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+6

Luddite

tomlang

danf

GP49

Bob Latino

Bugs

10 posters

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ST 70 Quad Cap (Long Post)

Bugs: Posts : 90
Join date : 2009-04-17

Post n°1

ST 70 Quad Cap (Long Post)

by Bugs Tue Sep 22, 2009 3:21 pm

I have two ST 70s that I have built to be indentical using VTA boards and upgraded quad caps. The only difference is that I have fitted one of Bob's triode/pentode switches to one and haven't got around to doing that in the other.

The amp with the triode/pentode switch developed a progressively louder hum over the course of a couple of weeks and along with degraded sound quality from the speakers. I suspected the input transformer and ordered a replacement. While I was putting in the transformer I also put in new tube sockets.

When I started up the amp with the new transformer the amp still hums quite loud. On initial start up the speakers also hum very loud and after 10 seconds or so quiet down. The hum in both speakers is equal. To me it sounds like a 60 cycle hum from both the amp and speakers.

Both channels bias properly. I have swapped all the tubes out with identical tubes from the other amp with no change to the hum. Switching from triode to pentode has no noticable effect on the hum from the amp or speakers.

So, having not properly diagnosed the problem in the first place, the transformer does not appear to be the source of the problem (= waste of time and money Rolling Eyes

Rolling Eyes

)and tubes do not appear to be the problem.

The quad cap is less than a year old and I do not see any leaks. Should I replace the quad cap and see what happens or are there some other diagnostics I can try? Please keep in mind that I am new to electronics so I've no doubt missed something obvious and at the same time I do not really want to electrocute myself.

Thanks,
Doug

Bob Latino: Admin; Posts : 3276
Join date : 2008-11-26
Location : Massachusetts

Post n°2

Re: ST 70 Quad Cap (Long Post)

by Bob Latino Tue Sep 22, 2009 5:25 pm

Bugs wrote:I have two ST 70s that I have built to be indentical using VTA boards and upgraded quad caps. The only difference is that I have fitted one of Bob's triode/pentode switches to one and haven't got around to doing that in the other.

The amp with the triode/pentode switch developed a progressively louder hum over the course of a couple of weeks and along with degraded sound quality from the speakers. I suspected the input transformer and ordered a replacement. While I was putting in the transformer I also put in new tube sockets.

When I started up the amp with the new transformer the amp still hums quite loud. On initial start up the speakers also hum very loud and after 10 seconds or so quiet down. The hum in both speakers is equal. To me it sounds like a 60 cycle hum from both the amp and speakers.

Both channels bias properly. I have swapped all the tubes out with identical tubes from the other amp with no change to the hum. Switching from triode to pentode has no noticable effect on the hum from the amp or speakers.

So, having not properly diagnosed the problem in the first place, the transformer does not appear to be the source of the problem (= waste of time and money )and tubes do not appear to be the problem.

The quad cap is less than a year old and I do not see any leaks. Should I replace the quad cap and see what happens or are there some other diagnostics I can try? Please keep in mind that I am new to electronics so I've no doubt missed something obvious and at the same time I do not really want to electrocute myself.

Thanks,
Doug

Doug,

The most common cause of a hum in both channels that does not change with volume is a leaky quad cap. This is easily diagnosed with a cap meter but most people don't have a cap meter.

Try this > Make sure the amp is OFF and has been OFF for a couple of minutes. Then use an INSULATED screwdriver and short each one of the quad cap's four terminals to the chassis. This will discharge the quad cap. Try measuring the resistance of each section of the quad cap with a multitester. What will normally happen is that you should see a rising (changing) resistance on your meter over a period of a minute or so. The resistance of each section should be over 1,000,000 ohms (1 meg ohm). If one or more sections only show 5000 to 10,000 ohms (or maybe even lower) the quad cap is probably bad and should be replaced. Even though your quad cap is only a year old it *could* have gone bad (developed a partial short).

Always test for hum with the input jacks on an ST-70 terminated. Plug in a set of RCA interconnects and either short the center pin to the outer shield on each interconnect on the OTHER END > or simply plug the other ends into your preamp (keep the preamp OFF).

