Need help with building the right amp ( update)

[chikov]

You said your transformers are not yet attached to the chassis, but the orientation you have them in is likely to allow magnetic coupling of the PT to the OT. You should rotate the OT 90 degrees and place them on the same center line to minimize that. With power applied to the PT you can listen to or measure the AC voltage on the OT secondary to find the best location for it.

I just did all what you've said and did not get much of a reading on the AC volts setting, but got a lot of reading on AC mVolts. You can see perfectly how when both transformers are paraller one to another it shows up to 15 mV and when I turn it perpendicular it goes down to 2 mV or so. Thank you for the great advice!

[chikov]

Tonight I finally finished my first amp from scatch and .... It did not work
I used this schematic: https://tubeamparchive.com/download/file.php?id=28211
I added the totem pole stack. There are no shorts but all I get is a hum...
Voltage on the imput is 117VAC. On B+ is 623V ( without a load usd to be 660Vac), or about 312 Vac to the centertap. At the node A it is 267VDC, screens voltage 263Vdc ( after the choke), on node C 224 vdc and node D 197vdc. On the first preamp triode plate(V1a) it is 119Vdc, on the second plate (V1b) 217VDc, on the third (V2a) 129Vdc, on the phase inverter plate 172Vdc. On the output tubes' plate the voltage is about 255Vdc. On the fixed grid (pin 5) on one 6V6 is 0.7 V on the other 1.4 Vdc. I need some help, please. Does anyone know what moght be happenning based on the info i just submitted? I checked all the tubes on the tube tester - there were no problems. Oh yeah, i installed two more diodes on 5Y3 rectifier' s pins 6 and 4, which is a little strange, because when i measured the B+ voltage between the transformer's wires it was full 623, but on the pins 6 and 4 ( past the diodes) it was only 91vac... I will be thankful for any help or ideas.

[martin manning]

You can see perfectly how when both transformers are paraller one to another it shows up to 15 mV and when I turn it perpendicular it goes down to 2 mV or so

Exactly as it should.

Your unloaded PT VAC is 660, or 330-0-330, right? The DC voltage at the A node should be pretty close to 400V, so that suggests a lot of current draw somewhere.

Looks like your bias supply is not working, and with ~0V on the grids, the power tubes will conduct a lot of current. If they are not red-plating I'm surprised. Pin 5 on the 6V6's should be somewhere around -30VDC. I suggest removing all of the tubes except the rectifier, and rechecking the power supply voltages A-D. Check the bias supply voltage at pin 5 and work back to the PT secondary connection to see where that has gone wrong. Make sure you can adjust the bias voltage in the required range with the bias pot, and leave it set for the most negative voltage.

I would also install 1-ohm resistors between the 6V6 cathodes and ground so you can easily check the current flowing in each one.

Connect a speaker load and put the tubes back in and measure voltages again. If the voltages seem ok, bring up the bias voltage until you have ~20mA per tube.

[chikov]

Thank you, Martin, for coming for my resque. After I unplugged all the tubes but rectifier, I got my voltages more normal. But first, I found one of the mistakes I made! I attached 100k grid bias resistor to the wrong side of my rectifier back up diode that i placed between pins 5 and 6. I soldered to the pin 6 ( cathode side of the diode) that was giving me DC... So I moved it back to the AC side ( pin 5) and i got my -41V on the OT grids. Yeay!
Now, I still do not have the guitar sound... It still hums but much quieter Here are my voltages. Node A- 400.7 vdc, node B ( past choke) - 400.3 vdc, node C -347 vdc, node D -313 vdc.
Plate V1a - 186 Vdc, V1b -309 vdc (????), V2a- 201 vdc and V2b - 266 vdc. The both plates on 6V6 have 400.1 Vdc.
The one on V1b seems to be waaay off. Do you have any idea why? I will go and continue checking my wiring and the two new 12ax7 tubes.

[John_P_WI]

Check your cathode voltages, especially v1b.

