So I had this left-over challenge with my friend, make an amp in one day, with leftover components without lookin' on the internet for data and sheets.. (Of all the sins possible in the tube amp world I probably did them all in this build)
So... It is build from old radio tubes and transformers, EF89, EL86, EZ80.
I know that EF89 should not be preamp but that's all I had and I was in hurry with everything, BUT i was MORE than careful with high-voltage. (NOTE: OT from the radio had a million plugs on it but I just connected EL84 plate, B+ and speakers on the other side transformer(secondary).
So, here's the schematic, It is really badly drawn but i hope it is readable.
So... It is build from old radio tubes and transformers, EF89, EL86, EZ80.
I know that EF89 should not be preamp but that's all I had and I was in hurry with everything, BUT i was MORE than careful with high-voltage. (NOTE: OT from the radio had a million plugs on it but I just connected EL84 plate, B+ and speakers on the other side transformer(secondary).
So, here's the schematic, It is really badly drawn but i hope it is readable.
Problems: Tube amp sounds farty, when I start playing, sound is barley alive, it fuzzes and farts, and one of transformers(not sure if PT or OT, didn't want to touch them while the amp is on) produces this crazy 50/60 Hz sound that shakes the whole cabinet. Sounds like something is dying inside.
Uuhhh... Fixed the problem, moved the EF89, redisigned the circut, put 6N1-P instead of EF89, amp plays, but it is very quiet and I can't get it to work louder, the circut was designed for EL86, but handles EL84 well and I have never put EL86 in it untill today and old radio EL84 output transformer didn't handle it well. I will do the measurements in a few days, maybe today but I don't feel so comfortable pokin' around 400v caps while the thing is working. If any of you can figure out what is that silent thing going on with the amp, I would very grateful.
Huh... OK I still haven't managed to fix the amp, currently doing measurements, I measured that the secondary of PT is 540V both live AC leads going to the rectifier EZ80, is that huge voltage normal for such a small rectifier?
When I saw multimeter showing 540V between both red leads and 270 volts between center tap and red lead I turned of the amp the SAME moment
.
Did not want to measure along because I think caps might blow and who knows what could go wrong. Voltages are HUGE! Does anyone of you have the idea how to measure the cathode output (DC rectified output) of the rectifier without connecting output tubes and capacitors, and does the rectifier need to have the load as other tubes do?
And I also made a complete schematic of an amp in its current state.
Please HELP and reply quickly because my winter break is over this week (And yea I'm 15 for those of you who don't know).
When I saw multimeter showing 540V between both red leads and 270 volts between center tap and red lead I turned of the amp the SAME moment
. Did not want to measure along because I think caps might blow and who knows what could go wrong. Voltages are HUGE! Does anyone of you have the idea how to measure the cathode output (DC rectified output) of the rectifier without connecting output tubes and capacitors, and does the rectifier need to have the load as other tubes do?
And I also made a complete schematic of an amp in its current state.
Please HELP and reply quickly because my winter break is over this week (And yea I'm 15 for those of you who don't know).
Yea, the transformer secondary was measured without load: just tranny to 220V on primary and voltage meter on secondary (transformer was not connected to the rectifier)... Is that why I get high voltage readings?
Let me precise this: you are referring to power stages. It's not the tube that must not run without load. The problem with power amplifiers is the large inductance of an unloaded output transformer. This inductance causes potentially high voltages when a signal is applied, which may lead to sparks inside the transformer, and/or the output tube(s) and the tube sockets. A transformer will probably be destroyed.
As long as the filter caps of your power supply stand the idling voltage, you are safe. The EZ80 is at its limit, but should be ok as far as voltage is concerned.
Thank you! You have made it clear for me! Actually found out what was "oscillating" it was the pot that made super weird sounds and squeals, or is it somethig other? Got the amp going, redesigned the schematic a little bit, still drawing it. Amp works now as far as I don't move the pot (it is not fixed to the chasis) but the amp is still a little bit silent, does not sound like 5 watter, more like 0,8W or so, and also, the chasis is wooden should that be a problem?
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It's normal to see high voltages under no-load conditions. Power transformers are designed to give rated output under a certain nominal load.
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