Hall VVR3 fizzy sound

General discussion area for tube amps.

Moderators: pompeiisneaks, Colossal

Post Reply
silverjet89
Posts: 6
Joined: Sun Dec 07, 2014 3:09 am

Hall VVR3 fizzy sound

Post by silverjet89 »

Hi all, I just installed a Hall VVR3 in a Weber Plexi clone. The amp sounds great at full volume. The VVR seems to work correctly until the pot gets to about 3/4 and then it gets very fizzy and thin sounding.

I re-did the installation test in the instructions and all the voltages are tracking properly, with or without tubes. BUT if I check the bias with a bias probe I notice that the bias does not seem to be tracking correctly even though the voltage from the VVR board does.

At full volume the bias probe reads about 40mA and the VVR bias voltage out is about 42v. As I turn the VVR it tracks until about 3/4 volume where it reads 36mA (38v). Then it drops drastically to about 16mA yet the VVR bias voltage out is about 36v. The amp sounds bad immediately. (Hope this makes sense)

Any ideas why the bias mA would drop so fast and not track with the voltage?

Thanks
User avatar
norburybrook
Posts: 3290
Joined: Mon Jan 06, 2014 12:47 am
Location: London
Contact:

Re: Hall VVR3 fizzy sound

Post by norburybrook »

have you got the right VVR3? there are two types, one for cathode biased and one for fixed bias amps.



M
Stevem
Posts: 4551
Joined: Fri Jan 24, 2014 3:01 pm
Location: 1/3rd the way out one of the arms of the Milkyway.

M

Post by Stevem »

Do you have a some type of master volume after the PI ?
As you Crank down the output stage plate and screen voltage you also need to reduce the drive signal to the grids of the output tubes and sometimes even if you are powerscaling the PI it will still deliver too much drive to the output tubes.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: Hall VVR3 fizzy sound

Post by martin manning »

Post plate and bias voltages tubes out and tubes in, as measured at the socket pins for 100%, 50%, and minimum setting on the VVR.
User avatar
Richie
Posts: 1174
Joined: Mon Jan 09, 2006 3:44 am
Location: Ky

Re: Hall VVR3 fizzy sound

Post by Richie »

did you add the cap and resistor on the input,if not at low voltages the amp can sputter or lose signal? And lead dress or where you mount the VVR can make a difference. If you run wires to or from it,so move them around and see which makes it better or worse.
aiyiadam
Posts: 1
Joined: Thu Oct 22, 2015 6:44 pm

Re: Hall VVR3 fizzy sound

Post by aiyiadam »

I had also built a fixed bias Marshall type amp with VVR (started out as a JCM800/Plexi but ended up rather like a Jubilee) . I had also built the VVR myself, but it musty be very similar to Hall's.

My amp also has this abrupt bias increase at about %20 pot position. I even thought that the pots were very unbalanced and measured them and this wasn't the case. There is a high volume increase at this point, but I don't hear a big effect of this on tone. And I stopped caring about that.

The problem might be somewhere else. I had farty bass and a fizzy tone at low voltage settings and realized that I was just hitting the starved PI with a very high signal. I reduced the PI grid resistor to 470K (or even 330K. don't remember now) and the PI signal input cap to 10n. In this way you do lose some lows. Putting a master volume before the PI could be the better approach.
silverjet89
Posts: 6
Joined: Sun Dec 07, 2014 3:09 am

Re: Hall VVR3 fizzy sound

Post by silverjet89 »

OK, finally got back into this thing.

this VVR is for fixed bias amps.

No PI master. This kit is supposed to scale the entire amp.

I didn't put the cap & resistor on the input but I did change 2 resistors on the board to keep the VVR from going to low voltages. This is in the manual. I won't be using the amp at volumes that low.

Voltages tubes in
Plate bias mA on bias probe
Full VVR 460 -42 33
80% 400 -40 28.5
50% 285 -29 6
25% 177 -17.5 4.4
0% 87 -7.6 5

Tubes Out
Plate bias mA on bias probe
Full VVR 498 -43 N/A
80% 420 -41
50% 338 -28
25% 215 -17.6
0% 89 -7.7

The voltage & bias drops seem fairly in line what is odd, if you notice, is the mA showing on the bias probe. It drops drastically below 80% full power. This is exactly where the fizzy thin crap sound starts.
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: Hall VVR3 fizzy sound

Post by martin manning »

The voltages seem to be tracking correctly, so the VVR seems to be working as it should. You are measuring these at the tube socket, right? It's not the bias probe is it, meaning the poor sound happens with or without the bias probe connected?
silverjet89
Posts: 6
Joined: Sun Dec 07, 2014 3:09 am

Re: Hall VVR3 fizzy sound

Post by silverjet89 »

Yeah, measured at the socket. it's the same problem with or without the bias probe.
User avatar
martin manning
Posts: 13080
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: Hall VVR3 fizzy sound

Post by martin manning »

The voltages and bias current just don't make sense. Are these measured on one tube only or both? Have you tried different tubes?
Post Reply