hello. i am getting an old Peak ST-100 amplifier back to making proper sound. using stereo 6GW8's and a 6CA4, it only has small output - in fact it is tiny. but after cleaning it up and doing the best i can to restore it to working, then hooking it up to my vienna acoustics mozart speakers, i simply cannot go back to my copland CSA-8. so i want to get this little old beast working properly, learn some things, and then move onto building a 2A3 SE design from the ground up, with a touch more power (and a touch more everything else i hope....)
i bought in on ebay and it was a complete mess. power cabling was insanely weird, pots replaced and missing knobs, most of the chassis missing.... and very noisy one channel, one channel completely dead. i cleaned it a little, replaced a couple of caps, buggered around with the tone control circuits a little, cut out a few needless pre-amp circuits, made less of a mess of the power cabling (it was actually pretty dangerous), and now i need some help in getting further. this is the basic circuit as far as i can draw it (excuse me - this is not my usual hobby!)
basically i want to remove the tone circuit, find some more bass in there somewhere if at all possible, put in new capacitors if needed, and box it all up nicely so it doesn't pose a serious spark risk to my girlfriend if she uses it.
one - how do i remove the tone control circuitry and create an flat tone across the internal preamp of the valve??? as it stands, the tone control is very problematic - the left channel can only be used set to "high" tone and there is some noise - it goes silent if set to "low" and distorts greatly in between - it seems some caps there are buggered? i would love to remove the tone section completely if possible. does anyone know how to do this? or should i simply replace all the associated caps, and the pot, or try cleaning the pot?
two - the bass is cut off pretty high, which i notice on the vienna acoustics pretty easy. would this be due to the transformers? or other components? is there anything worth trying out to regain some of that bass? or am i up for output transformer replacement to get any results?
three - there is quite a hum - is there anywhere worth looking to locate and cut it? any links to other articles would be great. this is really my first foray into tube work for some time (only once before 15 years ago fixing an old little valve amp very similar....)
four - should i consider replacing any other components? i have a new stereo alps pot to replace the dual mono volume controls, new valves, and a pair of 0.002uF oil caps for across the output transformers (one measured intermittently as short), but should i replace any other capacitors or other components?
....and if there is anything else i should worry about - please let me know - i am kind of new to all of this and generally good at talking as if i know stuff when i don't.... : )
cheers,
sean.
ps - this amp sounds sweet - got to say that it is real music, as opposed to the moderately expensive copland, which really should be in shame after sounding crap and tinny and artificial in front of this dusty old relic.... though it is gorgeously made and dead sexy, neither of which can be said for the old peak....
i bought in on ebay and it was a complete mess. power cabling was insanely weird, pots replaced and missing knobs, most of the chassis missing.... and very noisy one channel, one channel completely dead. i cleaned it a little, replaced a couple of caps, buggered around with the tone control circuits a little, cut out a few needless pre-amp circuits, made less of a mess of the power cabling (it was actually pretty dangerous), and now i need some help in getting further. this is the basic circuit as far as i can draw it (excuse me - this is not my usual hobby!)
basically i want to remove the tone circuit, find some more bass in there somewhere if at all possible, put in new capacitors if needed, and box it all up nicely so it doesn't pose a serious spark risk to my girlfriend if she uses it.
one - how do i remove the tone control circuitry and create an flat tone across the internal preamp of the valve??? as it stands, the tone control is very problematic - the left channel can only be used set to "high" tone and there is some noise - it goes silent if set to "low" and distorts greatly in between - it seems some caps there are buggered? i would love to remove the tone section completely if possible. does anyone know how to do this? or should i simply replace all the associated caps, and the pot, or try cleaning the pot?
two - the bass is cut off pretty high, which i notice on the vienna acoustics pretty easy. would this be due to the transformers? or other components? is there anything worth trying out to regain some of that bass? or am i up for output transformer replacement to get any results?
three - there is quite a hum - is there anywhere worth looking to locate and cut it? any links to other articles would be great. this is really my first foray into tube work for some time (only once before 15 years ago fixing an old little valve amp very similar....)
four - should i consider replacing any other components? i have a new stereo alps pot to replace the dual mono volume controls, new valves, and a pair of 0.002uF oil caps for across the output transformers (one measured intermittently as short), but should i replace any other capacitors or other components?
