays – no problem there. Lots of shielded wire in the preamp – that’s extra time and cost. It is better/cleaner wiring than I can do. Some of the solder joints looked borderline cold, but I’m also used to using leaded solder so could be I don’t know what non-leaded solder should look like. Also worth noting everything in the amp worked perfectly – nothing to “fix”. There were some interesting little details too – like the last electrolytic cap in the power supply for the first tube had a smaller poly cap in parallel with it – lots of hifi folks do that kind of thing in crossovers. I think the justification is to bypass the higher ESR in the electrolytic. Anyways who knows if that even does anything audible, especially in a power supply, but here they put it into the amp. Again extra time and cost to go the extra mile – a good sign.

Also worth noting – I have no means of assessing the quality of the output transformer, power transformer, and choke (this amp has all three) but they are all beefy and large, which is generally a good sign.

In terms of the amp itself, I’ll start with a block diagram as that is easier to understand than a schematic just from the get-go. Friendly reminder I’m doing this for fun, so could definitely be some errors in all of this (see attachment - VHT D50 stock block diagram)
https://ampgarage.com/forum/download/fi ... w&id=64326

Few quick notes before breaking this down:
- Basically all of the coupling capacitors between stages are 22n and big enough to pass everything (the one after 2a is ~15n but that’s still pretty big).
- The tone stack is basically the one described by Merlin B here: https://ampgarage.com/forum/viewtopic.php?t=12175
- I couldn’t figure out how the PAB (pre amp boost) works – it makes the output of the preamp hotter and subjectively the tone doesn’t change. I’ll have to look closer to see how it is implemented. I thought it would be a partial tone stack bypass but the tone controls all seem to work and the tone seems not to change – not sure. Maybe a voltage divider which is then defeated?
- There is some full bandwidth local feedback on 1b which shaves a few db and makes it even harder to overdrive
- 2a and 2b have small caps bridging anode to cathode, but these mostly bleed 10Khz+ so from a guitar perspective they don’t do much

Ok quick breakdown/analysis:
- There are two gain stages pre-master for the clean setup, and four when OD is engaged
- The “D” tone stack is very lossy, so even with the volume pot max’d out that is why I couldn’t get any clean preamp distortion. Modeling the circuit later confirms this. I’m guessing the local feedback on 1b is there to ensure you basically cannot overdrive the clean channel/reduce distortion
- Every gain stage in the whole amp have high value load resistors, which max out gain / output voltage range / linearity
- Every gain stage in the whole amp has the cathode resistor bypassed (more gain). The caps on my amp in the OD section are face down, so without pulling them out (I will later) I’m assuming they are also 4.7u. So every gain stage has max’d gain/linearity/output in the load resistor, and max’d gain from a bypass standpoint
- There is a ton of gain when you add in the OD section. Nowhere is the bass bled off (usually coupling caps or smaller bypass caps) which helps explain the tone I’m hearing – it is the fundamental distorting heavy.
o There is no EQ then after the distortion – I feel like this is a missed opportunity
o I can already tell I’m going to be taking out bass to get this to my taste
- The power section feedback level is a quick estimate, but at 12db it is on the higher end for a guitar amp is my understanding from Blencowe/Kuehnel’s books
o This explains the tightness and cleanness of the clean tone and why the power section OD was “abrupt” – lots of feedback
- Taking this all together, you have a very linear preamp and power section when clean. Almost solid statey? I’m not sure how different it would sound as solid state to be honest – I’m guessing distortion is pretty low. My guess is this is part of the D “thing”
- Just to throw it out there, the “ultra high gain” reference preamp in Merlin Blencowe’s “Designing Tube Preamps for Guitar and bass” has four gain stages with 100K load resistors, smaller bypass caps, and a switchable bypass cap in position 2 (though the tone stack is at the end). Just saying there is a huge amount of gain and bass in the stock circuit – usually as gain goes up bass is cut – this is unusual.
- Assuming I didn’t blow the part measurement, I’m confused by how aggressive the OD trim voltage divider is – max’d on the pot you would lose still 5/6 of your voltage, whereas the ones after that max’d out you’d drop ~1/2 the voltage. Anyhow not sure why they did this but you can still overdrive 2a.
- I’m not sure why they made 2a/b a mirror of 1a/b – guessing this is what “D” did – feels like a missed opportunity to bias hot or cold and do some funky distortion stuff
- I would have to model it out, but seems clear why everyone complains about the FX loop on the D amps, assuming this copied it – it is post a 1 meg pot. So in a perfect world your output impedance feeding the loop would be ~0 and here it is something like a few hundred K ohms depending on pot position. You get tone suck with long cables and a guitar’s output impedance of like 5-10k so no surprise this has problems (it makes an RC circuit, which is a low-pass filter).
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erwin_ve
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Re: VHT D-50 Mod Journey

