Need help with building the right amp ( update)

[martin manning]

Good point, Johnny. A quick way to test that theory would be to short the 47R at the V2a cathode to kill the NFB.

Re the OT primary impedance, 10k will work for 6V6, but in that case I would like to see the screen voltage around 350. You could put a shared 5-10k resistor between the screen node and the screen resistors, but dropping 50V might be a stretch. Miss-matching the speaker load to get 5k primary impedance would be too low, so I don't recommend that.

If you can't get the idle plate current up to 20mA or so with the bias pot then you'll have to adjust the bias circuit by reducing the 27k resistor going from the bias pot to ground.

[chikov]

Thank you Martin and Johnny for the help! I finally came around to working on my amp. I have several good news. First, I switched the primary leads on the OT and whole hell broke loose ( I think I have some hearing loss because of that:-)). That thing started sequel like a pig under a gate. I knew then originally I got it right and I restored it to the way it was. Second, I built in NFB loop on/off switch and it helped to hear the difference. Now, about the loud oscillations when the main volume button turned all the way up. I installed 5k resistor before the two 470R screen resistors and it helped, plus I reduced the 27k bias pot resistor to 12.5 k. Most of my oscillations are gone and I was able to bias it at 30mA plate currant, which with 357V plate voltage puts me around at 10W Plate dissipation. Unfortunately my bias pot is at its minimum setting, ad I would like to have some more room ( so I can go down to 20mA at list). I guess I should increase my bias pot resistor a little more...
BTW, I did install 1 ohm, 2 Watt resistors on both of my cathodes and now biasing is easy - Voltage drop value equals mA plate current - nice.

Well, with my NBF on, the sound gets louder, but when I turn my volume to 8-9 it starts oscillating/squealing/distorting/clipping. When I disconnect NFB (via my little toggle switch) it gets quieter, but when volume on 10 it makes this clicking/popping sound. Do you know what still might cause that?

PS. The amp, though, is starting to sound REALLY NICE

[chikov]

Interesting, once i lowered my plate current to 20mA, the popping (when vol on 10 and NBF open) stopped, but with NBF on it squeals and when I turn my Treble on all the way it squeals as well (according to my Fluke tester) voltage drop on my 1ohms resistor would go way up, when squealing takes place...
PS. My Plate voltage on phase inverter V2b triode is 247Vdc and on its Cathode is about 63vdc... Isn't it a little too high (the rest preamp plates are at about 175-185VDC though)?

[martin manning]

Interesting, once i lowered my plate current to 20mA, the popping (when vol on 10 and NBF open) stopped, but with NBF on it squeals and when I turn my Treble on all the way it squeals as well (according to my Fluke tester) voltage drop on my 1ohms resistor would go way up, when squealing takes place...

These are probably related to lead dress. It's normal for the current draw to jump when an oscillation kicks in.

PS. My Plate voltage on phase inverter V2b triode is 247Vdc. Isn't it a little too high (the rest preamp plates are at about 175-185VDC though)?

247V is fine. The cathode is at ~60 I expect.

[chikov]

Hmmm, what do you mean by lead dress? Should i use shielded cables the go to every grid pin on preamp tubes?
I read some other threads and some suggested that some of that stuff can be caused by undersized choke ( mine is only 155 ohms) or OT? Do you think mine are undersized?
The amp seems to be too loud IMHO...
The amp does have overall humm - i had to put a lot of parts in a small chasses afterall

[martin manning]

Lead dress is a general term for the arrangement of the components and the connecting wires. Running wires that carry signals that are in-phase close together can cause feedback and oscillations. Shielding the grid leads might help, especially if they are long. I wouldn't think the size of the choke is a problem. The inductance is more important than the DC resistance for filtering out ripple voltage.

[chikov]

Martin, you are the man! I started pocking around my wires with a bamboo stick and found the 2 culprits. The first was a wire that was connecting PI plate with a .1 uf coupling cap. I had to make it about 4" and it went over a few component. So i replaced it with a shielded wire with one end of the sield grounded and 50% of oscillations went away. The other two wires were a part of a NFB on/off switch that i had to make super long to go from the back of the chassis to the front . I just moved the whole switch to the back with about 3" long leads - now almost all junk is gone when the volume is at 10. However, when i turn my Trebels all the way on i still get that hiss... May be another wire needs to be shielded?

[chikov]

Yes, i belive I found another trouble maker. This wire goes from a plate of a pin 2 on the second preamp tube. I started moving it around and the hum dceased. I will shield it tomorrow, but, better yet, i will move my two teeminal posts with a bunch of V2 tube related parts far away from the Rectifier tube (dah!). It sounds great alrady, but i want it to be quet too

[martin manning]

Hey good work there. Some hiss at high volume settings is normal, but you might try some different tubes to see if you can find quiet ones, most importantly in the first position. No doubt you have learned a lot from this project already, which was one of your goals, right?

[chikov]

Oh yes, I am so glad I made all these mistakes! I realized how important lead dress is and the location of diffrernt components. I wanted to learn more about Biasing the output tubes - I got a good idea at this point what it's all about. My next step is to experiment with voicing this amp by adjusting the value of cathode bypass caps... Stay tuned, as they say...

[johnnyreece]

Glad it's working out for you! I'm curious to hear your opinion and see if it's what you were looking for, sound-wise.

[chikov]

Glad it's working out for you! I'm curious to hear your opinion and see if it's what you were looking for, sound-wise.

Yes, i definitely will, after I play a little with the values of the preamp tubes bypass capacitors.
I do have a question, my bypass switch is connected to the pin 2 of the rectifier tube, but many people say it should be pin 8. Does it make any difference?

[sluckey]

It will work either way. But, technically speaking, use pin 8. Older rectifier tubes like 5U4, 5E3, and others have a directly heated cathode. There is no separate cathode element. The cathode is actually a coating on the filament. These types can use either pin 2 or 8. Some later rectifier types such as 5AR4/EZ34 do have a separate cathode element that is heated indirectly by the filament. In order to maintain pin compatibility with 5U4, 5R4, etc., the separate cathode is internally tied to filament pin 8. But even with this type, either pin can be used, although I would use pin 8 since it makes direct connection with the cathode. My personal preference is to always use pin 8

[martin manning]

I do have a question, my bypass switch is connected to the pin 2 of the rectifier tube, but many people say it should be pin 8. Does it make any difference?

You mean standby switch, right? In the case of rectifiers with directly heated cathodes like 5Y3, no it doesn't make any difference. In rectifiers with indirectly heated cathodes, the rectified voltage should be taken from the pin that is connected directly to the cathode so the DC doesn't have to flow through the PT. On 5AR4's that is pin 8.

[chikov]

Glad it's working out for you! I'm curious to hear your opinion and see if it's what you were looking for, sound-wise.

Wow, it is hard to acurately describe my feelings about this amp, but I will try. It definitely sounds vintage - not very thick, smooth, transparent overdrive. High notes are very defined and loud and bright. The lows are not huge, but just right for me. It does not have a huge amount of head room, but I wanted some hair on my tone anyway ( that is why I used 3 stage preamp schematic). I like the rich and loud sound it produces (every bit of those 18 watts). I biased it at around 28mA and the OT plates have about 386VDC with the bigger 5U4 rectifier. Here is my crude iPhone demo of this amp at this stage:

https://www.youtube.com/watch?v=J976vW7d57U

what do you think? I know that the hum is a little too loud, but I work quite a bit on my lead dress and it is almost as much as I could do. Any ideas how to reduce this hum ( staying away from PT helps a lot, plus the pickups on this old Pan guitar are VERY microphonic)...