....and if there is anything else i should worry about - please let me know - i am kind of new to all of this and generally good at talking as if i know stuff when i don't.... : )
cheers,
sean.
ps - this amp sounds sweet - got to say that it is real music, as opposed to the moderately expensive copland, which really should be in shame after sounding crap and tinny and artificial in front of this dusty old relic.... though it is gorgeously made and dead sexy, neither of which can be said for the old peak....
1st things 1st. Remove the 1000 pF. cap. between the center tap (CT) of the power trafo rectifier winding and its primary. For very good reason, "Death Cap." is the appellation associated with that part. Safety also requires the installation of a proper, 3 wire, power cord.
Eliminating the FUGLY tone control is easy enough. The entire network of parts between the voltage amplifier triode's plate and the power pentode's control grid is removed. A 0.1 μF./400 WVDC cap. connects plate to grid and a 470 KOhm resistor connects the pentode's control grid to ground. I suggest a 716P series "Orange Drop" for the cap. and a 1/2 W. metal film part for the resistor.
The damping factor of that setup is VERY poor and it mates badly with your speakers. Part of the bass trouble comes from that fact. However, the bulk of the bass issue is due to the 100 μF. part in parallel with the 2 W./170 Ω resistor. Install a 220 μF./15 WVDC part in that position and the amp should get down to approx. 45 Hz.
As electrolytic capacitors literally dry out over time, all 'lytics in the unit require replacement.
Eliminating the FUGLY tone control is easy enough. The entire network of parts between the voltage amplifier triode's plate and the power pentode's control grid is removed. A 0.1 μF./400 WVDC cap. connects plate to grid and a 470 KOhm resistor connects the pentode's control grid to ground. I suggest a 716P series "Orange Drop" for the cap. and a 1/2 W. metal film part for the resistor.
The damping factor of that setup is VERY poor and it mates badly with your speakers. Part of the bass trouble comes from that fact. However, the bulk of the bass issue is due to the 100 μF. part in parallel with the 2 W./170 Ω resistor. Install a 220 μF./15 WVDC part in that position and the amp should get down to approx. 45 Hz.
As electrolytic capacitors literally dry out over time, all 'lytics in the unit require replacement.
Also try and do some basic distributed star grounding - where the mains protective earth goes separately to chassis; and the HT CT goes to new cap 0V, and that then goes to output stage 0V chassis.
eli - thanks so much for the reply - i will go through and sort this out. thanks for the tone control details and the electrolytic replacement - will suss it....
trobbins - just reading up on star grounding....
the three wire power cord was one of the first things i replaced - along with taking out the aluminium foil from the fuse holder (!!!)
i may be quite slow in getting parts, but i will post the progress.
trobbins - just reading up on star grounding....
the three wire power cord was one of the first things i replaced - along with taking out the aluminium foil from the fuse holder (!!!)
i may be quite slow in getting parts, but i will post the progress.
peak st-100 finally fiddled....
finally got around to restoring this old peak st-100 valve amp - i am listening to it in the morning sun with coffee and chocolate cake. and even besides all that goodness, it sounds sweet....
thanks to eli and others for suggestions - the amp has turned out nice. i intend to re-case it as i am a metalworker - hopefully something interesting and all stainless. it is nice to know that even a total fool can get something restored with a little help from others that know more - THANKS!
the tone controls were removed, replaced with a 0.1uF orange cap and 500k, electrolytic caps replaced, an alps volume pot put in, death cap removed, 0.002uF caps across the output, a decent power capacitor (held in with cable tie - classy....) and best of all nice tomiko output transformers - there is now bass again.
it sounds very articulate - clean and open, fast, bass is great and punchy, and it is all open and airy and full of imaging. not a great amount of power of course, but i am listening happily here even with some wharfedale diamonds and a big sealed 15" sub connected in series as well (crazy, but the amp is quite happy....) the only thing besides a new case is trying to get rid of the hum - it is persistent and while not invasive, it is there....
