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Preamp: Foreplay or 5687 or 12B4

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diyAudio Senior Member
Joined 2002
Hi,

Welcome to the forum.

May I kindly suggest to browse through the latest pages of this section of the forum?

There have been plenty of preamp discussions here some of them discussing the preamps you're considering, some about variations using either the 5687 or 12B4A.

If you have any particular questions, don't hesitate to ask.

Cheers, ;)
 
FWIW, Doug Piccard, AKA Bandersnatch on this "board", built a Foreplay style circuit around a pair of 5687s. Doug called his preamp Imposter.

IMO/IME, Doug's unit is GOOD (I've heard it). The gain of a 5687 section wired common cathode is less than that of a 12AU7 section and a 5687 section makes a MUCH BETTER cathode follower than the 12AU7 does.
 
Eli Duttman said:
FWIW, Doug Piccard, AKA Bandersnatch on this "board", built a Foreplay style circuit around a pair of 5687s. Doug called his preamp Imposter.

IMO/IME, Doug's unit is GOOD (I've heard it). The gain of a 5687 section wired common cathode is less than that of a 12AU7 section and a 5687 section makes a MUCH BETTER cathode follower than the 12AU7 does.


Hi,

Please give us a link for Doug's 5687 preamp, the search button is not doing it for me.

Thanks!
JojoD
 
Hey-Hey!!!,
...or you could just e-mail me!

Get the ForePlay manual from Bottlehead. It is a fairly adaptable circuit. The big change/imiprovement, aside from properly sized Iron was to make a bi-polar supply so the cathode follower load could be made larger( bigger R, and flow more current). You need lots of volts to do this( comparatively, speaking). I used a 166G6 which is 125-0-125 and put on a full bridge and gronded the center tap. +/- 180 volts was the result.

A sheet of aluminum plate and a drill press and I was off to the races. It is a big improvement over using the 12AU7, and offers the same options for active loads the FP does.

You'll probably want a DC filament supply too, LM317 will handle this quite well. You can do this a few ways from fast/cheap and easy to Iron intensive and expensive...
regards,
Douglas

drop me a note, I still have a few copies of the schematic I could mail you.
 
diyAudio Senior Member
Joined 2002
Hi,

the foreplay was more extended, .. but the 12b4 had more drive, especially in the bass and was more enjoyable to listen to, .. and IMHO was the better preamp.

Assuming you mean extended in the highs?
You could try this to correct it:

Remove the cathode bypass cap so the 12B4A is now a little degenerated. The downside is that the Zout will go up a little and the gain will drop a little too.
I don't think you'll miss the extra gain and if you're driving a tubeamp the higher Zoout won't be a problem.

Make sure the gridstopper resistor is as close to the grid as possible and that it's a carbon type of a 1/4W.
If that doesn't cure the highs try removing it altogether; the 12B4A doesn't exhibit a tendency to oscillate with this kind of topology.

Hope this helps, ;)
 
The Foreply comes in enough iterations that to compare it against something else doesn't mean much unless you state which iteration it was that you heard. The stock Foreplay is a nice introduction to tubes. Additions, like the C4S constant current sources and sinks, change its demeanor significantly. You can swap out the 12au7 for a 5687, or a 6SN7(with all of the requisite changes), and you take another step up.

If you feel that you may need support in the building process, it's a good deal. If you think that you have the experience to scratch build something else, there are a million possibilities.
 
fdegrove said:
Hi,



Assuming you mean extended in the highs?
You could try this to correct it:

Remove the cathode bypass cap so the 12B4A is now a little degenerated. The downside is that the Zout will go up a little and the gain will drop a little too.
I don't think you'll miss the extra gain and if you're driving a tubeamp the higher Zoout won't be a problem.

Make sure the gridstopper resistor is as close to the grid as possible and that it's a carbon type of a 1/4W.
If that doesn't cure the highs try removing it altogether; the 12B4A doesn't exhibit a tendency to oscillate with this kind of topology.

Hope this helps, ;)


Frank,

You may remember my 12B4 from a thread a month or so ago http://www.diyaudio.com/forums/showthread.php?s=&threadid=34619&perpage=10&pagenumber=1 . In case I haven't mentioned it yet, thanks to all for all the advice then
I was looking a quite a few options on CCS'ing the cathode or plate which came to nothing, . the stock preamp being wonderful as it was (and me being lazy)

i've noted the advice about the grid stopper,.. will keep in mind, whenever i dive into this piece again.

In my opinion, .. the highs on my 12B4 (RCA tubes) were correct, though a little soft., and in this particular application, sounded better than the foreplay (dual mono and a bunch of other tweaks I'm not sure of, since I haven't spoken to the builder yet). ...

