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Cheap Forum Amp by Captainbeefheart


MEH Synergy

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26 minutes ago, Shakeydeal said:

Now can you mention the lowdown on my amp…..

 

For a more refined taste, you have ridiculously efficient speakers so the amplifier sweet spot will be at a much lower output level. A higher powered single ended amplifier with some even harmonics is what the captain calls for.

 

Single ended Class A2

 

Full pentode operation both driver and output stage

 

Special grid drive circuit won't produce nasty sounding blocking distortion and bias shift

 

12 watts RMS no more than 1.5% THD

 

The amp will produce 20 watts RMS but at higher distortion levels. I always rate my amplifiers very conservatively so there is adequate headroom. For music reproduction playback which high power demands are typically short duration transients you have plenty of headroom.

 

Power tube 6550

Voltage amp stage 6SJ7

 

Since it's Class A and the load is fairly constant there is no need of regulation. With Pentodes not having the best power supply ripple rejection great care was taken in the power supply with the use of a Large Choke and lots of filtering to attain an extremely low ripple DC supply. Tube rectifier also adds damping making rectifier induced ringing typically found with Silicon rectifiers nonexistent.

 

Exotic wood chassis - Caribbean Rosewood

High end paint finish - BMW Mineral White Metallic with clear coat

 

 

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7 hours ago, Shakeydeal said:

Thanks. I know you and I discussed the details before, but I thought it might interest other forum members.

 

Yes you were correct it did pique the interest of other members and I got some private messages about it.

 

I'll respond to the inquiries but also answer them openly.

 

The circuit is a little different then the one originally posted. You can't tell how it will exactly sound from a simulation and calculations and I wanted to try different front end tubes. Although some gave better numbers including the original 6AU6a I eventually opted for the 6SJ7GT after listening to the breadboards. The 6SJ7GT sounded the best to my ears. It's possible the slightly higher harmonics added to the increased soundstage but I felt of all the pentodes I tried it was the smoothest and had the best imaging.

 

It takes a lot of time to find the right combination. I don't just try different tubes, I also try different operating points of each tube. With pentodes you have many options, run the screen very low for low current with a high load on the plate for starved mode operation, or a more typical operating point that datasheets steer you towards. Hifi you typically want center bias but sometimes shifting it slightly will give results you like. The output stage is the same.

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16 minutes ago, captainbeefheart said:

 

Yes you were correct it did pique the interest of other members and I got some private messages about it.

 

I'll respond to the inquiries but also answer them openly.

 

The circuit is a little different then the one originally posted. You can't tell how it will exactly sound from a simulation and calculations and I wanted to try different front end tubes. Although some gave better numbers including the original 6AU6a I eventually opted for the 6SJ7GT after listening to the breadboards. The 6SJ7GT sounded the best to my ears. It's possible the slightly higher harmonics added to the increased soundstage but I felt of all the pentodes I tried it was the smoothest and had the best imaging.

 

It takes a lot of time to find the right combination. I don't just try different tubes, I also try different operating points of each tube. With pentodes you have many options, run the screen very low for low current with a high load on the plate for starved mode operation, or a more typical operating point that datasheets steer you towards. Hifi you typically want center bias but sometimes shifting it slightly will give results you like. The output stage is the same.

Of all the sharp-cutoff pentodes, the 6SJ7 (and 12SJ7) seem to be the best for audio. I've heard a few in various homebuilt amps and preamps and they are quiet and sound good in pentode or triode mode. 

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Since the original thread is an awful resource for anyone trying to navigate information about the actual project I figured it best to just start fresh.

 

The original theme got put on hold due to being asked to use the circuit behind the build and make it into a high end build with exotic wood and high end paint chassis, top notch transformers, and hand wiring. I also got asked by another member to build them a unique high powered push pull amplifier. This thread will be about the Shakey's amp and the inexpensive version of it. Chris (westcoastdrums) will get a completely different thread to keep things clean and concise for each build. I am about finished with Shakey's amp and once westcoastdrums is done I'll start on the PCB board for the inexpensive version finally.

 

In the meantime if anyone wants to discuss and help with the selection of chassis, hardware, tubes, and transformers we can do that. I'll list the requirements for the transformers to help make a list of options.

 

I should say I do have the schematic for Tim's amp but even an inexpensive version may cost around $500. But it's capable of pushing 20 watts so we can relax the output power to make the amp more affordable, again another topic of discussion. There were a few people happy with the $250-$300 price tag and ok with less than 10 watts of power. There seemed to be more people that would rather spend the extra couple hundred dollars to get more output power.

 

Here is Shakey's amp, I'm still wiring up the inside for him.

P1010451.JPG

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Which OPT's are you planning to use. CuriousGeorge found a source for some SE transformers at a good price he is going to give a try. Check with George for details. It is a new OPT by Musical Power Supplies Inc for audio. Believe the price is going to be $38.

 

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5 hours ago, henry4841 said:

Which OPT's are you planning to use. CuriousGeorge found a source for some SE transformers at a good price he is going to give a try. Check with George for details. It is a new OPT by Musical Power Supplies Inc for audio. Believe the price is going to be $38.

