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Old 15th Sep 2006, 12:42 am   #1
adibrook
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Default output valve choice

I'm getting closer and closer to my eventual goal of building a really cool and original looking stage amp. However, i think the more i know about valves the more difficult it gets.

I have allredy got a transformer, which is 300-0-300 @170mA (but it looks big enough to do more...perhaps it can be pushed to 60W).

It also has a 5V winding and a 25.2CV winding. No 6.3V winding unfortunatly.

The HT supply is not TOO bad, and will probably be pretty loud. And with that 5V rect heater windong it means i can use my favourite rect, the 83. The 83 will do up to 225mA, so no problem there.

But what output valves do i use?

I thought 807's would be really cool. After all, they are suppsoed to be big tough etc. But after some prototyping i realised that an 807 actually doesnt look that amazing. They are big and stuff, but dont glow much.

Maybe i can use 4 smaller valves. That woudl also add a nice use for the 25.2V winding. But what ones? 6v6's and EL84's are too small, and i think will look stupid on an amp with a huge transformer and a 83 rect.

So, as usual, i need some suggestions for suitable output valves. Maybe some big triodes?

I guess a good alternative are 813's. Theya re huge, glow alot and have considerable power. But for them i will need another power transformer (microwave transformer possibly?) and allmost cirtenly a solid state bridge rect. They have 813's in valve and tube supplies for 15 each. The heater supplies are 10V @ 10A i think, which is abit big but nothign too bad for modern transformers.

Any other wierd and wonderfull output or tx valves out there that are worth considering?

Thanks.
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Old 15th Sep 2006, 6:57 am   #2
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Default Re: output valve choice

Hmm, in no particular order:
  • Older transformers should not be pushed. The wire insulation etc. is not up to the abuse of the modern counterparts.
  • The current rating of your transformer is valid when it is powering a resistive load, like ... a resistor. For a capacitor input filter, which I suspect you will be using, the average apparent transformer load is 2 to 5(!) times higher than you would expect from the output load current and voltage. <--- This is the reason why you often have people complaining about overheating mains transformers for their nice DIY class A audio amp designs. Few DIY websites and books explains about this issue, which is based in math and physics, not opinion. Bottom line: Transformers powering class A amps must have the power rating of their HT winding derated(!) by a factor of 2 to 5. The exact figure depends on the components used.

    Source: Radiotron Designers Handbook by RCA, Fourth Edition (1952). Page 1175, chapter 30, ' Rectification', section 2 (III).
  • As previously mentioned in another thread some time ago, then the 83 *will* *simply* *die* if you run it at high current in a common capacitor input filter in a PSU. Additionally, if you did use one 'properly' with a choke filter coil, the output voltage would be much less, reducing output and a suitable choke may be difficult to obtain. Mercury vapour rectifiers, while they can take high average current, can not survive the high peak currents found in a capacitor input filter.
  • Agree on the 807 and their ilk like 6V6, 6L6 etc. They don't really look that nice when powered. I have an 'open frame' 807 without the internal shielding and it glows pretty nicely, but that appears to be an uncommon type of construction.

    My choice for a push-pull stereo amp would be EL34s, even if the shape isn't that fantastic. At least they are large and available. They glow reasonably well too, but eats 1,5A of heater current a piece.
  • I wonder what the current rating of the 26V heater winding is? How much does the transformer weigh?
  • The problem with using triodes compared to tetrodes/pentodes is that - everything else being equal - you will draw less current through the triode. Read: Less power for equal anode voltage. The voltage gain of triodes may also be significantly lower than pentodes, so you may need an extra gain stage to really break down the walls with volume. Finally there is the Miller capacitance issue and the possible problems with having to run grid current. You may need a real power driver to push a pair of triodes.
  • Two things with 813s and similar higher voltage power valves: You won't easily find an output transformer, since they are worth their weight in gold (more or less) to the radio amateurs building classic style AM transmitters.

    Secondly, I at least won't help you use them. The voltages involved are simply too dangerous to be used in a transportable stage amp, and the mechanical construction required to make it 'road-and-audience' safe wouldn't be trivial. No open style chassis here would do IMHO.

Stepping off my

Frank N.

