VVR Placement Advice (Traynor YBA-1) Update

[Blindog]

I have my fixed bias VVR 2 board from Dana installed and fired it up for some quick testing.

It's in a '68 Traynor YBA-1, and I’m regulating the whole amp. I have not put in the extra caps and resistors in the preamp yet. It works, but I’m getting some bad hum when I turn down the voltage until I get to about 1/3 on the pot. I suspect it’s where I installed the board. As you can see from the pic, it’s in the hole where the standby switch used to be, but right on top of the power transformer secondary, and near the heater wires. The B+ in and out wires run right through there too.

Can anyone recommend a better place to install the board, and how long can the leads be from the pot to the board? I was thinking about the space in between the output tube and can cap, with the mosfet soldered to the board and the pot where the standby switch is.

Thanks!

Mark

[Deric]

You may be able to leave the board where it's at if you just work with your lead dress.

I've never done a VVR with bias tracking but in my (cathode biased) experience:

I had the exact same type noise - fine all the way up or all the way down but hummed in between. I fixed the issue by twisting the B+ wires to the VVR and keeping all VVR wires as far away from AC ( heater wires ) as possible.

Try twisting your B+ wires and running them up and over the heater wires.

[Richie]

I try to keep the mosfet on the board ,or close as possible. And i have run wires off the board to the pot. I've installed the baords in some fender bassman amps, and didn't have any problems,worked great.

In the bassman, i have the mosfet mounted on the side of the chassis like you have,and the board sitting behind the transformer. Then removed the ground switch, and put the pot there. the wires ran from the pot to the board, about 3 or 4". The mosfet had maybe a 1" piece added to connect it to the board..

Be sure to check you voltage and bias,and make sure the connections are correct.. Should work fine, with no hums or noises..

[Blindog]

Deric,

I'll try and twist those B+ wires and see if that helps. I'd like to keep the board where it is so the pot can be up front. I have a feeling I'm going to have to move it though.

Richie,

If I move the board to the side of the chassis close to the caps, I'll probably have double the lenght of wires to the pot that you had. I like that idea because the B+ wires will be very short. The pot will have to go to where the standby switch is now (nearest to the fuse) Do you think that will pose a problem?

Thanks for the responses!

Mark

[Richie]

No it shouldn't be a problem.. in the bassman, the pot i placed is in the ground switch hole. That is between the AC socket,and power switches. Some runs, to and from the bias are routed on the board side of the PT..And i didn't have any problems.

I also installed one in a plexi type amp,and the bias wire had to be routed in a certain way. It would cause a little hum if it was run in the wrong place. Looks like you have a fairly long run to the bias on that amp. try moving it or lifting it,see if it changes. You shouldn't have any hums etc..when its finished. So far all i have used are quiet,and no problems.

[Blindog]

Richie,

I did move the bias wires around when I first powered it up and it didn't seem to do anything. I think the noise is coming from the B+ and the mosfet wires.

What kind of wire did you use for the pot? I was thinking of some multi-conductor wire like CAT 5 cable or something like that. Do you think that would be OK?

[Richie]

I just used 20ga uni-strand. I don't know if the cat wire would have a high voltage rating.

[Stanz]

When KOC talks about these types of set ups, he says that any ground/noise issues that already exist before installing the "reducer", will only be made worse after it is installed. Not a bad idea to twist all AC wires and keep their path separate from signal or even other power supplies (heaters). Take a look at where your grounds are being tied in. Hard for me to tell, but is the ground from the VVR pot tied in at the same place as your pot grounds?

[Blindog]

Richie, I've moved the board to the side of the chassis with the mosfet soldered to the board. I mounted the pot to the rear where the ground switch used to be. Now the B+ wires are about 2 inches long.

Now when I take it off standby, at full voltage (no reduction) the mosfet gets too hot to touch fairly quickly. I turned it off when I found that out. From what I read that should not happen. Any suggestions?

Stanz, thanks for the input. I moved the board away from all that noise potential, but now it seems I have more issues...

[Blindog]

double

[Blindog]

Double

[Blindog]

OK, I rewired the pot and have the overheating problem fixed. I moved the mosfet to the board and relocated the board to the side of the chassis. Hum is all gone.

Now I have some operation questions. Is it normal for a 50 watter to get too hot to touch at about half way on the pot without additional heatsinking?

The tone gets fizzy sounding at about half and below. From what I've read it should not behave that way, correct? Is this maybe related to overheating?

Is it necessary to install the caps and resistors in the preamp?

Thanks to all for your help. I'm excited to get this baby dialed in!

Mark

[Blindog]

I've spent some more "quality time" (no kids or wife around ) playing the amp with different VVR settings. I also went to the Shack and got a TO-220 heat sink.

My initial reactions in the post above were wrong after playing with the different settings. I also found if I tweak the tone controls it really helps at lower VVR settings(below half way).

I think my only issue now is that with the pot set to at about half or below the mosfet gets into volcano heat territory. The whole section of chassis under the mosfet is affected. Any suggestions?

Thanks!

[UR12]

I've spent some more "quality time" (no kids or wife around ) playing the amp with different VVR settings. I also went to the Shack and got a TO-220 heat sink.

My initial reactions in the post above were wrong after playing with the different settings. I also found if I tweak the tone controls it really helps at lower VVR settings(below half way).

I think my only issue now is that with the pot set to at about half or below the mosfet gets into volcano heat territory. The whole section of chassis under the mosfet is affected. Any suggestions?

Thanks!

Your mosfet is going to get the hottest at between 1/2 and 3/4 power. A 50 watt amp is the max and it will definately require a much bigger heatsink that just a TO220 heatsink you get at Radio Shack. Once you reduce the voltage to say 1/4 your tubes won't be drawing as much current and the heat will go down from 1/2 voltage to it's lowest setting. I have a bigger heat sink that I can send you if you want to try it.

[Stanz]

You are going to get "fizzy" tone as you draw the power down but the signal that has not been reduced will now appear bigger to the reduced stages. If it is all of them, the guitar input needs to be reduced. Dana has mentioned that he reduces the signal with the guitar.

I have done this now for three amps, one cathode biased and two fixed bias. All three only have the power tubes reduced. The methods I have seen to compensate for this are to install some sort of master volume for systems that have only reduced the power tubes. I personally like the bootstrapped master, or you can install a PPIMV.