6L6 quad push pull monoblock project - diyAudio
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Old 14th April 2017, 06:59 PM   #1
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Default 6L6 quad push pull monoblock project

Dear all

After having finished my two bi-amp monoblocks with two quads of 6V6s on each monoblock, i think about a new project. I still have some tubes left, 6SJ7, 6SN7 and two matched quads of 6L6GC.

I would like to make two monoblocks with a 6SJ7 voltage amp, 6SN7 LTP and a quad of 6L6GC in push pull, run on approx. 400V on plate and little less on screen.

- as power tranny I would use a Hammond 378CX (400-0-400v 534mA, 6.3v 6A to heat 6SJ7 and 6SN7)
- for heater power of 6L6 quad a Hammond 266S6 (6.3v 10A)
- for rectifier heaters a Hammond 266S6 (6.3V 10A with a 0.2R/10W resistor to get down to 5V), since there are no 5V rectifier trannies available for 220V primary - or does anybody knows a 5V 8-10A rectifier heater tranny with 220-240V primary? Or go solid state rectification?
- Hammond chokes 193P 5H/500mA and 193Q 10H/500mA

The big question for me is the OPT, Hammond only has a 120W 1900ct OPT for push pull, they say it works with a quad of 6L6 on their web site.
Are there any suggestions for other OPTs with approx 120W and 3300ct (i would like trannies with bells for visible mounting).

Attached you will find a schematic for what I am planning. The LPT stage is exactly the same as I used it in my 6V6 quad amp. The quad stage has just adapted cathode resistors.

I would appreciate a review, especially of the 6SJ7 stage and the 6L6 quad stage cathode resistors.

As soon as I know that the 6SJ7 stage is correct, I want to add a feedback loop with adjustable feedbach to the 6SJ7 (proposed by rojoknox when dicussing my 6V6 amp).

Thanks a lot for your support.
Attached Files
File Type: pdf new 120W Amp_14_04_17.pdf (206.3 KB, 276 views)

Last edited by thomasweissbach; 14th April 2017 at 07:25 PM.
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Old 14th April 2017, 08:01 PM   #2
smbrown is offline smbrown  United States
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Hmmm. your B+ of 400v appears to be going through two paralleled series caps? I think those should be referenced to ground, will not pass much voltage as shown.
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Old 14th April 2017, 08:16 PM   #3
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Hi smbrown, you are right, of course. This is a mistake. Thanks. Unfortunately I can no longer withdraw the schematic with the mentioned mistake. I will replace it tomorrow. Thomas
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Old 14th April 2017, 10:14 PM   #4
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Location: Canby, Oregon

There is a couple errors in you schematic. One, the 6SQ7 is a triode and not a pentode as shown. The 6SQ7 also contains a couple of diode normal used for a detector which will not hurt anything but should be disabled. Second, the filaments for the two 5U4's should be in parallel and not series if being powered via a 6.3v winding with a dropping resistor.

A couple of other recommendations are to add a dedicated grid leak resistor to the control grid of the 6SQ7 and ground. This will keep the tube bias assured even if pot gets a little dirty over time and itermittently goes open. Also, do not ground both ends of the shield cable going to the input connector. This will help assure no ground loop currents at the front end.


Last edited by Mickeystan; 14th April 2017 at 10:18 PM.
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Old 14th April 2017, 10:22 PM   #5
djn is offline djn  United States
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where is B2 coming from?
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Old 15th April 2017, 06:04 AM   #6
kward is offline kward  United States
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Max power with quad 6L6's in PP AB1 config given conditions you've described will give you maybe 60 watts output with fixed bias and probably more like 50 watts with cathode bias. Anyway this might open up some other possibilities for output transformers.
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Old 15th April 2017, 07:17 AM   #7
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Dear all,
you'right, there are some copy paste mistakes in the schematic which I took from my other project...
I want to use a 6SJ7 as voltage amp, the 6SQ7 on the schematic is wrong.

I have changed the filament supply of the 5U4GB to parallel, as proposed by Mickeystan and changed the grounding od the shielded input cable.

The cap on B+ is now star grounded.

Concerning kwards input of the lower output of the 6L6 quad, could it be increased by higher B+? This would mean using a pair of 5AR4 instead of the 5U4GB, this would give some extra power on B+.
How should the set up be to get lets say 40W from a push pull pair and around 80 from the quad?

Thanks a lot for your inputs.
Attached Files
File Type: pdf new 120W Amp_15_04_17.pdf (207.5 KB, 82 views)
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Old 15th April 2017, 10:24 AM   #8
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If you don't mind a little sand in your amp, you can check out George's powerdrive experiments with a pair of 6L6GCs producing 90W in AB2: 500V anode, 400V g2, 3k3 load. Still 50W in triode mode AB2.
Universal PP Driver Board | Tubelab
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Old 15th April 2017, 01:03 PM   #9
MelB is offline MelB  Canada
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You have almost 15 Kg of chokes in your design. My advise would be be scale back on the chokes and add some more micro farads.
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Old 15th April 2017, 01:31 PM   #10
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Why use an extra transformer for heating the 6L6s when the main PT has enough capacity?
For what i have read about Hammonds, they need to be properly loaded or they'll put out more voltage.
Also, when you go for vacuum rectification, make sure the heater winding for the 5U4s or GZ34s has a DC rating as well. The winding will have the full rectified dc while the core is grounded.
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