i hope this one makes someone excited

[ee]

My friend and audio buddy that got me, i am gonna say turned on, to bottlehead last year said I could try to fix his foreplay. All I know about it so far is is that it powers up and the tubes glow. But I'm glad he's letting me tinker. I hope it makes someone here feel good seeing an ol' friend. I believe he bought it from you about 15 years ago. It is definitely about to get rubbed on. I may have questions on treating her right though.
Thanks
Eric

 

[Grainger49]

I know this one well. 

What is the problem, no sound from either channel?

Take some readings on the pins of each tube and post them.

 

[ee]

correct no sound I am told. He thinks it may be as simple as a bad vol pot. I haven't really looked in it yet but it looks like 2 pots with a selector in between. I also haven't had a chance to talk to him about it's history other than the vol pot possibly the issue. I picked it up at his house yesterday but he wasn't home yet. I'll post a pic when I take off the bottom for any glaring issues that may be seen. And I'll try to get the readings soon.
Thanks
Eric

 

[ee]

I did measurements for you (and me)
HV+ (term strip at last cap) 182.5
R channel pin
1  48
2  0.1
3  1.99
4/5 (soldered together) 55.9
6  182
7  48.8
8  55
9  55.9

L channel pin
1  46
2  0.2
3  2
4/5  56
6  182.1
7  46
8  52.7
9  56

It seems there is continuity from input>selector>pot>pin>output.
I am planning on putting in 1 stereo pot if that's ok.
Will it be safe to connect foreplay to stereomour (with pot open at 100k) to check foreplay output.
My friend thought the vol pot was bad but it seems strange that both would be bad and they don't seem tied together. Which would say selector switch to me at this point is functioning correctly.
Are the yellow auricaps inline from the cathode pin (8) to ouput standard practice or for smoothing or something? Also not sure why he decided to take the wire on the cap (L in pic) to the term strip and then to rca output instead of just straight to rca.
Thanks for you help!
Eric

 

[Grainger49]

I'll get upstairs and call up my drawing of the FP with voltages shortly. 

Yes, you can try the FP 2 with the volumes wide open into the Stereomour.  You should get a better idea of what is going on.  You may even enjoy the sound more when the FP is working.  This will also give you the opportunity to verify that all the inputs work by changing the cables around.

You can check the pots out by clipping a meter lead on the resistor lead going to a pot, place the other meter lead on the center pin of that pot and turn the pot.  As you turn it clockwise from the stop you will get less and less resistance.

If you put in a stereo volume pot you lose the ability to adjust balance.  Recordings seldom have the center vocalist or instrument truly centered.

The Auricaps replace what were called Green Meanies.  Those became brown by the time I bought.  They block the DC that is on the Cathode from going to the output.  DC on the output is a bad thing.  The audio signal passes through them, the DC is blocked.

The stock caps had short leads and went to the terminal strips then to the output jacks.  You can remove the short black wire and put the cap lead into the solder pot of the RCA jack.

Feedback on voltages coming after 7:00 EDT.

 

[ee]

Thanks a ton! No rush on the voltages. I've got nothing but time unless my wife or God say differently.

 

[Grainger49]

Ok, the voltages look pretty good.  Tube pins 6 are a little high, they come directly off of the power supply.  They should be 157V DC and you are getting 182V. 

So measure the terminals in the power supply.  That is T2, T7 & T10.  Terminal T10 should be the same high voltage you have on tube pins 6 of both tubes.

 

[ee]

volts are a bit different today about 3-5 less all around.
Volt
T2  190
T4  70.2
T5  70.4
T7  183.7
T10  177


Resist
T2-7  1k
T7-10  1k
selector switch pin "out" to 'in" terminal of both pots acorss the resistor = 91k

 

[ee]

ok - i spoke with my buddy and he said it did have output it was just that it would get really crazy with the sound and start popping, get shrill distorted stuff coming through, like a car crash.
I hooked it up to my computer speakers (luckily) before I spoke with him though. No harm done. The computer speakers are audioengine powered speakers. Specs say a 10k input impedance.
With the volume all the way up on the speakers the preamp vol is super jumpy. Meaning you barely move vol pot and it goes to full vol and a touch more starts getting very distorted. More turning results in car crash sounds .  I turned the computer speaker vol way down and could play the preamp and it sounded great. Any touching of the preamp or cables and you get the microphonic type sound like when you tap a tube.
So I'll go ahead and replace the tubes and see what happens then. He said they had been in there quite a while and this problem began and just got worse. I haven't hooked it up to the stereomour due to fear. a lot of fear. I could cut the volume all the way down and then dial it up. Just kidding I won't. So the closer the impedance of the preamp output was to the input impedance of the speakers it sounded better. That's opposite world, right?
Any other thoughts?
Thanks
Eric

 

[Grainger49]

So, don't tap the tubes.  I think all tubes will make noise if you tap them.

Try it with your Stereomour, it is safe if you are careful with the Stereomour volume pot.  Run the FP volume wide open, I always do, then use the volume control on the Stereomour.  The FP has a lot of gain.  That can be tamed by putting a 249k Ohm resistor between the selector switch output and the volume pots for each channel.  But do it only if he seems to have the "Dreaded Hair Trigger Volume Control" in his system.  Every system is different. 

The FP should not distort with the volume all the way up. 

Swapping tubes is a good first step.  If the socket is a little dirty just insert and remove the tubes 10 times.  If the volume pot cuts in and out he might need a new pot or two.

Post back.

