Power amp essentials?

[dogears]

That is basically what I do. Bob I was over and never was a believer in PI trimmers until I adjusted his with him watching and listening. Sometimes it takes the right PI tube as a baseline!

I'll add....Turning that trimpot makes a huge difference in the sound I hear.

Just strum the low e, turn the pot till the amp blooms!

I think its very obvious by ear where the amp gets louder, blooms out, and 'sounds right'.

Simple as PI (3.141519......)

[Mark]

Thanks for the replies, I do have a lot of work ahead of me, so I expect the thread to stall for a while.

Here is some info from the Valve Wizard site:

http://www.freewebs.com/valvewizard1/pp.html

http://www.freewebs.com/valvewizard/pentode.html

http://valvewizard1.webs.com/se.html

http://www.freewebs.com/valvewizard/acltp.html

http://www.freewebs.com/valvewizard/dcltp.html

http://www.freewebs.com/valvewizard/gridstopper.html

[Mark]

ust strum the low e, turn the pot till the amp blooms!

I think its very obvious by ear where the amp gets louder, blooms out, and 'sounds right'.

Simple as PI (3.141519......)

I gave it a try today. I broke out the 82 Strat with hand wound CBS style pickups.

I didn't hear a huge difference, just maybe there was a slight improvement. Though it could have been the angle I was strumming guitar?

I don't discount this method yet as I need to sort out the output valves which are 10mA apart when biased.

As previously mentioned I will have to install individual bias controls for each valve (two valves) and set up the bias & gain of the output stage.

Continue plodding!

[Mark]

I decided to put a signal in to the amp and have a look. I wasn't too scientific, I kept it all pretty basic.

I suppose the only real news from these photo's is that the pot offers a 2 volt adjustment in my amp. I am getting new tubes so it will be interesting to see if that makes a difference.

Is this what you guys are finding?

Oh yeah, the oscillator was R.G. Keen's quick and dirty oscillator. Which is quick and easy for fault finding, but not so good in this application.

[CHIP]

That is basically what I do. Bob I was over and never was a believer in PI trimmers until I adjusted his with him watching and listening. Sometimes it takes the right PI tube as a baseline!

I'll add....Turning that trimpot makes a huge difference in the sound I hear.

Just strum the low e, turn the pot till the amp blooms!

I think its very obvious by ear where the amp gets louder, blooms out, and 'sounds right'.

Simple as PI (3.141519......)

The right tube is the first adjustment. I try different balanced tubes, adjust and listen. I have found that the right tube will bring it. At least for me.

[Mark]

Dear Chip

Thanks for the reply. I hope I don't come across as being critical. As that isn't the case.

People have given me advice and I have tried it. Enough people are saying it works, so why doesn't it work for me?

The next step is to quantify what I'm seeing. I've found out that I may need to buy a sig gen as the R.G. Keen circuit has several flaws.

I think it is important know what the PI trimmer actually does. I found that I could get between 37vac and 39vac from the trimmer. So it wasn't a huge adjustment.

The disclaimer is that it was done very quickly and the limitations of the "test" equipment have to be considered.

I am curious about the observation of others who've used a CRO on the P.I.????????

[greiswig]

I am curious about the observation of others who've used a CRO on the P.I.????????

Mark,

I tried using a scope on mine. I have an old 2-channel Tek that I probably don't know how to use properly. What I did was to put both leads on one of the PI channels and adjust the gain and such so that both traces overlapped as closely as possible. Then I flipped polarity on one of the channels and put the lead on the other PI channel...with the theory that I could once again get the two traces to overlap if the PI were adjusted for this theoretical ideal.

Well, it sounded fine. But I couldn't get the PI trimmer to make a lick of visible difference in what I was seeing on the scope. I gave up, assuming that I was doing something wrong with the scope.

And I still don't think there's been an answer to the question I asked earlier: does adjusting to max bloom by ear do the same thing as adjusting the PI objectively for symmetry? Or are they accomplishing two different things?

