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Old April 9th, 2010, 10:56 PM   #1 (permalink)
TNO
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VHT Special 6 Mod thread

OK, maybe this is a bit premature since these amps just came out

I couln't resist pulling mine apart and having a look. Construction is very nice-pretty much on par with boutique stuff built in the US. The only thing I saw right off that I'd want to change would be to upgrade the coupling caps.

I'm a vintage Jensen guy so I installed a nicely broken-in Weber Sig10AS. I've tried a few higher-power ceramic Emis in other amps and always went right back to the Weber Sigs. In this case I much prefer the stock speaker. It's pretty even in the mids, plenty of highs and the bass is well suppported. Considering how loud the amp is it's probably fairly efficient. I'm not even tempted to try to upgrade it.

I was getting a lot of rf interference so I thought I'd open it up and check to see if it had a grid stopper resistor on the input. It didn't so I added a 68k. It worked-the amp is no longer a radio

For coupling caps the amp has .047s in the preamp and .1s in the power amp. I'm hearing a little more low mids than I like so when I upgrade the caps (to Mojo Dijons) I'll go with .02s in the preamp. For filters it has 22ufs in the preamp and 47s in the power amp. I'll drop the filters in the preamp to 10uf to loosen it up.

It's a fairly quiet amp but there is some hum even with nothing plugged in. It's not bad at all but I did notice that the preamp and power amp are grounded together. The next time I open it up I'll try grounding the preamp and power amp separately to see if it quiets it down any. Did this to a 5E3 I built and it made a huge difference.

All said, this is an amp that doesn't need mods to sound great, which is really amazing considering the price. VHT is going to sell a ton of these things.


Last edited by TNO; April 10th, 2010 at 07:13 PM.
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Old April 10th, 2010, 04:10 PM   #2 (permalink)
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I'll be keeping up with this thread -- just ordered one that should be here next week. Seriously, nothing can touch the price of these things!

The only modding I've done previously has been to vintage Champs, so this will be a fun project. I have a Weber Sig8 in my VC, are you saying you *didn't* like the Sig10 in this guy? How crazy would it be to not have to upgrade the speaker immediately on such an inexpensive amp!

Any opinions on the overdrive? From the videos I've heard, it's a bit nasal -- thoughts on getting it to open up a little? Tube swaps, perhaps?
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Old April 10th, 2010, 07:12 PM   #3 (permalink)
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I don't think it sounds nasally at all. I'm running an old "world logo" Amperex 12ax7 and a grey-plate RCA 6V6 in mine. After beating on it hard for a week it's sounding sweeter, probably due to the amp burning in and the speaker loosening up. It is crazy that the speaker, trannies and cab don't need upgrading.

It would be great if some of the more knowledgeable guys here could trace the circuit to see what it's based on.
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Old April 12th, 2010, 06:10 PM   #4 (permalink)
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I just checked the price on these over here in Europe, and I'm SO getting the head version of that, it even has 4/8/16 ohm taps! Yay!
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Old April 12th, 2010, 06:41 PM   #5 (permalink)
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Hey Walter, I really think you'd like it. Through an efficient cab it might be loud enough for small rooms with a restrained drummer.
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Old April 12th, 2010, 07:13 PM   #6 (permalink)
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Won't be able to get it until next month, but based on your recommendation, the price, and the fact it's tagboard, so very "mod-able", I will!
And I have a 4X10 cab I'm not doing much with right now, and an almost embarrassing amount of 10" speakers to choose from...I think I'll have a lot of fun with it.

I'd been thinking of something single-ended to record with anyway, so this is perfect - for recording I'll just go with 1X10", or the two 10"s in my Weber tweed Super clone...woohoo!
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Old April 28th, 2010, 06:47 PM   #7 (permalink)
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Here's the schematic (thanks Will Chen): http://www.mia-amps.com/images/vht_special_6.png
Any comments?
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Old April 29th, 2010, 07:58 AM   #8 (permalink)
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A few things I've noticed compared to a 5F2a-the bypass cap on the first stage of the 12AX7 is 4.7uf in place of the typical 25uf to tighten bass response. The other half of the 12AX7 has a 4.7uf bypass cap to raise gain and tighten bass. Coupling caps are .1s and .047s. There's an extra stage of filtering in the power supply. No negative feedback loop.
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Old April 29th, 2010, 08:00 AM   #9 (permalink)
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Loud

Is it more like a Tweed Champ than a Silver ?
How much headroom ?

