Preamp: Foreplay or 5687 or 12B4

[Toyman_PA]

Well. Looking to build my first preamp.

Foreplay kit (stock)

or

Simple 5687 (with Elna Cerafine, Auricaps, Riken resistors) website

or

Etude 12B4 (with Elna Cerafine, Auricaps, Riken resistors) website


Any opions/input/suggests/alternatives would be greatly appreciated.

Thanks,
Toyman

[fdegrove]

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,

[Eli Duttman]

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.

[JojoD818]

Originally posted by Eli Duttman
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

[Bandersnatch]

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.

[zobsky]

i recently compared a foreplay to my 12b4 preamp (built from the 12b4 schematic on the diyparadise site), . .. 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.

[fdegrove]

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,

[pedroskova]

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.

[zobsky]

Originally posted by fdegrove
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 yet another 12b4a . 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).

[stigaard123]

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..........