Post by erwin_ve »

Your preaching to the choir! :D
Anyway welcome, enjoy the journey of modding and if you have specific questions don't hesitate.
Any particular sound you're aiming? Ods style amps are very different from clean amps, like a Steel String Singer.
pdf64
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Re: VHT D-50 Mod Journey

Post by pdf64 »

I find that removing cathode bypass on clipping stage/s helps for a smoother transition into overdrive and smoother overdriven tone.
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Reeltarded
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Re: VHT D-50 Mod Journey

Post by Reeltarded »

The OD entrance is how you balance OD action against the clean setting. Please do this before anything else (and while thinking of it leave those cat-anode caps on V2.. you will discover why they exist when you do this next thing.

Settings: All switches off. Clean volume STRAIGHT UP. all tones STRAIGHT UP. OD1 STRAIGHT UP. OD2 STRAIGHT UP. Master 4-5. Pres STRAIGHT UP.

Engage the OD, smash the hell out of some chords and dial in the OD entrance. Ok, now click back and forth changing only OD2 for volume balance against clean. You should not have to move any control more than a number now.

I have watched 10 videos in a row of youtubers completely failing the settings test. Everything sounds best when it's near 50% rotation. I never smack the clean volume and OD1.. EVER. That would be pinched and miserable. My clean never goes above 4. (no LNFB)

What I could do with $39.95 and two hours getting wires off themselves on one of these and not have to kill anything expensive.
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bwest
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Re: VHT D-50 Mod Journey

Post by bwest »

Thanks @erwin_ve! No particular sound in mind - I figured I'd slowly mod from stock until I find "that thing" - I just know I don't want another marshall or tweed sound. I do really like that Petrucci liquid lead tone and the Andy Timmons type tone, and there sure is enough gain in there to do it, so will be interesting to see where we end up
bwest
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Re: VHT D-50 Mod Journey

Post by bwest »

@Reel - yes I started with everything at noon, and I started with all the gain backed off in the OD section - I just didn't like the sound that much. Got some recordings I'll post which are after mod1 but with OD section stock
bwest
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Re: VHT D-50 Mod Journey

Post by bwest »

Post 3:
Mod thoughts/goals:
- I don’t want to touch the core clean tone and tone stack – that is the “D” thing and I want it in my arsenal
- I plan to run the power section always clean, so I’m not going to bother with messing with it. Maybe I’ll put in a switch to drop the level of feedback for a different feel, but not high on the priority list.
- For the OD I want to get more of a singing mid-pushed lead sound, which is hopefully distinctive from my Marshall and tweed sounds. Feels like we could get into high gain territory here – there’s certainly enough gain on tap (liquid lead?)
- I don’t care about reverting to stock or resale value – we are burning the boats
- I don’t care about sticking to any classic “D” build or formula
- I want to do this in stages to see how it sounds
- I think I will play this amp clean with the OD being the “solo” sound, so they should make sense together.
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Reeltarded
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Re: VHT D-50 Mod Journey

Post by Reeltarded »

Fair ideas. If I had one of these today, I would swap out all the plates and signal caps, get the preamp wires away from each other, then simply follow the procedure for balancing the output and setting OD trigger. [after shoving EH 12ax7s in it]

The output doesn't sound good when it's lit up. I don't like them over 5. It's just something sweet for the preamp to hit.

Hurry! I want to hear it!
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bwest
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Re: VHT D-50 Mod Journey

Post by bwest »

Post 4 - Mod 1: Move the tone stack to the back before the master
Here’s my thinking on this:
- Since when playing clean nothing distorts in the stock arrangement, the placement of the tone stack doesn’t really matter
- If I move it to the end right before the master volume, that would preserve enough signal to distort 1b if I want for a dirtier clean (or if you want stock clean, just roll back the volume knob)
- Then the only question is whether the master will interact with the tone stack, but in the normal config the tone stack was followed by a 1meg pot – so it is kind of the same
- Also this would allow the tone stack to work post distortion, so you don’t just get a “fuzz” square wave tone – you can shape it (though for now clean and OD share a tone stack)
- So net net the way I view it is I have the ability to get a slightly compressed/hairy clean, and keep the stock sound, and actually have a tweed sound if I add a defeat on the tone stack later, and have more gain feeding the OD if we want it
--- Feels like a lot of upside
--- Only real downside is that there is less output potential to overdrive the power section (as the tone stack is lossy)
--- But since I decided I don’t plan to overdrive the output section this doesn’t matter to me, and I suspect you would still have enough output to overdrive it if you wanted to, especially if you lowered the amount of feedback

