VVR Install: Marshall 50 watt Plexi 1987x -- NOT SCALING

[cluster]

Hello All,  My first post here.  I just installed a fixed bias VVR into a 50 watt plexi (1987x circuit).  The amp came to me as a pre built clone with a PPIMV already installed.  I installed the VVR as per the VVR3 instructions.  All the voltages track as they should (I think).  However, at full VVR the lowest plate voltage I got was about 55 DCV (tracking from about 495 to 55 VDC).  Not sure if it should track to 0 or not (as per the SV1 kit sold by Kevin O' Conner).  Also, my guitar volume stayed the same throughout the range of VVR (very loud).  My PPIMV worked fine though. I mapped my VVR installation diagram to a schematic (see attached).  I've included voltages as well as some changes from the original layout the previous builder made (position of the standby switch, etc..).  If anyone here can see what I did wrong please let me know.  If you need more data, let me know.  Thanks.

Joseph

[kagliostro]

look at the image, it shows the relationship between Perceived Loudness and the Effective Power

Despite that, if you installed the VVR correctly, you must hear some difference dropping to 55v of B+

K

[PRR]

> the lowest plate voltage I got was about 55 DCV ...Not sure if it should track to 0 or not

The last few volts are useless. Even with bias-track, a 400+V tube starved down to 50V will be nearly off. You'd complain that the last tenth of a turn "does nothing". So Kevin leaves-out that very low volt zone.

> my guitar volume stayed the same throughout the range of VVR (very loud).

If the Plates really go down to 54V, it should be a half-Watt amplifier. Which can be pretty loud. But if it's making the 50+Watts (annoy the neighbors volume) at 500V, you really should hear a difference at half-Watt.

[cluster]

> the lowest plate voltage I got was about 55 DCV ...Not sure if it should track to 0 or not

The last few volts are useless. Even with bias-track, a 400+V tube starved down to 50V will be nearly off. You'd complain that the last tenth of a turn "does nothing". So Kevin leaves-out that very low volt zone.

> my guitar volume stayed the same throughout the range of VVR (very loud).

If the Plates really go down to 54V, it should be a half-Watt amplifier. Which can be pretty loud. But if it's making the 50+Watts (annoy the neighbors volume) at 500V, you really should hear a difference at half-Watt.

I agree.  I haven't been able to crank everything to 11 to truly test the VVR but you would figure even with the volume at 4-5 on a 50 watt amp, I would hear some volume drop with VVR.   

One funny thing though is I tried to jumper the raw negative bias output off the board to the grid leaks effectively shorting out the bias circuit on the VVR and was able to get zero volume on the guitar with the VVR on a 1/3 (about 300 dcv on the plates).  I'm assuming what happened here is that by shorting the bias circuit at the VVR, I was able to apply more negative bias to the tubes there by cutting them off sooner.  When the volume got low from applying a high VVR, the guitar tone got buzzy in the same way as if you turned off the power but left the standby on.

Joseph

[HotBluePlates]

One funny thing though is I tried to jumper the raw negative bias output off the board to the grid leaks effectively shorting out the bias circuit on the VVR and was able to get zero volume on the guitar with the VVR on a 1/3 (about 300 dcv on the plates).  I'm assuming what happened here is that by shorting the bias circuit at the VVR...

Don't do that. The bias is supposed to track with the reduced B+ voltage.

And yes, what happened was that with the bias voltage staying constant, and with the B+ dropping, you reach a point where the existing bias completely cuts off the output tubes. The buzzy sound you mention is the sound of extreme low plate current operation.

You'd be surprised how loud a Champ is through a 4x12 cabinet. I think the issue is you haven't been able to compare full-power loudness to the volume you get with the VVR reducing B+.

[cluster]

One funny thing though is I tried to jumper the raw negative bias output off the board to the grid leaks effectively shorting out the bias circuit on the VVR and was able to get zero volume on the guitar with the VVR on a 1/3 (about 300 dcv on the plates).  I'm assuming what happened here is that by shorting the bias circuit at the VVR...

Don't do that. The bias is supposed to track with the reduced B+ voltage.

And yes, what happened was that with the bias voltage staying constant, and with the B+ dropping, you reach a point where the existing bias completely cuts off the output tubes. The buzzy sound you mention is the sound of extreme low plate current operation.

You'd be surprised how loud a Champ is through a 4x12 cabinet. I think the issue is you haven't been able to compare full-power loudness to the volume you get with the VVR reducing B+.

You're right.  Yesterday I had the amp full on 10 all the way and then increased VVR.  The volume dropped some but what really dropped was the clean headroom.  That's when I adjusted the PPIMV down a bit to get the right headroom level.  The SV1 by KOC that I had in a super lead threw me off, since I could get that one down to bedroom levels without using much PPIMV.

I spoke to Dana on the phone and he told me that there are 2 100k resistors on the VVR that I could jumper to get the plates further down to 0 (same as the SV1).  Getting the plates on the VVR below 56DCV would probably get me into that bedroom volume range.  I'll probably try that next and report back.  Thanks for helping me work this out.

