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VHT Pitbull Classic

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2/27/2003 8:49 PM

VHT Pitbull Classic

I have a friend who wants me to modify this sucker to make it a little warmer and a little beefier. Does any know where I can find a schematic for this amp? Is anybody familiar enough with this amp to suggest a couple of modifications? I haven't looked inside it yet, but I commonly replace the tone caps in the tone stack on any current production amp (Marshall/Fender/Peavey) to give it a little clearer beefier sound. I know these are a little bit more of a boutique amp. The preamp tubes will be the first thing I try but I want to do a little more than that. Does anybody have any ideas? I appreciate your help.

Alan

2/27/2003 9:06 PM
Ray Ivers	Alan, I was briefly VHT's east coast service shop, and Stevie doesn't make his prints available to anyone outside of the factory. I've had several requests to modify these amps, but no one wanted to spring for my time to remove and draw out the (double-sided) board. Pull the chassis and check it out - I think you'll probably want a print before starting any mods. Ray Ivers

2/27/2003 10:40 PM

I worked on one of these amps a while back. The preamp distortion didn't sound good to me until I loaded the amp with Chinese 12AX7's. I tried several other types and didn't care for the character of the sound. I don't recall if the EH tubes were available yet but start with the tubes and see how they affect the tone. The distortion circuit isn't too different from a Marshall JMP/JCM800 MV circuit set up for higher gain. A few little tricks here and there but nothing too unusual. Draw out the circuit.

How else are you ever going to be able to work on these amps? I rarely if ever charge a customer for that. I also make a general habbit of not distributing them. They are for servicing within my shop only.

RE

2/28/2003 1:54 PM

I had one of these a while ago and poked around a bit inside. Although the amp is very cleanly build and designed, I think you you should reconsider any mods.

My first observation is the amp runs HOT! The only tubes that could even bias close to where I like them are JJ and Svetlana EL34's. My first suggestion is to get some real hard JJ's and try to get the bias down to 75% max plate dissapation. (This was not possible on some tubes. 1 quad of Mesa Sovteks for example idled with a B+ of 492V and 45mA at max bias!). Mike F. at VHT is lots of help if you can reach him, and he told me the amp was designed for Seimens tubes which are hard to find now.

Side note: At these plate voltages, when you switch to Class A mode on this amp with cheapo tubes, they'll last about 5 minutes.

Anyway, I found a hard quad of JJ's that actually tamed the amp down 472V B+ @ ~39mA idle current which sounded great. Try tuning your power section before attempting mods. The VHT classic is a great amp. Hard to find tubes for it though.

BK

3/3/2003 5:33 PM

Often times, my first intention is to try different preamp tubes because the user does not often know how important the choice of preamp tubes is. I think better preamp tubes might make enough difference for him to not modify the amp itself, thus saving him money and me time. Also, he wanted more bottom end and he may be able to obtain that by changing what speaker he is playing through.

I do not like to modify such boutique amps because they were designed that way for a very good reason. However, I do want to know about everything I am working with and you all have been quite a lot of help. I will try to find those JJ's which will help him greatly. Thanks everybody!

Alan

3/4/2003 4:36 AM
Drew Champion	What Brand is the Poweramp Like? I heard it was kind of like Hiwatt, but does that include the PI?

3/5/2003 7:21 PM

quote:
"I heard it was kind of like Hiwatt"

The one I worked on was the VHT Pitbull 50/12 Combo amp. It had (4) EL84 power tubes, SS rectifier, the PI was a cathodyne type, and the distortion preamp was very similar to a classic Marshall MV 3 stage design, 3 stages setup as a higher gain but not maxed out preamp. There is a switch in the dist. tone stack to switch between a Super Lead and Super Bass tone stack values. The clean tone stack may resemble a Hiwatt in some ways but I haven't really analyzed it that much yet.

RE

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