Building a Solid State Amp. Really?

Speedy454

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Back in the day when those power amp ICs came out, I messed with them quite a bit. I blew up a bunch of them, but back then they were pretty affordable and plentiful.
They do seem to need a good size heat sink if you plan on using more than a watt or two. But depending on the speaker, and sitting right on a desk, 1 watt could be pretty loud.

The FET preamp is the way to go. They can sound pretty good. You coud even copy some of the newer Fender solid state preamp designs and use the TL0-71 FET op amp chips to simplify things and make life easier. You may find them more stable, less noisy and more reliable than discrete FETs also.

From the previous preamp schematic, that design looks stable and clean. FET constant current sources driving fixed gain stages. On the one I currently use, I omitted the constant current sources and replaced the 1.2K resistor with a 200 ohm resistor in series with a 2K pot. By lowering the pot closer to zero and changing the drain resistor all the way down to 200 ohms, the first stage FET can get a little angry and aggressive. I had an old Univox amp with a FET input that had a Gain Boost switch that replaced the drain resistor with pretty much a short. It did boost the gain with somewhat tube like characteristics. But what did I know. That was 1977 when I was 17.
 

TequilaCaster

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chris m.

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You can simplify your JFET preamp by using the Wampler Plexi Drive circuit..

https://1.bp.blogspot.com/-rAWTuUNK1aM/TxDYkVflb9I/AAAAAAAAAg4/V81eRfQXHMY/s1600/plexi-drive.PNG

... to which the Benson Preamp is remarkably similar...

https://external-preview.redd.it/YR...bp&s=cf1d9b6ed1dbcc71e0f5d386f3943fa6b8b2e1ef

... and you can buy a PCB for either at PedalPCB.com for <$10.

Then Parts-express.com has a nice selection of inexpensive class-D amp boards to make it all super easy.
This is actually very interesting! People have been building tube amps from kits for quite awhile. If someone were to put out a really good sounding, affordable, light weight, analog solid state amp kit I know I would be super interested in building it!! Not such a big step up from building an effects pedal, which I have already done a few times.....
 

chris m.

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Or one could go ultra easy with the new EHX Howitzer amp pedal...
https://www.ehx.com/products/15watt-howitzer/
I already have that covered. I have an EHX 44 Magnum and I use a Joyo AC Tone as a pre-amp for it. Sounds way, way better than the Quilter SuperBlock US to my ears. And it's 44 watts.

I just think building kits is fun, but then again I used to build model planes and ships as a kid-- with some brain damage from all that Tester's glue, probably. Given the economies of scale associated with mass production these days it's almost just as cheap or even cheaper to buy it fully made by someone else at a low labor rate (China, robots). So the only real reason to build from a kit is just for fun and learning, IMO.
 

printer2

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I came up with a nifty design as a person in the Amp central Station was asking about a cheap amp to start on. The teal big cost was the power transformer. That got me thinking,

zBC3BiT.png
 
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TequilaCaster

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What exactly is the high voltage module black box on your schematic? Something you build, or buy?

And... if you are running 12VDC heaters, you MUST use both heaters in the second preamp tube methinks.
 
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printer2

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What exactly is the high voltage module black box on your schematic? Something you build, or buy?

And... if you are running 12VDC heaters, you MUST use both heaters in the second preamp tube methinks.
I am guessing each triode has a separate heater element in the cathode tube. They are in series and you parallel them up on 6V using pin 9, on 12V you run them in series only using pin 4,5. So you would use pin 9 and one of the others. But you have a reasonable question, I guess I should find out for sure. Looks to be separate, I'll have to set up a test circuit to make sure the one gets enough heat to operate correctly.

212231000000000-00-500x500.jpg


The HV module.