 

 

Yes Nick sent me the details the other day, they will be added to the list of possible transformer vendors for the build. One concern I have with them is they are guitar amp transformers rated down to 40Hz. I'm assuming this is for the rated 10 watts so myself or I believe Nick said he got 4 of them so maybe he can do some testing to see distortion vs output power at 20Hz with different DC bias currents. Another concern I have is the 50mA rating, for a 10 watt SE hifi output transformer I'd like to see at least 80mA or 100mA even better. I'm sure we can work around these concerns and still achieve good results but they designed those transformers around the guitar amp market. For the 6550 circuit we need a lower load impedance like 3k and it needs a minimum bias current of 80mA and preferably 100mA. We just need to pick a different power tube. Nick was thinking 6V6 which should work better with them, bias current of 6V6 wouldn't be more than the 50mA. BUT, and there is always a but, 20H inductance isn't really great for such high impedance tube like the 6V6 which is 50,000 ohms or higher, sometimes as high as 80,000 depending on operating point. At 20Hz the impedance of 20H is down to half the reflected (and optimal) load impedance of 5k at 2.5k. Basically the low inductance starts to dominate at low frequencies as the load impedance (2.5k) while mid band frequencies see 5k. It would be nice to see 40H of primary inductance for a 6V6 build. It's tricky because most tubes that want a 5k load will have high plate impedance and so need more than 20H, more like 40H. They are guitar amp OPT so I know they probably designed them this way on purpose. Maybe he can wind us a 2.5k load impedance with 20H that will work with an EL34 but we would need him to probably up the bias current a tad to 80mA. Even if they were $50 each that's still a great price. One of us should open up a correspondence with him and see if he would wind us something to our spec and price it out for us.

 

There are other options like parafeed. I have some 10 watt constant voltage PA transformers that are cheap as dirt. I was getting good results with them, easily 5 watts output with a gyrator load and AC coupled. I don't think anyone will complain about spending $6 on an output transformer. I'll see if I can get the full 10 watts out of one at 20Hz but I doubt it.

 

 

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Another reason maybe is he is trying to finish the amplifiers he has been working on and getting them shipped. I am sure when that is done he will give more thought to his inexpensive forum tube amplifier thread. At least I hope he does. I want to see what he comes up with. I hope it is with one of those NOS USA, British, eastern European tubes of old. They made the best of the best audio tubes back in that day. Other countries did not make many or any of them. No market in their country for audio tubes. Most could not afford a radio in their country and the ones that could bought them from the places mentioned above. 

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6 hours ago, Shakeydeal said:

Can we get an update on this thread? Captain?

 

 

 

I think Shakey wants me to post the details about his specific amp. Which could turn into the inexpensive amp but it won't be under $300. I'll get a lowest possible price for the same circuit used in his amp.

 

The amp is Class A2 Single ended Pentode, 12 watts at no more than 1.5% THD. It will do 20 watts but at higher distortion.

Tubes:  6SJ7GT, 6550

 

For the people asking details on the actual inexpensive amp designed on a printed circuit board for easy building we need to figure out the direction. I suppose finding out the actual lowest cost for Shakey's circuit will be the first step towards seeing if that should move forward.

 

There were a lot of people interested in high performing tube amp costing $250-$300. Personally I think if we are trying to get as much power as possible for as cheap as possible it would be better to go push pull. I was talking with Racer and I thought since PWK loved his Brook 12a I could make a poor mans version with triode strapped 6AV5GA's. Also replace the 6J5's with 12AU7's to cut down on tube count and chassis real estate. There is choke loads for driver tubes, I'll see if I can replace with CCS but a CCS can't swing past B+, I'll need to analyze the circuit more to see if that's going to be a problem. I don't think it is, I'm sure they just used chokes to keep DC resistance low while making AC impedance high to which a CCS will work better than the choke.

 

Things to consider while I'm finishing Shakey and Westcoastdrums amps.

 

First, we really need to settle on what "inexpensive" means and put an actual number to it. Some people say $500 is nothing for a nice tube amp and I agree but many may not want to dip their toes into a $500 tube bath.

 

Single ended or push pull? The latter will allow us to keep cost down while increasing power. i.e. price per watt is much lower.

 

I'll work on doing an absolute lowest cost for Shakey's circuit while you all discuss the other options.

 

 

 

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I hope all that are building or planning on building a tube amplifier jumps on board and do much as I am doing detailing their build with pictures. Members seem to enjoy following along and I guarantee even experienced builders will probably learn some tips from the other members that do post and describe their build. 

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Guys I have told ya'll about a really good sounding tube, the 6Y6, that you can buy cheap for an old tube made long ago and made specifically for audio. The general consensus from all that I have read is those old tubes sound better than the new production tubes you can buy. Most say if you install a set of new old tubes in any amplifier you own you are going to experience another level with your amplifier. I hope one of you will listen to an old man and design your circuit with your twist using this 6Y6 tube. Call it the Super Sweetie or Hot Rod Sweetie. I think you get the idea. The price of it is only going to go up never down and right now they are readily available at a cheap price. With the Deckert amp being so much in demand a little amp designed around an good old audio tube should be very desirable. 

 

This is about the cheapest price you will find for a NOS tube for Deckerts amplifier, $80. Let me see, 80 bucks or $10 for an old tube to build upon. 

 

https://www.tubedepot.com/products/6bq5-el84-usa

 

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