The best way to set the PI trimmer WITHOUT a scope?

[Joost]

I have been reading everything I can about this on the forum and can only find threads that involve scopes and very difficult setup procedures. I have a signal generator which I can insert in the return from my built-in dumbleator and measure the DC balance at the phase inverter, but I keep reading it's about the AC balance point and that doesn't coincide with the DC balance point.

So, how do I set it up with only my trusty fluke DMM and a signal generator?

[Structo]

Seriously you can do it by ear.

Set it to clean sound and turn it up pretty loud.

lightly strum the strings and adjust the trimmer just a bit.
You want it where the notes seem to bloom more and sustain.

Be very careful doing this. In fact a much better idea to have someone else strum the guitar while you adjust.

Because the guitar is grounded and if you slip inside the amp, could be lethal.

You may have to try several tubes to find one that works well.

[erwin_ve]

Hi Joost, already forgot the procedure since friday?

There are several ways to do this, I'll decribe 2, in both ways you have to use a load instead of a speaker to save your ears.;

1. Use the AMPEG SVT procedure. http://www.el34world.com/charts/Schemat ... T_RevA.pdf the procedure is explained there.
It uses the bias cathode measure points of the amp. In a dclone amp you have to tie the bias points of V4 en v5 together and also tie V6 and 7 together(In your case V5+V6 and V7+V8)., those 2 point have become your ac balance points, but you have to set your meter at DC (in mV) because both points are positive to ground.

2. Insert a 400Hz to 1kHz signal in the PI. Set your MM at high voltage DC range((around 300 v). Both triodes are measured in respect to ground so DC!!!)
Measure the Plate voltage to ground for both halfs.(2 MM would be ideal). Both should be set equal to each other.

The last part is to actually play with the amp+guitar and set the trimmer (should be very close to the measured setting)where the most harmonics appear. You have done this last friday so I think it's no problem for you.

Anyway: measuring brings you close to the sweet spot, dialing in while playing is where you can get the most out of it.
Other members have used other methods succesfully; I use these 2.
A balanced tube is a must.

happy easter!

Erwin

[Joost]

Hi Joost, already forgot the procedure since friday?

There are several ways to do this, I'll decribe 2, in both ways you have to use a load instead of a speaker to save your ears.;

1. Use the AMPEG SVT procedure. http://www.el34world.com/charts/Schemat ... T_RevA.pdf the procedure is explained there.
It uses the bias cathode measure points of the amp. In a dclone amp you have to tie the bias points of V4 en v5 together and also tie V6 and 7 together(In your case V5+V6 and V7+V8)., those 2 point have become your ac balance points, but you have to set your meter at DC (in mV) because both points are positive to ground.

2. Insert a 400Hz to 1kHz signal in the PI. Set your MM at high voltage DC range((around 300 v). Both triodes are measured in respect to ground so DC!!!)
Measure the Plate voltage to ground for both halfs.(2 MM would be ideal). Both should be set equal to each other.

The last part is to actually play with the amp+guitar and set the trimmer (should be very close to the measured setting)where the most harmonics appear. You have done this last friday so I think it's no problem for you.

Anyway: measuring brings you close to the sweet spot, dialing in while playing is where you can get the most out of it.
Other members have used other methods succesfully; I use these 2.
A balanced tube is a must.

happy easter!

Erwin

Thanks man, no I hadn't forgotten. I really appreciated last friday. Great results. I also switched the bass cap as per your suggestion and the bass is much more controlled and tighter. I have managed to score several matched tubes and I wanted to make sure I got it right. Then I got to reading and read everywhere that measuring DC is not the right way, so I wanted to make sure.

[erwin_ve]

Hi Joost, already forgot the procedure since friday?

There are several ways to do this, I'll decribe 2, in both ways you have to use a load instead of a speaker to save your ears.;

1. Use the AMPEG SVT procedure. http://www.el34world.com/charts/Schemat ... T_RevA.pdf the procedure is explained there.
It uses the bias cathode measure points of the amp. In a dclone amp you have to tie the bias points of V4 en v5 together and also tie V6 and 7 together(In your case V5+V6 and V7+V8)., those 2 point have become your ac balance points, but you have to set your meter at DC (in mV) because both points are positive to ground.

2. Insert a 400Hz to 1kHz signal in the PI. Set your MM at high voltage DC range((around 300 v). Both triodes are measured in respect to ground so DC!!!)
Measure the Plate voltage to ground for both halfs.(2 MM would be ideal). Both should be set equal to each other.

The last part is to actually play with the amp+guitar and set the trimmer (should be very close to the measured setting)where the most harmonics appear. You have done this last friday so I think it's no problem for you.

Anyway: measuring brings you close to the sweet spot, dialing in while playing is where you can get the most out of it.
Other members have used other methods succesfully; I use these 2.
A balanced tube is a must.

happy easter!

Erwin

Thanks man, no I hadn't forgotten. I really appreciated last friday. Great results. I also switched the bass cap as per your suggestion and the bass is much more controlled and tighter. I have managed to score several matched tubes and I wanted to make sure I got it right. Then I got to reading and read everywhere that measuring DC is not the right way, so I wanted to make sure.

Great to hear, that is a fast catch scoring several balanced tubes!
Probably because DC reading is often reffered to measure without a signal injected in the PI. Then you got DC balance.
HAD advised the AMPEG SVT method to several Dumble owners, it's a great way and tecnically makes sense.

[Structo]

Can you just use one meter and adjust for 0 volts between the cathodes of each power tube side?

[erwin_ve]

Can you just use one meter and adjust for 0 volts between the cathodes of each power tube side?

Yes actually that is the ampeg method about.

[BobW]

I usually adjust by ear, but want to try this method. Should I assume measuring from cathode to cathode, then adjusting for 0 VDC would require doing this in both 50 and 100W modes, then readjusting to a happy average if there are different readings in 50 vs 100W?

[boldaslove6789]

Indeed, I prefer using both a Volt meter, measure both sides of the Trimmer to 7V apart, then use my ear to fine tune the "Harmonic Feedback". Of course having a "Matched Triode" 12ax7 is crucial to balancing the PI.

[212Mavguy]

Just finished adjusting the PI trimmer by ear on my CT 50w HRM. Was just slightly different from as it came. Next did the HRM trimmers to taste, leaving the mids alone and tweaking the bass and treble. Then on to the board OD trim pot and reset it. Kept playing and tweaking and vintage tube rolling shootout for V1 and to a lesser extent V2 til my ears got tired. Was a blast. The amp is sounding stellar, sings at small room volumes without a C-lator (though I normally pair it with one) through a variety of cabs. Oh, the current winner for that V1? Sylvania long blackplate JHS12ax7. V2 is now Adzam 12ax7s, PI Siemens e83cc, RFT/siemens el34's for the power tubes. Thanks to ampgarage members for what I learned here and applied.