Tube Amp Archive

I would do it. Easy enough to add to the schematic. Could be a bit more challenging to actually install it in the amp. yes but at the same time I saw that a resistor was added in bias section, might it compensate on this side.. at the end this voltage drop is to control the power on the tube , can ...

Hi all any expert on the bass 400 + here ? I'm checking my own 400 +, some noises.. discovered 1 tube dead and some 680 ohm resistors out of specs and some far away I discovered that I don't have the zener 15V to drop the B+ .. strange even that it is on the schematic as bassmans for example running...

What is the resistive load value you’re testing with and what is the power supply voltage? I assume your testing into close to a 4 ohm load. Of the top of my head I got to say that getting 150 peak watts will take darn close to 500 volts. 220v.. on 4ohm yes. I reviewed the channel 1 back to stock n...

1/ In a AB amp, an increase in cathode / anode current with signal, ie higher than the idle level, is normal. Much of the additional current is transferred to the load, rather than dissipated by the valve. Hence the load gets hot etc. To measure 150W, surely the amp is being overdriven? The sine wa...

How's the bias voltage look on the control.grid? Sounds like the bias voltage losing control of a gassy tube. But since a new tube does the same I'd see if the bias voltage on the tube is changing. Maybe touch up the joints in the path until the bias is stable then put the tubes back in and check t...

Hi all, several questions in the same post, hope it won't be messing 1) on a blond, 5881 tubes new biased correctly, i was sending signal keeping checking the bias V on the 1 ohm resistors.. while increasing till full power (25v on output which is a lot by the way), the current increased in the tube...

What voltage do you get with the tubes removed and the standby on "play?" This will give you the correct no-load condition. With no reservoir capacitor connected you are measuring average DC of a FW rectified sine wave. i can't disconnect caps without power tubes 480 v. premap tube plugged. I loose...

Where exactly is your standby switch? When you are measuring the B+ in standby is there a reservoir capacitor connected at the rectifier output? Post a schematic of what you have built. no I have standby switch just out of rectifier I removed diodes now before rectifier, on startup I have 380 then ...

last update

on the 2 diodes 1N5408 i have DC drop 10v without the rectifier tube.. 20v with the tube.

I guess the diode have an issue.. diode tester give them at 0.5

I will remove and check.

unless I forgot in my electronic lessons.. I should not have more than 0.5 DC drop

A schematic with your mods might be more helpful here. Not assuming you've done anything "wrong" just, it's a whole lot easier to reference from. What is the voltage at the speaker output jack? With a signal and the amp dialed up to clipping. yes thanks for not assuming, but I first do assumed that...

Stevem wrote: ↑Sun Jul 28, 2024 11:16 am The low V+ is why I asked what is the idle current the output tube is pulling.

not much I have 16v on the cathode resistor.

so 300 - 16 / 250 ohm

I will test something I'm suspecting that maybe I lost 1 of winding in high voltage transformer (dead diode or bad solder or bad tranformer)

Stevem wrote: ↑Sat Jul 27, 2024 6:49 pm Can you post up several good pictures of the guts?

I will look but it is difficult to see around power tubes.. due to added switches on the rear

no comments on the low B+ voltage ?

martin manning wrote: ↑Sat Jul 27, 2024 2:06 pm

fredouille wrote: ↑Sat Jul 27, 2024 1:52 pm interesting where this 50hz could come from.. filament?

Most likely. Rectified DC voltage would be 100 Hz, and sawtooth in waveform.

yes.. so this 50hz is caught by induction from filament, still the level is low.. 0,0Xmv..

update for myself in did negative feedback issue solved by crossing wires debugging noises a bit. none of the B+ have any V AC, neither anything visible but see attached blue is output.. no signal injected, all the ground, volume a 0 and tone at the middle yellow is where I catch first time 50hz in ...

Hi all, I built directly a modulus kit with mods.. I have small issues on which your comments are welcome I moded the diode protection + diode switch.. the pre rectifing diodes make me loose a lot.. 400v with no 6v6 and 300 loaded !!!... 450 with diodes and 380 loaded.. a bit low no ? looks like not...