12AT7 and the Simple EL84

[haziz]

I have a ton of 12AT7 tubes lying around including some quite expensive exotica. I may wish to use them in this design rather than have to buy the 5687 or ECC99 tubes mentioned in this design/schematic. I also already have the EL84s

Do I have to change anything in the schematic or the values of the components?

This is my first attempt at a 'from scratch' tube amp so 'DIY Tube Amps for Dummies' level advice would be appreciated.

I am following the design mentioned in http://www.diyparadise.com/simpleel84.html
I will start with the most basic design and go from there.

Thanks.

Sincerely,

Hany.

[Svein_B]

I would expect the 12AT7 to work quite well as a driver for EL84.
If you print the 12AT7 data sheet and draw a load-line, I think you may want to adjust the cathode resistor, and maybe also the anode resistor to get an optimal operating point.

I do however have one suggestion; make room in your chassis for two driver tubes (one 12AT7 per channel). This way you have the option to easily change the topology to a more conventional driver circuit, if you find that the self-splitting output does not give the performance you desire.

SveinB.

[Merlinb]

Originally posted by Svein_B


I do however have one suggestion; make room in your chassis for two driver tubes (one 12AT7 per channel). This way you have the option to easily change the topology to a more conventional driver circuit, if you find that the self-splitting output does not give the performance you desire.

SveinB.

Agreed- a gain stage DC coupled to a cathodyne PI is more usual, and better balanced. The ECC81 (12AT7) operates well under a 47k anode resistor I find, which will improve linearity too.

[Eli Duttman]

Hany,

Build an "El Cheapo" variant. The design is quite simple and takes full advantage of the 12AT7's characteristics, including (sic) its distortion spectrum.

Other than a cathode bias resistor change to accomodate EL84 "finals", you can use the design unchanged.

"El Cheapo" schematic here.

[ME2]

Hi, I have built the simple EL84 and started with some 12AX7 tubes I had, cycling a few different types. I did have to change the plate and cathode resistors to suit those tubes(100K Ohm and 680 Ohm from memory). changed to some 12AU7 tubes next and found the sound was slightly more relaxed.

I then wanted to see if using EL84 tubes to drive the EL84s as suggested on the SEL84 page would work. I didn't like that too much though and just this week have changed to the JJECC99s.

The JJECC99 easily win for sound out of all the tubes I tried with least colouration and cleanest sound.

The rest of my system is a Rega Apollo and the Lynn Olson design ME2 speakers.

[haziz]

So which amp should I go for. Mr Duttman's "El-cheapo" vs the aforementioned "Simple".

I don't mind paying a little more for decent output transformers to begin with (??Hammonds), moreover NOS 5687s are only $3.95 at Tube Depot so even that objection to the original Simple design evaporates. This can also apply even if I opt for the El-Cheapo if there is any advantage for the better iron. My understanding is that the OPT has the greatest influence on sound in an amp. No?

Is the 5687 a better driver than an 12AT7? I did read somewhere that the 12AT7 was never designed for audio.

What do people think of the two amps sonically as well as ease of putting together?

So before I start the process of gathering parts which amp do people recommend. The fact that Mr. Duttman starts his posting on the Decware forum with a parts list icluding parts numbers (hurray) makes it even easier for a newbie like me.

Thanks.

Sincerely,

Hany.

[Eli Duttman]

Originally posted by haziz
So which amp should I go for. Mr Duttman's "El-cheapo" vs the aforementioned "Simple".

I don't mind paying a little more for decent output transformers to begin with (??Hammonds), moreover NOS 5687s are only $3.95 at Tube Depot so even that objection to the original Simple design evaporates. This can also apply even if I opt for the El-Cheapo if there is any advantage for the better iron. My understanding is that the OPT has the greatest influence on sound in an amp. No?

Is the 5687 a better driver than an 12AT7? I did read somewhere that the 12AT7 was never designed for audio.

What do people think of the two amps sonically as well as ease of putting together?

So before I start the process of gathering parts which amp do people recommend. The fact that Mr. Duttman starts his posting on the Decware forum with a parts list icluding parts numbers (hurray) makes it even easier for a newbie like me.

Thanks.

Sincerely,

Hany.

The 5687 is more linear than the 12AT7, but the 'T7 has a higher mu. The context of use determines which is "better". I would avoid the 'T7 in a SE circuit, given its distortion spectrum. However, the distortion spectrum of the 'T7 blends well with PP finals, where internally generated even order distortion products are cancelled. IMO, the proper perspective is to look at the net distortion spectrum of the entire amp.

If you can afford the expense, DynaClone Z565 O/P trafos are THE way to go. Dyna style "iron" is a price/performance champion. Better performance costs MORE, while less costly stuff exhibits noticeable degradation.

If you decide to go the "El Cheapo" route, purchasing parts is easy. Jim McShane has put together parts kits at favorable prices. Look here. All that's left is a chassis and "iron". Allied Electronics carries the: B-, main B+, and combined heater/B+ boost trafos. Triode Electronics carries the PSU filter choke and O/P trafos.

[haziz]

I do notice that the El-Cheapo is designed for the 12AQ5 tube. Any advantage apart from cost new? I do have around 32 EL84s lying around the house (in matched octets), and just planned to use those. What do I have to change in the schematic to do so? Total newbie warning here!

Thanks.

Sincerely,

Hany.

[Eli Duttman]

Originally posted by haziz
I do notice that the El-Cheapo is designed for the 12AQ5 tube. Any advantage apart from cost new? I do have around 32 EL84s lying around the house (in matched octets), and just planned to use those. What do I have to change in the schematic to do so? Total newbie warning here!

Thanks.

Sincerely,

Hany.

As you surmised, the 12AQ5 was chosen for its LOW price. However, the 'AQ5 is a member of the 6V6 family and 6V6 family tubes are incredibly linear triode wired. Don't worry, EL84s are quite good in triode mode too. AAMOF, there's something "magical" about "12" W. multi-grid power O/P tubes in general.

By all means, use the EL84 sets you have in the amp you build. The 2 "gotchas" to avoid are the increased heater draw of EL84s compared to 'AQ5s and changed values in the shared RC bias networks. EMail Jim McShane and he'll square you away on bias network values. I manage to foul that computation up far too often. The increased heater draw can be dealt with by moving the combined filament/B+ boost trafo up 1 VA size.

I suggest you wire the O/P tube sockets for the Russian 6p15p-ev. EL84s work fine and you gain an added "rolling" option. The data sheet for the 6p15p-ev is here.



Edit: get by the censor

[ME2]

Originally posted by haziz
So which amp should I go for. Mr Duttman's "El-cheapo" vs the aforementioned "Simple".

I don't mind paying a little more for decent output transformers to begin with (??Hammonds), moreover NOS 5687s are only $3.95 at Tube Depot so even that objection to the original Simple design evaporates. This can also apply even if I opt for the El-Cheapo if there is any advantage for the better iron. My understanding is that the OPT has the greatest influence on sound in an amp. No?

Is the 5687 a better driver than an 12AT7? I did read somewhere that the 12AT7 was never designed for audio.

What do people think of the two amps sonically as well as ease of putting together?

So before I start the process of gathering parts which amp do people recommend. The fact that Mr. Duttman starts his posting on the Decware forum with a parts list icluding parts numbers (hurray) makes it even easier for a newbie like me.

Thanks.

Sincerely,

Hany.

I have tried some super cheap 100V line transformers and now the Hammond 1608. While the Hammond iron is much better I find there is almost as much to gain in the driver tubes. For $3.50 a tube the NOS 5687s would be my choice.

The simple EL was very easy to build and I had the first version working in a weekend. THe El-Cheapo should not be much harder to build although I have not tried it yet. With both these amps being so simple they are easy to fault find and modify, which was a big attraction to me in the first place as I wanted a platform which I could play with.

Having the deisgner of the El-cheapo here to help is worth a lot so if I was starting again I would probably build and El-Cheapo and take full advantage of this fact