4x6GV8 proposed build - diyAudio
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Old 19th March 2014, 09:08 AM   #1
sangram is offline sangram  India
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Default 4x6GV8 proposed build

Hi there

I am a peniless musician (bankrupted by a build of the F5 Turbo) who's looking at cobbling together a small tube amp for practice and recording from leftover, scrap and local resources. And I've never built a tube amp before, so go easy on me

The speaker cab is made from scrap MDF and pine blockboard, and will have two $5 8" fullrange units, nothing worth talking about - very fat tone but low sensitivity. I shall finish the cabs for now and run it off this amp temporarily: 6moons audio reviews: Synergy Hifi 6F3

In my tests, the amp works well enough for its diminutive size, but the speakers are hardly distressed by the power, and the transformers saturate very early specially when hitting the last two strings. It sounds flatulent, and not nearly loud enough. Specially when playing acoustic guitar through it.

I do know I could use different speakers, but you don't really get instrument speakers easily/cheaply in India. I have 4/5 of these tubes in hand (apart from the ones in the amp above) and I was hoping to rustle them together into a little 15-20W amp that I could add into the cab at a later point in time.

So essentially I was thinking of 4 6f3 tubes in PPP config, switchable between triode and UL and a practice setting that would turn off the B+ on one output pentode pair. The triode sections would be used for preamplification, crunch channel and phase splitter duties. My design bible is the book 'Designing V-T amplifiers' by Charles Couch. The configuration is proposed to be flexible, where the amp output could be routed to an external cab, and the speaker be used just by itself if I wanted.

For power, I have a transformer with a single 200V, B+ winding and a couple of 12V heater windings, plus a smaller 12V service winding for use on relays and stuff like that. And a small 40V winding in case I opted for fixed bias. Basically series heaters, and a cap multiplier for the B+. Target B+ is around 250V.

The issue is the output transformer, and this affects pretty much the whole build. I have a custom-wound 13V, 60VA transformer with a UL-tapped primary. This is actually a mains toroid, which is the issue. Given that DC balance is practically mandatory, I was thinking of using a CCS in the cathode using a LM317 or 78xx, one per parallel pair of tubes or even one per tube. This forces Class A operation and somewhat limits the achievable output power, even in pentode mode. At 250V B+ I am seeing about 25mA per tube to stay under the 7W dissipation limit.

I also have a pair of 18V, 18VA EI units, which I could use with parallel secondaries and series primaries but am not sure how this works out in practice. The EIs have much lower bandwidth, I tried both on the output of a standard solid state amp and definitely prefer the toroid. Also the EIs are much smaller, I'm not confident they can take the kind of power required here.

Nobody in our country knows how to wind an OPT, far as I can tell. There are some manufacturers but the less spoken about their units the better. The quality of laminations is very doubtful and the wire is mostly aluminum or iron, not copper. You can forget about split bobbins and interleaved windings, and almost all EIs are so badly put together that they will toast themselves with zero load.

The toroid manufacturers use better materials, and follow specifications quite closely. EI manufacturers tend to be just people with winding machines and not much of a clue. This is the main reason I opted for toroids. I have been following the Van Der Veen publications, at least what is in public domain, to understand the compromises involved in using toroids as OPTs. Shipping in units from overseas is not for peniless musicians like me

Be glad for any advice and assistance, thanks in advance!
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Old 19th March 2014, 11:08 PM   #2
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Join Date: Oct 2004
Location: Adelaide South Oz
I assume you are think of this circuit with the output stage "doubled up".
DIY Audio Projects Forum • ECL85 (6F5P ) oddwatt clone

6GV8 = ECL85 Try searching on each to see what else turns up.

The pentode section of the 6GV8 was intended for use in television sets as the vertical amplifier - that means it should be very linear and hence be good for audio output.

For "inspiration" you might like to search for amplifier circuits using other TV vertical amplifier tubes, 6BQ6 comes to mind.

Cheers,
Ian

Cheers,
Ian
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Old 20th March 2014, 01:00 AM   #3
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This has a lot of possibilities. Keep us informed of your progress. Valve amp transformers in NZ are soooo expensive I only use 100v line transformers for output.
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Old 20th March 2014, 04:45 AM   #4
sangram is offline sangram  India
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@gingertube, I had seen that circuit but I was thinking of using a separate CCS for each tube. The problem is that the toroid will saturate with as little as 3-4mA imbalance between windings and though the balance pot can sort that out at one operating point it cannot compensate for differences in dynamic characteristics of tubes. I am given to understand that modern tubes can have very wide variances because of poorer quality control.

I did search, and there is a lovely circuit and build of a PP 6GV8 on a Japanese forum. I'll see if I can dig up a link. However, I wanted to learn a little more about designing from ground up so I'm trying to use a book and tube data along with inspiration from other designs to design the amp, along with the excellent postings by people like George here, really a goldmine for learning basics.

@djgibson, I will try and update the progress but this will be a very slow burn project. Expect a year or more to completion, if I get anywhere with it. My F5 turbo is in the oven and that will take the next two or three months, work on this will begin only then. However, the design and part sourcing has already begun. I actually only need buy the sockets and the passives.