Joseph

[aiyiadam]

Hello guys,

In the last couple of months I was busy with installing a VVR variant to my 1987x inspired amp and had bumped into this topic.

I had also done the same thing mentioned here. This amp was too loud because I didn't scale the PI with the power tubes.  You need to get the PI voltages down, in order to get it distorted at an acceptable volume.

The second thing is the high impedance bias source. When I turned the VVR knob, the bias voltage didn't always change as fast as my B+.  Since my tranny doesn't have a 50V tap, I have used a 5W 24V-220V transformer and connected it to my heater tap (as suggested by KOC somewhere else)

And the third thing. The PI design of 1987x doesn't seem to work well with poor negative feedback and under low voltages. I had a horrible distortion (probably blocking distortion).
I had previously made two VVR installations on my 18W TMB based EL84 and 6V6 amps. And the PI of the 18W TMB circuit worked with VVR perfectly.

Anyway, the following PI changes similar to the 18W circuit have solved the problem: Cathode resistor: 820 Ohm,  Tail Resistor:  50K,  Grid Ref. Resistors: 330K  Could someone explain, why this have worked?

I didn't have a chance to try the amp on high volume. The changes in the PI must have affected the sound of the amp on normal PI and EL34 voltages . There probably is a trade-off.

Thanks

Evren

[Willabe]

This amp was too loud because I didn't scale the PI with the power tubes.  You need to get the PI voltages down, in order to get it distorted at an acceptable volume.

You can put in a post phase inverter master volume (PPIMV) instead of VVRing the PI or do both.

The PPIMV will let you limit the drive to the output tubes and will keep the distortion down.


                         Brad   

[aiyiadam]

You can put in a post phase inverter master volume (PPIMV) instead of VVRing the PI or do both.

The PPIMV will let you limit the drive to the output tubes and will keep the distortion down.


                         Brad   

you are right. Actually I have both VVR and a Lar/Mar PPIMV.

The PPIMV alone (with B+ around 410V) does get me some clipping, but it doesn't bring my amp close to f.e. the Van Halen territory. And when I have the PPIMV under a certain level the tone isn't very usable.  Maybe my dual 250K alpha pot isn't balanced well enough (There is a 15K difference between the two single pots). I have ordered a couple of new pots. After measuring them I might change my PPIMV pot.

The PPIMV works very well with the VVR. Even when I scale the PI and the power tubes to ~36V, the amp is still too loud for bedroom levels. If I really need to be quiet, I keep my master volume between 6-7.

Evren

[jojokeo]

And when I have the PPIMV under a certain level the tone isn't very usable.  Maybe my dual 250K alpha pot isn't balanced well enough (There is a 15K difference between the two single pots). I have ordered a couple of new pots. After measuring them I might change my PPIMV pot.

Not sure that's enough of a variance to affect the tone negatively like you speak of? When turning the ppimv down you're putting a lot of resistance in line (in the way) of the signal path therefore losing high end and tone. Try putting two 100pf or so treble bleeders on the ganged pot like what's done with simple MV controls and I think you'll find this helps?

[aiyiadam]

Not sure that's enough of a variance to affect the tone negatively like you speak of? When turning the ppimv down you're putting a lot of resistance in line (in the way) of the signal path therefore losing high end and tone. Try putting two 100pf or so treble bleeders on the ganged pot like what's done with simple MV controls and I think you'll find this helps?

I think, I should check the resistance of the pots in different positions from 1 to 10  and then enter/compare the values on an excel table. I have read somewhere on the metroamp forum that the variance of the pots at different positions might matter more than the total difference between the two pots.

Putting capacitors is also something worth trying. I'll try to find more information about that. Thank you for the suggestion.

[jojokeo]

By the time you read, research, contemplate, & worry you easily could've just installed the bright caps and be done with it. Also the rotation deal is the same, people go out of their way to create imbalance on purpose with good results? You won't hurt anything except the the delay in finding out what you want to know if it matters or not and if you like it or not? Here's another thing to consider - you're talking about only a 6% difference and that's within the tolerance of the parts you're using on your circuitboard. Now ask yourself if it makes sense to overthink it or worry further? (Just trying to help & provide perspective).

[aiyiadam]

By the time you read, research, contemplate, & worry you easily could've just installed the bright caps and be done with it. Also the rotation deal is the same, people go out of their way to create imbalance on purpose with good results? You won't hurt anything except the the delay in finding out what you want to know if it matters or not and if you like it or not? Here's another thing to consider - you're talking about only a 6% difference and that's within the tolerance of the parts you're using on your circuitboard. Now ask yourself if it makes sense to overthink it or worry further? (Just trying to help & provide perspective).

I am not worrying about anything. I already have a good sounding plexi and I am messing with it just to make it a little better.  And yes, for me it does make sense to overthink and it is even fun. :)

Anyway, thank you for providing perspective.

[jojokeo]

kumbaya my friend...