hearing lots of things i haven't heard before, and even better, just smiling when i am listening - it is joyous - i am happy.
the schematics are below - the originals as it came to me (the amp was hacked up even before i got it - i am not a butcher of pristine equipment - just perviously hacked up stuff), and also a schematic as it stands and works....
will post pics when it is re-cased, and in the meantime i will listen with joy. valves rule, even little baby ones like this!
original schematics as it came to me -
and revised now and working well with new transformers -
finally got around to restoring this old peak st-100 valve amp - i am listening to it in the morning sun with coffee and chocolate cake. and even besides all that goodness, it sounds sweet....
thanks to eli and others for suggestions - the amp has turned out nice. i intend to re-case it as i am a metalworker - hopefully something interesting and all stainless. it is nice to know that even a total fool can get something restored with a little help from others that know more - THANKS!
the tone controls were removed, replaced with a 0.1uF orange cap and 500k, electrolytic caps replaced, an alps volume pot put in, death cap removed, 0.002uF caps across the output, a decent power capacitor (held in with cable tie - classy....) and best of all nice tomiko output transformers - there is now bass again.
it sounds very articulate - clean and open, fast, bass is great and punchy, and it is all open and airy and full of imaging. not a great amount of power of course, but i am listening happily here even with some wharfedale diamonds and a big sealed 15" sub connected in series as well (crazy, but the amp is quite happy....) the only thing besides a new case is trying to get rid of the hum - it is persistent and while not invasive, it is there....
hearing lots of things i haven't heard before, and even better, just smiling when i am listening - it is joyous - i am happy.
the schematics are below - the originals as it came to me (the amp was hacked up even before i got it - i am not a butcher of pristine equipment - just perviously hacked up stuff), and also a schematic as it stands and works....
will post pics when it is re-cased, and in the meantime i will listen with joy. valves rule, even little baby ones like this!
An externally hosted image should be here but it was not working when we last tested it.
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Uploaded with ImageShack.comAn externally hosted image should be here but it was not working when we last tested it.
Uploaded with ImageShack.comAn externally hosted image should be here but it was not working when we last tested it.
Uploaded with ImageShack.comAn externally hosted image should be here but it was not working when we last tested it.
Uploaded with ImageShack.comAn externally hosted image should be here but it was not working when we last tested it.
Uploaded with ImageShack.comoriginal schematics as it came to me -
An externally hosted image should be here but it was not working when we last tested it.
Uploaded with ImageShack.comand revised now and working well with new transformers -
An externally hosted image should be here but it was not working when we last tested it.
Uploaded with ImageShack.comHi guy!
You've already taken a very serious plunge in replacing the puny OEM O/P transformers. JMO, it's time to go "hog wild". 😉
The 6GW8/EL86 is a fine tube, but it's rapidly going the way of the Dodo. There are 3X 9 pin miniature (Noval) sockets on the chassis. Switch to SS rectified B+ and use 2X 6BQ5/EL84s and a Russian 6H2П-EB (6n2p-ev) as the tube complement. You will wind up with approx. 5 WPC.
The extra volts SS rectification of the B+ yields can be combined with regulated screen grid B+ to improve open loop linearity. Improve damping factor and overall linearity by adding a global negative feedback loop.
Scan the archives for posts about star and bus grounding schemes. Those methods are very well proven for holding residual hum to a dead minimum.
The cap. across the O/P trafo primary rates to be unneeded, now.
To avoid high frequency roll off, use 100,000 Ω volume controls. The resistance of the volume control combines with the Miller capacitance of the voltage amplifier triode to create a low pass filter. The "corner" frequency of that filter is out of the audio band, when the resistance at the I/P is held down.
You've already taken a very serious plunge in replacing the puny OEM O/P transformers. JMO, it's time to go "hog wild". 😉
The 6GW8/EL86 is a fine tube, but it's rapidly going the way of the Dodo. There are 3X 9 pin miniature (Noval) sockets on the chassis. Switch to SS rectified B+ and use 2X 6BQ5/EL84s and a Russian 6H2П-EB (6n2p-ev) as the tube complement. You will wind up with approx. 5 WPC.