 

[ee]

haha- i haven't tapped on the tubes. Well when I first got the tube amp awhile back I did but only once just to see how it sounded. Didn't know at the time "NO TAPPING". Like a kid and a cookie jar.
Yep I'll put the new tubes in when they arrive and get back so it'll be a few days. My friend has decided I can have it if I fix it. Woohahaha. Means I may have a preamp for the Stereomour.
If I decide to put the 249k resistor to decrease gain does that replace the 100k( that's roughly what I measure) that's there or in series with it?

 

[Grainger49]

Good Deal ! ! !  This is a simple device.  Other than higher voltage out of the power supply than expected nothing seems wrong with it.  The pots can be replaced easily.

Yes, if the hair trigger volume pot happens in your system replace the 100k with a 240k. 

My system has BHDAC, Eros and a tuner feeding my FP 2.  Then with a minor channel balance on one side the volume pots are wide open, that runs to a Creek OBH-10 remote volume pot.  The Creek feeds my Paramours. 

I got tired of running back and forth adjusting the volume.  I'm 65, I'm retired, and lazy.  So the Creek for $100 is a nice addition.  Today you can get something to integrate into your FP 2 for $50 that also selects from four inputs on ebay.

 

[corndog71]

Personally, I've always liked the dual volume controls.  I think I prefer them to a single volume and balance control.  Particularly when I want to match channel levels as close as possible.

You got a good deal with that Foreplay!  It's such a great and simple little preamp.  I still keep mine as a backup. 

You can tweak the sound by swapping those big yellow caps with other brands.  I noticed the right channel has the cap going to the terminal strip before going to the output jack.  Since you have the slack I would take that jumper out and connect the cap directly to the output RCA jack.  Oh, and flip it around so it matches the other cap with the colored leads.  Ok, I'll stop.  I've tweaked mine to death and back!  Too much fun.

 

[Paul Birkeland]

With the volume all the way up on the speakers the preamp vol is super jumpy. [...]
So I'll go ahead and replace the tubes and see what happens then. He said they had been in there quite a while and this problem began and just got worse. I haven't hooked it up to the stereomour due to fear. a lot of fear. I could cut the volume all the way down and then dial it up. Just kidding I won't. So the closer the impedance of the preamp output was to the input impedance of the speakers it sounded better. That's opposite world, right?
Any other thoughts?

You need to pad the inputs, a lot.  The original Foreplay had way more gain than anybody needed.  Try removing each wire that goes from the selector switch to the pot, then replace each piece of wire with a 100K resistor.  On each pot, connect the outer two lugs together with a ~20K resistor. 

This will significantly increase the control range of the volume pots, and make the preamp more useable, all for about $1 worth of parts. 

 

[ee]

yep Corndog71 there is a bit of symmetry missing. This was a first build for him many moons ago. The wires in the power supply could probably have a bit tighter twist if I take it down that much. I'm gonna clean the sockets carefully and before I put in new tubes when they arrive.
Sorry to ask and I'm very sure this has been asked as it seems like it is on every build about caps but I can't seem to find much. Other than some builds on the internet.
1 Regarding the 3rd cap at T10 would 200uf be ok? I have 2 mundorf evo/oil 100uf I would like to parallel at that spot. If best to go 220 or larger then I'll get another to parallel in also.
2 The bypass cap on the tube socket I can't seem to get a good measure on to figure out it's size. If you will please give a uf on that one. The small redish brown one.
3 I saw where the Foreplay is phase inverted- does this mean I need to do anything when connecting it to an amp like my Stereomour?
Yeah he's being super generous with me. He's gonna have second thoughts in a couple of weeks . He inherited a McIntosh preamp/tuner a couple of years ago around the same time the Foreplay started to act up so he set aside but loves it's sound. He also built the Straight 8s back when he did the Foreplay. They are beautiful and sound amazing!
Thanks
Eric

 

[ee]

Update - New 12au7's are in and the amp is going strong  . The volume control is pretty touchy. It is useable from 0 degrees to about 45 degrees in very small increments.
I will try adding the 100k resistors from the selector switch to the pot for all of those wires (+ and -)and putting a 20k across the outer two lugs which should be the out and ground.
Thanks for your help. I'll give another update when I get done.
As far as the Stereomour and me making it an amp. Do I put the 100k resistors on wires both the + (red) and - (black) from the rca or just a resistor on the + (red) wires?
thanks
Eric

 

[Paul Birkeland]

I will try adding the 100k resistors from the selector switch to the pot for all of those wires (+ and -)and putting a 20k across the outer two lugs which should be the out and ground.

The middle lug of the pot is the output.  Nothing connects there except the connection to the 9 pin socket.

Thanks for your help. I'll give another update when I get done.
As far as the Stereomour and me making it an amp. Do I put the 100k resistors on wires both the + (red) and - (black) from the rca or just a resistor on the + (red) wires?

Just turn the volume pot all the way up, then it becomes a pair of 100K resistors.

 

[ee]

I failed to mention I had already taken out the vol pot and selector out of the Stereomour. But in the process I have learned more about the vol pot functioning as a voltage divider. V out = V in (R2/R1+R2) helped me understand I was putting them in the right place.  It's soldered in and measurements and sound vol are equal to before. Added benefit from the Kiwame carbon film I put in is that it sounds really nice as initial impression. Milky smooth and a bit warm and bass improved. Perhaps the detail quality of the bass seems better.

 

[corndog71]

The cap at the tube socket is for the DC offset on the AC heaters.  No need to change it or increase it.

As long as we're chatting about this...  Do you know if that cap and voltage divider is necessary if one uses regulated dc for the heaters?

 

[Grainger49]

The bias is there to keep the cathode to heater voltage within spec.  That is if I remember this correctly from over a decade ago. 

Whether the DC is regulated or not the bias is a good idea.