Help ! VHT Special 6 Ultra. First mod o.k., yet glitch !

[tonequester]

Greetings to ALL !

I just performed my first simple mode on my VHT Special 6 Ultra. I added a Hammond 157M choke per Rob Robinette's website. It involved merely removing R43 and soldering the leads of the choke there. The mod itself got rid of almost all of my high gain noise(Ultra channel). Then, while tweaking my tone as I played, I noticed that I no longer had a functional Wattage control. The "effect" is "stuck" at the full wattage position. 3 wires lead away from the Wattage pot(red, brown, black). I have checked the continuity of all three, and I get just a few Ohms on each wire. The black wire does lead to one of my choke lead connections, but as i said, it checks out. Visual inspection had turned up no loose connections, bad looking solder, or burned spots.
I am not the best troubleshooter, especially when I can't hardly read the VHT schematic, cause it's so darned small print wise. So, I'm asking anyone who might be able to check this problem out, to let me know what I might check. I'm afraid that I don't know how to leave a quick link to the schematic. However, one can search VHT Amplification - Special 6 Ultra. Then click on manual, and go to page 15. I truly appreciate any help anyone may offer here. I have emailed VHT, but it may be days or weeks before I get an answer from them, IF I do.

Sincerely, tonequester

[Colossal]

I posted a zoomable schematic in your other thread, here: https://tubeamparchive.com/download/file.php?id=34950

Good move on adding the choke. Bummer about your wattage control. Have you measured the resistance on the wattage control pot (VR4 1MA) to ensure it hasn't failed? Place your DMM probes across 2 and 1 and see if you can sweep the full value of the pot.

Be sure the capacitors are drained before probing around with fingers in the amp.

[tonequester]

Greetings Colossal.

Thanks for your reply and the link to the schematic. I have written it down for future use. I just checked the pot as you suggested, and it sweeps just like it should. If you have any other ideas, please let me know. This is so weird, that I really don't know where to begin. I figured that since the Watts control went bad just when I had added the choke, I must have accidently disconnected something, or I made a (rare) bad solder joint. This is NOT the case, and as I mentioned, I am not the best troubleshooter, especially on such a radical(for me) amp. I'll try anything someone is good enough to suggest, while I wait on VHT to email me(I hope).

Thanks again my friend. At least I can give a quick link to the schematic, thanks to you ! tonequester

[jazbo8]

If the VVR control appears to be "stuck" at full volume, then the MOSFET could be bad. To verify, try take out the choke and put the 330R resistor back to see if VVR works, if it does, then you know you have not blown the MOSFET.

[frankdrebin]

Greetings to ALL !

I just performed my first simple mode on my VHT Special 6 Ultra. I added a Hammond 157M choke per Rob Robinette's website. It involved merely removing R43 and soldering the leads of the choke there. The mod itself got rid of almost all of my high gain noise(Ultra channel). Then, while tweaking my tone as I played, I noticed that I no longer had a functional Wattage control. The "effect" is "stuck" at the full wattage position. 3 wires lead away from the Wattage pot(red, brown, black). I have checked the continuity of all three, and I get just a few Ohms on each wire. The black wire does lead to one of my choke lead connections, but as i said, it checks out. Visual inspection had turned up no loose connections, bad looking solder, or burned spots.
I am not the best troubleshooter, especially when I can't hardly read the VHT schematic, cause it's so darned small print wise. So, I'm asking anyone who might be able to check this problem out, to let me know what I might check. I'm afraid that I don't know how to leave a quick link to the schematic. However, one can search VHT Amplification - Special 6 Ultra. Then click on manual, and go to page 15. I truly appreciate any help anyone may offer here. I have emailed VHT, but it may be days or weeks before I get an answer from them, IF I do.

Sincerely, tonequester

i would consider R48 as a "choke" placement,in between plate and screen.
Does that make sense?

[jazbo8]

i would consider R48 as a "choke" placement,in between plate and screen.
Does that make sense?

Not really, since the largest current fluctuation is at the plate not the screen grid. R43 is the recommended spot for a choke replacement.

[tonequester]

Greetings guys.

I just wanted you to know that the mosfet in my amps Wattage circuit is DEFINITELY bad. I appreciate ALL of your replies. I found one at MCM electronics for $1.31. Shipping is $6.00. Ain't that "ducky"? At least I should be back in business in about a week. I really appreciate your time and effort in posting. This is the first time in 33 years of soldering that I ever cooked a component, and I built many an Avionics board as an electronic assembler back in the "good old days" ! Even though the mosfet is heat sinked, I guess I better use a "clip on the lead" heat sink as well, when I install the new one.

Best Regards, and Best Wishes to you all ! tonequester.

[Leo_Gnardo]

Even though the mosfet is heat sinked, I guess I better use a "clip on the lead" heat sink as well, when I install the new one.

Could have been static electricity maybe. Don't think I've ever popped a FET that way but it doesn't take much.

[tonequester]

Greetings Leo_Gnardo.

Thanks for your reply. You know, it IS possible that static fried the mosfet. I had forgot the fact that when I worked as an Avionics assembler, we always had to be grounded with a wrist "strap". This is one good reason why I prefer to work with tubes ALONE. My soldering irons aren't the best. A good soldering station is on my "wish list", and maybe I should move it up in rank. I think that some of the good ones have static protection built in, plus one can control the heat better as well. Thanks for jogging my memory Leo.

Best of Regards to you.................tonequester.