Some questions with making a guitar amp.

[daddario]

Hello!
I'm building my second guitar amp on tubes and I'd like to ask some questions.
I have two schematics to take the power amp part from.

1) http://www.sugardas.lt/%7Eigoramps/a...matic_mod2.gif
From C19 to the OT.
2) http://drtube.com/schematics/laney/gc30v.gif
From R28 to the OT.

Note to the 1. schematic: The 6Н2П is 12AX7.

The 1. schematic is currently working on my table, though the OT is not 8 kOhm:8 Ohm, but (2x)20 Ohm:0,9 Ohm.
And a question: How powerful should be the R20?


But main question is: The phaseshifters on those schematics are diffirent. Is one a lot better than the other?


I'm not really sure, if this is the correct forum to ask about this(found it on Google). In our local Estonian electronics forum, there are not many people, who know about buliding tube amps(maybe 7-12), and most of them(sadly the smarter ones) don't really bother to answer to questions from newbies. They rather go to some russian electronics forum, but I don't speak a word russian.

[Enzo]

Hi, welcome. Yes, this is a good place to ask, and we all are happy to answer.

Your OT - Looking at your numbers I am thinking you measured the resistance of the transformer windings with a meter. Is that correct? That is not the impedance of the transformer. The impedances are determined by the load and the turns ratio of the transformer. You cannot measure the impedances with a meter.

Transformers don't have impedance themselves, they reflect the load impedance back to the tubes. SO in your example, the OT primary only is 8k IF there is an 8 ohm load on it.

R20 is the cathode resistor for the output tubes. I would think 5 watts would be enough. Certainly a 10 watt is more than enough.

[daddario]

Ok, I think i get it. How should I calculate the correct sec. coil for 4, 8 and 16 Ohm load?
The primary coil has 2* 308 turns of 0,28 mm wire. The transformer has EI pieces, the thicknes of one plate is 0,35 mm.
Size of the transformer:

Width: 72mm
Heigth: 60mm
Thickness: about 37mm

Size of the transformer windows:
Width: 12mm
Heigth: 36mm

How should I calculate, how many turns of how thick wire should I wind for the secondary coil?

I appologieze, for I may not know the exact technical terms in English.

[Enzo]

Try this, download this chapter of the RDH, and read through it a couple times. It is the chapter on transofrmers, and explains all the relationships.

http://headfonz.rutgers.edu/RDH4/CHAPTR05.PDF

[zhyla]

How should I calculate, how many turns of how thick wire should I wind for the secondary coil?

I had the same question a couple weeks ago, check out this thread for two methods to find out the impedance ratio of a transformer:

http://music-electronics-forum.com/s...ead.php?t=8640

[daddario]

Thanks! Those both links were very useful. It looks like the correct load for the 1st schematic with my transformer is 4.3 Ohms. Going to make a new secondary anyway. For 4,8 and 16 Ohm load. Maybe I should boil the transformer in paraffin later.

BTW: Did I understand correctly, that the transformer's primary is mesaured between the outer connections, not from center to one of the outer taps?

[Rubbertoe]

hey bro i have built a few tube amps .. a good site to see is www.hoffmanamps.com full layouts and lots of tips and stuff good luck to the amp building

[daddario]

Ok! Now little about the preamp. There will be 2 channels with seperate preamps.
http://www.mhuss.com/MyJCM/JCM800_2204.gif
The channels split from the top connector of the "low" input, so the V1B will be common for both channels. The "drive" channel will be axactly as shown on the schematic and ends at the "master volume" pot. About the clean channel: Is it a good idea to take the the same schematic, but throw the V1a out? So the "preamp volume" pot goes directly to the grid of the V2a.

[tubeswell]

http://www.mhuss.com/MyJCM/JCM800_2204.gif
About the clean channel: Is it a good idea to take the the same schematic, but throw the V1a out? So the "preamp volume" pot goes directly to the grid of the V2a.

You get much the same effect in gain reduction as just plugging your guitar into the "lo" input (which bypasses V1b), albeit that the voicing would be different than if you had V1a bypassed (because of the different cathode resistor/bypass cap combinations in V1b and V1a, and the different padding between the stages).

If you were going to go to all that trouble to set it up, you might as well look at just splicing on different pre-amp configurations that already have channel switching.

[daddario]

Okay. About the OT now:

*Reality:
The primary coil has 616 turns. Let's put 25 turns for the secondary for testing.
Let's measure:
We will give 6V AC 50Hz on the secondary coil. Now let's measure the primary: 226V
Let's calculate:
226V / 6V = 37.6666667:1
37.6666667square =1 418.77778
8000 Ohms / 1 418.77778 = 5.63865611 ~ 5.6 Ohms.

*Theory:
1.1 * square root 5.6 Ohms / 8000 Ohms = 0.0291... ~ 0.03
Turns for the secondary coil:
616 turns * 0.03 = 18.48 ~ 18.5 turns.

So for a 5.6 Ohm load reality gives 25 turns, but theory says 18.5 turns.

[Albert Kreuzer]

Hi daddario,

in theory, theory and reality are the same

Did you check if your transformer has enough inductivity to handle 50Hz?
Maybe you like to re-measure the whole thing with say 500Hz and report back.

Cheers,
Albert

[daddario]

I suspected that. The problem now is - where can I get a 500Hz signal with enough "power". 50Hz comes right from the "wall".

[daddario]

Okay. One problem less. I will get a 27VDC > 36VAC 400Hz unformer(motor + generator) from a soviet military plane.

[Albert Kreuzer]

That should do the trick
But you don't need much power to test an unloaded (!) transformer. Almost any NF tone generator will work. Just expect the output to "sag" a little. Or put a small SS amplifier between generator and transformer.

Cheers and good testing
Albert

[daddario]

Hi again. Finished the amp long time ago, but there was a problem. It was noisy. I don't know the exact word to describe it. It was not a hum. It was something like white noise at random places. The time further, more it was. I blamed it for bad assembly and crappy soviet tube socets. I rebuilt it from square one. New sockets and alot better assembly.

The schematic is:
Power amp and PSU: http://www.sugardas.lt/%7Eigoramps/a...matic_mod2.gif
Starting from C19
Preamp: http://mhuss.com/MyJCM/JCM800_2204.gif
Ending with the master volume pot.

But no diffirence. The noise comes from the preamp. I have tried diffirent tubes (all are soviet NOS).

Picture: http://www.foto24.ee/album/45410/s1