Will This possibly work?

[Trout]

Gezz I am ambitious lately..

Anyways, I have another project started now, Want to use this for tape and CD playback.

The Million dollar question is will this work or even come close?
schematic

Looks close to me,, But hey,I am no designer, I just enjoy the torture building stuff in the 89 degree ,85% humidity we have been seeing all summer.

Gene

 

[Eli Duttman]

Problems.

For lowest distortion, screen grid B+ should be regulated. A VR150 should get the job done.

The O/P impedance of pentodes is HIGH. That means a LOW damping factor and poor voice coil control. Loop NFB is needed to lower the O/P impedance.

BTW, Eli is cheap. A $4 7Z4 instead of the 6CA4 is good for 100 mA. of DC.

 

[planet10]

The O/P impedance of pentodes is HIGH. That means a LOW damping factor and poor voice coil control.

Which is excellent if you want to do the current amp thing with some efficient FRs.

dave

 

[hacknet]

why not try some local feedback, theres been lotsa talk bout it and i`m waiting to try it.

 

[Trout]

BTW, Eli is cheap. A $4 7Z4 instead of the 6CA4 is good for 100 mA. of DC.

Eh, I have a dozen 6CA4's laying around of various brands, So to me they are a good choice, Plus, I have no loctal sockets laying around.
I am only looking for 4-5 watts tops here.
I am concidering NFB loop but I am really not up on how to modify the current schematic yet.

Hopefully I will have this built by friday night, Chassis is done and I am mounting sockets Trannies Ect already.
Once again I am packing this into a 6X10X2 Hammond chassis as I had several here to putz with.
Gene

 

[dr._sleep]

Down here in Louisiana 97 degrees/ 95% H, we DIY in the AC.

Here's nifty feedback circuit for the EL84 operating as a pentode. I don't think you'll care for the sound without NFB, something about the non-linearility at midband to high frequencies.

[IMGDEAD]https://img.photobucket.com/albums/v310/tubeman/feedback.gif[/IMGDEAD]

Looks like you're pretty close. Ignore the 4.7 ohm 5 watt to ground off the OT. Float the OT's without reference to ground.

Take a look at the Taboo circuit at www.decware.com under articles.

dr._sleep

 

[planet10]

why not try some local feedback

After going thru a number of EL84 SE, we ended up with a variation on the RH84.... a couple fairly lengthy threads here covering that one.

dave

 

[Trout]

Down here in Louisiana 97 degrees/ 95% H, we DIY in the AC.

Here's nifty feedback circuit for the EL84 operating as a pentode. I don't think you'll care for the sound without NFB, something about the non-linearility at midband to high frequencies.

[IMGDEAD]https://img.photobucket.com/albums/v310/tubeman/feedback.gif[/IMGDEAD]

Looks like you're pretty close. Ignore the 4.7 ohm 5 watt to ground off the OT. Float the OT's without reference to ground.

Take a look at the Taboo circuit at www.decware.com under articles.

dr._sleep

Now, 2 Dumb questions time, The 10K pot, #1 Is this an adjustment I will be needing to make often? Or #2 should this pot be easily accessible?

After going thru a number of EL84 SE, we ended up with a variation on the RH84.... a couple fairly lengthy threads here covering that one.

I did a search on that, Oh My thats alot of reading! 70% of which is brail to me, So I will need to study a bunch before going to far as I hate rewiring to much inside such tight chassis.
But I am sure the results will be well worth the efforts.
Gene

 

[Trout]

Just A Thought,

After searching a few RH84 treads, One recently used a simplier(to Me anyways) Schematic to achieve nfb.
Here



Given the chassis limitations and simplicity of this circuit, Wouldnt this give me about the same results?

Gene

 

[planet10]

Given the chassis limitations and simplicity of this circuit, Wouldnt this give me about the same results?

No

That is no simplier than the RH84. And the RH84 feedback is much more local.