Let us know what you find ??

Bob

Bugs: Posts : 90
Join date : 2009-04-17

Post n°3

Re: ST 70 Quad Cap (Long Post)

by Bugs Tue Sep 22, 2009 5:31 pm

Thanks Bob, When I check for resistance on the quad cap, do I check between each terminal and the ground or between terminals?

Thanks,
Doug

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Post n°4

Re: ST 70 Quad Cap (Long Post)

by Bob Latino Tue Sep 22, 2009 5:56 pm

Bugs wrote:Thanks Bob, When I check for resistance on the quad cap, do I check between each terminal and the ground or between terminals?

Thanks,
Doug

Doug,

Measure the resistance between each of the 4 terminals and the chassis ground one terminal at a time.

Bob

Bugs: Posts : 90
Join date : 2009-04-17

Post n°5

Re: ST 70 Quad Cap (Long Post)

by Bugs Tue Sep 22, 2009 9:27 pm

All of the quad cap terminals tested at around 3.5 M ohm.

Still humming along. A couple other observations are that I can noticably feel the amp vibrate from the hum while the quiet amp does not have any vibration what-so-ever. When I discharged the cap I did not see or hear any spark jump.

Do I need to disconnect the wiring and resistor from the quad cap before doing this test? It seems like at least three of the cap section are hooked up in parallel.

I'm thinking I should check my wiring one more time or something else?

Thanks,
Doug

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Location : Massachusetts

Post n°6

Re: ST 70 Quad Cap (Long Post)

by Bob Latino Tue Sep 22, 2009 9:55 pm

Doug,

If all the quad cap terminals measure in at 3.5 meg or higher then the quad cap is *probably* OK. You don't need to disconnect the resistors or the choke from the cap.

Next > Recheck all the ground connections in the amp.

> Do a voltage check on the amp. The voltages you should get for an ST-70 with a VTA driver board are below.

* Note # 1 – Make sure you have your meter set to AC or DC as mentioned below. Ground is any point on the chassis frame. Place the black or negative probe on the chassis and the red or positive probe on the point mentioned. All tubes should be plugged in, inputs shorted or connected to your preamp, speakers connected and no signal should be running through the amplifier. Be careful not to cross two pins with the positive probe !
* Note # 2 – Readings very slightly above or very slightly below the range are not normally a sign of a problem. Line voltages vary slightly throughout the country. Differences in the GZ34 rectifier tube can cause variations also. The use of a solid state rectifier in place of the GZ34 may cause some voltages to be higher.
GZ34 – Pin 2 to ground – 420 - 450 volts DC
Pin 8 to ground – 420 - 450 volts DC
Pin 4 to ground – 355 - 380 volts AC
Pin 6 to ground – 355 - 380 volts AC

Any EL34 – Pin 1 or pin 8 to ground – approx .400 volts DC (depends on bias setting)
Across pins 2 and 7 – 6.0 – 6.5 volts AC
Pin 3 to ground – 400 - 435 volts DC
Pin 4 to ground – 400 - 435 volts DC (usually 1 or 2 volts higher than pin 3)
Pin 5 to ground – minus 30 to minus 40 volts DC
Pin 6 to ground – minus 30 to minus 40 volts DC
Pin 8 to ground – approx .400 volts DC (depends on bias setting)

Quad cap –-- Section # 1 (square symbol) –------ 420 – 450 volts DC
Section # 2 (half circle symbol) --- 420 – 445 volts DC
Section # 3 (NO symbol) ------------- 420 – 445 volts DC
Section # 4 (triangle symbol) ------ 380 – 420 volts DC

Central 12AT7 – Pin 1 or pin 6 to ground > 140 – 165 volts DC
Two outer 12AT7’s – Pin 1 or pin 6 to ground > 250 – 280 volts DC
Any 12AT7 – across the two incoming heater wires > 6.0 - 6.5 volts AC
“-50 VAC” on driver board > 48 – 55 volts AC
“B+” on driver board > 380 – 420 volts DC

Bob

Bugs: Posts : 90
Join date : 2009-04-17

Post n°7

Re: ST 70 Quad Cap (Long Post)

by Bugs Wed Sep 23, 2009 1:04 am

Okay, low DC voltages throughout. Plus I could not find a pin-out diagram for the 12AT7s. Switches were set for pentode.

