• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Another compactron PP output

I'm planning another stupid project to recycle some of the odd parts I've got. The output tubes are 33GY7A, the power transformer is from a big solid state combo organ wreck (quite a beefy iron), the output transformer is toroid 15V with 120+120V primary. I have only one pair of tubes and one OT so this is going to be a guitar amp (mono = guitar, stereo = hi fi). Fortunately, I don't have compactron preamp tubes so I'm using some of the other semi-odd tubes I've got.

The preliminary schematics :
compactron_amp_v1_0.jpg


comments welcome!
 

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kevinkr

Administrator
Paid Member
I'd probably use AC to heat the filaments of your 33GY7A, the half wave rectifier based filament supply you have come up with probably is not going to work too well.

I note that the filaments use 450mA so I would put them in series across the entire secondary to reduce IR losses in the windings.

I'm not too crazy about your half wave voltage doubler either, but I think that running a full wave doubler on the same winding with the filaments might be pushing your luck in terms of filament to cathode insulation ratings, but you might just squeak by..

One other thought is put the fuse before the power switch, and only switch the hot side for safety. (I am assuming you are using the standard Israeli polarized ac plug?)
 
Volume and Gain controls should be 1Meg ...else they will load down the previous stage....
The screen resistors should be inscreased...since this is for guitar...when you go into square wave clipping the output stage...the tubes will act like diodes...and you would like the screens to stay lower in potential then the plate, or else you will start arcing over ...
Is the 1K and 47uF properly decoupling??? Is the amount of attenuation to the 120Hz adequate??
Those 250V filter caps....what will the voltage be durring start-up ??? Will those caps survive start-up how many times??

Chris
 
Amplifier with 33GY7A Compactron

Goodnight everyone!

This is my first post here on the site .

I always liked electronics, in particular for tube circuits (amplifiers) and decided to set up this scheme originally posted here:
http://www.diyaudio.com/forums/tubes-valves/120959-another-compactron-pp-output.html

compactron_amp_v1_0.jpg


My questions are:

1) Power supply: The potential drop in the use of them is of bridge rectifier 21 V it? Soon I will need a power transformer 150V, 130V for the input? The values ​​of capacitors are adequate to supply a good filter?

2) Excluding the gain and volume pots that will be exchanged for 1M, what more could be done to improve this circuit?

3) 6C4 The valve can be replaced with a 12AU7 (as I intend to build a stereo amplifier avoid using 2 6C4). There is an alternative to the valve easy to purchase 5670?

4) The dual capacitor / resistor that comes out of the cathode 33GY7A correct?
 

SY

diyAudio Moderator Emeritus
2002-10-24 10:19 pm
Chicagoland
www.SYclotron.com
To get any significant current running through the tube, the grid has to be biased up to a positive voltage. Let's say you want the idle current through the tube to be 1mA (which is pretty low). The grid resistor is 47k. The grid is likely to be a couple volts negative of the cathode (see the tube characteristics on the datasheet), so by Ohm's Law, it will need to be biased 45V or so above ground. That means capacitive coupling and bootstrapping if you want to keep the volume control in front of that stage..

If you don't have a copy of Morgan Jones's "Valve Amplifiers," run right out and get one. He treats these issues in great detail.
 
Top rectifier cathode may peak at 192VDC, but heater inside is tied hard GND.
Component spec sheet warns that 200V heater to cathode differential is max.
That leaves only 8V before something ugly might or might not happen....

I'm just saying, if you disconnect heaters from hard GND, and reference them
+75VDC or thereabouts, you get 75 extra volts of safety to play with. Others
may say a slight positive bias keeps the heaters from emitting electrons that
migrate to cathodes and become noise, I don't know how much of the latter
might be a fact or myth. For rectifiers, its only about the safety...
 
Last edited:
Top rectifier cathode may peak at 192VDC, but heater inside is tied hard GND.
Component spec sheet warns that 200V heater to cathode differential is max.
That leaves only 8V before something ugly might or might not happen....

I'm just saying, if you disconnect heaters from hard GND, and reference them
+75VDC or thereabouts, you get 75 extra volts of safety to play with. Others
may say a slight positive bias keeps the heaters from emitting electrons that
migrate to cathodes and become noise, I don't know how much of the latter
might be a fact or myth. For rectifiers, its only about the safety...
kenpeter what do you suggest me to correct this circuit so it will work perfectly.

Thank you.

Paul.