Carlson Stromberg 6v6 Vintage amplifier repair/ ceramic capacitors

[Siemens EL34]

Hey People... i have a very old little 6v6 mono amplifier. I am going through some troubleshooting before I plug in and make some noise with it. Came to me in a very pretty shape. All the tubes and paper caps are Carlson Stromberg. Full with ceramic capacitors at the tone and volume controls but also, on both 6v6 anode/screen to the output transformer. These little purple ceramics are off and my DMM can not recognize the values. Still soldered in but all the other ceramic caps soldered in gave me the correct spot on values except these ones:

C11 and C12 on the schematic attached. What exactly these caps for in this circuit? I did see many other 6v6 designs but no filter caps on this locations. Is it wise to replace them with foil ?

Thank you for your suggestions. :)

[Williamblake]

Are you saying the schematic fits your amp? There wouldnt be any DC on G2s and zero output as far as i know. // Never mind, didnt see the screen power supply.

[shooter]

What exactly these caps for in this circuit?

you can replace them with about any cap you'd use in the signal path, you can leave them out and be pretty sure they won't cause "issues"

as to the purpose, I don't recall so it'd be a wag 

[sluckey]

You can't read the value of those caps because there is a 5600Ω in parallel. You must disconnect one end to be able to read correctly. I really doubt they are bad.

The caps are there to attenuate some high frequencies.

[Siemens EL34]

Thanks guys, i really appreciate your help. I think for now, i will leave these caps in then. There are a couple of drifted resistors I am going to change and will fire it up and see whats going on with it... :)

[Latole]

I'll replace all plate's resistors, they are often noisy after so many year
R6, R10 and R14

Drift resistors are rarely a issue, open or short resistors, yes

Filter caps and other 'lytic caps could be dry.

As sluckey wrote, you must disconnect one side of resistor or caps for reading.

[Siemens EL34]

Thank you guys for the suggestion and responses. I did clean the amp, replaced and re soldered drifter resistors and filter caps.

I am having spot on voltages across the tube except ONE place and I don't know why is that. Its the V2,  12AX7 Plate 1 and 2. Instead of 90V I am getting a good 195V. It is odd.. plate 2 measures around 218V. It is sort of OK, the schematic calls for 210V.

I did replace the R10 and R14 plate resistors to a brand new ones. Swapped tubes, same sort of voltages...

I am not sure where I am getting this high DC's from?

Attached the schema...

[shooter]

I'm calling it a drafting error a 1 in front of the 90 makes it work fine 

verify the 4700 isn't 47K though, I case I'm wrong 

[66Strat]

Thank you guys for the suggestion and responses. I did clean the amp, replaced and re soldered drifter resistors and filter caps.

I am having spot on voltages across the tube except ONE place and I don't know why is that. Its the V2,  12AX7 Plate 1 and 2. Instead of 90V I am getting a good 195V. It is odd.. plate 2 measures around 218V. It is sort of OK, the schematic calls for 210V.

I did replace the R10 and R14 plate resistors to a brand new ones. Swapped tubes, same sort of voltages...

I am not sure where I am getting this high DC's from?

Attached the schema...

I'm calling it a drafting error a 1 in front of the 90 makes it work fine 

verify the 4700 isn't 47K though, I case I'm wrong 

Shooter called it..... DRAFTING ERROR.

IMO, the 195 Volt reading appears reasonable and more believable than the 90 volt value indicated on the schematic. V2A cathode voltage per schematic is 2.2 volts. Cathode voltage of 2.2 volts divided by cathode resistor of 4700 ohms results in 0.47 ma tube current. Tube current of 0.47 ma times the V2A plate resistor value of 220,000 ohms results in a voltage drop of ~ 103 volts across the plate resistor. Supply voltage of 310 volts less 103 volts dropped across the plate resistor results in an expected value of 207 volts.

[Latole]

 IMO, the 195 Volt reading appears reasonable and more believable than the 90 volt value indicated on the schematic. 

I agree. 90 volts look toooooo low.

[Siemens EL34]

Thanks so much guys. Lesson learned, really appreciate your comments and advises. Really thank you... now whats left is the good old hum... as always.. I feel that I am cursed of something....    I have never had a tube amplifier which did not hum and I can not believe that there is a tube amplifier there dead silent (joking..i know there is...).... I will see if I feed this amp with some sort of input source will the hum go away or not, also this hum has a higher pitched buzz too.. I am doubting that is the main filter cap but will check that too later... with a set of other tubes.

