Ceriatone Forum

Hello all
Almost finished with OTS FM50 build minus 1 part on the way ( internal pot ). Once received I will be ready to power amp up and start tweaking.
How is the internal pot on main component board setup ( Phase input pot )? In other words are there voltage readings to be achieved or is this adjusted via ears? Also is there a good starting point like ( set to 11:00 and then adjust from there )?
Also any other startup recomendations you can give this newbie will be very helpful!
Sorry if this is has been covered
Thanks

Todd

I don't see a manual online for this variant of the ots. There are 2 trimmers in the layout - the FET one and the phase inverter one. The FET one you set to taste I think - start out with it in the middle. The Phase inverter one is set with a DMM once you have everything else running, and it isn't critical where it is when you power up the thing, so set it in the middle for now. Email Nik for specific instructions about how to set the phase inverter for this amp, although I suspect it's pretty much the same as the other amps - look at the standard ots manuals.

All the info you need on setting the PI trimmer here:

 http://ampgarage.com/forum/viewtopic.php?t=13993

Thanks Sduck and boldaslove6789
very informative, wish I had an Oscilloscope and a weathergirl now LOL.
Since I do not have either I will probably just set it to 12:00 and then tweak it gradually by ear.
I did ask Nik this same question, his response was much shorter than the amp garage. But he didn't include pics of his weathergirl!

just set it to the middle.

This one, you wont kill anything, really even if set to extremes.

Then adjust it so that it's slightly off than each other, or to your liking.

Thanks again
Todd

I have long thought my OTS doesn't exhibit that 'bloom' you hear on good ones and I do know my PI is not very well balanced, when I set the trimmer I couldn't get it to closer than 12volts. I now have a Sovtek 12ax7LPS, sold as balanced so I need to do this again.

It seems setting it by DC voltage is not the right way to do it. AC voltage on the PI plates is what Andy Fuchs was suggesting. Others suggest that is the right way to get it in the ballpark then do it by ear. The information about maximising the second harmonic was very useful and it should be easy to set by feeding a signal tone into the PI and checking the output using a spectrum analyser (my hotplate will suffice for this, a dummy load and a line out).

I've just  found this one

http://www.sillanumsoft.org/prod01.htm (http://www.sillanumsoft.org/prod01.htm)

and run it on my laptop and it works fine. I already had a 440Hz sample file on my laptop (for fooling my Canon 7D into not using the AGC  whilst videoing) and I got a nice clean spike at 440 a smaller one at 880 and a similar one at 1320. I'm not sure whether thats the right frequency to use but I might also try 1000Hz.

I've been reading the same PI balancing thread as you Hywell.

I had originally fitted a 'balanced' tube from Watford Valves, think it was one of their standard Harma (JJ) type.

I changed it last week to a Harma Retro ECC83, not specified as balanced and I have to say that I noticed the difference and prefer this new valve without doing any more PI adjustment - my pot is pretty much at the midpoint.

I have read that some rave about the Long plate Sovtek in the PI position so I've bought one of those to try as well, again the non balanced version.

I got mine from Hotrox and I asked them for a well balanced one. Not fitted it yet so we'll see whether it is or not. I do hope to generate some of that 'bloom' that I hear in other OTS's

I found this post on that thread at Ampgarage which seems to sum it up in very non technical way.

I think starting with a matched/balanced tube is probably the best place to begin. Atleast I hoping cuz I just received my tubes today, which I spent extra 5 bucks to get a balanced one.

Got all Tung Sol 12AX7's and TAD 6L6WGC-STR I hope they sound good!! 8)

Them tungsols are shoty at best. When there good though they sound great.

When you say "shoty" do you mean microphonics or something else? I normally use JJ's in my other amps, but it seems they are not as popular with the dumble clones. we'll see, I guess. :-\

http://www.guitaramplifierblueprinting.com/files/Phaseinverter.pdf

I found this interesting

 Sorry what I mean is that a lot of them I've tried have malfunctioned or simply didn't work (even when I bought them from a place that tested them). I had to pick through a bunch to get one that  sounded decent & wasn't microfonic. When I got that good one it sounded great but it isn't worth the hassle. I then switched to the Mullard reissue ECC83's which are consistent and sound great in just about all position's even in the PI. I've been using them ever since.

I wish I hadn't read that ampgarage thread. I haven't messed with my PI trimmer yet, and was just going to use Nik's method to set it, maybe next week or whenever I can find the time. But now... do I just follow Nik's method, or do I worry about injecting a signal, testing the AC current, etc??? Hmmm...

BTW, I just grabbed a bunch of tubes from the local music store when I was building my Bluesmaster 50w - JJ 6L6GC's and new TungSol 12AX7's. Figured they'd get it going, and I can mess with the tubes later. However, these tubes sound great! Absolutely no problem with the tungsols. Although admittedly I haven't tried other tubes yet - can it be even better???!?

Before posting on that ampgarage thread and exposing my utter ignorance I thought I post here and see if I am even in the right ball park. Any help apreciated.

I set up to generate a 440Hz signal from (yes I know) a file played on my mobile phone (mylaptop wouldn't do signal out and in concurrently, I need my external sound module back from my son!) in at FX loop return. Presence to 0.  Speaker out to my Hotplate set to load and the line out of the Hotplate to my laptop running that software I mentioned in an earlier thread. Line level set to minimise distortion.  This gives me a nice spectrum analysis of the amp output. I am able to capture the spectrum and save it. So far so good.

So I got this Sovtek LPS, supposedly balanced. DC across pins 1 and 6 measures from 4.5 to -2.3. When I measure AC i get 3.1volts. The trimmer makes no effect on the AC balance, however the trimmer does make a difference to the waveform, the second harmonic being minimised at 4.5 volts DC( trimmer full anticlock)

Next I try a Harma, DC 21.7 to 14.5. AC nearly in balance at 0.3volts. Second harmonic the smallest of all the valves I test.

Various other JJ tubes give quite varying results but no nearer than 2.2 vAC. In all cases the second harmonic is lowest at trimmer set fully anticlockwise. I now wonder whether I need to change one of the resistors?

So I try by ear and it does seem the HArma has something the Sovtek doesn't but its minimal and I now am beginning to suspect I'm chasing shadows and I too am wishing I hadn't read the ampgarage thread.  >:(

I suspect the Sovtek and now need to ask more questions at Hotrox about what they mean by balanced. It seems transconductance balance is whats required here but I'm not sure thats what I got. The Harma seems to be getting closer to AC balance but the voltages are way different from what I was lead to believe was correct (Nik says 6vDC, others 6-10) I also check the two 3.3k grid resistors and find 3.66 on V4 and 3.46 on V5. Way off spec, by 10% and 4% so I wonder if I need to change those at least to get them to match?

Where to go next? Any guidance  welcome, including 'stop bothering', and 'you're out of your depth' or 'take it to a tech'  ;D

Since I started this topic and found the use of O-scope was needed beacause ears could not detect the sweet spot, I knew it was out of my capacity, but.
I called a amp tech from the ceriatone site and asked what he would charge to setup PI
He said 1 hr or $45 and that he had the equip necesary which is pretty cheap so I might end up doing that.
He did say once you setup PI if you change PI tube you will obviously have to reset PI trim pot again.
So far my OTS FM50 sounds good to me , it is just set in 12:00 position or middle.