GZ34

Pin 2--- 329.2 VDC
Pin 8--- 329 VDC
Pin 4--- 387.5 VAC
Pin 6--- 384 VAC

KT 66 (bias ~ .43 VDC

Pin 1--- V2 .440, V3 .440, V6 .45, V7 .451 (VDC)

Across Pins 2 and 7--- V2 6.75, V3 6.72, V6 6.70, V7 6.72 (VAC)

Pin 3--- V2 312.6, V3 313.0, V6 313.0, V7 312.3 (VDC)
Pin 4--- V2 315.5, V3 315.2, V6 315.3, V7 315.3 (VDC)
Pin 5--- V2 -30.53, V3 -31.02, V6 -29.68, V7 -29.71 (VDC)
Pin 6--- V2 -30.67, V3 -31.08, V6 -29.78, V7 -29.72 (VDC)
Pin 8--- V2 .441, V3 .441, V6 .451, V7 .452 (VDC)

Quad Cap

Sec. 1--- 318.5 VDC
Sec. 2--- 318.6 VDC
Sec. 3--- 283.9 VDC
Sec. 4--- 332.6 VDC

12AT7

I could not find a pin-out diagram. No readings.

"-50 VAC"--- 59.5 VAC
"B+"-------- 284.4 VDC

Bob Latino: Admin; Posts : 3276
Join date : 2008-11-26
Location : Massachusetts

Post n°8

Re: ST 70 Quad Cap (Long Post)

by Bob Latino Wed Sep 23, 2009 5:58 am

Doug,

The B+ DC high voltage is low. Most likely either the rectifier tube is bad OR the quad cap is bad. There is also a smaller possibility that there is a problem on the driver board.

Try a new rectifier in there OR try the rectifier from the other amp that you have in there FIRST and recheck the DC high voltage on pin #8 of the rectifier tube. See if the voltage rises on pin #8 with the new rectifier and the hum goes away. If it doesn't then most likely the quad cap is bad.

Bob

Bugs: Posts : 90
Join date : 2009-04-17

Post n°9

Re: ST 70 Quad Cap (Long Post)

by Bugs Wed Sep 23, 2009 9:48 am

Using the other rectifier tube, both pin 8 of the rectifier tube and the B+ dropped a few volts.

Pin 8 went from to 329.2 to 326.6 (VDC)

B+ went from 284.4 to 281.2 (VDC)

Both rectifier tubes drive the other amp just fine.

Should I order a quad cap and try that?

Bob Latino: Admin; Posts : 3276
Join date : 2008-11-26
Location : Massachusetts

Post n°10

Re: ST 70 Quad Cap (Long Post)

by Bob Latino Wed Sep 23, 2009 10:50 am

Doug,

It almost has to be the quad cap. For some reason it won't hold the charge properly. It may be just one section but that's all it takes. Order a new quad cap and replace the old one ...

Bob

GP49: Posts : 792
Join date : 2009-04-30
Location : East of the sun and west of the moon

Post n°11

Re: ST 70 Quad Cap (Long Post)

by GP49 Wed Sep 23, 2009 11:06 am

Bugs wrote:

"All of the quad cap terminals tested at around 3.5 M ohm."

But Bob wrote: "Try measuring the resistance of each section of the quad cap with a multitester. What will normally happen is that you should see a rising (changing) resistance on your meter over a period of a minute or so. The resistance of each section should be over 1,000,000 ohms (1 meg ohm)."

Bugs, did you see the GRADUALLY RISING RESISTANCE happening as each capacitor section charged up after you had completely discharged it? If not, that particular capacitor section could be OPEN and thus not doing any filtering at all; it would be as though it were not in the circuit at all.

