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What do you think of this mashup amp?
What do you think of this mashup amp?
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Old 8th February 2018, 02:50 AM   #1
zacster is offline zacster  United States
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Default What do you think of this mashup amp?

I'm looking to build this amp out of the parts I mostly have on hand (a few ICs are missing). It is a mashup of my 6B4G Dynaco based output section, the Impasse voltage regulator and CCS for the driver, and the Amity for the signal path via input and interstage transformers. It would all be built onto my ST-70 chassis where much of the circuit already exists. I would mostly replace just the driver board, and even there I would keep the VR since the Curcio driver I have was adopted for the Impasse as well. The driver tube itself would be the cheap and plentiful 6N6P, also already being used in the Curcio board in place of a 6DJ8. It would all fit, with the big transformers on the top and the little 1544a underneath.

I still have to determine some resistor values but that is about it. It all seems too simple to me, but I've been looking at these designs off and on for years without actually building anything. Truth be told I'm still happy with what I have but need something to do. The additional cost to build won't exceed $50, so that isn't an issue.

Please excuse any misuse of symbols as I've never used the software before. I couldn't find a DHT symbol either. Also I've excluded the power supply, but it is still an ST-70, with maybe some small mods that Curcio included in his quad cap replacement board. No selenium rectifier either, that was for bias IIRC, and I'm using cathode bias.

Any comments/advice?
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Old 8th February 2018, 04:37 AM   #2
john_tracy is offline john_tracy  United States
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I used to use 6N6Ps in my differential transformer coupled line stage (50mA@150V) and they would last ~6 months. Switched to 6N30s, last 2 years and sound better. Granted they are more expensive and you will also have to screen for matched pairs (same as with the 6N6Ps). Also, a neg. grid bias of ~6.5-7V creates more dynamic headroom.
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Old 8th February 2018, 01:05 PM   #3
kodabmx is online now kodabmx  Canada
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Considering the OP of the 6N6P is 28ma@120V I'm not surprised they only lasted 6 months... That's 7.5W from a tube rated at 4.5W or 8W both sections!
I've been using them at 22ma@140V in a headphone amp, and they've lasted for years so far.

Last edited by kodabmx; 8th February 2018 at 01:07 PM.
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Old 8th February 2018, 04:00 PM   #4
john_tracy is offline john_tracy  United States
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That was 50mA total or 25mA/section. I used a cathode CCS "tail" with the B+ CCS fed shunt reg. The tubes would initially have ~-3V grid bias. When they were "done" it had dropped to less than -1V. 6 months to a year max.
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Old 8th February 2018, 04:46 PM   #5
audiowize is offline audiowize  United States
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The CCS in series with the CT of your interstage transformer isn't serving any purpose. You could use the CCS to feed a VR 75 + VR150 regulator tubes in series to make +225V regulated for your driver.

If you have two 6V windings and you use two separate cathode bias resistors/caps, then you get nice DC current balance through your OT.

You could dump T3, ground one grid of the 6N6 and put a simple CCS under the common cathodes. This will improve amplifier performance and drop the cost considerably.

You are running the 6N6 too hard. Increasing the bias voltage would give more compliance to a CCS/differential amp, so that's worth considering.

If the voltage regulator and CCS are all crammed onto a module and you must use them, you could setup the 6N6 as paralleled voltage amplifier that's CCS loaded then use the interstage transformer as a parallel feed phase inverter.
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Old 8th February 2018, 04:59 PM   #6
Lingwendil is offline Lingwendil  United States
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What do you think of this mashup amp?
Quote:
Originally Posted by audiowize View Post
The CCS in series with the CT of your interstage transformer isn't serving any purpose. You could use the CCS to feed a VR 75 + VR150 regulator tubes in series to make +225V regulated for your driver.

If you have two 6V windings and you use two separate cathode bias resistors/caps, then you get nice DC current balance through your OT.

You could dump T3, ground one grid of the 6N6 and put a simple CCS under the common cathodes. This will improve amplifier performance and drop the cost considerably.

You are running the 6N6 too hard. Increasing the bias voltage would give more compliance to a CCS/differential amp, so that's worth considering.
Agree with all of this. Separate cathode resistors will be better at keeping things balanced versus a single anyway.
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Old 8th February 2018, 05:37 PM   #7
john_tracy is offline john_tracy  United States
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If you want the highest quality sound you need to worry about AC balance also. IMO the CCS belongs on the cathode were it can enforce AC balance. That will entail a negative supply and require matched tubes. As to a CCS fed shunt regulated B+ supply: VR tubes? Yuck! Look at the Salas shunt reg. or the K&K Audio version. It should be noted with this arrangement that as the tubes age the grid bias voltage will drop cutting into signal headroom. With the CCS on top as drawn the bias current and grid voltage are fixed; the current source will set the plate voltage and as the tube ages it will rise.
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Old 8th February 2018, 06:27 PM   #8
audiowize is offline audiowize  United States
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Quote:
Originally Posted by john_tracy View Post
That will entail a negative supply and require matched tubes.
Driver or output? AC balance with a CCS negates the need to have matched triodes in the driver half. If 10V of bias is used, I would debate the need for a negative rail.

Quote:
Originally Posted by john_tracy View Post
It should be noted with this arrangement that as the tubes age the grid bias voltage will drop cutting into signal headroom. With the CCS on top as drawn the bias current and grid voltage are fixed; the current source will set the plate voltage and as the tube ages it will rise.
The grid voltage is too low as it is. Setting it closer to 10V will reduce plate dissipation a bit and provide some flexibility as the tubes age.
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Old 8th February 2018, 11:56 PM   #9
john_tracy is offline john_tracy  United States
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The CCS I use for 50mA current requires ~30V. You can run the 6N6P at lower bias current, but I do not care for the sound.
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Old 9th February 2018, 12:45 PM   #10
zacster is offline zacster  United States
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Let me ask my question a different way. Will it work? It sounds like I can then refine it from there.

For VR tubes, I have a quad of 0C3 that I could use (I was going to build the Amity from scratch, which uses these). And what was recommended above goes back to the same concept except slightly different values.

I don't have separate 6.3v windings to separate the cathodes of the power tubes, what I drew is what has been working for 10 years now.

I already have the transformers, so I will use the input phase splitter. I've been watching the Swap Meet and eBay and snapped them up when I saw them.

Since it took me years to get to this point, other priorities and all that, and this was to be my project for the winter and it is already February, maybe I'll just stick with it the way it is. I'll just give it a try on a breadboard and see how it sounds and take it from there. There really aren't that many parts.
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