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Old 24th November 2002, 02:05 AM   #1
SHiFTY is offline SHiFTY  New Zealand
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Cool Single-ended: Triodes or pentodes?

I am looking at building a decent single ended amp on the cheap (I'm a poor student!) and was looking at the prices of triodes.

I have a PP amp which sounds fantastic, but, I guess you can never have too many amps

The thing is, I have access to a large quantity of cheap 807s (like a 6L6) and possibly some unused Mullard EL37s, some other kind of output pentode. It would be cool to use these instead of buying a whole lot of expensive tubes.

For transformers, I am looking at the Hammond 125ESE, and I have a big old power tranny 425-0-425 @ 200mA.

Now my questions: Is there some benefit that only triodes have? Will I still get the "SE sound" despite using a pentode/beam tube, and a little feedback? Could I get that sound with triode strapped pentodes?

Also If I use the primary centre tap of the output transformer as a 50% UL tap, will this sacrifice much in the way of power?

cool.


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Old 24th November 2002, 08:37 AM   #2
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Default Re: Single-ended: Triodes or pentodes?

Quote:
Originally posted by ShiFtY
I have a PP amp which sounds fantastic, but, I guess you can never have too many amps
So why do you want an SE?
Quote:
The thing is, I have access to a large quantity of cheap 807s (like a 6L6) and possibly some unused Mullard EL37s, some other kind of output pentode. It would be cool to use these instead of buying a whole lot of expensive tubes.
The 807 is a tube I'm kind of fond of, especially some of the earlier types. The EL84 is another. Really excellent and comprehensive datasheets for both of these at www.retrovox.com.au . I have no experience with the EL37 but have heard it referred to highly elsewhere. I"d try the 807 myself.

There was an article on an SE amp using one of these written by Gordon Rankin in an old Sound Practices. I can't find it on the web, but maybe someone will have a copy and email it to you. Would prove a very handy starting point as it's apparently a well thought out and developed design.

For all their simplicity SE amps can sound dire if not executed well IME. There is no inherent magic in any particular topology, that poor design and execution can't royally stuff.
Quote:
For transformers, I am looking at the Hammond 125ESE, and I have a big old power tranny 425-0-425 @ 200mA.
Might be good, might not be. Depends on the design you choose and operating conditions. More info neccessary.
Quote:
Now my questions: Is there some benefit that only triodes have? Will I still get the "SE sound" despite using a pentode/beam tube, and a little feedback? Could I get that sound with triode strapped pentodes?
DHTs are more linear than triode connected pentodes and often have lower Rp. Some IHTs are less linear than trioded multigrids. For all types a rule of thumb is 1/4 of the rated plate dissipation is max power output. With an 807 Pdmax ~25W, so max power out in SE will be 6W roughly. Is this enough?

What is SE sound? We all hear so differently, and topologies, designs executions etc vary so much I dunno what exactly you mean. I've head a couple of SETs (with 845/211) sound quite good, but generally, they sound like mud to me, I've never found them at all magical, and almost without exception, all go to hell with complex passages. Part of the reason for the renaissance of SE amps amongst the DIY fraternity, is it's easy to make a two tube SE amp work with little understanding of electronics. But working, and being really good, are two different things.
It will be interesting to hear your experiences.
Quote:
Also If I use the primary centre tap of the output transformer as a 50% UL tap, will this sacrifice much in the way of power?
You can't use a PP OPT in a SE amp. In a PP OPT, the two halves of the primary are wound opposite, so the magnetic flux generated by the two halves cancel. In an SE OPT, there is an airgap introduced into the core to allow for the static DC bias current. Run a DC current through half or all of a PP OPT primary and it will have saturated before it reaches more than a couple of mA. You need to use a proper SE OPT.

Good luck
Cheers
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Old 24th November 2002, 10:32 AM   #3
JohnR is offline JohnR  Australia
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You know, if I had some unused Mullard EL37s I think I'd be sorely tempted to put them on EBay...

Anyway, this might help:

http://tinpan.fortunecity.com/saints...mer/index.html

JohnR
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Old 24th November 2002, 10:32 AM   #4
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Default SE 807

Hi,

The article from SP Brett refers to:

SOUND PRACTICES

Both the 807 and EL 37 belong to the family of 6L6s,if you can get a number of mint EL37s at a reasonable price and would then put them on sale it would pay for a decent SE OPT.

Working with the 807 will require a pair of UX5 sockets and anode caps.

When used as a pseudo triode they are pretty linear.

If you use the SE amps as the top amp of a bi-amped system (keeping your PP for the bottom) you avoid much of the deficiencies Brett talks about.

GR uses a paralelled 12AY7A/6072 as the driver,most of the rest of the article is about the math to get there.

In case you need it I can have it scanned in at a local shop and e-mail it to you.

Cheers,
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Old 24th November 2002, 11:00 AM   #5
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Default SCHEMATICS

Hello,

Here are some diagrams for general guidance on the SE 807

CIRCUIT & PSU:

Click the image to open in full size.

Copyright Alexander Kitic 2001.

Cheers,
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Old 24th November 2002, 09:31 PM   #6
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Default Re: SCHEMATICS

Quote:
Originally posted by fdegrove
Here are some diagrams for general guidance on the SE 807
I too have a whack of 807 (and variants). This is (reputably) an under-respected tube, probably due to the top cap.

dave
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Old 24th November 2002, 09:41 PM   #7
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Default 807

Hi,

Better put them to use before they're gone.

This a darn fine sounding tube with decent specs.
If you need more inspiration just look at the sheets with them trioded.

Imagine them without top cap and you're as close to the KT66 as you can get.
BTW,having the topcap there is a technical advantage although a bit of a scare for the DIY.

One major manufacturer used them extensively and having heard them at least a hundred times it still remains a fav of mine.

Ciao,
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Old 24th November 2002, 10:07 PM   #8
ststone is offline ststone  United States
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Default 12 uur

I think it is my task to put you guess to bed.
Now Go, or I will send Gina
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Old 24th November 2002, 10:15 PM   #9
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Default GINA SLEEPIN'MAN

Hi Steve,

Alles O.K.?

Ciao,
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Old 25th November 2002, 03:10 PM   #10
Joel is offline Joel  United States
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Default Re: Single-ended: Triodes or pentodes?

Quote:
Originally posted by ShiFtY
For transformers, I am looking at the Hammond 125ESE...
A good choice. I was skeptical, even after reading Joseph Esmilla's review of it. But then I used them in my 71-A amp and I think they sound fantasitic.

As far as the "SE sound", it's mainly in the transformers, although switching to a true triode will be subtly audible. I used a triode strapped EL84 amp prior to building the DHT SET.
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