The low power supply voltages suggest that there is inadequate power supply filter capacitance, for whatever reason or defect.

Also, the "progressively louder hum over the course of a couple of weeks" would seem to rule out a transformer problem, which would likely have come on catastrophically. It does suggest either a progressive deterioration of a section or sections of the quad cap, or a cold solder connection somewhere, that has gone bad. Is the shell of the quad cap well and truly grounded to the chassis? Have you verified the integrity of every connection, in addition to verifying that each wire is going to where it is supposed to go? You may consider reheating each solder connection and reflowing the solder on it, if you have not already done so.

I suppose the internal ground connection of one or more sections of the quad cap could be defective so that there isn't a really good connection internally to the shell. That could cause your symptoms even if the shell of the quad cap is properly grounded.

Bugs: Posts : 90
Join date : 2009-04-17

Post n°12

Re: ST 70 Quad Cap (Long Post)

by Bugs Wed Sep 23, 2009 4:07 pm

GP49, thanks for the input. I went back and rechecked the resistance on the quad cap and it does ramp up over 10 or 15 seconds. I also rechecked the solder joints on the cap, reflowed the joint, and added an extra ground the the shell while I was at it. Still no change after power-up. I'll take another look at the wiring diagram while I'm waiting for the new quad cap to arrive.

The last thing I can think of is when the new transformer arrived it was missing one of the nylon shoulder washers. I substituted a couple of the old fiber washers to make up the difference. If I had potentially secured the bolt through the transformer off center and it was touching the side of the plates, could that cause the problems I'm seeing?

Thanks guys,
Doug

Bugs: Posts : 90
Join date : 2009-04-17

Post n°13

Re: ST 70 Quad Cap (Long Post)

by Bugs Tue Sep 29, 2009 9:55 pm

New quad cap and no hum. DC voltages are way up, in most cases over spec. Section 3 of the quad cap looked odd.

Should I hook it up or take a look at the high voltages?

Thanks,
Doug

GZ34

Pin 2--- 479 VDC
Pin 8--- 479 VDC
Pin 4--- 388 VAC
Pin 6--- 390 VAC

KT 66 (bias ~ .43 VDC

Pin 1--- V2 .429, V3 .426, V6 .427, V7 .418 (VDC)
Across 2 & 7 --- V2 6.76, V3 6.75, V6 6.65, V7 6.66 (VAC)
Pin 3--- V2 460, V3 460, V6 460, V7 460 (VDC)
Pin 4--- V2 463, V3 463, V6 461, V7 461 (VDC)
Pin 5--- V2 -50, V3 -50, V6 -49, V7 -49(VDC)
Pin 6--- V2 -50, V3 -50, V6 -49, V7 -49(VDC)
Pin 8--- V2 .428, V3 .426, V6 .426, V7 .426 (VDC)

Quad Cap

Sec. 1--- 462 VDC
Sec. 2--- 462 VDC
Sec. 3--- 410 VDC
Sec. 4--- 474 VDC

12AT7

I could not find a pin-out diagram. No readings.

"-50 VAC"--- 60 VAC
"B+"-------- 409 VDC

Bob Latino: Admin; Posts : 3276
Join date : 2008-11-26
Location : Massachusetts

Post n°14

Re: ST 70 Quad Cap (Long Post)

by Bob Latino Wed Sep 30, 2009 5:18 am

Bugs,

The voltages are a little on the high side but are within a normal range. Sometimes if your AC line voltage are a little high you can get the readings that you posted in your last post. Just adjust the bias and play the amp ...

Bob

Bugs: Posts : 90
Join date : 2009-04-17

Post n°15

Re: ST 70 Quad Cap (Long Post)

by Bugs Wed Sep 30, 2009 7:14 pm

Bob, thank you once again for all the help and support. You are officially my hero Shocked

Shocked

.

GP49, thank you too for the help.

I biased the tubes and hooked up the amp and it sounds great. This is one of the ST 70s I'm using to vertically biamp my speakers. So far the system sounds great. I'll have to listen a little more to determine if there is really a difference over using one amp.

