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Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members. |
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16th Apr 2021, 9:58 am | #21 | |
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: 807 tube amp, any ideas?
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Your right, they are a very hard tube to kill, Ive just gone through a bunch of used ones and most tested like new and only had to throw away 2 or 3 out of the whole box! Good news is they are still made in China https://www.tubedepot.com/products/s...er-vacuum-tube Do you think I would need to do much mods if I dropped them into a mullard 5-10 circuit? |
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16th Apr 2021, 10:14 am | #22 |
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Re: 807 tube amp, any ideas?
I don't honestly know.
If you wanted to find out, you'd have to do pretty much all the design work needed to calculate the output load line and transformer ratio, as well as to re-calculate the drive swing needed at the grid. In practical terms, if you just dropped them in, it would work to an extent. If it's a bit off of optimum, then you won't get the full output power potential, the forwards gain will be down a bit, and consequently the amount of feedback benefit you get will be reduced. You might decide that these effects aren't significant. On the whole the 807, 6L6, 6V6 family has softer shaped curves, a bit lower gain and a bit higher output Z than the EL34, EL84 family Mullard based their designs on. David
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16th Apr 2021, 11:41 am | #23 | |
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: 807 tube amp, any ideas?
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I have no idea on what impedance the UL taps are supposed to be, but I will probably ask the transformer winder to figure it out for me. I think I will run the tubes at 450V. |
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16th Apr 2021, 12:31 pm | #24 | |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
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Re: 807 tube amp, any ideas?
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That was a huge bit of practical and theoretical research and fettling, Andy. I remember when you were building it. Really pleasing result and and massive kudos to you!
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Al |
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18th Apr 2021, 7:53 am | #25 | |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,637
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Re: 807 tube amp, any ideas?
Thanks Al, kind words.
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You then need to draw a load line, see - http://www.valvewizard.co.uk/pp.html but before that read and understand the common triode gain stage. If you want to design from scratch your going to have to learn quite a bit. The basic procedure is work out what you want first, how many watts, what are you going to amplify, how much dosh are you willing to chuck at it, how much are you going to build yourself? these are only a few. You then find an OPT after doing some research as above. Stick it on the bench and after drawing a load line to find Rg1, Rk etc build a prototype OP stage. You then have to build a power supply, something else you'll need to learn about. Power it up, then bench test. Andy.
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20th Apr 2021, 9:32 pm | #26 | |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
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Re: 807 tube amp, any ideas?
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You need to drive the grids from a low-impedance source that can deliver significant power, because you're driving them into significant grid-current. It's a design recommended by RCA - see page 3 of this: http://n4trb.com/AmateurRadio/RCA_Ha...amtips0702.pdf 120 Watts output, minimal component-count - what's not to like? |
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30th Apr 2021, 8:17 am | #27 | ||
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: 807 tube amp, any ideas?
Quote:
I have been advised on the DIY Audio forum that 6.6K primary should be about right. I have winding gear and all the materials to make a good amount of transformers, so I will have a go at making my own. I think 30W is the maximum amount of power I can expect from a pair of 807s in AB1. But looking at the datasheet it suggests its around 36W and 140mA for a pair. I think running in UL will bring this down a bit however? I am currently tossing up whether to go with the dynaco version of the mullard 5-20 circuit which I have been advised will likely work OK with 807s or else this schematic here, which is designed for 6L6 so should work fine with 807. http://www.seekic.com/circuit_diagra...l_circuit.html Im also considering making this amp have the function to switch between UL mode and tetrode mode with a regulated 300V screen supply. Am interested to hear everyones thoughts on this. Also need help with rectifier options, do I just go and put 2x GZ34 in parallel? It looks like I will need close to 300mA. Im tempted to use a couple of 866A. I cant find any dual wave rectifiers that can alone deliver enough current that I can tell from the chart ive looked at. |
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30th Apr 2021, 9:11 am | #28 |
Banned
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
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Re: 807 tube amp, any ideas?
Use sand rectifiers!!! Unless you have a goodly supply of 5AR4's
My only problem with todays rectifiers is that they are basically "crap". My amps use fast recovery silicon rectifiers, because I just cant get reliable old valve jobbies. Its your call however. Joe |
30th Apr 2021, 1:01 pm | #29 | |||
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: 807 tube amp, any ideas?
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Quote:
Got a whole box of 866a tubes and dont know what to do with them, only hassle is the heat up time for them but they look cool. I just like using tubes just to make use of vintage tech more than anything, I know silicon rectifiers are much better, but i just like filling the thing full of tubes! |
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1st May 2021, 8:04 am | #30 |
Hexode
Join Date: Oct 2019
Location: Weymouth, Dorset, UK.
Posts: 473
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Re: 807 tube amp, any ideas?
Joe[/QUOTE]
I know the modern tubes are crap for sure, i have a ton of NOS tubes and a bunch of good used ones around, I have a couple of GZ34/5AR4 and some 5U4, 5X4, etc Got a whole box of 866a tubes and dont know what to do with them, only hassle is the heat up time for them but they look cool. I just like using tubes just to make use of vintage tech more than anything, I know silicon rectifiers are much better, but i just like filling the thing full of tubes![/QUOTE] The 866A is a high voltage half wave mercury vapour rectifier. A pair in a full wave configuration feeding a choke input filter can produce 500 mA of DC at 3,000 Volts. Serious power and the rectifiers will produce damaging UV radiation and must be screened.
