EL84 Monoblocks

for the DIY ST35, the Dynakit and every other PP EL84

EL84 Monoblocks

Postby corndog71 » Mon Aug 10, 2015 9:13 pm

I've been enjoying my custom ST35 but find myself in the tweaking mood again and wonder about making monoblocks. Layout wise I was thinking of building sort of mini-Mk3's. I was wondering if simply splitting the ST35 in half would be doable. I have an extra PT.

Searching the forum has unfortunately not been helpful. I'm not the first to propose this but many seem to discourage the idea. It's a shame because I think it would be a cool project. Considering how simple these circuits are I would think it would be pretty easy.

If anyone has any helpful advice I would surely appreciate it.
some people call me Rob
corndog71
 
Posts: 105
Joined: Thu Feb 03, 2011 10:45 am
Location: midwest

Re: EL84 Monoblocks

Postby EWBrown » Tue Aug 11, 2015 3:22 pm

I suppose that a couple of the Poseidon boards could be used as drivers, just re-calculate the plate and cathode resistors for operation with 325-365 VDC B+

An LTPI will play well with a PP pair of EL84s [:)

/ed B
Real Radios Glow in the Dark
User avatar
EWBrown
Insulator & Iron Magnate
 
Posts: 6389
Joined: Wed Mar 19, 2003 6:03 am
Location: Now located in Clay County, NC !

Re: EL84 Monoblocks

Postby corndog71 » Wed Aug 12, 2015 2:17 pm

At first I was thinking I would simply split the St35 driver in half and retain the 12AX7 for voltage amp and 12AU7 for phase splitter but basically only using half of each tube.

Then I thought surely this could be accomplished with one tube. Could a 12AU7 or 6BQ7 work for both sections? I understand it has less gain but I have too much gain with my preamp anyway. I imagine changing the resistor values might be a good idea but I don't quite understand how to calculate this.

Also, since I still want to use the PA774 that means changing the power supply resistors as well. I read that those need to be doubled. I was also considering a choke after the first cap similar to the other Dynaco circuits. Would the C354 or Triad C24X still work in a mono configuration or do I need to find a bigger one as well?
some people call me Rob
corndog71
 
Posts: 105
Joined: Thu Feb 03, 2011 10:45 am
Location: midwest

Re: EL84 Monoblocks

Postby cdmawolf » Wed Aug 12, 2015 3:39 pm

LMAO, the "Original" Dynaco circuit used a 7247/12DW7 tube, which is 1/2 of each!!
cdmawolf
 
Posts: 37
Joined: Thu Nov 29, 2012 3:07 am

Re: EL84 Monoblocks

Postby corndog71 » Wed Aug 12, 2015 6:29 pm

cdmawolf wrote:LMAO, the "Original" Dynaco circuit used a 7247/12DW7 tube, which is 1/2 of each!!


I know, I know. I would prefer either a tube that's not as popular or a tube people currently make and not some long lost nos tube. It seems like an old practice to use a high gain tube for the voltage amp and a low gain tube for phase splitter. If I have to do it that way so be it but it seems rather unimaginative.
some people call me Rob
corndog71
 
Posts: 105
Joined: Thu Feb 03, 2011 10:45 am
Location: midwest

Re: EL84 Monoblocks

Postby cdmawolf » Wed Aug 12, 2015 6:47 pm

6AV6 and a 6C4?
cdmawolf
 
Posts: 37
Joined: Thu Nov 29, 2012 3:07 am

Re: EL84 Monoblocks

Postby EWBrown » Thu Aug 13, 2015 8:56 pm

That would work, 6AV6 is like half of a 12AX7, 6C4 is like half of a 12AU7. They also have the advantage of being relatively cheap tubes

/ed B
Real Radios Glow in the Dark
User avatar
EWBrown
Insulator & Iron Magnate
 
Posts: 6389
Joined: Wed Mar 19, 2003 6:03 am
Location: Now located in Clay County, NC !

Re: EL84 Monoblocks

Postby Writer Frog » Sun Aug 16, 2015 7:08 pm

corndog71 wrote: I would prefer either a tube that's not as popular or a tube people currently make and not some long lost nos tube.

If I am not mistaken, JJ 12DW7 is a current production tube. (???)
/Matt
User avatar
Writer Frog
 
Posts: 95
Joined: Mon Mar 06, 2006 9:17 am

Re: EL84 Monoblocks

Postby EWBrown » Sun Aug 16, 2015 7:53 pm

Also look for J/J ECC832, which is Euro-version of 7247 / 12DW7, as the number suggests, it is half ECC83 and half ECC82.