You are somewhat lucky in that you actually get output transformers in NZ. We have no reasonable sources here. Trying to make it work with a toroid is challenging, but if I can it's a decent template for future builds. I am planning on a hybrid tube/ss amp, with a ss front end and kt88 outputs. I already got the transformers that build :-)
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Old 20th March 2014, 04:53 AM   #5
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If you use a CCS for each tube then the CCSust be bypassed with a bypass capacitor.
Look up Baby Huey to see the CCS I used for this purpose.

Will be interested in that Japanese Design.

6GV8 are "common as mud" here in OZ from tube TV set says. I probably have 30 or 40 on the shelf. No -one wants them, perhaps you will make them "fashionable". As I said they should make a good amplifier because one of the requirements for a TV Vertical Deflection Tube was good linearity.

Cheers,
Ian

Cheers,
Ian
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Old 20th March 2014, 06:25 AM   #6
sangram is offline sangram  India
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Thanks Ian, I will look up the baby Huey thread. Your words are very encouraging. The tubes are not available here, but there are plenty of sellers on eBay. Unfortunately a large number are in Ukraine, which hopefully sorts itself out soon. I just happen to have the Chinese equivalents on hand. How linear those will be is anybody's guess :-) in SE mode in that amp, the have a non-objectionable, if somewhat plasticky sound. I got some Amperex tubes for cheap, those sound much nicer.

I will dig up the jap amp link when I get to the computer. I have the pictures on my tablet, but the browser does not work with DIYA correctly. Don't hold your breath though, it's a very standard circuit and no CCS, but the construction is real nice. Hoping to be able to get close to that level of cleanliness in a p2p build.

Last edited by sangram; 20th March 2014 at 06:28 AM.
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Old 20th March 2014, 08:19 AM   #7
sangram is offline sangram  India
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Here's that link: LINK!

The text is in Japanese, but the links (in blue, like the old HTML days) work fine Looking at it now and after reading up a bit, seems the heater wiring could be improved upon!

Last edited by sangram; 20th March 2014 at 08:21 AM.
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Old 24th March 2014, 05:14 AM   #8
sangram is offline sangram  India
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The Baby Huey is a piece of art, Ian. I am really interested in how you came up with the idea.

I'm really excited by it, and I do have enough tube sections to complete it with doubled outputs and two input channels. If you wouldn't mind, I have a few questions.

In the wiki text, you note that the ECL86 would work. I'm guessing the ECL85 might do just fine then, with slightly higher distortion and lower power output? Not a problem for a guitar practice amp.

If that is so, could I use a single CCS to operate two power tubes? I was trying to work out the math and arrived at B+ of 200V with 25 to 30mA per tube, so each CCS will need to deliver about 60mA. I may be using a 7805 to perform CCS duty here because it is simpler to implement. Any problems that I may run into while doing that?

As it turns out, I cannot turn off one pair of tubes as the load impedance falls handily below what a single pair of tubes can actually do. Therefore, each tube does not need its own CCS, as I had initially thought. Being this is the first time I've ever tried to design with tubes, I still have plenty hoops to jump through :-)

I also wanted to ask about the point designated '0V at ECC803'. It refers to a ground point for all the voltage references, so basically, system ground, correct? Except physically close to the input tube for better tracking of the CCS? Can the CCS be built on a veroboard and located a few inches away, or does it need to be cozy up against the tubes? The problem with multi section tubes is that there is significant physical distance between input sections :-)

Last question, you have a voltage doubler on a 5V tap to provide the power to the input pair CCS. I assume this results in about 9V after smoothing? Any possibility of using a 9V alkaline battery for this? Or is the current draw too high?

Sorry for the pestilential questions and thanks for your assistance!

Last edited by sangram; 24th March 2014 at 05:43 AM.
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Old 24th March 2014, 01:12 PM   #9
sangram is offline sangram  India
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Actually Ian, I see you have answered all of the last three questions at various points in time. I will work out my life from hereon in. Thanks for a very inspiring design :-)
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Old 24th March 2014, 10:11 PM   #10
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Sangram,
For a Guitar Practice Amp then there is no problem using a Voltage Regulator based CCS except for the CCS bias method itself.

Of Fixed Bias, Cathode (Auto) Bias and CCS bias (which is a form of fixed bias) the CCS bias has the worst overdrive recovery of any of them. I've actually seen this, in continuous overdrive the bias voltage of the cathode rapidly rises to about 2 x normal bias and it takes time to bleed (discharge the bypass capacitor) back to normal.

In a HiFI Amp which is seldom overdriven this is'nt a problem but with a guitar practice amp it maybe. I also design and build tube guitar amps and while I've been tempted to try the Baby Huey as the Power Amp section, my GUESS is it would need to be the fixed bias version not the CCS biased version.

Don't let me disuade you from trying it but be prepared to change the bias method if the overdrive recovery is a problem.

The Baby Huey design idea came from Yves M. at Dissident Audio.
Here is his original damned clever idea.
Push Pull ECL86/6GW8

Cheers,
Ian
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