Let's Mod my Allen 75's! Please help make them sing.

Fasterdamnit

Fasterdamnit

Super Member
OK-
Now that I bit the bullet and will be rebuilding a pair of Allen Organ 75's (Thanks, Redboy!!) I have questions about the mods I have found so far on the net. No serious change to topology, just good parts to max it's potential. I will post schematics at the end of the post.

Input->12AY7 gain->EF86 splitter->6SN7 drivers->6550 outputs

Removing the 12Ay7 to back off the gain- Anyone try a 12AU7 for the lower Mu? Otherwise it looks like an easy mod to bypass and go straight to the gain pot R7 and the EF86. And once it is gone you can drop the parts count both in the audio circuit and the power supply. I may also drop the gain pot entirely and just put in a fixed resistor. I run a ladder attenuator for my pre-amp.


C3- In the feedback path is an electrolytic 30uf cap.
1. Would there be any benefit to changing to film and/or changing the value?

2. One mod schematic I have shows a change from the EF86 pentode to a 6CG7 triode and C3 changed to 250uf 'lytic and changing R14 from 2150r to 820r. Would this be the more beneficial choice sonically than #1?

EF86/6267- Recommendations on Mfr for quality sound?

I am working on a project list at Mouser. Ordering metal film resistors and they will be bumped up in power rating.


Any other recommendations gladly welcomed!

Stock Schematic:

AllenType75amp.gif


Mod to remove 12AY7

Allen75-1mod.jpg


Power Supply mod, no 12AY7

allen_75_supply_NEW.jpg


Change EF86 to 6CG7

allen_75_signal_NEW.jpg
 
Register to hide this ad
I would definitely retain the EF86 and ditch the AY7. Feedback circuits are IMO never given the respect they deserve. To me, the feedback is just as important as the signal path. I always use the highest quality cap and R that I have. I bought all aluminum 25 watt Vishays for that bank of power resistors. They have fins for cooling and just look so much nicer. I would bypass all the PS caps with film and oil units. A 30uf lytic would get a .33 film, and a .033 oil. You can take that out farther too and include styrene and silver micah too, if you want. I would agree to get rid of the volume control if not needed. The 6CG7 is a 6SN7 in a 9 pin bottle, so to me, that would be too much of one tube number, the 12AU7 just isn't a very good gain stage tube, but the EF86, the sky's the limit, I prefer Mullards with the screen.
 
Last edited:
Oh, and get rid of the serialized diodes in favor of some large heavy duty jobs, say 6 amps min, 1200v p inv. v. Cree makes a dandy, but they are a bit spendy at 10 or 12 dollars each, but you would only need four. AES has some nice freds that will do just as good a job for around half the price.
 
I looks like there are both 115V and 125V connections for the AC input. You might consider using the higher tap and keep all the voltages just a tad lower depending on your line voltage. Mine usually runs right at 120-121V.

Great amps for a project.

John
 
jaymanaa said:
I would definitely retain the EF86 and ditch the AY7. Feedback circuits are IMO never given the respect they deserve. To me, the feedback is just as important as the signal path. I always use the highest quality cap and R that I have. I bought all aluminum 25 watt Vishays for that bank of power resistors. They have fins for cooling and just look so much nicer. I would bypass all the PS caps with film and oil units. A 30uf lytic would get a .33 film, and a .033 oil. You can take that out farther too and include styrene and silver micah too, if you want. I would agree to get rid of the volume control if not needed. The 6CG7 is a 6SN7 in a 9 pin bottle, so to me, that would be too much of one tube number, the 12AU7 just isn't a very good gain stage tube, but the EF86, the sky's the limit, I prefer Mullards with the screen.
Click to expand...

So go from 30uf to 0.33? How about a teflon bypass for the signal caps and polyprop for the PS? So far, I have not liked PIO's- lose detail to my ear. Those are the resistors on my list.
Thanks!
 
jaymanaa said:
Oh, and get rid of the serialized diodes in favor of some large heavy duty jobs, say 6 amps min, 1200v p inv. v. Cree makes a dandy, but they are a bit spendy at 10 or 12 dollars each, but you would only need four. AES has some nice freds that will do just as good a job for around half the price.
Click to expand...