Have a great evening guys... cheers

[shooter]

whats left is the good old hum... as always.. I feel that I am cursed of something...

 
did you work in large magnetic fields as a career, I can still fuzz out radios

[Siemens EL34]

whats left is the good old hum... as always.. I feel that I am cursed of something...

 
did you work in large magnetic fields as a career, I can still fuzz out radios

  ha ha... that's a good one... no, no magnetic field nearby..   ( i am just playing the drums thats all...and also with tube amplifiers as an amateur)

[66Strat]

Thanks so much guys. Lesson learned, really appreciate your comments and advises. Really thank you... now whats left is the good old hum... as always.. I feel that I am cursed of something....    I have never had a tube amplifier which did not hum and I can not believe that there is a tube amplifier there dead silent (joking..i know there is...).... I will see if I feed this amp with some sort of input source will the hum go away or not, also this hum has a higher pitched buzz too.. I am doubting that is the main filter cap but will check that too later... with a set of other tubes.

Have a great evening guys... cheers

Where is the rest of the schematic that contains the power supply? This could be helpful in diagnosing the source of the hum. The frequency of the hum would also be helpful.

[Siemens EL34]

Thanks so much guys. Lesson learned, really appreciate your comments and advises. Really thank you... now whats left is the good old hum... as always.. I feel that I am cursed of something....    I have never had a tube amplifier which did not hum and I can not believe that there is a tube amplifier there dead silent (joking..i know there is...).... I will see if I feed this amp with some sort of input source will the hum go away or not, also this hum has a higher pitched buzz too.. I am doubting that is the main filter cap but will check that too later... with a set of other tubes.

Have a great evening guys... cheers

Where is the rest of the schematic that contains the power supply? This could be helpful in diagnosing the source of the hum. The frequency of the hum would also be helpful.

Thanks JT, attached. Many thanks.

The frequency of the hum is 50/60 Hz but there is a high buzz on the top of that...like two different sound sources. The hum is at present when the volume control is down, hum starting as soon as the valves starting to heat up. The hum responds to the tone control treble and bass. In this amp, the heater wiring is a single lead, no twisted lead. No power switch. no light bulbs. Changed the two pin connector socket and a two prong cable is new, fresh Y2 cap added. The multi section capacitors are checked, seems OK on my LCR and the voltages ( i have the full list) seems to be OK, but the 6av6 needs to be re checked, as I did not pay attention to that yet). The multi section cap was replaced by someone before me, as it was badly re-soldered. That someone clipped off the previous capacitor clips, left everything soldered to that piece and took that peace and re soldered to the new cap pins with s.loads of solder... i did change / re soldered that.

Filaments 6.23 VAC all the way more or less...

Thanks for the suggestions.... i am hearing exactly the hum as on this video, the first one 50Hz with a high pitch buzz as on the video

[shooter]

but the 6av6 needs

you can just pull it and listen

60hz is typically filaments, or sensitive wires picking up filaments.  might need/want to re-do to a 2 wire arrangement.  guessing you have a noisy V1 or v1 parts though

[Siemens EL34]

but the 6av6 needs

you can just pull it and listen

60hz is typically filaments, or sensitive wires picking up filaments.  might need/want to re-do to a 2 wire arrangement.  guessing you have a noisy V1 or v1 parts though

Thanks... in the meantime I did pull out the V1. Did not change anything... i have tried the same pulling out the 12ax7 , same thing..nothing changed.

Attached the bottom view. The heater wiring goes into the V1 and then distributed further. Do I need to ground one heater wire at every single tube socket? What is the best way doing that? I have never done that tho..

[jjasilli]

Always suspect tubes first.


If all tubes are proven hum free, then that source of hum is excluded.


Other sources of hum:  Filament supply; Power Supply; Bias supply.


As stated above, 60Hz hum tends to indicate the source to be heater supply.  One test: disconnect the stock filament suppl and test the circuit with an outboard clean regulated 6VDC supply.  A lantern battery or similar with enough current handling capacity would work. 


IOW, systematically test specific circuits or components to rule them out as the cause, until the cause is discovered.  But. . .


Use repair Flowcharts.  There are documented procedures to discover & eliminate hum.   