The extra volts SS rectification of the B+ yields can be combined with regulated screen grid B+ to improve open loop linearity. Improve damping factor and overall linearity by adding a global negative feedback loop.
Scan the archives for posts about star and bus grounding schemes. Those methods are very well proven for holding residual hum to a dead minimum.
The cap. across the O/P trafo primary rates to be unneeded, now.
To avoid high frequency roll off, use 100,000 Ω volume controls. The resistance of the volume control combines with the Miller capacitance of the voltage amplifier triode to create a low pass filter. The "corner" frequency of that filter is out of the audio band, when the resistance at the I/P is held down.
eli,
you are speaking in a language that makes me feel like i need to read a LOT more - so i will....
i will remove the cap across the transformer - thanks.
i have a 50k volume pot - i will look into a 100k version....
just looked at the price of 6gw8 tubes - wow - lucky i bought 4 when prices were low.... this whole wild hog business may have to wait while i get my brain into gear for the mods you suggest.... one step at a time!!!
you are speaking in a language that makes me feel like i need to read a LOT more - so i will....
i will remove the cap across the transformer - thanks.
i have a 50k volume pot - i will look into a 100k version....
just looked at the price of 6gw8 tubes - wow - lucky i bought 4 when prices were low.... this whole wild hog business may have to wait while i get my brain into gear for the mods you suggest.... one step at a time!!!
That's fine! Over 100K, as was the OEM value, is where trouble can start.
RC filters follow a simple enough law, which follows immediately.
0 = (2П) (Fhertz) (Cfarads) (Rohms)
If you know any 2 of the variables, you can compute the 3rd.
Hi. I just play with tubes and Eli has helped me out a ton. It looks like a cap to ground at point A on your schematic might help your hum issue. Whenever I've had a series resistor in the power supply and then a tap to a stage with no cap to ground I've had some hum. Hopefully someone will confirm.
I had a 6bm8 amp for years and just loved it. Came out of an old reel to reel. Looks like fun. -Fred
I had a 6bm8 amp for years and just loved it. Came out of an old reel to reel. Looks like fun. -Fred
OOPS!! 
I misstated the RC Filter Law. The correct formula follows below.
1 = (2П) (Fhertz) (Cfarads) (Rohms)
I misstated the RC Filter Law. The correct formula follows below.
1 = (2П) (Fhertz) (Cfarads) (Rohms)
ha - thanks eli!
and thanks also - i will have a look at the cap - but i think there is one to ground from A...
i think (and i am very unknowledgeable, but it kind of makes sense to me), that the hum may be due to ground loops - the chassis is a mess as you can see, and it needs to be combed through and the grounding looked at. it was fairly random grounding, doing it as i went - most things were pulled out and replaced, but much was moved and forgotten....
will be a nice thing to do - though grounding seems so arcane when you first come across the idea!
thanks again everyone for your input! any other ideas and thoughts gratefully welcomed....
and thanks also - i will have a look at the cap - but i think there is one to ground from A...
i think (and i am very unknowledgeable, but it kind of makes sense to me), that the hum may be due to ground loops - the chassis is a mess as you can see, and it needs to be combed through and the grounding looked at. it was fairly random grounding, doing it as i went - most things were pulled out and replaced, but much was moved and forgotten....
will be a nice thing to do - though grounding seems so arcane when you first come across the idea!
thanks again everyone for your input! any other ideas and thoughts gratefully welcomed....