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Valve equipment repairs since 1968 https://jonsnell.co.uk |
1st May 2021, 8:31 am | #31 |
Hexode
Join Date: Oct 2019
Location: Weymouth, Dorset, UK.
Posts: 473
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Re: 807 tube amp, any ideas?
Here is my take on a popular selling power amplifier, the power supply uses a 5U4.
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Valve equipment repairs since 1968 https://jonsnell.co.uk |
1st May 2021, 1:27 pm | #32 |
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: 807 tube amp, any ideas?
The 866A is a high voltage half wave mercury vapour rectifier. A pair in a full wave configuration feeding a choke input filter can produce 500 mA of DC at 3,000 Volts. Serious power and the rectifiers will produce damaging UV radiation and must be screened.[/QUOTE]
I dont get the whole UV thing over mercury rectifiers, the glass will pretty much stop all the UV-C (which is the most dangerous) thats why UV-C lamps are made of quartz glass, since it lets the UV pass through. IDK if much UV is even generated to begin with, besides no one is going to be staring at the things all day. They certainly will provide enough current which I need. Anyway, I have been going through all this transformer gear i acquired and I have a bunch of transformers with it. I think ive found one thats pretty much the very voltage I need. It looks like it has a beefy 6.3 winding and has a 5V supply for a tube rectifier. The secondary is 675V with a centre tap. RMS would be around 470V looking at some rough calculations, so with the right tube rectifier, the voltage drop would probably be about right where I want it to run around 440-450V Would I be better off with a pair of 5U4 in parallel or something perhaps? Any other rectifier suggestions out there? I see some people are using TV rectifiers such as EY88. |
3rd May 2021, 12:11 pm | #33 |
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: 807 tube amp, any ideas?
Any thoughts with this schematic?
I will likely go with this, or else use a mullard 5-20 design. |
6th May 2021, 10:13 pm | #34 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,527
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Re: 807 tube amp, any ideas?
It'd be more self consistent with a 6J7G or EF37A instead of the EF86
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9th May 2021, 12:13 pm | #35 |
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: 807 tube amp, any ideas?
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9th May 2021, 1:32 pm | #36 |
Octode
Join Date: Dec 2017
Location: Morden, Surrey, UK.
Posts: 1,552
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Re: 807 tube amp, any ideas?
EF6 - EF36 - EF37 - EF37A - EF40 - EF86 - same valve apart from the base and some later tweaks to improve microphony and hum.
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10th May 2021, 11:47 am | #37 | |
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: 807 tube amp, any ideas?
Quote:
I have a few EF37A and the odd EF36 or two. I believe that there are less microphonics issues with the EF37A than there is with the EF86. Not sure how 6J7 compares, as thats the american counterpart. I have a couple of 6J7 and they seem easier to find. IDK if there is any advantage to the EF37A with its red painted metallic shield? |
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10th May 2021, 2:18 pm | #38 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
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Re: 807 tube amp, any ideas?
Look at the prices of EF37A!
They are very much 'in' with the hifi crowd, and also there is the rebuilt Colossus computer at Bletchley park needing supplies to keep it going. There's also the 6SJ7 in similar ilk, with its metal envelope screening it nicely. There seems to be plenty around. But with the EF86 etc, there's probably more gain than you really want if you are going to design your amp to run at modern signal levels. I'd consider a double triode as a balanced input stage ECC83 or equivalent and into either another one or an ECC82 driving the 807s So the input stage and the phase splitter are both balanced and the ECC82 would allow lower anode resistors and less intruding phase lag. A pair of pentodes as the second stage would be self-cascoding and reduce Miller capacitance. Feedback would go the the grid of the other part of the double triode first stage. David
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11th May 2021, 3:13 am | #39 | |
Pentode
Join Date: Feb 2016
Location: Tauranga, New Zealand
Posts: 132
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Re: 807 tube amp, any ideas?
Quote:
I thought with 807 tubes they needed a bit to drive them anyway so more sensitivity was a good thing? Anyway, I have plenty of EF37A that test near new. |
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11th May 2021, 8:43 am | #40 |
Banned
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
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Re: 807 tube amp, any ideas?
I would think you wouldnt drive a power amplifier directly in any case.
Some sort of "pre-amp" would be required, especially if a magnetic cartridge is employed. You will need RIAA filtering to make it work correctly. FAT high gain pentodes are sort of out of favour, although I use them in my phono stage. 6BR7,s. Modern ( ish) especially designed and constructed for low noise and low microphony. They have been changed out for CV4006. Same valve just different manufacture ( still Brimar UK ) but much lower noise again, at least in my application. The Hi-Fi phoolies ( yes I am one ) havent discovered 6C4's yet. Very common, very cheap and are half a 12AU7A. VERY low noise too. They work superbly in SRPP or CASCODE stages in Hi-Fi setup's. If you want to play hi-gain, try 6AT6 or even 6AV6 single triodes. LOTS of gain, but with large gain you get large noise. I am still building my power amp up, but that will be rapidly followed by my pre-amp, which will be cascode, and using RF twin triodes that were designed specifically for low noise black and white TV tuners way back, AND designed with cascode amplifiers in mind. Yes a tad higher frequency than Hi-Fi, but they have a screen between triodes in the same bottle. wound heater coils, and specifications showing which triode works for the top and bottom halves of the circuit. Check out 6BQ7A's to start with. If you want big fat ( octals ) valves only, I think you will pay big fat prices for anything decent and GENUINELY new old stock. My above hints on valves is because they are common, cheap, and available at hobbyist prices ( I am an old age pensioner, so read peasant). Joe |