• 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.

SE OTL amp - What do you think?

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi There.

I found this circuit on the web and I consider to give it a go. Have anny of you made before? What dou you think of the circuit. I was thinking of trying to use it to drive a set of Quad ELS.
Anny input is welcome

Thanks Peter Jensen



  • se otl.jpg
    se otl.jpg
    49.5 KB · Views: 955

A 2000uF/200v is bound to sound ugly whether you like cathode followers or not. And why a low voltage OTL for the Quad? You'll still be using it's step-up transformer which i never felt was briliant. If you really want to improve on the Quad go for either some dangerous high-voltage OTL or a custom o/p transformer which eliminates the Quad step-up.

best regards

Hi again

Well if you study the internet, severat OTL amps are listed as good amps to drive the quad´s. I know that the capacitor in the output stage is a critical component, but i still think the amp looks interesting.
If you forget about the quads and look at it as an amp for driving a dynamic speaker, it might be interesting to use it as a mid and or tveeter amp. The capacitor in the crossover vill remove the DC and you can forget about the 2000uf 200v beast. This only geos for a 3-vay system if used for both the tveeter and mid section.

Still not anny of you that have actually build this circuit?

MVH Peter Jensen
Why don't you just try it? I doubt if a 6SN7 will properly drive a 6080, but breadbording will take you an hour or so and at least you'll be able to make up your mind if it's worth building with blackgates at the o/p :) And the cathode follower... at least it's choke loaded.


diyAudio Senior Member
Joined 2002

I took a look at your circuit and have some remarks.
First of all I can say the 6SN7 will drive the 6080's with ample reserve,I have built a similar amp were it drove 8 6080's per channel.Also I 'd replace the 6SJ7 with a 12AX7.You will also need more power, 40 watts would be a good start.
A couple of years ago a well thought out OTL was offered in Glass Audio by Bruce Rosenblitt.I developped it a bit further to be used as monoblocks with twice the number of output valves,it has no dc blocking cap in the output ( you can still put it there if you're uncomfortable with the idea ) and has been working flawlessly ever since.
Also I prefer an electrostatic such as the Quads ( ESL 57's I presume ) to be driven by a push-pull amp rather than single-ended.Remember that in the very unlikely event of dc on the output (if you study the circuit and it's valves you KNOW this can not happen in the Push-pull OTL I suggest) the stepup transformer of the stats will block it nicely.
On the other hand people tend to overreact when they see such a huge electrolytic in the signal path but in this application it won't do that much harm.Moreover the Quads also have some in their signal path.Just see if you can hear the difference.


I also very take interested about this SE OTL Amp.
I send a e-mail to the web which post this circuit and they give me some comment :
1. use an EF86 for the input tube
2. use 4 -6 output tube. to devlep 8-12 watts
3. you can eleimate the 1mH & 600mH coil . They are used suppress RF interference

They told me it is a very good sounding amplifier.

Anyone really build it before ?

We will have reached Nirvana when someone builds an amp that dissipates over a kilowatt, has a source impedance of more than 20 ohms, puts out less than a watt, has more than 5% distortion, gets worse with reactive loads, and blows up on a regular basis. This is a step in the right direction.
The overrated capacitor...

A 2000uF/200v is bound to sound ugly whether you like cathode followers or not

This capacitor doesn't need to be 200 Volts rated...the choke will have a very low DC resistence to ground more the 20 Ohms resistor...we will end with 21 ou less Ohms to ground!

Even if you pass 1 Amp across it they don't have more than 20 volts across it...so being even is the safe side a 50 volt rated capacitor will do the job nicely...
diyAudio Senior Member
Joined 2002
Re:The overrated capacitor.


This capacitor doesn't need to be 200 Volts rated...

It does have a rating of 200V to protect the speaker in the very unlikely event of an output tube going totally south, i.e a complete short.

This is most unlikely to happen with a 6AS7 since the tube itself is fused internally.

What would be the output into 16ohms?

With my eyes closed, I'd say about 25W.

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.