Bugs: Posts : 90
Join date : 2009-04-17

Post n°16

Re: ST 70 Quad Cap (Long Post)

by Bugs Thu Nov 12, 2009 12:29 am

Life is a mystery. As I said in my last post, I replaced my quad cap and everything was singing along splendidly.

Tonight my ST 70 started to hum very loud again. I pulled the bottom cover off to check the voltages and found a puddle of electrolytic on the inside of the cover.

I'm okay with replacing the quad cap again, however it has only been operating for six weeks Rolling Eyes

Rolling Eyes

. Could there be something else happening in my amp that is causing the quad caps to go bad? My original cap was a new replacement and it lasted about six months. So I'm sitting at two blown caps in about eight months.

Was it just luck of the draw, am I a statistical blip in the quad cap quality control chart or do you think there is soemthing else going on? I'm perplexed.

Thanks,
Doug

Here is a thought, the only other change I have made to the system is to put in a Weber Copper Cap rectifier about a week ago. My voltages were a little high when I tested with the new quad cap after installation. Could the Cooper Cap rectifier have increased the voltages enough to have blown the cap?

Bob Latino: Admin; Posts : 3276
Join date : 2008-11-26
Location : Massachusetts

Post n°17

Re: ST 70 Quad Cap (Long Post)

by Bob Latino Thu Nov 12, 2009 7:10 am

Doug,

The use of the Weber Copper Cap does raise the B+ voltage maybe 10 or 12 volts but even that should be at least 70 volts BELOW the handling capacity of the 525 volt quad cap in an ST-70. Sometimes lightning does strike twice in the same place. There was something on the news the other day about a New Hampshire couple that won the NH lottery TWICE. (See link below)

http://www.lotterypost.com/news/203373

I know that this fact doesn't help your situation but stranger things have happened. What I would do in your case is switch over to the Dynakitparts 550 volt quad cap which will give you an extra 25 volts of power handling capacity. Link below ..

Dynakitparts 550 volt quad cap

Bob

Last edited by Bob Latino on Wed Nov 18, 2009 9:44 am; edited 1 time in total

danf: Posts : 58
Join date : 2009-01-19

Post n°18

Solid State Rectifier and high turn-on voltages

by danf Thu Nov 12, 2009 9:30 am

The original GZ34 has the excellent feature that it begins conducting (rectifying) slowly, so that the output tubes are warmed up and drawing current before the voltage ramps up completely. If you use a solid state rectifier or even a directly heated cathode tube, the turn on voltage (before the output tubes warm up) will be considerably higher than the operating voltage. Add to this the higher voltages with a solid state rectifier and you are putting a huge strain on the filter capacitor. I would bet that the voltage surges well above 550V during turn-on. I would ditch the copper cap rectifier or get capacitors rated at 600V or higher. I like solid state rectification but it cannot be just dropped in without re-engineering.

Bob Latino: Admin; Posts : 3276
Join date : 2008-11-26
Location : Massachusetts

Post n°19

Re: ST 70 Quad Cap (Long Post)

by Bob Latino Thu Nov 12, 2009 10:01 am

danf wrote:The original GZ34 has the excellent feature that it begins conducting (rectifying) slowly, so that the output tubes are warmed up and drawing current before the voltage ramps up completely. If you use a solid state rectifier or even a directly heated cathode tube, the turn on voltage (before the output tubes warm up) will be considerably higher than the operating voltage. Add to this the higher voltages with a solid state rectifier and you are putting a huge strain on the filter capacitor. I would bet that the voltage surges well above 550V during turn-on. I would ditch the copper cap rectifier or get capacitors rated at 600V or higher. I like solid state rectification but it cannot be just dropped in without re-engineering.

Dan,

The Weber Copper Cap does have a short (3 or 4 second) delay before applying high voltage to simulate the warm up of a GZ34 tube rectifier. Yes - It's not as long as a GZ34 (maybe 15 seconds). I have used a Weber Copper Cap for two years now in my own ST-70 amp with no issues. This amp does use the CE quad cap. If someone does intend to use use solid state rectification then the Dynakitparts 550 volt (600 volt surge) *may* stand up better to a SS rectifier and might be a better choice in an ST-70 with SS rectification.