[66Strat]

Thanks so much guys. Lesson learned, really appreciate your comments and advises. Really thank you... now whats left is the good old hum... as always.. I feel that I am cursed of something....    I have never had a tube amplifier which did not hum and I can not believe that there is a tube amplifier there dead silent (joking..i know there is...).... I will see if I feed this amp with some sort of input source will the hum go away or not, also this hum has a higher pitched buzz too.. I am doubting that is the main filter cap but will check that too later... with a set of other tubes.

Have a great evening guys... cheers

Where is the rest of the schematic that contains the power supply? This could be helpful in diagnosing the source of the hum. The frequency of the hum would also be helpful.

Thanks JT, attached. Many thanks.

The frequency of the hum is 50/60 Hz but there is a high buzz on the top of that...like two different sound sources. The hum is at present when the volume control is down, hum starting as soon as the valves starting to heat up. The hum responds to the tone control treble and bass. In this amp, the heater wiring is a single lead, no twisted lead. No power switch. no light bulbs. Changed the two pin connector socket and a two prong cable is new, fresh Y2 cap added. The multi section capacitors are checked, seems OK on my LCR and the voltages ( i have the full list) seems to be OK, but the 6av6 needs to be re checked, as I did not pay attention to that yet). The multi section cap was replaced by someone before me, as it was badly re-soldered. That someone clipped off the previous capacitor clips, left everything soldered to that piece and took that peace and re soldered to the new cap pins with s.loads of solder... i did change / re soldered that.

Filaments 6.23 VAC all the way more or less...

Thanks for the suggestions.... i am hearing exactly the hum as on this video, the first one 50Hz with a high pitch buzz as on the video

The 50 hz is heater related. The schematic shows a one wire heater wiring scheme. Changing to a two wire scheme may help out with the 50 hz. A badly mismatched pair of 6V6 tubes could also contribute to the 50 hz noise. Try pulling one 6v6 tube and then measure the voltage across the cathode resistor. Pull this tube and then reinsert the first tube and measure the voltage across the cathode resistor. Compare these two voltage readings. The closer the readings the better. Use the same socket for taking the readings. The high buzz that you hear is an oscillation that could be anywhere in the signal path or riding on the B+ rail. The schematic shows capacitor connections between the 6V6 screen grids and plates. These are there to address oscillation. I would rethink this and address oscillation earlier in the circuit by a 47pf to 100pf capacitor across the PI plates. I wouldn't trust the LCR reading of the filter caps unless you performed a leak down test, and then I still wouldn't trust it. Replace the filter caps.

[shooter]

Replace the filter caps.

+1
unless it's a "legacy" amp, if caps are >5yrs old AND I have hum, they get swapped

[Siemens EL34]

Thank you guys for this info's, there is a lot to do.

First , and apologies if it is a stupid question but if the filter cap would be leaky or bad, would I still get spot on voltages across the circuits? 

I did measure the voltages across the bias resistor as JT asked, using the same socket. I have 2 pairs of 6v6GTs and did measure all. The ones came with the tubes are two carlson branded 6v6gts measuring 17,25VDC and 17,53 VDC. I have a tung sol measuring 17,25 VDC too so these three are pretty close.

The "capacitor connections between the 6V6 screen grids and plates" are two ceramic disc's i think either Erie or Centralab ceramic's. Should I remove these or replace these? Or as JT suggested, and I hope I understood correctly a 100pf cap at the 12ax7 across pin1 and 7?

I will change the heater wiring if this will make this amp sound better or hum free... but after everything else is checked i guess. (?)

I have tapped every single connections, soldering, capacitors, resistors, moved tubes within sockets, nothing is loose or badly soldered. I assume my set of 6v6s are good as I did try two pairs, i have 3 x 12ax7's did swap and tried them all.

I don't have another rectifier( but again, assuming if the voltages measuring as required, it is performing ok?) and I don't have another 6av6.

I did try and hooked up a microphone at the input. Sound is coming out of the speaker, but with that hum and buzz.

Should I change those ceramic's or start re wiring the heaters? which would be the wisest move from here?


 

[shooter]

I did pull out the V1. Did not change anything

that sorta eliminates it.

2 wire filaments have been "standard" for a few decades now........
If you go that route, think about the "route" for the fil.  you want them tucked against the chassis, away from ALL sensitive signals (anything preamp)

the "PI Cap" usually goes from plate to plate, look at Diverted GA20hum thread, look at the schematic to see how it's done

[66Strat]

Thank you guys for this info's, there is a lot to do.