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Old April 29th, 2010, 09:20 AM   #10 (permalink)
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It's got a ton more headroom than a tweed Champ. Been a while since I played a SF but form memory I'd say the VHT probably has more gain and is beefier when dimed.
I tried separating the pre and power amp grounds and it didn't make the amp any quieter.
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Old April 29th, 2010, 09:52 AM   #11 (permalink)
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Hey guys! Can't believe I found that schematic. I'm cosidering my first mod (please consider that I'm a total noob to tube amp modding): replace the tone pot with a push pull bright switch to allow brighter tones at lower volumes. Your thoughts?
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Old April 29th, 2010, 09:58 AM   #12 (permalink)
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I tried separating the pre and power amp grounds and it didn't make the amp any quieter
From the gut shots I saw on the VHT site I would hardly consider this a "boutique" wiring job as someone had suggested. You may want to clip the ties and move some wires around and away from each other, like the heater run from the pilot light for starters (BTW: is that an LED in the schematic?). Considering the price point, the country of origin, and the probable age of the assemblers building these things, I would check all solder joints as well. Wouldn't hurt IMO

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Old April 29th, 2010, 11:33 AM   #13 (permalink)
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Bright switch sounds like a good idea. I'd probably stay with a modest value, like 47pf?
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Old April 29th, 2010, 11:41 AM   #14 (permalink)
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Where are you ordering these!!!????!!!

Every search I do with the main online retailers they don't show up.

GAS!!!
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Old April 29th, 2010, 12:16 PM   #15 (permalink)
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Bright switch sounds like a good idea. I'd probably stay with a modest value, like 47pf?
I was actually thinking of soldering a small bright cap permanently in as I like the tone close to dimed with a much larger cap on the push/pull to get a brighter response for humbuckers.

I'd also like to mod the boost to be more of a mid range gain boost, kinda like hitting the front end with a TS9 to get more of a traditional dual channel vibe from the amp. But gotta walk before I run...

Also, what would adding a negative feedback loop sound like? Would wiring it to a pot function as a presence control?

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Where are you ordering these!!!????!!!

Every search I do with the main online retailers they don't show up.

GAS!!!
Don't think any of the big retailers are VHT dealers at the moment. Unfortunately, all of The Music Link's products are more mom and pop type lines (AXL, Recording King, Johnson, etc). They've got a shipment coming in May.
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Old April 29th, 2010, 01:58 PM   #16 (permalink)
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I started this thread hoping some of the more knowledgeable guys would jump in so take my opinions with a grain of salt...I generally like the tone of these little single-enders with negative feedback. Sounds less raw. I figured the Special 6 would have one but I don't see it on the schematic (could I'm missing it). I think the amp would benefit from a subtle hard-wired bright cap.

The Music Link is putting out some awesome product. I've got a Recording King ROS627 that sounds and plays like it cost several thousand dollars. The newest Loar archtops are carved spruce, lacquer finish, great feel and tone for around $600 with case. Good news for the smaller shops that can't afford to carry Fender.
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Old April 29th, 2010, 02:08 PM   #17 (permalink)
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Do any of you guys know what the combo cab is made from? birch ply? mdf?

Thanks.
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Old April 29th, 2010, 03:27 PM   #18 (permalink)
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Baltic birch. Everything is locked down tight to help prevent rattles.
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Old April 29th, 2010, 03:32 PM   #19 (permalink)
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TNO, where did you get yours? Thx.
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Old April 29th, 2010, 03:42 PM   #20 (permalink)
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Baltic birch. Everything is locked down tight to help prevent rattles.
Great news. Thanks TNO!
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