Working off another set of notes I found I was going to order 4- UF4007 diodes and one CREE Schottky 100V 4A diode in series to the choke. Would bypassing the diodes with .01uF film caps be advised?
 
nerdorama said:
I looks like there are both 115V and 125V connections for the AC input. You might consider using the higher tap and keep all the voltages just a tad lower depending on your line voltage. Mine usually runs right at 120-121V.

Great amps for a project.

John
Click to expand...

Great catch, thanks! :yes:
 
Hiya,

Jim already knows this .. but I am throwing in a vote towards the Tele EF86's followed by the Mullard 6267 grey plates. Also throw a bone at nickle plates as I remember some of them being magical.

I spent quite some time back in the old days screwing around with MicroPhonic EF86's and remember dimly that the Mesh plates were awesome when right but you went through so many to get quiet ones.

Of course not having built or modded an actual Allen 75 my experience was mostly with other gear that used EF86's back in the olden days.

Frannie
 
Fasterdamnit said:
Working off another set of notes I found I was going to order 4- UF4007 diodes and one CREE Schottky 100V 4A diode in series to the choke. Would bypassing the diodes with .01uF film caps be advised?
Click to expand...

The caps in post 5 sound fine, but I'm not understanding this post (the cree in series with choke). As for diode snubbers, I use .01 ceramic 1200v jobs, 1 across each diode. You can also add a high value resistor with them too.
 
Fasterdamnit said:
Not with the choke. to the choke.


Bridge->CREE->Choke.
Click to expand...

Interesting, I imagine it's purpose would be to reduce ripple? You can easily measure the AC component when you finish the supply and see if it helps. I sometimes use diodes off the different RC stages of power supplies. In my feeble mind it helps decoupling somewhat.
Do your Allens have those wonderful red IRC resistors in them? They have to be the nicest sounding metal films I've ever heard, and 1% if memory serves.

What kind of coupling caps are you thinking about?
 
jaymanaa said:
Interesting, I imagine it's purpose would be to reduce ripple? You can easily measure the AC component when you finish the supply and see if it helps. I sometimes use diodes off the different RC stages of power supplies. In my feeble mind it helps decoupling somewhat.
Do your Allens have those wonderful red IRC resistors in them? They have to be the nicest sounding metal films I've ever heard, and 1% if memory serves.

What kind of coupling caps are you thinking about?
Click to expand...

Yes, but I don't have evidence to prove it actually achieves cleaner DC. I don't see red resistors. :worried: I have Panasonic polypropylenes that I plan to bypass with Russki teflon. :thmbsp:

allen75021.jpg
 
Fasterdamnit said:
Yes, but I don't have evidence to prove it actually achieves cleaner DC. I don't see red resistors. :worried: I have Panasonic polypropylenes that I plan to bypass with Russki teflon. :thmbsp:

allen75021.jpg
Click to expand...

Damn, that's a pretty looking amp under the hood! I love the old-school cable lacing they used. I need to learn to do that on my amps!

-D
 
Here's a pic of the 50+lb amps for those that did not see the BT thread- :D

allen75003.jpg


Jay-
I understand the bypass theory for the 30uf, 'lytic, but would there be any reason to not replace it with a film cap?
 
Fasterdamnit said:
Here's a pic of the 50+lb amps for those that did not see the BT thread- :D

allen75003.jpg


Jay-
I understand the bypass theory for the 30uf, 'lytic, but would there be any reason to not replace it with a film cap?
Click to expand...

No, film would be an upgrade IMO. My SET 2A3 doesn't have any lytics, all film. Solen is good for power supply use, and makes some handy sizes. I tried them for coupling once and didn't care for them. They seemed harsh, but maybe I didn't let them burn in long enough. That's a nice clean pair, and look exactly like mine.
 
Looks like the Solens only go to 400V. I will go with 'lytics bypassed with film for the filters and the very affordable Axon film cap for the 30uf. I agree on the Solens for coupling. I also am not big on Auricaps, either.
 
In your revisions to the power supply, and particularly the rectifiers, pay close attention to the PIV requirements provided for in the original build. With a true choke input filter, that type of design can have a particularly nasty kickback from the choke at shut down, requiring much greater PIV capability. I would make sure to use at least as much total PIV rating in whatever diode type you end up using.

Best of luck with your project!

Dave
 
You must log in or register to reply here.
Back
Top Bottom