Last edited by YC-156; 15th Sep 2006 at 7:05 am.
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Old 15th Sep 2006, 7:13 am   #3
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Default Re: output valve choice

What about colour TV line output bottles. PL509/519 or EL509/519. Heater supply may be easier for the EL types. They are rugged and relatively cheap.
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Old 15th Sep 2006, 10:20 am   #4
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Default Re: output valve choice

Adi, I can only echo Frank's advice. As one, who built a 30W EL34 amp from scratch many years ago, I can testify to how much it hurt when I came in contact with the 400V HT one day !
Once you're in the hi power push pull league, high HT's and lots of heat is unaviodable
I will happily support you in your endeavours, and can supply bits, valves etc. but safety must be considered of the utmost importance. I built a wooden cabinet for my amp in the end as, due to the transformer used, the HT connections were above chassis.

As to choice of OP valve, you need to work backwards from the output transformer selected or available - this will dictate the choice of valves.

Don't underestimate the cost of a decent componenent - a flick through SOWTER's website will give you a flavour. Even secondhand, these things still go for fairly steep sums. For example, you may choose to use "TV" valves as Jefferey suggests as you have the 25 V heater secondary. The problem here is its quite difficult to get data for the correct anode load required for audio, as these were not published.( although it could be worked out ) This may lead to a figure for which no suitable transformer is avialable -
You see the problem Hope this makes sense ??

If you already have an OP transformer, the choice of valves will then be based on the transformer specs, and secondly on the heater volts available.
Is your mains transformer 25 volt secondary of reasonable capacity to support valve heaters ( it may be a 50 mA bias windning etc. )

Sorry to cloud the thought process, but this is no easy decision ( or project !! ) but it's still possible

Andy
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Old 15th Sep 2006, 11:17 am   #5
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Default Re: output valve choice

I also think that EL34s would be the most efficient for this purpose. In the May '64 issue of PW there is a circuit for a guitar amp using 807s which delivers 35W, but for that the mains transformer employer is a beast delivering 450-0-450 v at 250ma! Still, a 5U4 does the business as a rectifier.
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Old 15th Sep 2006, 11:30 am   #6
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Default Re: output valve choice

I really don't think you need that much power for a backamp which is going to be miked through the PA anyway. You can concentrate on making it look and sound nice and not worry too much about the output power - even 5W is likely to be adequate. This will massively reduce the cost of, and strain on, crucial components.

Paul
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Old 15th Sep 2006, 12:19 pm   #7
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Default Re: output valve choice

I agree with Frank on the choice of valves for a stage amp, Top caps are not a good idea. As for a cheap supply of transformers 760V at 200mA+ and EL509s, there are usually a lot of very cheap (20-50 Euros) S.H. Zetagi CB Power amps on the Italian Ebay.
A healthy respect for the high voltages in these types of equipment must always be maintained.
cheers,
Mike.
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Old 15th Sep 2006, 12:23 pm   #8
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Default Re: output valve choice

One other thing that I forgot to mention is that on the Italian Ebay there is very often a company that sells spares for Marshall amps. They have recently had O/P transformers for EL34s at 29,90 Euros and PSU transformers for 49,90 Euros.
Mike.
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Old 15th Sep 2006, 12:33 pm   #9
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Default Re: output valve choice

Thanks.
Some good advice there.

I dont wanna go down the EL34 route because its too normal. Pretty much every fair sized Marshall out there uses push pull EL34's, and i really dont like the marshall sound.

This is why i'v been trying every single valve i can get my hands on as an output valves. I want to find one that doesnt sound that usual

So far the only sound i'v found that i like is the QQV series. They sound abit colder than el84's/34's and distort rougher.

What paul said about power is true. This is why i gave up trying to create a big amp. After all, even tho the guitarist might laugh at me, it should still work. I think 20W upwords should be good. Just becaus i could have a BIT of fun with it offstage too.

So no serious power raiting is required.

Another valve i'v found that sounds good is the KT61. However, i have a pair of Marconi valves which have been pushed so hard i think theyre emitting xrays . They do sound very good when pushed that hard tho.

I'v experimented with triode power amps (true triode, not a triode strapped pentode) and i think theyre not too bad. So they need abit more drive, but not too much problem there. And the miller capacitence doesnt seem to effect it too badly. But then again i'v only tried a 2.5W triode.