As an aside > I have talked to CE Distribution in Arizona where the CE quad caps are made. They know the voltage "surges" during turn on and regularly test their caps at voltages higher than the 525 volts steady state handling capacity at which the cap is rated.

The fact that this happened to Doug TWICE also leads me to believe that their *could* be some other issue inside his amp that caused this to happen BUT maybe not?

Bob

tomlang: Posts : 36
Join date : 2009-08-12

Post n°20

Re: ST 70 Quad Cap (Long Post)

by tomlang Thu Nov 12, 2009 10:21 am

How does the Weber Copper Cap do the delay?

Bob Latino: Admin; Posts : 3276
Join date : 2008-11-26
Location : Massachusetts

Post n°21

Re: ST 70 Quad Cap (Long Post)

by Bob Latino Thu Nov 12, 2009 10:26 am

tomlang wrote:How does the Weber Copper Cap do the delay?

Hi Tom,

I am not exactly sure how they do the delay? Maybe there is a thermistor inside the cap? Your question should probably be directed to Weber who makes the cap. If they give you a reply maybe you can post it here in this thread ...

Bob

Bugs: Posts : 90
Join date : 2009-04-17

Post n°22

Re: ST 70 Quad Cap (Long Post)

by Bugs Thu Nov 12, 2009 8:05 pm

Thanks Bob,

I ordered the 550 V quad cap today. I'll put in next week and keep my fingers crossed.

Everything in the amp is new except for the choke and output transformers. Is there any wiring I should pay close attention to while I have it open?

I do have a triode switch installed and have used the switch at low volumes only. Should the amp be off when using the switch?

Best

Last edited by Bugs on Thu Nov 12, 2009 10:12 pm; edited 1 time in total

Bob Latino: Admin; Posts : 3276
Join date : 2008-11-26
Location : Massachusetts

Post n°23

Re: ST 70 Quad Cap (Long Post)

by Bob Latino Thu Nov 12, 2009 8:31 pm

Bugs wrote:Thanks Bob,

I ordered the 550 V quad cap today. I'll get put in next week and keep my fingers crossed.

Everything in the amp is new except for the choke and output transformers. Is there any wiring I should pay close attention to while I have it open?

I do have a triode switch installed and have used the switch at low volumes only. Should the amp be off when using the switch?

Best

Hi Doug,

Just check all the high voltage wiring to pins 3 and 4 of the output tubes and at the new quad cap that you put will put in.

Re > the triode/pentode switches. My advice is never to make the switch at high volume levels but switching at normal volume levels is OK - at least on a Dynaco EL34 or KT88 based amp. On the Dynaco ST-70, Mark II, Mark III and Mark IV amps the voltage on the plates and the ultralinear screen tap are almost the same on these amps. There is virtually no "voltage gradient" (the relative difference between two voltages). It is when you swing between two very different voltages that you can get a transient spike if you flip a switch. On a Dynaco ST-70 if the plate voltage (pin 3 of each output tube) is say 425 volts the ultralinear screen tap (pin 4 of each output tube) will be only 1 or 2 volts higher - maybe 427 volts. A two volt voltage gradient is almost nothing - less than 1 % of the total voltage. On other brands of amps (non Dynaco), depending on how the circuit is set up, this may not be the case. I once had a pair of ASL Hurricane monoblocks with triode/pentode switches. You had to turn the amp OFF to make the switch between triode and pentode (as it said in the manual). If you tried to do it while the amp was running you got a God awful noise through the speakers. I only did it once (nothing happened to the amp in question thankfully) but I never made the switch again with these amps with the amp running.

Bob

Bugs: Posts : 90
Join date : 2009-04-17

Post n°24

Re: ST 70 Quad Cap (Long Post)

by Bugs Tue Nov 17, 2009 11:13 pm

Bob, I found your comments in this forum on the CE quad cap orientation before I wrote everything below the line. I believe that all I need to know is which lugs the 2.2K resistor goes between and which lugs to jumper together.

