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6L6 quad push pull monoblock project
6L6 quad push pull monoblock project
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Old 19th July 2017, 07:33 AM   #41
thomasweissbach is offline thomasweissbach  Switzerland
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Location: Meyriez
Hi Johan

Thank you for your reply. Your layout sound very interesting too.

I have not yet finalized the build, so there might be some room for changes in a second step.

Voltages are assumptions, I get an unloaded 550V DC when using the 240V tap of the power transformer, and 600V when using the 220V tap, both in combination with the paired 5AR4 rectifier tubes and my psu. These are measurements currently done on the psu. The screen voltage is regulated to 430V with Zeners. So I could come quite close to your specs. My negative bias should deliver -30 to -60V under load, I have changed the layout to using a seperate 55V DC /0.8A tranny for each section.

Do you have a schematic of your amp for comparison reasons?

Best regards and thanks for your reply.
Thomas
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Old 23rd July 2017, 09:08 PM   #42
DAK808 is offline DAK808  United States
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6L6 quad push pull monoblock project
Quote:
Originally Posted by thomasweissbach View Post
Just started building my amp.

Voltage regulator (for the output tube screens) and bias regulators will be placed in the upper compartment.

The upper compartment will be built to make it possible to exchange the tube sections to make it possible to use other tube types later, all DC and AC connections will be made with connectors.
Hi Thomas, nice looking trannys! Did you decide on the design of your screen regulator? I am looking for suggestions in that department for my next build. Good luck with your build and hopefully, no shocking developments
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Old 25th July 2017, 10:52 PM   #43
Johan Potgieter is offline Johan Potgieter  South Africa
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Hi Thomas,

Apology for forgetting your request for a schematic.

However, my topology is somewhat different from the normal fixed bias schematic, but that does not influence the figures given.

(The 'non-round' figures for the voltages given come from the fact that my 'fixed bias' is by way of an adjustable constant voltage (current sink) circuit in the cathodes. The purpose was to get the grid returns at earth potential for reasons of control not relevant here. Taking an average value of 44V for cathode bias, it leaves the actual supply to the anodes at 600V and the screens (and rest of the amplifier) at a regulated 500V. This arrangement does not influence the basics of the circuit, which will give 110W after a high-efficiency output transformer.)
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Old 11th August 2017, 05:44 PM   #44
thomasweissbach is offline thomasweissbach  Switzerland
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Default got it running

Dear all,
I managed to get my 6L6GC stereo amp running, but from my point of view, the design is not yet optimized.
Compared to the initial plans, I had to make some modifications.
The planned 5AR4 rectifier tubes were arching and got burned during the first run, I replaced them with 5U4GB (accepting higher voltage loss, but B+ still well above 500V).
Instead of the planned 6SJ7 tube for the voltage amplification stage, I had to change to a 6SQ7 (since the 6SJ7 did not deliver enough gain).
The planned voltage regulation for the screens, using zener diodes (a first try with 4x 110V/10W in series and a second try with 2x 200V/50W in series) with using a 1.2k/10W respectively 800R/50W resistor lead to burned zener diodes
Leaving me without material to regulate the voltage -> I just tried to run the tubes on plate and screen at 515V, with success (at least for the moment).
Bias voltage currently is about -68V per tube.

General impression:
the amplifier works quite well. The sound (rock, classics) is clear, good mids and good highs, bass is ok (probably a bit week), compareable to my two 6V6 amplifiers made in the beginning of this year.

What needs improvement:
- the amplifier could be more powerful, I loose power/do not get the possible full gain somewhere in the 6L6GC output section (does the connected negative feedback consume all the power?)
- bass could be improved (probably change the first decoupling cap to 10nF?
- does anybody have an sample schematic with tubes or solids for a voltage Regulator for the screen voltage (440V at approx. 50mA per tube, means going down from 520V to 440V at a total of max. 200mA)?

I am open to any suggestions for improvement
Thomas
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File Type: pdf 6L6 quad AMP 11_08_17.pdf (377.5 KB, 85 views)
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Old 13th September 2017, 05:37 PM   #45
thomasweissbach is offline thomasweissbach  Switzerland
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Default nearly finished, one high frequency issue left

Dear all

i have nearly finished my 6L6WXT+ project, see attached pictures and schematic.

One side of the stereo amp works properly, the other is making an extremely high frequency tone (on the edge of my ability to hear). This tone comes up when the volume poti is turned up, with no signal attached (the same happens too when there is a signal). The tone is increasing in intensity with volume turned up and decreases and disapears with volume turned down. When the tone appears and the volume is turned up higher, the 6L6WXT+ tubes on this side of the amp start arching (not always the same tube, it happens randomly).

Unfortunately I have no scope to detect the source of the tone.