Unfortunately I didn’t make any recordings before doing Mod 1. But Mod 1 is done and it performs as intended. The stuff spanky clean is there, but roll up the clean preamp gain and it gets glassier and a little hairy, which is just right for me. One additional downside is the OD level control basically doesn’t work anymore – I’m guessing this is because of the loading of the tone stack post-pot – need to investigate.
bwest
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Re: VHT D-50 Mod Journey

Post by bwest »

Ok two quick uploads of the post Mod1 tones.

First - clean - just rolled back the clean gain to 9 o clock or so to show that you can get the same clean tone out of this setup.

Second - pushed clean. Clean gain about 12 o clock.

Both are recorded with the amp going to a custom loadbox (reacts just like a real speaker) and the line out from that box sent to a Focusrite Scarlett interface. Only FX was to use a Cab IR from redwirez (the Tweed Deluxe Blue / SM57 cap 1in). All samples have this same cab and I'll hold that constant through the tests (unless stated otehrwise).
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bwest
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Re: VHT D-50 Mod Journey

Post by bwest »

Third recording - this is through Mod1 but with the OD as stock. Its funny - I recorded this yesterday and was hating the tone the whole way, but listening today (not playing). It doesn't sound that bad. Yes the playing is rough but hey - caveat emptor :-). There's a mix of bridge and neck pickup on here - interestingly they don't sound super different through this OD. The start is bridge with gain rolled down, then rolling it up on the guitar, then neck (super flubby and thick sounding). OD trim was at 9 o clock, OD drive 10:30 or so.
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Reeltarded
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Re: VHT D-50 Mod Journey

Post by Reeltarded »

Buzzy to me. I think it's all about the loadbox though. I like the clean clean.

If you are just modding an amp, generally, moving the TS is ok, but it is very important to be between the early stages if you are shooting for dumble of any sort.
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bwest
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Re: VHT D-50 Mod Journey

Post by bwest »

Yes it's interesting - it sounded almost like the input of the interface was clipping, but I looked and it wasn't. I don't get that sound out of the real amp - not sure what the issue was
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Re: VHT D-50 Mod Journey

Post by Reeltarded »

The eq action is now dominant.Stages have their own sounds.. three stages come after the eq in an ODS.. This matters, here is more about that..

The first stage hugely overdrives the second now without any band-pass excepting the Miller effect if you have a grid stopper on stage 2. 4u7 cathodes can be virtually ignored. They go way down. The tone stack used to be a variable coupling network there. Two stages then eq using ODS values and just swapping order is going to sound like that. You could change to Marshall values in the tone stack but it won't be lots better. There is no interstage attenuation.

Stage 2 depended on the massive gain loss of the eq.

Stages after eq obscure the action of the eq with roll-off we percieve as 'warmth'.

This is a deep philosophy dive. It's about how to build amps with a goal in mind.

What sort of speaker do you have? I ask because the next problem is you will never hear the truth until you play through some fair reference speaker and cabinet. Anything you do on a load will sound like a fork scraping china because you try to compensate. Loads are never ideal; they are a compromise we make to do something else. The first compromise we make is deciding we want 50 and 100 watt tube amps. That's going to cost something everytime you change venue.

I have played load rigs since the 1980s with probably 20+ variations of loads including dead resistive loads, Harry Kolbe, Palmers... I have played through every forgotten attenuator ever made. I do this at home, live, and even sometimes in studios. I do this to channelize and effect several amps through a mixer and keep my cabinet count down to stereo pairs. It's all one thing for another, and pulling any amp from the rig and going straight into a cabinet reveals a dynamic truth that is only imagined with 100w squeezed down to mic level. There is a big learning curve on when one would be trying to compensate with control changes and the knowing that is impossibly fleeting.

The defining things about an ODS are incredible sustain, a world class clean, and the ability to do these things just over talking volume. I could be happy with a straight ODS on the second floor of a three story apartment building, no load.

I literally have 12 hours more on these subjects. You aren't actually modding/upgrading an ODS. You are wagging and about to throw kitchen sinks at tail-chasing. I know this from experience.
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
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