Output Transformer Questions

strato17 · Post by **strato17** » Fri May 30, 2008 3:25 am

I have one of the newer models of toneslut's OT's in my amp. This one does not have two different OT impedances. It only has three wires on the primary of the OT, a blue, purple and green. The blue is the center tap, and the purple and green are the inputs from the power tubes.

When I turn my amp on, I have having really bad squaling with the volume all the way down. I read the troubleshooting manual and it said if there is squeeling, disconnet the NFB wire from the impedance switch. This did indeed fix the squeeling. He then recommend that one thing that could cause the squeeling is that the OT primarys are backwards!

I never had any information on which primary should go to which power tube. Is this probably the problem?

wallybob · Post by **wallybob** » Fri May 30, 2008 4:02 am

Reversed OT XFMR primary leads were the cause for the squeal on my last build.

lewilson · Post by **lewilson** » Fri May 30, 2008 5:09 am

Thats right.You are getting positive feedback from the output tube to the phase inverter. you want negitive feedback .you must reverse the wires from the output transformer to the output tubes.

skyboltone · Post by **skyboltone** » Fri May 30, 2008 5:22 am

Yeah really no big deal. I have to change them on exactly half of my builds. Just like flippin' a coin.

Dan

strato17 · Post by **strato17** » Fri May 30, 2008 5:50 am

I switched em, and it did fix the problem!
Lead dress isnt 100% francesca now since i had already cut the wires, but I now have no sqeal or self oscillation... atleast not until the volume is about 12:00. Thats something I will work on later

With a guitar plugged in, I do get sound! the cleans are nice, but full volume on the guitar is a bit over the top, even with volume on amp only at 9:00. I can attribute that to crap tubes and not having the best bias voltage. I will put my NOS Mullards and NOS Brimars 12AX7's in when I know everthing is working right.

However, I have some other issues to address now....

My chassis seems horribly microphonic. Just tapping on it with a guitar pick can cause all sorts of crazy stuff to happen.

Also, some of the pots dont seem to be working correctly. Either my middle or bass pot(i cant remember which...) when turned past noon, the signal drops out completely. Maybe this is a ground issue with the copper bus bar? Perhaps a bad solder joint is causing pot not to work, as well as microphonics from the chassis?

I need to properly set my bias. Right now, my bias voltage is about -33 VDC, which I think is ok, but might not be optimum. Since I dont really know what I am doign with biasing, I might just drop the change on a weber bias right. Or is it easier to do than I think it is?

declan · Post by **declan** » Fri May 30, 2008 10:38 am

strato,

Biasing is easy, if you built the amp you can bias it, there is loads of bias info here and in Rons guide. A separate tool is not necessary. Just be VERY careful with the placement of your probes if you choose to do the transformer shunt (fastest method), unless you like a firework display.

Double check ALL your wiring and component placement against a layout diagram, it is so easy to leave something simple out, and do a voltage chart , again there is a blank chart in Rons guide. That will also help you troubleshoot and see if voltages are within expected range

Do all the above before risking your Mullards

Microphonics: Is your input wire and the wire from V1 to Vol pot shielded? you're also probably going to have to try a number of different preamp tubes to find one that works in a wreck

strato17 · Post by **strato17** » Fri May 30, 2008 4:18 pm

declan wrote:strato,

Microphonics: Is your input wire and the wire from V1 to Vol pot shielded? you're also probably going to have to try a number of different preamp tubes to find one that works in a wreck

The wire from the volume pot to V1 is not shielded. I know that KF didnt do this, but most recommend it, correct?

I will have to read up on the biasing stuff.

Thanks!

skyboltone · Post by **skyboltone** » Fri May 30, 2008 7:03 pm

strato17 wrote:I need to properly set my bias. Right now, my bias voltage is about -33 VDC, which I think is ok, but might not be optimum. Since I dont really know what I am doign with biasing, I might just drop the change on a weber bias right. Or is it easier to do than I think it is?

There are lots of ways to measure bias as you'll find by searching the forum. But...if I had the extra change I'd be buying one of the three or four products like the bias right. There is one, I don't remember which, that has a digital display that tells you everything you ever thought about knowing about the tube's performance. It's more expensive but folks laud it's performance. Anyone remember what that thing is called?

lewilson · Post by **lewilson** » Fri May 30, 2008 7:03 pm

You must try many tubes in the first position to find one that is not microphonic. I think thats the trouble there. Its a common problem with this amp that has high gain.

You must turn the volume on the guitar down and the amp up, and find the best clean sound, then adjust the guitar up for distortion.its easier with a guitar that does not have hot pickups, at least for me.

Also make shure if you are using a 6.6 primary ot with el34 tubes that you run a speaker load that is half of what the transformer specs says it needs . example run a 4 ohm load on a 8 ohm ot. sec.
its seems mushy to me if I don't do this

skyboltone · Post by **skyboltone** » Fri May 30, 2008 7:06 pm

lewilson wrote: Also make shure if you are using a 6.6 primary ot with el34 tubes that you run a speaker load that is half of what the transformer specs says it needs . example run a 4 ohm load on a 8 ohm ot. sec.
its seems mushy to me if I don't do this

EL-34's into a 3.3K load? There must be something else wrong.

lewilson · Post by **lewilson** » Fri May 30, 2008 7:49 pm

Yes, but how do you explain hooking up the tone controls? Once thats done maybe my advice will help.

lewilson · Post by **lewilson** » Fri May 30, 2008 9:10 pm

Find out how to bias an amp by current not voltage. Get a multimeter and do it that way. 50ma is what I set mine to. its easy

rooster · Post by **rooster** » Sat May 31, 2008 8:27 am

Hey, yall. lewilson, you say two things that are interesting to me. First, that speaker load thing. You know, you are probably right, but I don't see other people talking about that here. Is this thinking just a given? I wonder if Glen does what you do? I really hadn't thought about it, good point. Anybody else want to chime in? I have run the 16 ohm amp setting into an 8 ohm cab, and also the 8 ohm setting into the same cab. I couldn't really hear that mush you are talking about but I will get into it tomorrow when I have time. Right now I just run the 16/16, 8/8. 4/4 kind a thing. Hmm. Funny, you have a great point but I don't see anybody talking about this here. Damn!

OK, point #2, and to the point: 50ma? I run mine at 40 ma and consider things pretty covered. I am more blues than rock so I suppose I go for the lessor thing. Wow, what does your plate voltage read at that setting? Allyn says his plates are 380VDC, mine are 400. Could it be that he is doing what you are doing? At 50ma that would really bring the plates down, for sure. So what VDC do you have on your plates?

BTW, I just got the word that Eric has shipped my head case for my wreck, which means I can start taking this thing out. I am pretty excited, actually.

strato17 · Post by **strato17** » Sat May 31, 2008 5:50 pm

my plates are 400 and 404... I run close to 43 mA..... thats is a safe bias, correct?

Andy Le Blanc · Post by **Andy Le Blanc** » Sat May 31, 2008 7:26 pm

the load determines the reflected impeadance that the tube sees
ie...... 6.6k and a 8 ohm load is a ratio 825 to 1..... so at 4 ohms the
reflected impeadance is 3.3K which may show sonic benifits of a lower impeadance
like a wider bandwidth..... or the sweet spot for the tube type
or the preferd tone for the ear of the beholder

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