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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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28th Mar 2023, 4:10 pm | #41 |
Nonode
Join Date: Jun 2010
Location: Spalding, Peterborough, Cambridgeshire, UK.
Posts: 2,851
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Re: 6V6 ECC83 push-pull amp project
If you read my post #5, it is explained there.
With that part number, IT IS A MODULATION TRANSFORMER! The 94 ohms winding is for modulating the high voltage dc supply to the vhf tx pa stage. The low z loudspeaker winding you are using here in your amp would have originally been designed to work with a low z public address horn loudspeaker mounted on a vehicle. Eg. Water board vehicle warning the water will be cut off shortly. Rob
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28th Mar 2023, 4:21 pm | #42 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
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Re: 6V6 ECC83 push-pull amp project
You can get the turns ratio by following the instructions here: https://how-to.fandom.com/wiki/How_t...rs_turns_ratio
Then use an online calculator to convert this to impedance ratio. The hardest part is finding a low voltage ac source. Having said this, I struggled to get very accurate measurements when I tried this myself with a multimeter. Remember to put the multimeter on the AC setting. If you are tempted to put 6.3v AC in the secondary, remember that it will translate to quite a high voltage on the primary. |
28th Mar 2023, 4:56 pm | #43 | |
Tetrode
Join Date: Feb 2023
Location: Shoreham-by-Sea, West Sussex, UK.
Posts: 88
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Re: 6V6 ECC83 push-pull amp project
Quote:
Modulation Transformer (9 K anode to anode and 3.5 K ohms secondary at 75 mA DC) The amp is a bit of a hotch potch of components - which does work! But definitely scope for upgrades, and like I said, Jim told me the output transformer was a little under-specced which is why the bias is lower than it would otherwise be. Could that explain why those grid leak resistors are wired in the way that they are? |
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28th Mar 2023, 4:58 pm | #44 | |
Tetrode
Join Date: Feb 2023
Location: Shoreham-by-Sea, West Sussex, UK.
Posts: 88
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Re: 6V6 ECC83 push-pull amp project
Quote:
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28th Mar 2023, 5:46 pm | #45 | |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
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Re: 6V6 ECC83 push-pull amp project
Quote:
Cheers, GJ
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29th Mar 2023, 8:23 pm | #46 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
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Re: 6V6 ECC83 push-pull amp project
That's a great idea GJ. Thank you
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29th Mar 2023, 9:32 pm | #47 |
Heptode
Join Date: May 2017
Location: Konongo, Ghana
Posts: 510
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Re: 6V6 ECC83 push-pull amp project
Did you take notice of posts #17, #23, and #33?
For use as a guitar amplifier the first stage has to have cathode bias which it doesn't have in your schematics (in them the cathode of the first stage is grounded).
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30th Mar 2023, 12:47 am | #48 |
Tetrode
Join Date: Feb 2023
Location: Shoreham-by-Sea, West Sussex, UK.
Posts: 88
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Re: 6V6 ECC83 push-pull amp project
I've incorporated a number of changes in the attached schematic. These include:
6V6 ECC83 p&p v3.pdf Last edited by RogerLLL; 30th Mar 2023 at 12:53 am. |
30th Mar 2023, 3:34 am | #49 |
Heptode
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
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Re: 6V6 ECC83 push-pull amp project
The MV pot is inside the feedback loop - which won't work as you intend - suggest you place it in front of the feedback stage (ie. after the treble pot).
Do you have a scope and a signal generator? That can help check when the output signal gets clipped or asymmetric or blocked from overloading - which may be related to the crackling sound effect. Also may be worthwhile swapping the 12AX7's around to see if that changes the crackling effect. |
30th Mar 2023, 6:48 am | #50 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
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Re: 6V6 ECC83 push-pull amp project
It may be worth disconnecting the negative feedback loop from the secondary and seeing whether this makes things better or worse. I had an amp that sounded like yours once and the problem was instability. It could be that the negative feedback loop is connected the wrong way round (positive feedback). Given that the feedback amount is tiny anyway it may not be enough to cause overt oscillation.
Another quick test is to measure the voltage (AC) on the secondary using your dmm and a resistor load and no signal and seeing if you get a reading - also suggestive of oscillation. If everything fails and you are stuck, try disconnecting your opt and running the signal through the opt I sent you. You can do this easily using croc clips without physically having to mount it. Obviously be careful regarding the high voltages. I've had a opt oscillate at 50khz once for no apparent reason (I think it was underrated for current) and the only option was to replace it Last edited by Gabe001; 30th Mar 2023 at 7:01 am. |
30th Mar 2023, 9:02 am | #51 | |
Tetrode
Join Date: Feb 2023
Location: Shoreham-by-Sea, West Sussex, UK.