I have 3 big o/p transformers, one from a 6L6 ultra liniar power amp, one from a EL34 power amp and an unknown one that looks big anyway.

Maybe i shoudl try to find some old directly heated valves of about 10-15W plate dissipation.

The current raiting thing is not somethign i'v herd about before. Thats strange. So most transformers are actually grossly overloaded?
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Old 15th Sep 2006, 1:08 pm   #10
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Default Re: output valve choice

Adi what about trying some El38's nice big bottle with a top cap designed as a video amp but work well as audio amps and pack a punch in push pull have a google their is plendy of info on them on the net.

Jay
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Old 15th Sep 2006, 1:26 pm   #11
adibrook
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Default Re: output valve choice

The EL38 looks great. I have a pair of EL37's which are pretty simular but not as nice.

We may have a winner here....

Are EL38's easily avalible?
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Old 15th Sep 2006, 1:51 pm   #12
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Default Re: output valve choice

I have some 6BG6s which are very similar, but a bit less powerful so they're physically smaller. You are welcome to 4 for 10 if you want them. They're basically a 6L6 with a top cap. They're branded Zaerex but appear to be Russian.

http://www.mif.pg.gda.pl/homepages/f...37/6/6BG6G.pdf

Paul
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Old 15th Sep 2006, 1:55 pm   #13
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Default Re: output valve choice

The 12E1 could be a nice choice, at 37W max it's a pretty sizeable beam tetrode, with top-cap and classic coke bottle shape. It will still operate well at the lower voltages you've got (less than 450V).
By the way, assuming you'll be running class AB, the ripple current should not be an issue providing you don't run the transformer right on its max spec. Ripple current is high, but the average current stays the same, at whatever you set it as.
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Old 15th Sep 2006, 9:54 pm   #14
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Default Re: output valve choice

I have a lovely pair of NOS 13E1's, they glow nice but are very hungry for heater power, needing a PC 20 Amp 5V supply to light each one!
Les
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Old 15th Sep 2006, 10:53 pm   #15
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Default Re: output valve choice

Hi

How about KT88s ?

Richard.
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Old 15th Sep 2006, 11:01 pm   #16
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Default Re: output valve choice

WOW. Now wer getting somewhere!

On TDSL it says that they need 13V @ 2.6A, which is alot but not impossible. 67.6W heater power for a pair. They definetly look the part, and the anode voltage doesnt look too bad. If theyre 500V than they will run ok off a 350-0-350 transformer...right? Maybe not full blast but at a decent power level.

If you ever decide to get rid of that pair please give me a shout.

They may not be the classic coke bottle shape, BUT they do look cool anyway. Even cooler than coke bottle valves perhaps. @ 33W of heater power i bet they glow ALOT too.

EDIT: Just saw the post about the KT88's. They seem to be very sought after by audiophools, and therefore overly expensive.

Last edited by adibrook; 15th Sep 2006 at 11:02 pm. Reason: another post
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Old 15th Sep 2006, 11:10 pm   #17
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Default Re: output valve choice

adi i may have a pair of EL38's i definetely have one but i am sure i have a pair, i am working away from home for a coupple of days when i return i'll have a hunt through the valve stock.

Jay
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Old 16th Sep 2006, 1:07 am   #18
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Default Re: output valve choice

Sorry Adi, I think I was getting the heaters mixed up with my other FAT valves i.e. a number of NOS QY4-250's, mmmmmmmm Solid Graphite Anodes wrapped in a Pyrex shell
5V @ 14 Amps with a max Anode Voltage of 4 kV
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Old 16th Sep 2006, 1:40 am   #19
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Arrow Re: output valve choice

If you take sensible precautions with the HT, a pair of TT21 in push-pull (class AB1) should look impressive. And deliver a fair few watts as well. Mind you, you'll have to do a bit of experimenting with anode and screen voltages to get a reasonable impedance match to any existing O/P transformer, since these bottles were originally designed for power RF applications up to about 10 Mc or thereabouts.

Just a thought.

Al.
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Old 20th Sep 2006, 11:21 pm   #20
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Default Re: output valve choice

What about Mazda Pen45's in push-pull..............lovely shape, sound great................................unusual...... ...ianj
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