Lar/Mar PPIMV

[loicdulux]

I will have two bias probe to hands, they look pretty simple kit to mount but I'll test the connectivity before using it


So you would also do the measure of the voltage (450v) with the pin3 and ground? (so not using the bias probe, which is taking the pin 8 and 1)

[martin manning]

I believe the bias probe is inserting a 1 ohm resistor between the tube pin 8 and the pin 8 in the tube socket mounted in the amp chassis. This allows you to measure the voltage drop across that resistor, which will be numerically equal to the (cathode) current flowing through it (per Herr Ohm, 1mV is needed to get 1mA of current through 1 Ohm). You also need the plate-cathode voltage to calculate plate dissipation as described above. In this case pin 3 voltage to ground is fine. The bias probe may also give you access to pin 3 so that you can get the measurement from outside the chassis.

[loicdulux]

THANK YOU THANK YOU THANK YOU!

My plate voltage is 475 v, I could perfectly balance and set the bias to 44.2 ma to both tubes, and the master volume works perfectly, this is just AMAZING, a new amp is born! Master volume at 4, volume at 7, I have a great sound I never had in ten years I own the amp! Then I mixed both channel, volume of bass channel 10, this is so nice! THANK YOU!

[img:180:135]http://s20.postimg.org/hyf1yex21/image.jpg[/img]

[img:180:135]http://s20.postimg.org/55qxyhlg9/image.jpg[/img]

Edit: with the tubes in, the voltage between pin3 and ground is 433v. I set the tubes to 38mA.

[img:135:180]http://s20.postimg.org/mw6gcafo9/image.jpg[/img]

[img:200:134]http://s20.postimg.org/btrzu9gt5/IMG_8087.jpg[/img]

[img:200:134]http://s20.postimg.org/qogl8fqe1/IMG_8080.jpg[/img]

[martin manning]

You are most welcome, Loic. Good to hear it is such a success, and nice work there too!

[loicdulux]

Hi Martin,

Hope you're doing well!

I played a lot in my new sounding amp, very nice saturation I have never heard before!

But I have a question. When I roll down the master volume knob to the minimum, I still have plenty of sound, like if it was on 1/10 on simple volume. Is it normal that the sound doesn't cut off entirely? Would it be possible to decrease the volume to zero? I would have then possibility to play even at lower volume with maximum volume on front.

What do you think?

Best regards

[martin manning]

I think there is enough signal to overwhelm the ability of the bias supply to maintain the DC bias. DId you replace the bias supply filter caps already? I would try adding another filter cap to the supply, and move both second filters to the output of the balance pot as shown below. It would probably be a good idea to add 3-lug terminal strips to support the bias caps, and I see two empy holes in the chassis that would work for mounting those. More Shopping!

Edit: Revised drawing and text for adding terminal strips.

[loicdulux]

You are a master Martin, but you already know.

Great, more shopping to come. Would you recommend to change any other cap in the meantime?

I was also advised (not sure it was you) to replace the diods with better ones so that I can remove half of it. Not sure in what that would sound better.

I see so many brands for caps, is it affecting the sound at that place?

Thanks again for your time!! I will also order a book or two on tube amp, I think I got the mod hobby under my skin now!

[martin manning]

Well, let's see if moving the filter connections works...

If you haven't replaced the main filters (in the "dog house" under the chassis) in 10-15 years, then they are probably due. There isn't a significant difference in sound between brands, but get good quality parts with at least 85C rating, 105C is even better. If the filters are old, you will notice an improvement in the sound quality with new caps. Replacing the diode is a good idea for reliability; no sound impact.

Edit: I'm quite sure now that this modified circuit will work correctly (I should have seen the problem coming), and you should be able to get the volume down to almost nothing. I did some more doodling to show a more solid mounting arrangement for the pair of second bias supply filters and edited the drawing above. You might be able to use the lead from the existing cap (it is soldered directly to the chassis, I believe) for the ground connection. The center lug of the lower terminal strip will also work for a ground since it is bolted to the chassis, but if you have the soldered ground there you might as well use it.

[angelodp]

Martin, I know this a very old thread. I am planning a PPIMV for my AB165. Its pretty much stock, with upgraded cathode caps. Would you suggest staying with the 100k dual gang pot/2.2m. I have 250k on my Rocket build.

Upon closer inspection, I believe this is an AA864 amp. Just tried the Type 3 mod with a 250K pot and its pretty good. I'll source a 1M soon to try the suggested value.



Ange