I agree with you about removing the cathode bypass cap, .. I'm guessing it's responsible for the slightly flabby bass. I will get around to it one of these days, .. was actually thinking of replacing it with an IXYS CCS (battery bias seemed too intimidating considering the preamp was biased to provide 20mA current per channel), .. any opinions.on this idea . What's the difference / pro / con of implementing the CCS on the plate vs. the cathode, ..or can both be used simultaneously (don't think it's practical due to potential imbalance, but i had to ask)

(oops, sorry for hijacking the thread).
 
Foreplay becouse

IF it is your first tube project I would advise buying the Foreplay Kit. Buying audio parts can be time consuming, they NEVER have all components I would like to buy in one shop. Then making a box of a kind can also take a lot of time. The Foreplay have a manual, and even better, the there are a forum for their products with a lot of kind people.

The sound is said to be good, there are several tested mods. The other amps mentioned probably also sounds good, and can be tweaked also, but then you have to figure out how to do yourself.

If it was my first amp, I would go for the Foreplay and buy a C4S mod with it. Now I have a lot of parts, but I always have to buy a few things extra and pay a lot of postage. Becouse of the postage I often order a few more items for future plans, which then get changed and..........:bawling:
 
Basically if we could use a control group(same amps and same speakers) in which to use as a compare we could actually answer this question. That is of course if we had 100 blindfolded listeners.

I have listened to the Forplay and was not impressed so I didn't purchase the kit. I built the 5687 and used VR's in the power supply. The result was a very nice sounding linestage. I listened to a friends 12B4 and was blown away. I built it(12B4) and compared it to the 5687 and found it to be more natural sounding.

If I was to choose between the 5687, the 12B4, and the 5692 from Frank it would probably be a tie between the 5692 and the 12B4. Note* The 12B4 has yet to receive power supply upgrades (VR's) like the 5687 and 5692 have.

Note* My opinion here*****

I believe that when more time is spent in the design of the power supply the result is a better sounding piece. I believe in using VR's
in power supplies and I believe in regulated(Frank's circuit) as the best answer to power supply regulation. There is a night and day difference in the sonics when using good supplies.:smash:


Thanks Frank for the heads up on supplies.


Joe
 
Thanks for all of the responses guys.

I was originally going to go with Joel Tunnah's 6sn7 design. But after hearing about and seeing Frank's design, I may go with that.

As far as the PSU goes I might do something different. I did like the simplicity of Joel's design (first project for me). Here I modified it by increasing the resistors to 680/3W to get the voltage down to approx. 249-250V and the second cap to 100uF. Does anyone see any problem with this?

An externally hosted image should be here but it was not working when we last tested it.


The attached txt file is in fact a PSU file for you to look at via Duncans PSU designer.


Thanks all,
Toyman
 

Attachments

  • psu250.txt
    181 bytes · Views: 178
Okay. I changed my mind again for now. Can anyone recommend (schematic) a PSU to use with Frank's 12B4 linestage?
I would like to use a full wave rectifier LCLC. I saw Frank's new PSU designs and they are a lot over my head. I am a newb ya know. Something similar to this would be great:

An externally hosted image should be here but it was not working when we last tested it.


Thanks guys,
Toyman
 
I used a 20uf 20H 200uf which was dead silent for my 12b4 preamp.

Doing this again, I would use a 20uF 20H 200uF coupled to two R-C stages to decouple the two channels somewhat. (R-C coz it's cheaper)

This would be a good place to start, .. try modeling it in PSUD to find optimum values, .. and to get your preamp working with a minimum of fuss

Then, you can play with regulation and all the other goodies. If you plan to do so, give a good thought to layout beforehand. In fact, I suggest building the PSU and the preamp in seperate chassis (less interference, more space, and you can always build new power supplies if you are a tweaker type)
 
diyAudio Senior Member
Joined 2002
Hi,

close,.. but also include a smaller (10 - 20uF) cap before the choke followed by the 200uF and the RC stages , .. ( i think the rule of thumb was C -> L -> (10 x C) -> ..... )

Well, if it's a CLC filter that you want then, yes...
However, IMHO the LC filter type is the better suited one for audio purpose, YMMV.

Hammond Transformers as a nice overview of PS filter types with effeciency numbers:

HAMMOND

Frank any suggestions?

Since you ask...If you'd use chokes iso of the Rs to split the PS you'd come pretty close to the ideal without actually using dual mono power PSs IMO.

Cheers,;)
 
Frank,

Honestly I am just looking for a simple PSU putting out ~260V to run into your 12B4 linestage. I am not looking for anything specifically (CLC, RC, LC). Since I can't find a schematic for vacuum tube rectified LCLC to get 260V (all that I can find are 250V), I tried my own on post #14 by taking an existing 250V and trying chokes with diff resistance ratings to get to ~260V.

To make it short, in your opinion will the power supply in post #14 work well with your 12B4 linestage? By working well, I mean good sound, not necessarily as good as your new PSU designs. But something that I can learn on and expand on in the future.

Thanks again for your time,
Toyman
 
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