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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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 Post subject: Re: Baxandall circuit
PostPosted: 26 May 2019, 20:35 
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Joined: 23 Feb 2017, 02:02
Posts: 555
If any resistor blows thats likely the schematic or installation plorbem, resistor defects are extremly rare, let along say cheapo transitors from china cost you $1USD for 200-400pcs, never had any defects or plorbems from these cheap like for free transistors


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 Post subject: Re: Baxandall circuit
PostPosted: 05 Jun 2019, 07:53 
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Joined: 07 Oct 2010, 14:15
Posts: 10
Hi
Just checking ....I have 4S I built some time back in storage which worked perfectly. I drifted to vintage Solid State over the years as I found I needed EQ but am now drifting back to valves/tubes. I don't need a defeat ....so is the first circuit posted in this thread still valid to mod my 4S to add tone controls?
Thanks
Mike


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 Post subject: Re: Baxandall circuit
PostPosted: 05 Jun 2019, 09:44 
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Joined: 13 May 2016, 02:02
Posts: 131
Location: Javea, Spain
I've been playing with variations of this circuit for nearly a year now. The best circuit is on page 3 of this thread. If you use one tube for the 2 buffers and one tube to gain back your missing 20 dB on each channel, you can still tube roll. I rewired my 4S rather than build from scratch,and it worked fine. I didn't wire in a defeat switch on my last build.


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 Post subject: Re: Baxandall circuit
PostPosted: 05 Jun 2019, 09:58 
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Joined: 07 Oct 2010, 14:15
Posts: 10
Thanks John!


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 Post subject: Re: Baxandall circuit
PostPosted: 06 Sep 2019, 06:02 
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Joined: 13 May 2016, 02:02
Posts: 131
Location: Javea, Spain
A bit of help please. This is the third tone stack pre-amp I've built so I should know what I'm doing, but it appears I don't.
I have a 4S pre-amp and a 6V6 Lacewood V2 built in one chassis and I've rewired the 4S pre-amp as a tone stack pre-amp.
I've wired up the tone stack without a defeat switch as per the schematic below.
The outputs go directly into the grids of the 6SN7 in the 6V6 Lacewood, as I have two volume controls on the tone stack.
I did all the normal checks for voltages and fired it up. Nothing.....
I've fed in a 1000Hz signal at 2 volts and this what I can see on my 'scope. Why on earth would the output voltage be so low. I expected to see it drop in the tone stack and then return to 2 volts at the output.
The 6V6 Lacewood is working fine and I've checked the wiring numerous times. Changing valves doesn't make any difference and I've buzzed out the valve base to make sure all connections are good.
Anyone got any ideas?


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 Post subject: Re: Baxandall circuit
PostPosted: 06 Sep 2019, 09:52 
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Joined: 13 May 2016, 02:02
Posts: 131
Location: Javea, Spain
More info....
I used a valve base that sits inside a copper fitting. All leads are soldered on and then heat sleeved, then the whole thing is potted in araldite. Before I used it, I checked for continuity and shorts. All good. Could this problem be something to do with this? As I can see 125mV right up to the grid on the second valve in the circuit, does that guarantee the problem is after this voltage reading?
I also haven't included the 33uF bypass capacitors, but that should only drop the gain a little?


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 Post subject: Re: Baxandall circuit
PostPosted: 06 Sep 2019, 15:36 
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Joined: 23 Aug 2008, 06:51
Posts: 31
What input impedance is your 'scope? I would say it needs to be 10M at least


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