[Mark]

Dear Greiswig

I suppose I'm trying to find out under what conditions does this blooming occur?

Surely if it can be done by ear, it can be done with test experiment?

What new production tubes work in the PI stage of a Dumble amp?

Where do you get 12AX7 with matched triodes?

My observations are that balanced triodes were quite common several years ago, but not so common now. However for all that the old Groove Tube 12AX7M's that I have been using is fairly closely matched.

[washburnmemphis]

Dear Greiswig

I suppose I'm trying to find out under what conditions does this blooming occur?

Surely if it can be done by ear, it can be done with test experiment?

What new production tubes work in the PI stage of a Dumble amp?

Where do you get 12AX7 with matched triodes?

My observations are that balanced triodes were quite common several years ago, but not so common now. However for all that the old Groove Tube 12AX7M's that I have been using is fairly closely matched.

I've been lurking here for a while, I recently finished putting together a Ceriatone Bluesmaster 100W. Just thought I would give you my $.02.

If you are looking for matched triodes most established tube sellers (Tube Store, Doug's Tubes etc) sell matched triodes for an additional fee.

I believe I managed to get bloom (at least on the G string), using a matched Sovtek 12AX7 LPS and also a non-matched JJ ecc803s. Apparently the long plates help.

I also tried a JJ ECC83S, EH 12AX7, Shuguang 12AX7-A, and Shuguang 12AX7-B all had matched triodes but I wasn't able to get anything resembling bloom. Both Shuguang's sound terrific in V1.

The approach I took was to turn the amp up as loud as possible w/o feedback on the clean channel with PAB on. I would then play the Ab, A, Bb starting at the 13th fret on the G string, while adjusting the PI trimmer. I'm using a Les Paul Traditional on the neck pup. The speaker is a single CRex in a 2X12 (I'm still trying to find it a mate, possibly another CRex)

[Mark]

Dear WBM

Thanks for your observations.

I have some new pre-amp tubes coming, they are the JJ803 (what is the deal with the long plate?)
Two JJ83S tubes. I will see how it goes when I get the tubes, but I'd like to sort out the output stage in the mean time.

[Structo]

The problem I see is unless you play in PAB mode a lot, why would you tune the PI to bloom or feedback in PAB?

I would rather have the amp bloom and flip in normal OD mode which for me is Rock switch on and OD slightly louder than Clean.

[washburnmemphis]

The problem I see is unless you play in PAB mode a lot, why would you tune the PI to bloom or feedback in PAB?

I would rather have the amp bloom and flip in normal OD mode which for me is Rock switch on and OD slightly louder than Clean.

My point was to try to at least see if I could get it to happen, regardless of what my normal playing conditions might be.

I plan to go back and try it without the PAB. I also plan to try some more non-long plate 12AX7's.

[greiswig]

The problem I see is unless you play in PAB mode a lot, why would you tune the PI to bloom or feedback in PAB?

I would rather have the amp bloom and flip in normal OD mode which for me is Rock switch on and OD slightly louder than Clean.

I guess the other question is why would it make a difference? I.e. is there a different ideal for PAB versus non? Intuitively, if this is only affecting the PI and power amp, I'd expect the sweet spot to be the same for both.

[talbany]

I too prefer the LP's in the PI as well.. It seems to help keep thing a bit tighter especially in OD and adds some girth to the tone..
In my adjustments I notice bloom in all modes
Hope this helps..

Tony

[Mark]

Could we recap on this bloom effect?

1. Long plated tubes are preferred.

2. Some people are tuning it by ear others are setting the PI for symmetrical output.

3. If the PI trimmer is set up by ear then the amp has to be loud.

Okay, the significance of the long plate eludes me. I assume the amp has to be set loud so the source is distorted in the output stage.

The questions that I ask myself is; the bloom must be a product of distortion within the PI or output stage, so if the amp is turned down does the bloom disappear?

As the OD stage generates so many harmonics does the bloom effect become less apparent when in OD mode?

Is there any footage on You Tube demonstrating the bloom effect?

Got to go, the toddler calls.