• 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.

GU-81m tube amp schematics???

There are no need to push voltage more than 700-800V in any way. Or try to obtaine "more" power. 40-45W is huge enough for SE A1 class. (The guy who measure the tube in triode mode wrote that the abs max for G2 is about 700-750V.)
The key point for the GU-81M tube in triode mode IS LOW Req to drive low Primary inductance Output transformer.
And it is about 300 ohms !!!
From the sims it is 15Hy of the primary inductance needed for very very low end - read truly low bass without distortion and phase shift. Still has to be big and from quality, because of the higher current, and more gap again because of the current... This OT will be with thick wire, very low Rdc, Secondary even more tick wire, less Rdc... This leading to significantly higher damping factor than ususal SE designs. Relatively small number of turns will produce lower capacitances of the transformer so the HF will be good as lF too.
But is still more friendly to make than for other higher Ri tubes with significantly less undistorted and clear output power.
.
Bare in mind the capacitors voltages and rectifiers voltages and current, so it is the goal to make ALL tube power amp. So the "lower" voltages are better in this case.
.
One thing is more important - to obtain high input signal in Vp-p for power tube?
In my example is abot 280Vp-p value. And should be capable to cope the dynamic capacitances of the power tube. Generally low Req impedance driver. How low could be calculated and simulated...
.
If we want say 2Vrms (5Vp-p cca. ) as input to this power amp, than we need in this case, 280/5=56 X amplification gain pre-output tube... And with low distortion, less than power tube has for max power, otherwise there is no point... Maybe more input voltage because it is a power amp and should be used with Tube preamp?
.
Also should be designed to use same power supply without much voltage drop... To keep the dissipation and heat low...
 
Last edited:
814 in triode mode can be good driver for GU-81M. And probably it is possible to find the obtainable tube from desired -A, higher Va, and with a Cap? To have all tubes with a caps... :)
.
For rectifiers 3B28 is good. Or some similar.
.
I think that 813 can be used as driver tube also?
.
So bare in mind about DHTs. Active circuits for DH deserves more space and own bigger heatsink for pass transistors devices. This is a high current DHTs.
So the most rational way to heat the GU-81M is with DC (RC nets). There will be the less dissipation on the Rs...
 
Last edited:
to do triode operation with low output resistance is:
G2 and G3 connected to anode vith 300 to 600v supply 100mA current
G1 some negative bias, or maybe 0v bias for class A2
ideal output transformer ratio 100:1?
since I will use this amp only for bass up to 400hz my plan is the following:
use a microwave oven transformer MOT (230 to 2000v) as output transformer,
connect the speaker to the filament winding and anode to the 2000v winding
or connect the speaker to the 230v primary directly or via a second toroidal transformer 230v to 24v
making an actual ratio of 2000/24= 83:1
 
Last edited: