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SoLow Watt 12A_7
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10thTx



Joined: 30 Dec 2006
Posts: 1063

PostPosted: Sat Mar 17, 2012 2:52 pm    Post subject: SoLow Watt 12A_7 Reply with quote

I previously shared my SoLow Watt design using ECL84 tubes and a 5879 pentode in V1. The ECL84 is a triode/pentode tube.

I decided to rewire V1 to use a 12A_7. The result is a smoother & creamier overdrive with a more transparent clearer tone ....... and sweeter harmonics.

There is a thread with more info here:
http://www.el34world.com/Forum/index.php?topic=13438.0

I think this is a fantastic sounding low wattage amp that sounds HUGE using a delay pedal.

With respect, 10thtx
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LeftyStrat



Joined: 14 Jan 2005
Posts: 3010
Location: Seattle, WA

PostPosted: Sat Mar 17, 2012 7:22 pm    Post subject: Reply with quote

Interesting. I have a small Blackheart Head cab I've been looking to do something with. What kind of output transformer are you using?
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10thTx



Joined: 30 Dec 2006
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PostPosted: Sat Mar 17, 2012 7:32 pm    Post subject: Reply with quote

The OT was given to me. As I understand it, it was probably from a PA system that had maybe 10-12 watts and probably used 6V6's.

Maybe use something like one of these:
http://www.musicalpowersupplies.com/3.html

It was small enough to fit inside this amp head cab which is pretty small.
The chassis now has two less pots on the front, but it's the same chassis and head cab.

or a Princeton Reverb type trannie?


With respect, 10thtx
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LeftyStrat



Joined: 14 Jan 2005
Posts: 3010
Location: Seattle, WA

PostPosted: Sat Mar 17, 2012 8:09 pm    Post subject: Reply with quote

10thTx wrote:
The OT was given to me. As I understand it, it was probably from a PA system that had maybe 10-12 watts and probably used 6V6's.

Maybe use something like one of these:
http://www.musicalpowersupplies.com/3.html

It was small enough to fit inside this amp head cab which is pretty small.
The chassis now has two less pots on the front, but it's the same chassis and head cab.

or a Princeton Reverb type trannie?


With respect, 10thtx


I noticed your using the 4 ohm tap on an 8 ohm speaker. So I assume 16k primary is what the ecl84's like to see?

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10thTx



Joined: 30 Dec 2006
Posts: 1063

PostPosted: Sat Mar 17, 2012 8:31 pm    Post subject: Reply with quote

Quote:
I noticed your using the 4 ohm tap on an 8 ohm speaker. So I assume 16k primary is what the ecl84's like to see?


That is my understanding. It sounds excellent to me and I've had NO issues with it.

With respect, 10thtx
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passfan



Joined: 10 Jan 2010
Posts: 1230
Location: Central Florida

PostPosted: Sun Mar 18, 2012 12:35 am    Post subject: Reply with quote

Wow, thanks for sharing that. I've got the tubes and parts laying around to set up a couple of these and have been wondering what to do with them. Is it loud enough to require a master volume ?
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10thTx



Joined: 30 Dec 2006
Posts: 1063

PostPosted: Sun Mar 18, 2012 12:50 am    Post subject: Reply with quote

Quote:
Is it loud enough to require a master volume ?


Yes, it is for what I want from it.

I think cranked this amp is probably somewhere between 3-5 watts? About the same as the VibroChamp (with 10" speaker) that I used to own except this is push/pull.

Nice thing about the PPIMV is that you can crank the preamp and lower the PPIMV to get a nice overdrive tone at moderate volume levels........... OR
lower the volume pot settings and open the PPIMV for a cleaner tone.

3-5 watts can be pretty loud, IMO.

I find the PPIMV very useful on this amp.

with respect, 10thtx
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LeftyStrat



Joined: 14 Jan 2005
Posts: 3010
Location: Seattle, WA

PostPosted: Sun Mar 18, 2012 3:50 am    Post subject: Reply with quote

Well now it looks like I might be building a whole bunch of ECL84-based amps. I did a "buy it now" on NOS Russian Mil-spec tubes from the Ukraine, and somehow ended up mistakenly ordering 24. Shocked But less than a dollar each. I think shipping might end up being almost as much.

Might try using a quad of them in an amp. It seems a lot of low powered amps leave me wishing for better bass response. That would bring down the z-out to 8k. And those two extra triodes I could do the post-PI cathode follower thing like an SSS.

Anyone up for a 10 watt SSS? Very Happy

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10thTx



Joined: 30 Dec 2006
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PostPosted: Sun Mar 18, 2012 11:53 am    Post subject: Reply with quote

Quote:
It seems a lot of low powered amps leave me wishing for better bass response


When I built this amp (w/5879) last Nov '11, I commented several times on the Hoffman forum how much bass response these tiny ECL84 tubes have. The ECL84 seems to accent the bass and lower mids reasonably well. I have used a Cannabis Rex and Red, White & Blues speaker with the amp.

Honestly, using a delay pedal, this amp sounds HUGE for a tiny low wattage amp.

And it seems like the ECL84 tube is pretty easy to overdrive.

And because of the bass response of the tube, I found it important to keep some of the coupling cap and cathode cap values fairly low.

Quote:
Anyone up for a 10 watt SSS?


Regarding the idea of a D-inspired low watt amp .................
I thought about that idea also! I think it could be done. You'd have to use smaller cathode and coupling cap values to make it work, IMO.

I've attached a schematic on how I would approach a D-style amp using the ECL84 tubes. Tweed BluezMeister ECL84

I probably would NOT use the triodes on the ECL84 for the LTPI. Instead, I would use a 12A_7 tubes. That way I could use a lower gain 12AU7, 12AV7, or 12AY7 tube for the LTPI to ensure not overdriving the ECL84 pentodes.

with respect, 10thtx


Last edited by 10thTx on Sun Mar 18, 2012 4:15 pm; edited 2 times in total
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passfan



Joined: 10 Jan 2010
Posts: 1230
Location: Central Florida

PostPosted: Sun Mar 18, 2012 12:44 pm    Post subject: Reply with quote

10thTx wrote:
Quote:
Is it loud enough to require a master volume ?


Yes, it is for what I want from it.

I think cranked this amp is probably somewhere between 3-5 watts? About the same as the VibroChamp (with 10" speaker) that I used to own except this is push/pull.

Nice thing about the PPIMV is that you can crank the preamp and lower the PPIMV to get a nice overdrive tone at moderate volume levels........... OR
lower the volume pot settings and open the PPIMV for a cleaner tone.

3-5 watts can be pretty loud, IMO.

I find the PPIMV very useful on this amp.

with respect, 10thtx


Oh, I didn't realize it had that much wattage. What effect do you think a higher plate voltage would have ? The hammond AO-44 reverb chassis I have is running a PT with a 270-0-270 secondary.

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Structo



Joined: 17 Oct 2007
Posts: 14209
Location: Oregon

PostPosted: Sun Mar 18, 2012 3:15 pm    Post subject: Reply with quote

Here is a BIN of 10 tubes for $12

But $12 shipping, I hate it when they do that.

But there are some deals for that ECL84 tube.

http://www.ebay.com/itm/MIX-Triode-Pentode-6F4P-ECL84-6DX8-6DQ8-Lot-10-/230109396535?pt=LH_DefaultDomain_0&hash=item359396be37#ht_3100wt_1059

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10thTx



Joined: 30 Dec 2006
Posts: 1063

PostPosted: Sun Mar 18, 2012 3:15 pm    Post subject: Reply with quote

You may need a 5Y3GT rectifier and then drop a few volts using a VVR or a zener to not exceed the max ratings.

Although, I would guess you'd be OK and end up with maybe 297v on the ECL84 plates?

with respect, 10thtx
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10thTx



Joined: 30 Dec 2006
Posts: 1063

PostPosted: Sun Mar 18, 2012 8:23 pm    Post subject: Reply with quote

A quick one take demo of the amp. It think the overdrive is reasonably smooth. OD stands out more towards the end of the tune.

I don't there is any lack of bass tone especially when you consider it's only a small pentode only producing 3-5 watts.

http://www.soundclick.com/player/single_player.cfm?songid=11515063&q=hi&newref=1

With respect, 10thtx
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LeftyStrat



Joined: 14 Jan 2005
Posts: 3010
Location: Seattle, WA

PostPosted: Sun Mar 18, 2012 11:48 pm    Post subject: Reply with quote

Nice. Like you said, the overdrive is very transparent. What guitar/pickups did you use?
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10thTx



Joined: 30 Dec 2006
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PostPosted: Mon Mar 19, 2012 2:27 am    Post subject: Reply with quote

Thanks for the nice remark.

I build my own guitars. Tele with hollow body and no f-holes.

DiMarzio AirNorton bridge
DiMarzio Fast Track 1 mid
StewMac humbucker neck

Maple over Swamp ash body. Rosewood fretboard.

With respect, 10thtx
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