61V5UdkttAL.__AC_SX300_SY300_QL70_ML2_.jpg



Module Properties: Non-isolated step-up module
Input Voltage: 8~32V input(the default is 10~32V input.), two input voltage range selectable, input1; DC8V-16V input 2 (jumper selectable through the back of the PCB)
Input Current: 5A (Max)
Quiescent current: 15mA (12V liter 50V, the output voltage, the higher the current will increase too quiet)
Output Voltage: ± 45-390V continuously adjustable (default output ±50V), This module has regulator function, after adjusted output voltage is stable and unchanging, not with changes in input voltage changes
Output Current: 0.2A Max(with input, output pressure related,the higher the output voltage, output current is smaller)
Output Power: 40W (Peak 70W)
Working Temperature: -40 ~ +75 ℃ (ambient temperature is too high, please enhance heat dissipation)
Operating frequency: 75 KHz
Conversion efficiency: up to 88% (efficiency and input and output voltage, current, pressure-related)
Short-circuit protection: Yes (input 10A fuse, please do not directly short-circuit the output arc! Direct short-circuit arc may damage the rectifier device)
Over current protection: Yes. (Input current exceeds 4.5A, reducing the output voltage)
Over voltage protection: Yes. (Output voltage exceeds 410V, lowering the output voltage)
Input reverse polarity protection: Yes (non-self-healing, reverse burning fuse, do not reverse.)
Installation: 4 x 3mm screws
Wiring: free welding output terminals
Module Size (L x W x H): 60 x 50 x 20 mm
 

TequilaCaster

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I am guessing each triode has a separate heater element in the cathode tube. They are in series and you parallel them up on 6V using pin 9, on 12V you run them in series only using pin 4,5. So you would use pin 9 and one of the others.
A filiament is basically a wire resistor. If a single filament runs 300mA at 6.3VAC, then it will run ~600mA at 12VDC... maybe burn up. When you run the filaments parallel, each filament is getting 6.3V, and when you run them in series at 12V, each filament is only dropping 6V across itself. That's how I see it.

Thank you for the HV module info. I wonder how much noise it will introduce into the amp? Maybe need to add a choke.
 
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printer2

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A filiament is basically a wire resistor. If a single filament runs 300mA at 6.3VAC, then it will run ~600mA at 12VDC... maybe burn up. That's how I see it.

Thank you for the HV module info. I wonder how much noise it will introduce into the amp?
No, you do not run the single heater element on 12V. A 6V6 draws 450mA. If you pit a 12A*7 in series with it you need an additional 150 mA as the 12A*7 draws 300mA. One triode heater uses 150mA, add that to the 300mA of the first 12A*7 and you have 450mA, the same as a 6V6. So you can put the 12*7's in parallel and then in series with the 6V6. This will give you 450mA across 12V. Actually a little less as the expected voltage is 12.6V, but still at 5% of the ideal voltage. The only thing that might cause issues is if the 6V6 or the triodes heat up before the other. But the one heating up early would cause more voltage to be developed across the one that is heating up slower which would then encourage it to heat up faster. Lose anyone?

I have read people using the module and the noise was not an issue. On a high gain amp I would not know. I think I tried it at some point but can not remember anything special about it. The nice thing about the module is the voltage is adjustable so someone can turn it down to 45V and slowly increase the voltage and check how things are going and if something starts acting funny they can stop and do some troubleshooting before going to full voltage. A built in variac.
 

TequilaCaster

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Cool. That's clever. I missed the part about it being in series with the 6V6 filament. My conceptual understanding of filaments has been corrected. Thanks again.

Rephrasing...

You have half of a 12Ax7 filament (using pin 9, and either pin 4 or 5 but not both) in series with the 6V6 filament, running 12V, so each filament or filament section sees 6V across it. The 6V6 uses 450mA and the half 12AX7 uses 150mA for a sub-total of 600mA.

Then you have the second 12AX7's full filament (using pins 4 and 5), in parallel with the 6V6 - half 12AX7 filaments series combination, also running @12V, and using 150mA.

Total filament draw is 750mA.
 
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TequilaCaster

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Yikes! Maybe I still don't get it.
You want the three 12AX7 filament sections in parallel (like you posted) so that when they are in series with the 6V6 (like you posted) and 12V is applied to the series, the current draw is relatively balanced at [email protected] for the 6V6, and [email protected] for the parallel 12AX7 filament sections, and the 12VDC current draw is 450mA.
A picture is worth a thousand words... add the filament circuit to the schematic please. Thanks.
This Ohms law stuff... sheesh! The elusive obvious!
 
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printer2

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Yikes! Maybe I still don't get it.
You want the three 12AX7 filament sections in parallel (like you posted) so that when they are in series with the 6V6 (like you posted) and 12V is applied to the series, the current draw is relatively balanced at [email protected] for the 6V6, and [email protected] for the parallel 12AX7 filament sections, and the 12VDC current draw is 450mA.
A picture is worth a thousand words... add the filament circuit to the schematic please. Thanks.
That is why I love schematics. I wanted to confirm the heater arrangement first.
 




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