VHT D50 Minimum Mods

[njolly]

I recently picked up a VHT D50 for an excellent price. I knew lots of people were doing various modifications to these amps to bring them more inline with a Dumble clone. I was looking for the most bang for your buck kind of mods. I thought it was quite likely others would want something similar so I thought I would detail what I did.

1) Eliminate C6
2) Replace C9 with a 390p
3) Replace C1 with a jumper
4) Replace C20 with a 4.7u 25v
5) Install some good tubes
6) Set Bias
7) Adjust PI balance

After doing the above work I was rewarded with some fine tones for minimal time/money investment. Hope someone else finds this useful. Stock schematic attached for reference.

[Da Sting]

Interesting - you have any before / after sound files? Can you describe what the mods do? Did you also disconnect the LFB at V1b? Thanks!
Cheers, Frank

[njolly]

1) Eliminate C6 - RF filter. I thought these caps might eliminate highs at the input. I am a fan of simple and these simply aren't needed IMHO.
2) Replace C9 with a 390p - More mids in the treble circuit like the original #183
3) Replace C1 with a jumper - This cap filters out low end going into the OD section of the preamp. There's already .O5 prior that does the job. More gain without C1
4) Replace C20 with a 4.7u 25v - A bit more lows and therefore more gain. Similar to the original #183 as well
5) Install some good tubes - Can't go wrong with some good tubes. I used two Tung-Sol 12AX7s in V1 and V2. A JJ 803S in the PI. Two SED Winged C 6L6s for power tubes
6) Set Bias - Probably the single best thing you can do. Mine was biased way cold
7) Adjust PI balance - introduces more second order harmonics in the power section. Really makes it sing if dialed in correctly.

After I had posted this I carried on doing some more modifications. I did eliminate the feedback circuit on V1b. That adds a bit of distortion in the preamp which gets you more harmonics. If you like pristine cleans then you may want to leave the feedback circuit as is.
I also replaced the .1uf coupling caps, C35, and C36, in the power section with .02uf. This tightens up the low end.
I changed C7 on V1a to a 2.2uf to reduce a bit of low end. That also reduces the gain slightly on the first stage making up for the feedback circuit removed earlier on V1b
I'll attach some iPhone clips. Sorry about the playing
Original

Reed Road 57.mp3

Clean post mods

Reed Road 60.mp3

OD post mods

Reed Road 58.mp3

[Richard1001]

C5 is not there to eliminate highs at the input. It is there to block RF from entering the amps circuit by creating an ac ground to chassis directly at the input. You see these caps in amps that have isolated input jacks / star ground to chassis.

[Reeltarded]

I did one of these. We swapped all the resistors and caps and tubes and pots and the ot.

It is still an unremarkable thing. heh It is better but..

[njolly]

C5 is not there to eliminate highs at the input. It is there to block RF from entering the amps circuit by creating an ac ground to chassis directly at the input. You see these caps in amps that have isolated input jacks / star ground to chassis.

That's what I suspected the purpose was. I edited the original post to reflect your information. Poor value choices perhaps? They really do seem to muffle the high frequencies. Thanks!

[njolly]

I did one of these. We swapped all the resistors and caps and tubes and pots and the ot.

It is still an unremarkable thing. heh It is better but..

I saw some of your posts. Thanks for all the work you put in! Bang for buck is pretty good in one of these. I haven't been spoiled by playing a real dumble, or a real good clone, so I can enjoy this more

[Reeltarded]

Make sure you try many deep cabinets. The owner has an EV12L in a shallow Fender 1x12 and it sounds like an AM radio. A very clear AM radio, but zero extension. It works pretty good with a Marshall 800 4x12.

[njolly]

Make sure you try many deep cabinets. The owner has an EV12L in a shallow Fender 1x12 and it sounds like an AM radio. A very clear AM radio, but zero extension. It works pretty good with a Marshall 800 4x12.

Mine sounds pretty good with my 12" closed back ported cabs. Works well with my 2 12" Fender open back cab as well. There seems to be considerable variation in these amps. Some sound good and some sound terrible.
Thanks!

[Richard1001]

C5 is not there to eliminate highs at the input. It is there to block RF from entering the amps circuit by creating an ac ground to chassis directly at the input. You see these caps in amps that have isolated input jacks / star ground to chassis.

That's what I suspected the purpose was. I edited the original post to reflect your information. Poor value choices perhaps? They really do seem to muffle the high frequencies. Thanks!

C5 is not in the signal path. It's just a shorter path to the chassis for ac. C6 (100p) however is in the signal path. This cap is parallel to the inputs hot to ground, loading the pickup of your guitar. A good thing you removed this one.

I listened to the mp3. Nice sound!

[njolly]

C5 is not there to eliminate highs at the input. It is there to block RF from entering the amps circuit by creating an ac ground to chassis directly at the input. You see these caps in amps that have isolated input jacks / star ground to chassis.

That's what I suspected the purpose was. I edited the original post to reflect your information. Poor value choices perhaps? They really do seem to muffle the high frequencies. Thanks!

C5 is not in the signal path. It's just a shorter path to the chassis for ac. C6 (100p) however is in the signal path. This cap is parallel to the inputs hot to ground, loading the pickup of your guitar. A good thing you removed this one.

I listened to the mp3. Nice sound!

Last night I was giving your comment some thought. I decided exactly the same thing. I reinstalled C5 but left out C6. Still sounds great. Not that I need the RF suppression out here in the mountains where I live. I can't even get a radio station with a radio

Thanks for all the help!