But it does not appear when either the 6SJ7 tube is being removed (and the rest stays) or the 6SN7 tube is removed and the 6SJ7 and the quad stays inside. But at the same time, moving the 6SJ7 to the other, working amp side does not bring the tone over too.

I therefore assume that it is no the 6SJ7 tube itself (or the 6SN7 tube).
What I haven't checked untill now is moving the 6L6WXT+ quad to see whether the tone is linked to these tubes (which could probably be too when one is somehow defective).

The 6SJ7 has approx. 180V on the plate and 80or so on the screen, the 6SN7 has 250V on the plate.
The 6L6WXT+ tubes have 500V on the plate and 440V on the screen, negative bias with 20mA per tube (measured across the 1R cathode resistor.

Does anybody know a possible cause to this very high frequency tone with unknown source which seems to be amplified by the 6SJ7 voltage amplifier, then passed on to the 6SN7 phase splitter, and to the 6L6WXT+ quad? The tone appears with no cable and no signal attached to the input socket.

Thanks for all replies.
Thomas
Attached Images
File Type: jpg IMG_0832.JPG (96.6 KB, 229 views)
Attached Files
File Type: pdf 6L6 quad AMP 13_09_17.pdf (384.9 KB, 64 views)

Last edited by thomasweissbach; 13th September 2017 at 05:42 PM.
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Old 13th September 2017, 06:05 PM   #46
smbrown is offline smbrown  United States
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I'd put grid stop resistors on the driver and phase splitter
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Old 13th September 2017, 06:24 PM   #47
thomasweissbach is offline thomasweissbach  Switzerland
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hi smbrown
In the Input grids of the 6SJ7 and 6SN7?
What value would you propose?


Beside that high frequency tone, the amp in the good side works great. There is no hum or anything. It has a very strong bass in my eminent speakers, maybe even too much. Mids and highs are great and clear, but different from my 6V6GC set up with 6SQ7 amplifier and 6SN7 phase splitter. Maybe a little less clear.

Thomas

Last edited by thomasweissbach; 13th September 2017 at 06:34 PM.
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Old 14th September 2017, 01:06 AM   #48
kward is online now kward  United States
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Looking good there, Thomas.

Try maybe a 10K grid stopper on the 6SJ7.

Also, you may be nearing or at the onset of HF instability due to the characteristics of the output transformer and the amount of feedback applied. You may need an additional network to reduce HF extension of the amp around the feedback loop. That typically looks like a cap in series with a resistor from the plate of the 6SJ7 to ground. If you need this, start with maybe 100 pF + 10K and adjust from there. The exact values are kind of important to get right. You would normally use a scope and signal generator to test for HF stability under various speaker loading conditions, and to find those proper component values.

Try the grid stopper first. See what that gets you.

How much feedback is applied?

Also, if you made the upper cathode resistor in the 6SJ7 stage 2.2K, and the lower cathode resistor 100R, you could probably do away with the 100 uF/50V cap in the feedback loop. A 10K FB resistor relative to 100R in that voltage divider is 1%--it will be negligible in skewing the bias of that stage. But that means you'd also have to adjust the FB resistor size to keep the same level of feedback, and perhaps adjust the FB capacitor slightly also to keep the same HF characteristics. I'd have to put pencil to paper to figure out the new resistor size to keep FB level the same, relative to a 100R resistor in the FB voltage divider.

Last edited by kward; 14th September 2017 at 01:21 AM.
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Old 14th September 2017, 04:20 PM   #49
thomasweissbach is offline thomasweissbach  Switzerland
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Hi kward

I have an adjustable negative feedback on the 6SJ7 cathode, using a 1k8 resistor and a 500R bournes poti, giving a total of 2k3 on the cathode.

The high frequency tone appeared with no feedback applied (or maybe 5R or less, poti at Minimum), and it only appears on one of the two mono sections (the amp has 2 identical mono sections), the other section works properly.
All resistors have been checked, all caps are according to my schematic, so the cause should exist in both sections. It does not, so this means I am somewhere in the edge to encounter a high frequency tone, generated by my Set Up.

In the good side I can turn the Feedback up to max and back to min at full Volume without any HF tone.

I will try the grid stopper first and see what happens.

If this does not help, the next step will be the HF ground in the 6SJ7 plate.

Thanks in advance.

Thomas
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Old 18th September 2017, 01:45 PM   #50
thomasweissbach is offline thomasweissbach  Switzerland
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Default Final Schematic of a running amp

Dear all,
to close this thread, I hereby attach the final schematic of the 6L6WXT+ stereo amplifier.
The bias setting of the amp is currently 20mA per tube.

The amp is completely humm free and has a clear sound, good bass (without the 20Hz high pass filter after the input signal poti, the bass could be increased), goods mids and highs.
Thomas
Attached Files
File Type: pdf 6L6 quad AMP 18_09_17.pdf (384.9 KB, 104 views)
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