From what I read the half circle goes to the rear and has the two OPT leads and one choke lead going to it.

The square gets the rectifier pin 8 lead and a choke lead.

The unmarked lug gets the B+ lead

My previous quad cap was also a CE and it was not hooked up like this.

My other ST70 has an OP Manufacturing quad cap and seems to be working fine with the square lug to the rear. Am I lucky or is this okay with that cap?
___________________________________________________________
I'm thinking I may have lost my quad cap because I had the quad cap orientation wrong. Bob would you staighten me out on the connections. I had the square 40 mf lug facing the rear of the chasis towards the transformers with one choke lead, two output transformer leads and a lead jumpered to the 80 mf lug. Is this the right orientation?

I also have a 2.2 kohm resistor between the 80 mf lug and the 20 mf lug.

Okay, I found my VTA instructions.

Section 1, Square 40 mf - Right OPT, left OPT, and choke out
Section 2, Half Circle 80 mf - one side of a 2.2K resistor
Section 3, Line 20 mf - B+ from VTA board and the other side
Section 4, Triangle 30 mf - Choke in, pin 8 from rectifier
of the 2.2K resistor

The VTA instructions say to jumper between Sections 2 and 3.

As I continue to do my home work I'm begining to think that different quad caps have different symbols. I'm using the cap from Dyanakit.

Thanks,
Doug

Bob Latino: Admin; Posts : 3276
Join date : 2008-11-26
Location : Massachusetts

Post n°25

Re: ST 70 Quad Cap (Long Post)

by Bob Latino Wed Nov 18, 2009 8:30 am

Bugs wrote:Bob, I found your comments in this forum on the CE quad cap orientation before I wrote everything below the line. I believe that all I need to know is which lugs the 2.2K resistor goes between and which lugs to jumper together.

From what I read the half circle goes to the rear and has the two OPT leads and one choke lead going to it.

The square gets the rectifier pin 8 lead and a choke lead.

The unmarked lug gets the B+ lead

My previous quad cap was also a CE and it was not hooked up like this.

My other ST70 has an OP Manufacturing quad cap and seems to be working fine with the square lug to the rear. Am I lucky or is this okay with that cap?
___________________________________________________________
I'm thinking I may have lost my quad cap because I had the quad cap orientation wrong. Bob would you staighten me out on the connections. I had the square 40 mf lug facing the rear of the chasis towards the transformers with one choke lead, two output transformer leads and a lead jumpered to the 80 mf lug. Is this the right orientation?

I also have a 2.2 kohm resistor between the 80 mf lug and the 20 mf lug.

Okay, I found my VTA instructions.

Section 1, Square 40 mf - Right OPT, left OPT, and choke out
Section 2, Half Circle 80 mf - one side of a 2.2K resistor
Section 3, Line 20 mf - B+ from VTA board and the other side
Section 4, Triangle 30 mf - Choke in, pin 8 from rectifier
of the 2.2K resistor

The VTA instructions say to jumper between Sections 2 and 3.

As I continue to do my home work I'm begining to think that different quad caps have different symbols. I'm using the cap from Dyanakit.

Thanks,
Doug

Doug,

You have the wrong information here ... This is the way that the CE cap should be hitched up with the VTA driver board. The CE cap should have the 80 section with the HALF CIRCLE facing the back of the amp.

40 section (SQUARE symbol) - has a wire from pin #8 of the rectifier tube and one choke lead connected.

80 section (HALF CIRCLE symbol) - has the other choke lead, the two red wires from the output transformers and a jumper wire leading to the 20 section connected.

20 section (NO symbol) - has the other end of the jumper wire from the 80 section and one end of the 2200 ohm resistor connected.

30 section (TRIANGLE symbol) - has the other end of the 2200 ohm resistor and a wire leading to the B+ connection on the driver board.

Bob

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Re: ST 70 Quad Cap (Long Post)

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