[chikov]

I tested my tubes and found that V2 (12ax7) was bad. It is brand new J/J tube - i guess I fried it. Anyway, something strange just happened. After I tested my tubes, I started checking voltages and now I have 404.3 Vdc on centertap of OT, 404.1 on Screens, 403.6 on node C and 402.7 on D... How can it be? I have all these 18k resistors that suppose to drop my voltage much more than that... I did remove my preamp tubes, because now I am afraid to fry them...

[chikov]

Check your cathode voltages, especially v1b.

Right now it is 5.4 Vdc
BTW, once i installed the preamp tubes, the plate voltages and node ones went back to normal except for V1b - it is still 309 Vdc.

[TUBEDUDE]

Sounds like you lost the B+ ground return. The tubes aren't conducting, so the voltage is not dropping. Check the ground connection on your main filter cap, or the connection between the totem pole caps.

[chikov]

Oops, on the Boost circuit, I missed the ground. Let us see what will happen now...after I restore it.

[TUBEDUDE]

Oh, didn't realize you had removed the tubes and not reinstalled them.

[chikov]

Hurray !!! It is working, but, my master volume pot is wired backwards, so when it is to 10 - it is silent, on 8 - very loud, anything below that - loud ocillation. Let me reverce it, see what happens...

[chikov]

After swapping wires around on the volume pot, i get a prpper volume increase from 0 to 7 and after that it goes in occilation. I checked my treb and bass pots - they are also wired backwards... May be ocillation will go away after i rewire the other two pots. Although, I am not too proud of all the wires that hang in there:

[IMG:640:480]http://i409.photobucket.com/albums/pp171/alexjohnne/a81d338a-f7a0-4700-9872-6889ff647a80_zpsu8lswhlx.jpg[/img]
I was going to just learn and use this project as a prototype...

Another strange thing, when i tried to measure the plate currant between the pin 2 of the rectifier and the pin 3 on either tube, i get about 265 mAmps... I can adjust it between 246 and almost 500 mA, but it seems too much...

[martin manning]

Another strange thing, when i tried to measure the plate currant between the pin 2 of the rectifier and the pin 3 on either tube, i get about 265 mAmps... I can adjust it between 246 and almost 500 mA, but it seems too much...

Are you using the OT primary resistance to measure current? To do that you measure the DC resistance of each side of the OT primary with the power off, and then measure the DC voltage drop across each side with the amp on and idling. The numbers for each side will be something like 100 ohms and 2 volts, and then the current would be V/R = 2/100 = 0.02A = 20mA. Perhaps you have misplaced a decimal?

Target current should be something like 0.7 * Pa max / Vplate, in your case 0.7 * 12 / 400 = 0.021A or 21mA. If the current is too high, you may need to adjust the bias voltage range, which you can do by increasing the 27k resistor going to ground from the bias pot.

Oscillations are always a possibility due to unwanted coupling and feedback within the chassis. You may have to reroute some wiring to eliminate that problem. Did you add screen resistors? If not that could help.

[chikov]

Thanks for advice, I added it to my shopping card.
I was hoping to get some advice on the output transformer... After studying it for a little more discovered that chances are the output transformer is made for 16 and 8 ohms speakers. Because according to the rules, the voltage on the secondary should increase by square root of 2 from 4 to 8 to 16 ohms. My two readings were .85 V and 1.2V - exactly matches the theory. Plus if you calculate 16 ohms times 25 (ratio) squared equals to 10,000 and the next tap (8Ohms) gave me about 35.3 ratio. If you square that and times 8 it equals about 9,980... Good. But the question is, will it work with a pair of 6V6s with the output transformer generating about 660 Volts (330V to the tap on B+)?
Anybody?

I wonder if my OT matches 6v6 tubes with its almost 10,000 ohms impedance on primery?
Yes, i did install 470 ohms screen resistors...
I got Ot primary resistance to the center tap 104 ohms on the center tube and
The 113 ohms on outer one. The voltage drop was 1.22 and 1.3 respectively. So, that gives me about 11 mAmps n each tube. Which means that , with plate voltage beng 397 Vdc, Plate dissipation is about 4.4 - a little low for 6V6...

[johnnyreece]

Did you try swapping your OT primaries to kill the oscillation? Sorry, didn't see it mentioned.