Structo....question 4 U

[iknowjohnny]

I was looking at an old thread where i had asked about low end tightening and you graciously made a very long reply for me. I don't recall what i did with that info, as it was a while back, but at one point you said the following about tightening the lows in the PI and PA....

PI and output tweaks - I saved the best for last - you can really make or break an amp in this section. For max headroom try and use the smallest PI entrance cap you can (a lot of Fenders used .001!) and larger output tube grid resistors (I like 5k1s better than 3k3s) Screens on 6L6s sound better to me at 500 rather than 1k. If you have a trim pot to balance the plates make sure you can adjust them to within 5 volts of each other if not change one RP until you can, start at 5 volt difference and then very slowly increase as you strum a chord through the amp - you are trying to get maximum overtones - you'll know it when you find it.

I'm particularly interested in the text in bold and was hoping you could clarify this for me. When you mention a trim pot, can i assume you mean to balance the voltages on the 2 plates in the PI tube, and if so can you point me to schematic with that? And can i assume you are saying that the closer the plate voltages the tighter the low end? You mention overtones so i'm not sure if you mean this is a method to gain more harmonics or tighten the lows. Thanks.

[Marvel]

I know my name isn't Structo, but...

Attached you'll find the schem. I don't think you balance for equal voltage with a DMM. You balance for richest harmonics with a guitar. Usually you'll end up with 5V difference. I have no idea about the bottom end though...

Regards, Marcin

[iknowjohnny]

Thanks, that answers some of my questions, and i will try it for the harmonics. Still waiting to see if structo can tell me whether this also relates to low end tightness. I think i WILL try the 1000PF coupler into the PI tho.

[andyhardy]

That post was actually from Henry aka heisthl

https://tubeamparchive.com/viewtopic.ph ... t=tweaking

[dobbhill]

Here's a good one about PI trimmers..
https://tubeamparchive.com/viewtopic.ph ... ht=balance
D

[Structo]

Hi Johnny,

Not sure when I posted that but yes, I believe it is something Henry posted in a thread that I repeated.

As far as I know larger swamper resistors, making grid couplers smaller, larger plate filtering, more negative feedback can also tighten bottom.

Screen resistors can also affect it as well, but is subtle until you are blasting.

The PI trimming is more about harmonic overtones and sustain.

[Bob-I]

PI trimming is about the AC swing, not the DC voltage. I've had some tubes where the DC was quite far off, 10-12V but the AC balance was good.

Use the adjustment method in the thread above and you'll get not only max headroom but harmonics as well.

[Structo]

Can you measure the ac voltage across the plates to get it close?

[Bob-I]

Can you measure the ac voltage across the plates to get it close?

check out the thread dobbhill pointed to. It takes a dummy load resistor and a signal generator. The other method is to use dogears, he'll make the adjustment by feel and be right on the money. I double checked his adjustment and it couldn't be more balanced.

[Structo]

I don't have dogears


Hey, but my Rottweiler does!


Lilly! Lilly here Lilly!

What do ya think girl?

[img:400:300]http://lh4.ggpht.com/_MHnI-B0pJ8k/TMIpz ... G_0663.JPG[/img]

[img:400:372]http://lh4.ggpht.com/_MHnI-B0pJ8k/TMIpz ... G_0664.JPG[/img]

[img:800:658]http://lh6.ggpht.com/_MHnI-B0pJ8k/TMIqp ... G_0660.JPG[/img]

She's no help......

[Dr-Joned]

Is there any loading caused by the output section? From this I am asking once you balance the PI section, does the balance stay effective even if you switch power tubes?

[iknowjohnny]

Cute pooch to be sure. By the way, in my post where i said i'd try a 100pf cap, i meant to say 1000pf. I just edited it but just wanted to mention it so you don't all think i'm even more stupid than i probably appear.