• 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.

Tube mod a HT rcvr?

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I was recently given two HT receivers one is an Onkyo TX-SV525 which appears to be fully functional except that the vol control buttons on the remote are not working. It only has prologic but the power amps seem pretty decent as far as HTs go.

The other one however is a Sony STR-DE597 which probably has pretty crappy PAs but it is giving a protect error anyway so something in the output stage is probably toasted anyway. The nice thing about it is that it supports more of the digital surround formats.

As I was deciding whether to repair the Sony I was musing over my desire to build a HT and stereo system which uses tubes as much as possible. I have always lamented the lack of affordable decoding. Well it struck me today that I may hold in my hand the answer to the problem (or at least a first pass at it).

Why not suck it up and buy a schematic/service manual for this thing and just remove all of the offending analog sand and replace it with tube line stages and maybe a stereo PA for the mains. I could still use the digital control circuitry for the remote and the volume control on the surrounds as well as the decoding functions. I think that it uses a motorized pot for the main volume but I am not sure exactly ow things are implemented.

Has anyone ever tried something like this.

at the very least you're going to need space for the much bigger transformers and the tubes themselves. if this home theatre boxes are hard steel cases you may be able to mount all of this above, but figuring out how to interface with the existing circuits will be hard. Some of those will have silicon in the signal path, without a doubt. Your best bet on this ambitious project would be to replace just the power amp board, if it's seperate, with a standard stereo tube amp. It would be easier to have a seperate box for this, and run leads from the preamp/decoders.
planet10 said:
Probably best to consider just making the receiver into an HT pre-amp. Removing the heat sinks and o/p stage should give room for tubes & appropriate power trafos for LS. Then use whatever poweramps you want and change at will.


Totally new PSU circuitry is probably unnecessary. ;) The 6GM8 twin triode operates at a low voltage and makes a decent cathode follower. 6GM8 data sheet here. Regulate the existing bipolar PSU rails down to the voltages needed by 6GM8s and the supporting CCSes. AES catalog # P-TB9-2-2 is a 2X 9 pin mini socket plus terminal board that might be very useful for this job.
You are probably right Planet10. More flexibility that way for sure. I liked the mental picture of a Sony HT receiver with a bunch of tubes sticking out the top. What a hoot. Maybe I should make up a picture in GIMP. :)

That 6GM8 looks like an interesting tube but it sure can't swing much input voltage. I suppose one could attenuate the input first if you were using it as a CC. Of course I guess as a CF you would not need a lot of swing there since the cathode will be following the grid. Presumably with 25V on the plate one would be able to swing enough at the output for a line level buffer.

Are these tubes commonly available (I presume NOS and used only)?

They might find some use in other projects where high voltage is undesirable such as teaching my grandkids about tube circuits.

Are these tubes commonly available (I presume NOS and used only)?


The 6GM8 is out of production, but unused specimens are available at reasonable cost. Jim McShane's site shows 9X new old stock (NOS) 6GM8s @$9.50 each. AES and RES probably have 'em too.

30 V. on the positive rail, 35 V. on the negative rail, and a cascode constant current sink (CCS) load set to 3 mA. should be fine. Cathode follower O/P impedance is approx. 385 Ohms, which is definitely respectable. As long as interconnect cable capacitance is kept reasonably low, driving just about anything downstream appears OK.

Use stoppers on 6GM8 grids. The type is closely related to the 6DJ8 and can oscillate quite easily.
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