My FIRST tube amp (and it's working!)

[rankot]

My FIRST tube amp, PCL86 based, and it's working from the beginning! There is some hum, but I hope I will be able to fix it. 



Amp is near output transformer, top left, and below it is 5670 based preamp.

[Marcos - Munky]

That's nice! I'm working in a PCL84 tube amp right now, using the PCL84 triode as the preamp, those tv tubes sounds very good. It's great when the amp works from the beginning.

[rankot]

What puzzles me is B+ voltage - without tubes, it is 250V, when I put them in, it falls to 150V.

[statzern]

Having the B+ sag with installing tubes is pretty normal behavior - you are loading the power transformer, so some drop is expected. 100v (almost 40% of the unloaded B+) seems like a lot, though. I usually would expect around 10% or so.

Some reading here: https://music-electronics-forum.com/showthread.php?t=23852

[printer2]

What puzzles me is B+ voltage - without tubes, it is 250V, when I put them in, it falls to 150V.

Do you have a schematic?

[rankot]

Certainly, here it is:



Preamp is taken from here: http://meatexz.com/engel-sound/5670preamp.html

[rankot]

Huston, we have a problem!!!

After initial testing with small 2W 8 Ohm speaker, I connected 100W 8 Ohm speaker and amp started to oscillate - there's a high pitch squeal and sound is distorted. Squeal gets quiter if I touch output transformer's core. It is dedicated OTF for SE tube amps, 5k2/8 Ohm, but it seems that it is oversaturating somehow. Where to look next? What could be the cause?

[Marcos - Munky]

Just by curiosity, did you tried it without an external preamp? I mean, you already have a triode working as the preamp. And you can add a cap in parallel to R2 to have a little bit of gain on that stage.

For the oscillation, does it change as you turn the pots? Does it change as you roll down your guitar volume? I'm having a similar issue with my PCL84 guitar amp, which got solved by putting a small cap from pins 1 and 8 (actual pinout of your PCL86) to ground. I have 470pF for both caps, but probably you'll have to play around with the values (if it's the same issue as mine).

[rankot]

No, I didn't try it w/o external preamp, but I will, first thing in the morning

I'm not sure about oscillation, if I remember correctly it wasn't affected by volume pot settings (although tone controls did change the sound), but I will try it in the morning too, I'm too tired now.

What's puzzling me is why it worked fine for the first time, and then everything goes wrong???

[MJ_Sound_Cubed]

Hey man, Well done .

Its super satisfying to have the glowing tubes for the fist time!

For how much power is the power transformer rated for? Volts? Amps?

Are the tubes new or used? If used did you test them?


Can you write in the schematic the voltages you get in the power tube's anode, screen and cathode?

[anotherjim]

My ECL86 based amp could squeal until I added NFB. I based the feedback circuit on a Mullard hi-fi scheme that included a bass boost control which is quite useful. Without NFB, you'll probably have to stand in a pentagram and drip goats blood on it to keep it stable.
http://www.r-type.org/articles/art-003g.htm

Back driven mains transformer drop quite a bit on load I've found. I think that efficiency losses and the high ohmic resistance of the primary winding of 220v transformers might probably be the causes?

Anyway, yes I also think they make nice little amps. If use a simple tone control, the one tube alone has enough to make a good little guitar amp. I've got a 7586 Nuvistor triode in front of mine with a full Fender tone stack.

[thermionix]

The schematic is incomplete in that the power supply isn't depicted.  But did you really hook your B+ up like that?  I would connect it at the junction of R6/R9/C7/R3.  It's unusual, perhaps ill-advised, to have a resistor between B+ and OT primary.

Probably not related to squealing though, that may be a lead dress or layout issue.  C4 seems kinda large.  Your input impedance is very low.  Most designs put the volume control after the first gain stage.

[anotherjim]

Yes, I'd put the OT primary on the rectifier output too, together with another of those 47uF caps.

[Vikt0r]

Hi

The 100V voltage sag is way too much, especially for a solid-state power supply - this means the power transformer has insufficient power to feed the amp, or the secondary
coil impedance is way too high. Your PT must be heating well.
For the self-oscillation - did you ground one side of the OT secondary coil? Also, it is possible, that the amp falls into heavy oscillation in the ultrasonic frequency range, which
you can't hear, but it degrades the sound quality and sags the voltage due to heavy load - you'd need an oscilloscope to find this out. Looking at the picture I would say this is easily
possible.

[rankot]

You are right, I forgot to ground one side of secondary coil!  I'll try that too, but I can't do it before next Friday, I'll be on a trip abroad.

I used back to back PT, 220:14V/3.5A, then 12V/1.25A:220V. Maybe the second one is insufficient? It will provide 68mA on the secondary, but maybe that's not enough?

[anotherjim]

May need minor, but tedious tweeks. Pentode cathode resistor may need to be higher. I have 180R. BTW, wire wound is recommended for that resistor.
According to the web, those pentodes like 240v B+ and 7v cathode bias. Mine comes nowhere near to that. It sounds right and is loud enough for what I want.

[rankot]

OK, I've connected speaker minus to ground and there's no squeal any more, but I don't get any sound out of the speaker. It seems that OTF produces sound when I strum strings

Never mind, it will have to wait till I return from a trip.

[printer2]

Going through the 470 ohm resistor is no big deal, just gives better filtering. The datasheet says that at 150V the current to the plate can be 18 mA, the screen at 3mA, triode say 2mA for 23mA. Ohm's Law says the voltage across the 470R resistor is 470 x 0.023 = 10.8V. So either you are drawing a tone of current to get a 100V drop or you have the wrong resistor value in there. The datasheet says that the grid of the pentode should be biased at 2.1V, Ohm's law again, 2.1 / 120R = 17.5mA. So that resistor value is appropriate. It would be good if you take dc voltages on the tube pins and for good measure across the resistors.

Just as a guess, you might have the input of the pentode shorted and it is drawing maximum current, heating up the OT. We need those voltages to eliminate a number of possible faults.

[thermionix]

Going through the 470 ohm resistor is no big deal, just gives better filtering.

Yeah it'll fly in a LOW powered amp, but you want the most filtering at the preamp stage, where noise will be amplified more.  Fender (and everybody else) did it right.

If grounding one side of the OT secondary killed the output, probably grounded the wrong side.  Some are grounded to case internally.

[MJ_Sound_Cubed]

You are right, I forgot to ground one side of secondary coil!  I'll try that too, but I can't do it before next Friday, I'll be on a trip abroad.

I used back to back PT, 220:14V/3.5A, then 12V/1.25A:220V. Maybe the second one is insufficient? It will provide 68mA on the secondary, but maybe that's not enough?

I can see you have a complex layout More parts more work to achieve good tone, but  I admit very creative never thought about building one that way

All depends in the efficiency of the tranformers, lets say you have decent ones, industrially produced, so 85%.

14x3.5x0.85=41Watts

12x1.25x0.85= 12,75Watts

12.75/220=58mA

Which kind of rectifier are you using?