Wednesday, November 24, 2010

VHT Special 6 Harp Amp Project – Step One

Today the Special 6 project amp spent some time at Mission Amps in Denver for testing and first-step harp mods. Here is what I can tell you:

-The amp produces its rated power of 6 watts with about 5 percent distortion at two o'clock on the volume control (it is not numbered). Feedback is well controlled. The amp is very well made, with a hand-crafted circuit similar in design to a blackface Fender Champ from the mid-60’s era.

-Changing to a 6L6 power tube in the amp does nothing to increase the power or improve the tone. It sounds and performs better with a 6V6. Also, the magnet on the stock ceramic speaker makes many 6L6 tubes a very tight fit.

Right out of the box this is a very good harp amp. It is among the most harp-friendly guitar amps I’ve seen. A harp player could buy this amp and do nothing to it, and get great tone from it.

But the amp does have a few compromises that can be improved upon to make it more suitable for outstanding blues harp tone:

-The slope of the tone stack is too trebly. It needs to be shifted downward.

-There is a slight reedy quality to the tone that detracts from its warmth.

So, we decided to make a few simple modifications to the tone stack, making it more like the circuit in a tweed Bassman amp. Each of these mods was tested by playing the amp to ensure the tone was improved in the way we expected. We sometimes had to try several different component values to get the tone just right.

We changed the tone stack slope resistor, the bright cap, and the cathode bypass cap, and changed the plate voltage on the preamp tube.

The result is a smoother, warmer tone without the reedy quality. The voice of the amp has been shifted downward, and its tonal girth has been enhanced. It retains the nice crunch and grit. It is a very fine sounding blues harp amp.

Remember, at this point the amp still has its original tubes and speaker, and I am in no hurry to change them. I suspect the speaker will break in well, and the stock tubes sound great. I’ll try other tubes later, after I’ve played it more and made certain the circuit mods are as they should be.

UPDATE 11/26/11: The amp is substantially louder now. Same room, amp settings, mic, and harp. I suspect the speaker got broken in during all the hard playing at Mission Amps while we were proto-typing the mods. The tweaks to the tone stack also focus more of the amps energy on the harp spectrum.

Jump to review of this amp with mods


. said...

I have done some VHT mods including gain, master volume pots, lower plate voltages and totally different tone controll (fender tone stack was gain eater). I can send you my schematic just to compare. At the beginning VHT head with 112 cab had practically no gain and bass response. I was wery disapointed playing harp on stock amp. Now it has a much better tone controll.
All the best from Poland, Adziad

sir nugallot said...

Just picked up a barely used one of these and am quite impressed. I don't see the spec.s of the mod's you or Bruce did - are they posted somewhere or could you spell them out? The Nick Clark youtube VHT spec. 6 clip credits you with the mods to that amp. I know the kids got tone up the wazoo regardless of equipment - but the amp in that clip sounds really good.

I have been looking at this page by JIMDRAMPS
and it is quite detailed. - I'd love to know what you / Bruce did to compare notes before sniffing the solder.

thanks, - noam (bonedog569)