First , and apologies if it is a stupid question but if the filter cap would be leaky or bad, would I still get spot on voltages across the circuits? 

I did measure the voltages across the bias resistor as JT asked, using the same socket. I have 2 pairs of 6v6GTs and did measure all. The ones came with the tubes are two carlson branded 6v6gts measuring 17,25VDC and 17,53 VDC. I have a tung sol measuring 17,25 VDC too so these three are pretty close.

The "capacitor connections between the 6V6 screen grids and plates" are two ceramic disc's i think either Erie or Centralab ceramic's. Should I remove these or replace these? Or as JT suggested, and I hope I understood correctly a 100pf cap at the 12ax7 across pin1 and 7?

I will change the heater wiring if this will make this amp sound better or hum free... but after everything else is checked i guess. (?)

I have tapped every single connections, soldering, capacitors, resistors, moved tubes within sockets, nothing is loose or badly soldered. I assume my set of 6v6s are good as I did try two pairs, i have 3 x 12ax7's did swap and tried them all.

I don't have another rectifier( but again, assuming if the voltages measuring as required, it is performing ok?) and I don't have another 6av6.

I did try and hooked up a microphone at the input. Sound is coming out of the speaker, but with that hum and buzz.

Should I change those ceramic's or start re wiring the heaters? which would be the wisest move from here?

Re the filter caps. Think if it this way, if you would happen across a garage find 1958 XK-150 Drophead Coupe, would you attempt to drive it without first flushing the fluids and swapping out the tyres?

Filter caps go bad over time. One of the first signs of bad caps is line noise.

Remove the 6V6 screen grid to plate caps. They're too late in the circuit to provide any benefit. Screen grid resistors would be a better solution. The 100pf PI capacitor should go across pins 1 and 6. 

[Siemens EL34]

Thank you guys for this info's, there is a lot to do.

First , and apologies if it is a stupid question but if the filter cap would be leaky or bad, would I still get spot on voltages across the circuits? 

I did measure the voltages across the bias resistor as JT asked, using the same socket. I have 2 pairs of 6v6GTs and did measure all. The ones came with the tubes are two carlson branded 6v6gts measuring 17,25VDC and 17,53 VDC. I have a tung sol measuring 17,25 VDC too so these three are pretty close.

The "capacitor connections between the 6V6 screen grids and plates" are two ceramic disc's i think either Erie or Centralab ceramic's. Should I remove these or replace these? Or as JT suggested, and I hope I understood correctly a 100pf cap at the 12ax7 across pin1 and 7?

I will change the heater wiring if this will make this amp sound better or hum free... but after everything else is checked i guess. (?)

I have tapped every single connections, soldering, capacitors, resistors, moved tubes within sockets, nothing is loose or badly soldered. I assume my set of 6v6s are good as I did try two pairs, i have 3 x 12ax7's did swap and tried them all.

I don't have another rectifier( but again, assuming if the voltages measuring as required, it is performing ok?) and I don't have another 6av6.

I did try and hooked up a microphone at the input. Sound is coming out of the speaker, but with that hum and buzz.

Should I change those ceramic's or start re wiring the heaters? which would be the wisest move from here?

Re the filter caps. Think if it this way, if you would happen across a garage find 1958 XK-150 Drophead Coupe, would you attempt to drive it without first flushing the fluids and swapping out the tyres?

Filter caps go bad over time. One of the first signs of bad caps is line noise.

Remove the 6V6 screen grid to plate caps. They're too late in the circuit to provide any benefit. Screen grid resistors would be a better solution. The 100pf PI capacitor should go across pins 1 and 6.

  Thanks JT, what a great reference.. I wish I could find a garage with that..but point taken. I am already on rewiring the heaters with twisted cables. Will post a picture when I am done with it. I did remove those ceramic caps of the 6v6s and added a 100pf at the PI.

The 6av6 seems to be microphonic...while tapping is i am hearing a ringing tap in the speaker, as i don;t have another one I bought a pair as it was only 7 quid each. Worth it. I will need to wait with the filter cap. The space on this little amp is so limited and not easy to get hold of a 4 section cap... or even 3 section...

Thanks again and will post some pic of the progress... Have a nice day there !

[Siemens EL34]

OKay... here it is. The heaters changed up until the output tubes. What do you think? Acceptable? First time ever I have done this and the limited space makes it a real nightmare.... a couple of "grounding" questions...

1. 6AV6 pin 3 was grounded to the socket/chassis. I de-soldered this as the heater wire ground went there.. is that correct?