Hi Sean, How are you going with this project? I've just bought the same model amp and found you guys here when searching for info. Mona is correct - there should be a cap from A to ground. If you look at the cap you removed you'll see it is actually two caps in one can. Did you replace it with a single cap or two? If you used only one, this could certainly create all sorts of hum.
hi,
i am not that great with all this, so don't take anything i say as making sense....
the double cap, which i replaced with a slightly larger one, has the neg to earth (common to both sides of the double). the positive is spanned by a 820ohm 10W resistor.
that is right, yes?
it is not ridiculous hum - it sounds more like earthing subtleties, but as said, i am not great with all this....
not much grunt, fancy that, but i am making some little speakers with some efficiency for this to drive in my study....
if you have any suggestions, please do not hesitate to tell me!!!
cheers,
sean.
i am not that great with all this, so don't take anything i say as making sense....
the double cap, which i replaced with a slightly larger one, has the neg to earth (common to both sides of the double). the positive is spanned by a 820ohm 10W resistor.
that is right, yes?
it is not ridiculous hum - it sounds more like earthing subtleties, but as said, i am not great with all this....
not much grunt, fancy that, but i am making some little speakers with some efficiency for this to drive in my study....
if you have any suggestions, please do not hesitate to tell me!!!
cheers,
sean.
ps - if anyone would know what value cap to put from a to ground, i would give that a try for sure!
Hi Sean, it sounds you soldered it up correctly re the double cap. So positive pin1 on the cap is connected to the rectifier, you have the 820r to pin2, then pin 2 becomes "A".
As far as the grounding in the amp goes - it sure is a mess, no wonder this thing's noisey! I was tempted just now to uproot all the chassis ground points and install a star grounding system, but it would probably work out just as easy to rebuild the entire thing with different valves as per suggestions from page 1.
I removed the caps across the outputs and mine started screaming like crazy. Poor thing has all sorts of issues. Next stop later in the week will be some feedback I think.
As far as the grounding in the amp goes - it sure is a mess, no wonder this thing's noisey! I was tempted just now to uproot all the chassis ground points and install a star grounding system, but it would probably work out just as easy to rebuild the entire thing with different valves as per suggestions from page 1.
I removed the caps across the outputs and mine started screaming like crazy. Poor thing has all sorts of issues. Next stop later in the week will be some feedback I think.
just plugged mine into my main speakers again - nice. surprisingly, one watt can do a lot. it can even make me smile - the music is alive.
the cap across the output is gone on mine and there is no screaming, so i'm not sure what is with yours mr tropical..... maybe somebody who knows something can pipe in?
i must repeat that the output transformer swap was a real amazing difference - there is now bass, and it is all fast and clear (up to a point, where it starts to clip, getting carried away with that volume knob....)
nice amp.
the cap across the output is gone on mine and there is no screaming, so i'm not sure what is with yours mr tropical..... maybe somebody who knows something can pipe in?
i must repeat that the output transformer swap was a real amazing difference - there is now bass, and it is all fast and clear (up to a point, where it starts to clip, getting carried away with that volume knob....)
nice amp.
Maybe copy first part of (google for manual download) the Eico HF-12 PS as it uses the same rectifier tube and PS trans B+ specs. It's also low DCR with no resistance to the B+ so get's rid of that resistor you have in yours which is a big plus sonically.
Since it's an SE amp you could add a small low DCR choke (around 20 ohms or less) 1-2H Edcor would help of around 250 mA capacity for low cost in place of the resistor after the first cap. Maybe you can find something similar over there. You can get bigger 4H+ (will work better on an SE amp) chokes from other makers like Hammond that are low DCR , but they are big and double to triple the price.
Since it's an SE amp you could add a small low DCR choke (around 20 ohms or less) 1-2H Edcor would help of around 250 mA capacity for low cost in place of the resistor after the first cap. Maybe you can find something similar over there. You can get bigger 4H+ (will work better on an SE amp) chokes from other makers like Hammond that are low DCR , but they are big and double to triple the price.
I read this post with interest, just got myself PEAK ST-100, haven't try it yet because rectifier tube is missing. Orangealpaca, in your new diagram, there is volume control missing...also isn't there supposed to be resistor and capacitor connected to cathode of pentode section, as it is to triode? Have you done any other mods, what is over all impression on sound after all this time...mod I will try to implement (apart from those already mentioned) will be to install two caps and two small chokes in output of power supply...any comments from other members having same amp welcome...
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