Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ed_Chambley on October 03, 2011, 11:45:41 am

Title: JTM-45 and VVR
Post by: Ed_Chambley on October 03, 2011, 11:45:41 am

Anyone ever added a VVR to a JTM-45 build.  I am doing one now and intend in doing it without adding a Master Volume.  I was wondering if it will act similar to a master volume.  It will only be connected between the Stand By and plates.  I will max out the bias on the VVR and use the balance pot in the amp.

I do have a question.  When adding the bias wires to the VVR, where should the break in bias supply be?  I have attached a copy of the layout I am using.

Title: Re: JTM-45 and VVR
Post by: zendragon63 on October 04, 2011, 07:35:23 pm

Hey Ed, a VVR on a JTM45 is probably a good idea--they are pretty stinking loud. I think that they traditionally are setup (as you have yours) fixed bias so at full voltage on the plates, your bias might be setting at say -45V.

It will only be connected between the Stand By and plates.  I will max out the bias on the VVR and use the balance pot in the amp.

If I am reading your intentions correctly, you plan setting the bias to the max and just pulling the plates down.  If you don't track the bias down, I would think that you will be still be at -45V with the lowered plate voltage; any reason you wouldn't want to bring the screens down as well? And unless you tracked down on the bias, you would continue to travel further into cutoff than would sound so good.

You mention breaking the bias line but...only if you have a regulation device scaled to do it.

Just a thought though: you could either build/utilize a version of the VVR tracks the bias down--either a UR12 VVR2 type or powerscaling equivalent or (the heresay of!) cathode biasing a JTM45 and go with a straight VVR.

I apologize that this is wordy--hopefully I understood you intentions. There are other folks fluent in VVR speak that might chime in. Good luck. Regards

dennis


 

Title: Re: JTM-45 and VVR
Post by: Ed_Chambley on October 05, 2011, 05:10:28 pm

Dennis,
Thanks for your reply.   I have a VVR from Hall Amplification which has bias tracking.  The instructions say you need a master volume for the install, but I cannot understand why.  This is the reason for the question of where to break into the bias supply.  I can install a master using the LAR MAR style.  I have a dual 250k log and some shielded wire, but I cannot understand why it is necessary, but according the Hall Amp instructions, it is needed.

Any thoughts?

Title: Re: JTM-45 and VVR
Post by: Willabe on October 05, 2011, 05:43:54 pm

As you dial down the over all volume on the amp with the VVR, it takes less and less signal to drive the OP tubes. Using a master volume lets the user adjust this. Some guys don't put the master in, but the tone changes after a certain point of dialing down the volume. I guess part of it depends on how much you want to drop the over all volume.

At least that's how I understand it.


                      Brad      :think1:   

Title: Re: JTM-45 and VVR
Post by: LooseChange on October 06, 2011, 05:09:56 am

Willabe's got it right. Here's a bit more.
The preamp will overload the power tubes at low wattage settings and sound crappy (IMO). The master helps to avoid this issue.

Title: Re: JTM-45 and VVR
Post by: Ed_Chambley on October 06, 2011, 11:48:52 am

Thanks for the replies, I got it.  You will have full voltage on the preamp section hitting reduced output section.  If the output section is reduced below around 150 volts the preamp will overdrive the output and there is no way to control it.  Master volume here we come.  I am assuming the LAR MAR style is best to use for the JTM 45.