In our experience, the global feedback pentode wired amp (as per your map) doesn't work as well as the same amp triode wired with no feedback, and that doesn't sound as good as wired RH-style. The RH also has twice the power as the triode version.

dave

 

[dr._sleep]

The shafts for the pots would be mounted through the top of the chassis.

The feedback circuit is voiced to control linearity at any point in it's adjustment window. There is a control for each channel that allows you to fine tune the feedback on the fly for any given recording, although the most common use is to find that magic spot where the amp becomes synergistic with a given set of speakers and then leave it there.

Perhaps a novel way to apply NFB.

dr._sleep

 

[Trout]

No

That is no simplier than the RH84. And the RH84 feedback is much more local.

In our experience, the global feedback pentode wired amp (as per your map) doesn't work as well as the same amp triode wired with no feedback, and that doesn't sound as good as wired RH-style. The RH also has twice the power as the triode version.

dave

I Like This one the best, Not as many extra parts and now I can proceed without trying to find another spot to mount more pots.

I have got everything mounted already, Power supplies built, Filaments wired Ect. Now I will move on to the actual circuitry as you posted.
Thanks for all the help, I should have pictures tomorrow as I am moving at a good pace.

Its FINALLY cooled down here and we even got a little rain. My lawns in terrible condition (dead/dormant) as we have been in severe drought for 2 months now.
But it should be weeks before I will have to stop tinkering to mow haha.

Once again,, Thanks!!
Gene

 

[Eli Duttman]

There is an elegant solution to the NFB issue, but the "iron" is costly. Use O/P trafos with 20% cathode feedback tertiary windings. The "finals" will be LINEAR and the damping factor will be good. The drive voltage requirements for the "finals" is high.

 

[Trout]

No

That is no simplier than the RH84. And the RH84 feedback is much more local.

In our experience, the global feedback pentode wired amp (as per your map) doesn't work as well as the same amp triode wired with no feedback, and that doesn't sound as good as wired RH-style. The RH also has twice the power as the triode version.

dave

Dave, I have this wired up, Now I have a few questions.
#1 Expected output would be 4-5W ?
#2 Input source should be from? Portable CD Player? or Line out from say my laptop or Tape player?

I have powered it up, Its Hum Free and nothings smoking which is always a good sign, It appears to be just about right, I did put in my poorest condition tubes as to no risk any of my NOS stuff.
Gene

 

[planet10]

#1 Expected output would be 4-5W ?

Yes

#2 Input source should be from Line out from say my laptop or Tape player?

Yes.

dave

 

[amperex]

Audio tranny solution as mentioned......

Find two old vintage Hammond quad 6V6 amps on eBay with the 6J5 tube, not the newer versions. The PP audio transformer can be used for SE up to 8-watts without core saturation issues and it has that important cathode winding. These trannys have a following at AA.

Use the power transformers & two chokes for a killer LCLC power supply. Change that stock 5U4 for a Sylvania 5R4GYB smooth black plate to get the voltage down to a reasonble level. Radio Daze in NY has the somewhat rare NOS 5R4 for $13 each.

 

[amperex]

I forgot......

the regulated screen is a great idea. I would use a gas tube regulator shunted with a .1 uF cap to remoce braodband noise and feed that to a 10HY or so 500 ohm choke and use a 20uF poly cap to bypass the screen.

 

[Trout]

OK,
I Missed Something or at least maybe my parts are. At best I am getting about 1/2w maybe less output.

Very Very clean sound, just not enough of it. I havent had a chance to run the voltmeter around it since its been built, So as soon as time allows, I will get that info.

The OPTs are magnavox salvaged originally EL84 SE, The PT was from same unit.

Gene

 

[Trout]

Update,

Sounds Great! Found the output issue was actually a source issue.

I do get a very very low level hum, Not even noticable even at very low settings while cd plays. I suspect easily cureable .

Very Impressive sound quality though far better than the donor the transformers were extracted from.
Its A Keeper!!

Gene

 

[planet10]

Its A Keeper!!

pictures?