/ed B
Real Radios Glow in the Dark
User avatar
EWBrown
Insulator & Iron Magnate
 
Posts: 6389
Joined: Wed Mar 19, 2003 6:03 am
Location: Now located in Clay County, NC !

Re: EL84 Monoblocks

Postby 20to20 » Mon Aug 17, 2015 7:48 am

corndog71 wrote:
cdmawolf wrote:LMAO, the "Original" Dynaco circuit used a 7247/12DW7 tube, which is 1/2 of each!!


I know, I know. I would prefer either a tube that's not as popular or a tube people currently make and not some long lost nos tube. It seems like an old practice to use a high gain tube for the voltage amp and a low gain tube for phase splitter. If I have to do it that way so be it but it seems rather unimaginative.



Back in the day Ed Laurent published a 7199/7189 PP circuit in Radio Electronics. Since the 7189 is just a robust version of the EL 84 and the 7199 has been been replaced by the 6GH8A in the ST70 type II it seems like a slam dunk option for your monos to use the 6GH8A which you can find for $5 or less. I got a ton of them in sleeves for $2 a couple of years ago.
Headed for Tishomingo to sing in a can...
User avatar
20to20
KT88
 
Posts: 566
Joined: Wed Apr 21, 2010 1:48 pm
Location: W-S, NC

Re: EL84 Monoblocks

Postby cdmawolf » Mon Aug 17, 2015 11:10 am

A 7199/7189(EL84) circuit would be a Dynaco SCA-35, the Integrated amp version of the ST-35.
cdmawolf
 
Posts: 37
Joined: Thu Nov 29, 2012 3:07 am

Re: EL84 Monoblocks

Postby Geek » Mon Aug 17, 2015 6:32 pm

Eico ST-70 used the 7247/12DW7, Dynaco ST-70 the 7199.

Each have their demons and charms [:)
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
User avatar
Geek
KT88
 
Posts: 3585
Joined: Sun Oct 21, 2007 3:01 am
Location: Chilliwack, British Columbia

Re: EL84 Monoblocks

Postby corndog71 » Mon Aug 17, 2015 7:37 pm

Yeah, I may just suck it up and go with the JJ 12DW7. At least they're cheap. I'm looking at using a 10x6 box.
some people call me Rob
corndog71
 
Posts: 105
Joined: Thu Feb 03, 2011 10:45 am
Location: midwest

Re: EL84 Monoblocks

Postby Writer Frog » Tue Aug 18, 2015 5:48 am

I am wondering if it is possible to simply use a 12AX7 instead of a 12DW7/7247/ECC832.
Does driving 6BQ7s require so much power that a 12AX7 cannot supply enough current in the Concertina phase splitter?

I know a few designs use a 12AX7 in the para-phase or the floating-para-phase splitter to drive 6BQ7s.

How about the Concertina set up with a 12AX7?

Just curious.
/Matt
User avatar
Writer Frog
 
Posts: 95
Joined: Mon Mar 06, 2006 9:17 am

Re: EL84 Monoblocks

Postby paart » Thu Aug 20, 2015 6:12 pm

Plugging a 12AX7 into a 12DW7/7247 will work (the amplifier will produce sound), but not very well. The phase splitter (the *au7 half) is setup to draw 4.5 ma, and the 12AX7 is designed to operate at less than 25% of that. If you feel the need to substitute another tube, the 12AT7 is a better choice.

The circuit could be re-designed to use the 12AX7 for the phase splitter, but since the circuit uses both direct coupling, and positive feedback, it’s a complex job, and it’s questionable if any improvement would result.
However replacing the 12AU7, where possible, (the DIY circuit) with a 12BH7 or 12FQ7 (requires a slight heater wire change), will make a measurable improvement, particularly if the B+ to the VA/PI is raised from 320 VDC to 365 VDC.

BTW; the 12FQ7 is electrically identical (other than heater voltage) to the 6SN7, which is two 6J5’s in one bottle. Sorry, they are both “octals”! ;)
paart
 
Posts: 95
Joined: Mon Jul 17, 2006 8:36 pm
Location: Central PA

Next

Return to stereo 35

Who is online

Users browsing this forum: No registered users and 3 guests