Posts: 88
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Re: 6V6 ECC83 push-pull amp project
Quote:
I'll relocate the MV as you have suggested 👍 |
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30th Mar 2023, 9:04 am | #52 | |
Tetrode
Join Date: Feb 2023
Location: Shoreham-by-Sea, West Sussex, UK.
Posts: 88
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Re: 6V6 ECC83 push-pull amp project
Quote:
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5th Apr 2023, 11:59 am | #53 |
Tetrode
Join Date: Feb 2023
Location: Shoreham-by-Sea, West Sussex, UK.
Posts: 88
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Re: 6V6 ECC83 push-pull amp project
Had a very busy few days, and finally had time today to make some mods to the circuit, as in the attached. The aim was to simplify the circuit by removing the low gain input and incorporate just the master volume for now. However, I'm getting no output. I've attached the schematic. Have I made any errors in the circuit? I've checked voltages and all seem ok, around 130v HT and 90-100v at the plates. Any ideas?
6V6 ECC83 p&p v4b.pdf |
5th Apr 2023, 2:57 pm | #54 |
Tetrode
Join Date: Feb 2023
Location: Shoreham-by-Sea, West Sussex, UK.
Posts: 88
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Re: 6V6 ECC83 push-pull amp project
OK so that was a bit embarrassing, I missed the small ground wire on the master volume pot, and was using that tag for the signal... Now reversed and all is as it should be. Master volume works well and allows more control of the amp. Still have the crackle, but a better system to test the amp with.
One thing I'm not sure about is whether the MV should go to ground or to the junction between the 100k resistor of the NFB and the 2.7k to ground, as this is what is depicted in this link: https://robrobinette.com/5e3_Modific..._Master_Volume Next will be the OT swap and trying disconnecting the NFB. |
5th Apr 2023, 3:46 pm | #55 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
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Re: 6V6 ECC83 push-pull amp project
You have the grid DC potential of some valves coming entirely via pot sliders. That's a strong recipe for crackles. DC blocking capacitors and fixed resistor for bias paths combats this.... even if the DC being blocked looks like ground volts. Similarly with switches which cause floating or jumps in thermoelectric DC. Sounds silly or trivial, but it's enough.
David
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5th Apr 2023, 8:49 pm | #56 |
Tetrode
Join Date: Feb 2023
Location: Shoreham-by-Sea, West Sussex, UK.
Posts: 88
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Re: 6V6 ECC83 push-pull amp project
I tried disconnecting the NFB, but the volume dropped considerably, so I'll just leave it connected.
I swapped out the OT. Immediate improvement in the overall quality of sound. Break up is much more even, smoother and less harsh sounding. The "crackle" sound is still there. I'm convinced this is actually crossover distortion, having listened to a couple of YouTube clips demonstrating this. I know for a fact that the bias is set quite low due to limitations of the power transformer, and this could be why. I've ordered a digital oscilloscope so will check the waveform when this arrives. |
6th Apr 2023, 5:56 am | #57 |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
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Re: 6V6 ECC83 push-pull amp project
Volume dropped significantly when you removed nfb? It's connected the wrong way round then (providing positive feedback) which makes the amp unstable.
You could try increasing the current through the output valves and see how hot the transformer gets after 20 mins |
6th Apr 2023, 8:24 am | #58 |
Tetrode
Join Date: Feb 2023
Location: Shoreham-by-Sea, West Sussex, UK.
Posts: 88
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Re: 6V6 ECC83 push-pull amp project
Could it be that the NFB was wired this way intentionally to compensate for the power transformer? Can it be wired differently so as to provide negative feedback? Would this likely also reduce the volume? I know very little about NFB loops so will research this.
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6th Apr 2023, 10:16 am | #59 |
Tetrode
Join Date: Feb 2023
Location: Shoreham-by-Sea, West Sussex, UK.
Posts: 88
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Re: 6V6 ECC83 push-pull amp project
Another test I've just done was to try plugging into the 12" speaker of my Sessionette 75 amp, which is a great solid state amp in itself. It's a Celestion G12H-100 8 ohm, so a pretty decent speaker. No more crackle. I just didn't imagine it could have been linked to the speaker. The crackle sound certainly wasn't the coil rubbing or anything else to indicate a bad speaker. I have read that the right speaker can make a world of difference, makes sense, but wasn't expecting that. The speaker I was using was from a Fender Bullet 15W 8 ohm, so in theory up to the job, but there you go.
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6th Apr 2023, 10:26 am | #60 | |
Octode
Join Date: Oct 2019
Location: Stockport, Greater Manchester, UK.
Posts: 1,612
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Re: 6V6 ECC83 push-pull amp project
Quote:
If you take the feedback out and you're not satisfied with the gain, with the feedback removed, you can add a bypass capacitor across the 2.7k cathode resistor of v2a (as per v1a) |
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