2. 12ax7 pin 4-5 tied together and the negative/ ground of the heater wire went to pin 9 ... correct?

3. Now the output tubes... pin2s are grounded to chassis, i will de solder this and make this similar to the previous other tubes, but the output phono/rca socket is also grounded there. Should I leave this grounded there or somewhere else? I have attached a pic so far of the work done.. :)

[shooter]

do one thing at a time, so leave grounds alone for now and test after filament re-wire

IF there's still hum, I would take all the grounds in red and connect them to ONE point only, near the input jack

[Siemens EL34]

do one thing at a time, so leave grounds alone for now and test after filament re-wire

IF there's still hum, I would take all the grounds in red and connect them to ONE point only, near the input jack

Roger that. Sadly it still hum's. Basically nothing has changed, same thing. Finished the heater wiring, attached. Will go through on these grounding points, but at the first sight it is pretty much at the same location/ ground point except on section of that filter cap, the 40uF one circled in red. Will confirm this later...

I know, I still did not replace the filter cap and it was suggested, and also a new 6av6 is coming to see if there is something... if I tap the C3, my speaker pops.. not sure it is related to the issue..

[Siemens EL34]

Hey Guys,

Just a quick update and help again.. please :) I did change the filter cap. Still having this high buzz and hum at the same time.

Interesting that if I tap the chassy with my finger I hear this pop on the speaker... i think there is some low AC in the chassy.

I will get a new rectifier tube for sure to exclude that but would you suggest anything else I can do ? Those ceramic caps on the pots... is it worth swapping them? of these are most likely not responsible for this hum/buzz I am having..? Please let me know your thoughts :)

As I did change the heater wiring for twisted pairs, do I need to wire the output tubes in phase or out of phase? I did wire them in phase... is this correct?

Than you :)

[shooter]

phasing twisted wires is fine, done correctly with all other wires there's no need for twisted pahsing


verify ALL your grounds, buzz/hum, changes with chassis........

[Siemens EL34]

phasing twisted wires is fine, done correctly with all other wires there's no need for twisted pahsing


verify ALL your grounds, buzz/hum, changes with chassis........

Thank you for checking in and the advise... I re checked all the grounds are solid. I did mark red where I am tapping with a chopstick I can hear a pop in my speaker. I have marked also where the star ground is originally fitted on this amp. I can not connect any other grounding point to that as the parts a far away from this grounding point.

I did also re solder the old solderings at some resistors at the output tube.

I did order another rectifier tube. Not sure if that will solve anything but I am at the point where I am running out of my knowledge/ options again... maybe the rectifier tube, maybe not.. I really hope it is tho :)

[shooter]

I am tapping with a chopstick I can hear a pop

Indicates something's not solid, either a cracked Resistor you're not seeing, a bad solder flow that's not obvious.  so re-flow, replace stuff associated (plate R's, remove old solder, add new.....)

[66Strat]

Hey Guys,

Just a quick update and help again.. please :) I did change the filter cap. Still having this high buzz and hum at the same time.

Interesting that if I tap the chassy with my finger I hear this pop on the speaker... i think there is some low AC in the chassy.

I will get a new rectifier tube for sure to exclude that but would you suggest anything else I can do ? Those ceramic caps on the pots... is it worth swapping them? of these are most likely not responsible for this hum/buzz I am having..? Please let me know your thoughts :)

As I did change the heater wiring for twisted pairs, do I need to wire the output tubes in phase or out of phase? I did wire them in phase... is this correct?

Than you :)

I think that the AC on the chassis may be caused by the two prong plug. The chassis is not grounded. Swap the two prong plug out for a three prong plug. It will be a lot safer and may help with you hum and noise.

[Siemens EL34]

Thank you guys for all the input, i am really thankful for this, I think I have a 90% success with the hum here. The high voltage AC lead coming in from the PT was way to close to the controls and the R9 resistor. As soon as moved this lead away, the buzz disappeared. I have a little warm hum from the still but that is i believe acceptable for this amplifier. Re soldered everything, again, also re soldered the old soldering. Getting better. Sadly the PI plate capacitor 100pf leg broke off and I don't have another one with the same value or anything close. I did put a 5nf there but i think it is way to much as the audio now is distorted. Will get another 100pf cap for the PI plates and try again.

So far, it is looking promising and happy about this. Thank you so much again all your help.

  have a wonderful weekend.