John Broskie's Guide to Tube Circuit Analysis & Design

Letter from Enro Borbely

 29 Nov 2002

Dear Editor,

I have received a copy of pages 20-27 of Vol. 3, Number 2 of your magazine from one of my customers. It is related to the Hybrid Tube/MOSFET amp design, that your contributor attributes to an Italian guy by the name of Generoso C. I am afraid he was criticizing the wrong guy for the design, because I designed the original Hybrid Tube/MOSFET circuit; he just copied my topology 100%. My circuit was published in issue 1/98 of Glass Audio , see copy of the original article on my homepage under ARTICLES ( Nothing is wrong copying my circuits for private use, but it is illegal/unethical to produce/sell somebody else's design. I complained to Editor Ed Dell about this and he sent several notes to Mr. C, but received no answer.

Just a couple of comments re your contributor's mod note. He is referring to my MOSFET design in Audio Amateur and calls the circuit the LINDER circuit. The guy's name was LENDER (Rudolf Lender, we were working at Motorola at the time, he was a Field Application Engineer in Germany and I was working at the Central Application Lab in Geneva, Switzerland). As for the modifications he suggested, I tried to make a very simple circuit, which I think worked very well with the current mirror. I think this was the simplest way of generating enough current to drive the input cap of the MOSFET. I left the generation of the second harmonics to the SE common source second stage, which also worked well, :-)) I guess I prefer a controlled/acceptable amount of harmonics, even of the second harmonic variety. As for the suggested N-channel MOSFETs as the active ones instead of the P-channel, I am using Hitachi and Toshiba MOSFETs and have not seen any major difference between them in my designs. This might be the case with the IR MOSFETs, however they don't sound as good as the Hitachi or Toshiba in my opinion anyway. By the way, I have just designed a headphone amp using the same topology, however, I replaced the input tube with a dual JFET, see description on the homepage.

Best regards,

It is an honor to meet you, even if it is over the net. I have read all your articles in Audio Amateur and I have enjoyed and learned from them greatly. Twenty years ago, I even built a pair of your 60 watt MOSFET amplifiers for a friend (very nice sounding; we used a 400 VA toroidal power transformer).

A thousand apologies to Rudolf. I have built a complex I-to-V converter based on his design, which worked quite well.

The sincerest-form-of-complement version of your circuit is popular and I bet it must madden you to see someone else spoil your design and make money by doing so. But then, I am not the one ask about circuit design ownership or patenting, as I am not sure that patenting of topologies should be allowed anymore than I believe sentence structure should be copyrightable. Now, using your name would be an altogether different case and thus we can see the genius of Ray Dolby copyrighting the name, not the circuit. (I have been told that some of the remakes of famous amplifiers which were printed in this journal have been faithfully implemented — without any credit to me, of course.) In general, I believe designs and topologies are not as valued as much as is hype. I just read in a business book at the bookstore yesterday that many people make the mistake of putting 90% of their effort into creating a better product and only 10% into marketing, rather than inverting the ratio! This philosophy sickens me, but it is probably closer to success than I wish to acknowledge.

Single-ended hybrid amplifier with DC-servo loop

 Power supply for amplifier above

As for your design, I am glad to see someone using the 6GM8. (I wish there were more grid-frame tubes.) Below is the circuit I would try to build today, if I had the time. The advantages of DC coupling are retained and the higher rail voltage for the tube could easily be derived with the voltage tippler circuit I outlined in the that issue of the Tube CAD Journal you received. As for the headphone amplifier, I bet the FETs are much more quiet. About a year ago, I received a schematic and two pairs of matched FETs from a kind reader. The simplest HP amp possible:


It works quite well; the IDSS is about 10 mA and it sounds great with 300-ohm headphones. (If only there were some P-channel depletion-mode MOSFETs.) If 32-ohm headphones need to be driven, then the following circuits might be better choices.


More email on Erno’s amplifier topology

I had been collecting parts to build the tube-MOSFET hybrid amp for a while and came across your article in the March 2001 issue. I can't pretend to understand it but given the thought put in to each individual aspect of how and why this circuit works it would make more sense for me to build your revision and start from there. There are several parts that you have not identified on your final remake. Can you please give me the value of C1 and the unlabelled cap and diode in your revision? I have collected too many parts now to not build it. Also, would there be any component value changes should I opt to use 6 output devices instead of 4? Thank you for the time you put into explaining these circuits.

In all honesty, I cannot recommend building any variation of the circuit that was stolen from Erno Borbely (visit The two tricks that made the circuit attractive are low plate voltage and direct coupling. The problem, however, with the circuit is low plate voltage and direct coupling. The low plate voltage is incompatible with low distortion and the direct coupling is incompatible with safety, as at startup, huge DC offsets are likely. My recommendation is to build an amp that uses a separate high-voltage PS for the tubes and AC couple the tube to the MOSFET. Alternatively, you might experiment with a simpler version of the original circuit, one that forgoes using the current mirror. Below is a DC coupled amplifier that might prove safer, if the time constants are set to match the tube’s delay in heating up.

But a better plan might be to use a more elaborate scheme wherein the output will not slam negatively when the tube fails or is removed from the powered circuit. In the circuit below, the bottom MOSFET relies on the tube to be conducting to energize its auto-bias circuit. If the triode is pulled out of its socket, the top transistor stops conducting and the bottom MOSFET loses its turn-on voltage.

The simplest and possibly the best plan might be to do away with the active constant current source altogether and use an inductive load instead, as the efficiency of the amplifier will double and the amplifier will not destroy your speakers, if the tube is jiggled or removed from its socket. (If the choke’s DCR is higher than a third of an ohm, a coupling capacitor might be needed at the output.)


The end
Well, before I get the urge to draw anymore schematics, let’s stop here. Let me know if you like or dislike this blog style. (I expect quite a few of you will be disappointed that you cannot as easily print what you read.)





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In general, I believe designs and topologies are not as valued as much as is hype.






















































































The simplest and possibly the best plan might be to do away with the active constant current source altogether and use an inductive load instead, as the efficiency of the amplifier will double





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