help with an old Peak ST-100 - single ended 6GW8

orangealpaca · 27th October 2012, 09:38 AM

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....)

P1130258.jpg

P1130259.jpg

P1130264.jpg

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!)

schematic.jpg

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....

Eli Duttman · 27th October 2012, 02:50 PM

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.

trobbins · 27th October 2012, 09:13 PM

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.

orangealpaca · 27th October 2012, 11:24 PM

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.

27th October 2012, 09:38 AM	#1
orangealpaca diyAudio Member Join Date: Apr 2008	help with an old Peak ST-100 - single ended 6GW8 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....) P1130258.jpg P1130259.jpg P1130264.jpg 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!) schematic.jpg 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....

27th October 2012, 02:50 PM	#2
Eli Duttman diyAudio Member Join Date: Apr 2004 Location: Plainsboro, NJ	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. __________________ Eli D.

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27th October 2012, 09:13 PM	#3
trobbins diyAudio Member Join Date: Feb 2009 Location: Melbourne, Oz	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.

27th October 2012, 11:24 PM	#4
orangealpaca diyAudio Member Join Date: Apr 2008	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.