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GU-46 push-pull & single-ended amplifiers
GU-46 push-pull & single-ended amplifiers
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Old 9th December 2017, 08:53 AM   #1
thomasweissbach is offline thomasweissbach  Switzerland
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Default GU-46 push-pull & single-ended amplifiers

Dear all

I am currently thinking about building a GU-46 push pull amplifier (two monoblocks).

As OPT I am thinking about a 5k Ohm 200W transformer, custom wound by ogonowski.eu, he already made several transformers for me. I am very satisfied with the quality of his work.

Can anybody comment in the impedance of 5k Ohm for a push pull OPT for a pair of GU-46? I have seen one other (russian single ended) project using a 5k OPT.

Intended anode voltage 1000-1100V (0.5A), grid 2 600V, neg. bias around -120V (to be evaluated).

I think about using the following set up for loading of the GU-46
First stage: phase splitter 6SN7
Second stage: for each phase a 12hg7 voltage amp (something like pmillet)
Feed into GU-46
I plan to use coupling capacitors between each stage.

I have not yet decided on the PSU.
The high voltage cirquit for the GUs shall have its own transformer (approx. 800-0-800V 1.2A), the rest of the tubes shall fed by a smaller (approx. 350-0-350V 0.3A, 6.3V CT for the heaters).
The heaters for the GUs requires 8.3V at 16A for a pair (will need to be decoupled in pp) coming from an LED Power supply most probably. The negative bias will have its own transformer (approx. 100V 0.3A).

I would love to use paralleled mercury rectifier tubes for the high voltage rectifier, but this would need additional iron. Alternatively the use of bridge rectifiers at 1600V/30A or so could be used. The lower voltages shall be rectified by one 5U4G per block and the neg. bias by diodes.

I have not yet drawn a schematic, but would be very interested in your professional feedbacks. This Project is a bit bigger than all I did before, early Input is very welcome.

Greetings from Switzerland.
Thomas
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Last edited by thomasweissbach; 16th December 2017 at 06:22 PM.
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Old 9th December 2017, 11:10 AM   #2
Eli Duttman is offline Eli Duttman  United States
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Thomas,

Hybrid bridge rectify the GU46 B+. 800 V. is not nearly as dangerous as 1600 V., end to end. Two of this Schottky and a pair of 6CJ3s should get the job done for you.

Oops, 4 6CJ3s, in paralleled pairs, as that transmitting type is a definite brute.
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Last edited by Eli Duttman; 9th December 2017 at 11:15 AM. Reason: added paralleled 6CJ3 remark
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Old 9th December 2017, 12:07 PM   #3
thomasweissbach is offline thomasweissbach  Switzerland
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Hi Eli
I haven't seen your only now. I have drawn a couple of schematics, your Input not yet included.

Schematic for one mono amp, one schematic for the GU-46 anode voltage PSU, one for the rest of the tubes, and one for the negative bias supply.

You are proposing to use a hybrid rectifier composed of the above mentioned Schottkys and two pairs of 6CJ3s for one chanel. I wrote this down in my notes.

Greetings, Thomas
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Old 9th December 2017, 02:38 PM   #4
Ketje is offline Ketje  Belgium
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You made some errors and probably better to take 300W for the OPT.
Mona
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Old 9th December 2017, 03:47 PM   #5
thomasweissbach is offline thomasweissbach  Switzerland
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Hi Mona
Thanks a lot for your corrections and the Input
Greetings, Thomas
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Old 9th December 2017, 04:26 PM   #6
banat is offline banat  Serbia
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Hi Thomas

Here`s one very good way to obtain B+1,2KV and Ug2+0,6KV for OPS , that 0,6KV line can be used to supply input and driver stages to, HV secondary is just 2x450VAC , S-By switch need to be one very rugged HV type ,same as B+ HV fuseholder and fuse ,
have no experience with GU46 power pentode but it look as one very interesting tube to me ,
personally I will try with simple three stage amplifier based on slightly modified Mullard 5-20 topology , maybe one 6J5 on input stage DC coupled on two EL84 as phase inverter/driver stage ,and further AC coupled on AB1 class biased GU46 PP-OPS ,
any way many very precision calculations is need to be done before you start making this very interesting and powerful amp.

Regards
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Old 9th December 2017, 04:45 PM   #7
Eli Duttman is offline Eli Duttman  United States
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Thomas,

Mona drew a full wave center tapped B+ setup. Nothing wrong with that, but 1600 V. end to end is exponentially more dangerous than 800 V. That's why I suggested a hybrid bridge. The Schottkys form the connection to ground and the damper diodes form the "hot" connection. The B- supply in "El Cheapo" is a hybrid bridge. You would invert all the polarities, for a B+ supply.

As far as vacuum rectifiers go, forward drop in damper diodes is low. Damper diodes are extremely slow starting, which allows the signal tubes to get fully warm.
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Old 9th December 2017, 07:33 PM   #8
Ketje is offline Ketje  Belgium
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I agree Eli, but the TS stated "The high voltage cirquit for the GUs shall have its own transformer (approx. 800-0-800V 1.2A)".What Banat shows is much better, hope the TS will change his idea. Mona
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Old 9th December 2017, 10:58 PM   #9
Eli Duttman is offline Eli Duttman  United States
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Some time ago, Pete Millett publicized a method for obtaining 2 full wave rectified rails from a single CT winding. Only 5 diodes are needed. By using a combination of choke and cap. I/P filters, it is possible to produce rails that are other than n V. and n/2 V.
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Old 10th December 2017, 07:09 AM   #10
thomasweissbach is offline thomasweissbach  Switzerland
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Dear all,
I am adapting my PSU Idea to the proposal of Banat.
This schematic seems to be relatively simple, and since the trannies will need to be custom wound anyways, it will make sence to combine these in one single large tranny per monoblock.

I will also go for AC heated GU-46, as proposed by Banat.

Does anybody have any comments in the amp schematic itself? Banat proposed using Mullard 5-20 topology, from other sources I have the Info that the GUs should be loaded with low impedance. There was once a thread in this forum of a guy using pmillets schematic with 12gh7 as first stage, but in a SE topology.

Greetings, Thomas
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