How to take voltage readings ? Tweed Princeton

[Boone]

Hello everybody , I am new to the forum and have minimal experience with tube amps. I have one amp tech in my area. He seems to be not up to par so I am hoping you folks can guide me through my amp concerns : I have a Tweed Princeton clone with a Silvertone 1481 OT and a std Fender Princeton PT. The schematic for 5F2-A calls for a 10k and 22K voltage drop resistors. Tech must have misread value and placed a 100K and 22K respectively . 1st question : will the 100K result in tubes going bad prematurely due to incorrect voltages? 2nd question : how do I take voltage readings on the 6V6GT and 12ax7? I have built a mission 5E3 but am clueless on the workings of tubes. In light of that, please refer to pin#'s and not ( grid, plates etc...). I have a boatload of NOS tubes and just want to make sure the amp is not putting incorrect voltages on both tubes. Thanks for your time and help, it is greatly appreciated.

[sluckey]

1st question : will the 100K result in tubes going bad prematurely due to incorrect voltages?

No. But the amp probably wont be all it can be with that 100K.

2nd question : how do I take voltage readings on the 6V6GT and 12ax7?

Set your meter to read DC volts. Connect the black lead to chassis ground. Now connect the red probe to each tube pin. Just hold the probe on the pin, taking care not to slip and short out adjacent pins. The pins you need a reading for on the 12AX7 are 1, 2, 3, 6, 7, 8. The pins you need a reading for on the 6V6 are 3, 4, 5, 8.

Be careful!

[Boone]

DC volts: is that the setting with ~ sign ? Also- I will post readings soon to get advice. Thanks for the help!

[Boone]

Note: rec tube is 5Y3 and cathode resistor is 470.  Readings: 6V6- #3(408V) #4(183.2V) #5(.973V...this reading varied .875-.975V)#8(12.56V).            12ax7: #1(108.3V)#2(8.5mV)#3(.722V)#6(106.8V)#7(17.5mV)#8(.6mV).....any thoughts?

[sluckey]

DC volts: is that the setting with ~ sign ?

No it's not. ~ means AC. What brand and model meter do you have? Post a pic if you can.

any thoughts?

Yes. Replace that 100K with a 10K 1 watt (or higher wattage). That will get the 6V6 pin 4 reading up to where it should be for a 5F2A amp.

[Boone]

Any thoughts on the readings re 6V6/12ax7? 408V on pin3?!!!

[sluckey]

408V on pin3?!!!

While that may be a little high, it will not cause any problems. Change the resistor and that will get the 6V6 conducting harder which will probably bring that 408 number down.

Another easy way to lower that voltage is to use a NOS 5Y3.

[Boone]

I have a JAN Phillips 5y3 in there now. Have tried my JAN Hytron and GE 5y3 as well. Maybe a diff PT is in order here?

[punkykatt]

Replace that 100k resistor with a 10k as sluckey suggested and retake all your voltage readings and post them.

[Boone]

I will replace with 10K and keep you posted.......thanks

[Boone]

I swapped in 10K : 6V6gt is now red plating. Readings: 6v6- #3(364.5V) #4(315.5V)#5(3.7V)#8(23.85V).  12ax7- #1(167.5V)#2(28.8mv)#3(1.428V)#6(169.7v)#7(1.0mv)#8(1.398v). Note: after I got amp back with 100k in there (where 10K is now) amp broke up a lot sooner and did not red plate . Any thoughts with new readings ?

[Boone]

I may have to also point out: this amp only has 3 filter caps from left to right:20uF,10uf and 8 uf. On board between 20 and 10uf is the new 100K. Between the 10uf and 8 uf is the 22K. Thanks for all of your help folks!

[Boone]

Sorry re above reply: between 20uf and 10uf is new (10K)......

[punkykatt]

Pin 5 of the 6V6 should have 0 volts.  what is the actual ohms of the cathode resistor connected to pin 3 of the 6V6? The tube is biased way hot. You may need a 680+ ohm resistor?

[Boone]

I have a 10watt 470 in there with 10uf 250V in parallel.

[punkykatt]

Try a 680 ohm  5 or 10 watt resistor if you have one and see what happens.

[Boone]

With regard to all readings or just #5 on 6v6?

[punkykatt]

Can you adjust your meter to not read mV in the DC setting? The new voltage readings look to be normal except pin 5 on the 6V6. 

[Boone]

Excuse the wet behind the ears.... VOM directions say to set meter to auto- I did that. What would rc look like : one setting has ~ (AC) I see, and - sign with mv at bottom left hand corner.

[punkykatt]

What make and model meter are you using?    Do you have a 680 ohm 5 or 10 watt resistor to replace that 470 ohm cathode resistor thats in there now?

[Boone]

Meter is a piece of poop cheapo type Radio shack. I know......I just ordered a 10 watt 680 and 700 ohm clarostat power resistors. Here's my concern, the amp with the 100K (1st power resistor) sounded pretty good. I read somewhere that some other deal in the 6v6 should be balanced or not to exceed the plate voltage). So I was concerned that the screwy 100k was putting undue stress on tubes. I agree with the previous post that bias is up there now. I guess it's just impossible to get this thing to work at correct voltages,amp and bias with the 470 in there. Is it a common practice to increase to 680 and still have all pins and bias fall into correct specs? Bear with me here, I am obviously an amature at this . I also have a boat load of NOS tubes that I inherited and would like to use them for audio not a light bulb. Thanks for the thoughts everybody!!!

[Willabe]

I also have a boat load of NOS tubes that I inherited and would like to use them for audio not a light bulb.

If you have a bunch of different 6V6's then start popping them in 1 by 1 and you might find 1 that doesn't red plate.

All tubes even from the same manufacturer will draw a different amount of current.


                Brad    

[Boone]

How do I figure bias? I have a 1 ohm I can to one of the 6v6 pins...I don't which one to ck bias with.Amature here: step by step please- also what are parameters for hot/safe bias and cold/safe bias? Thank you

[Boone]

Hey Willabe, the tube the was not red plating before the resistor swap red plates after 10k soldered in......

[HotBluePlates]

How do I figure bias? ... what are parameters for hot/safe bias and cold/safe bias?

Well, it's a cathode-biased amp, so you shouldn't have to figure anything. The tube settles in on whatever idle current it decides as a result of the cathode resistor value.

For a single-ended amp like yours, "hot, safe bias" is redplating but just enough cooler to stop the redplating. Or 100% of the plate dissipation rating, which is 12w (or 14w if you choose the later number for the 6V6).

"Cool, safe bias" is no current, stone dead. But that wouldn't be a good amplifier, so anything above 0mA of idle current that doesn't sound nasty at low volume to you.

Most people idle at 100% plate dissipation, because in a single-ended amp anything less just gives you less output power.

How do I figure bias? I have a 1 ohm I can to one of the 6v6 pins...

Unsolder the cathode resistor from pin 8 of your 6V6. Solder one leg of the 1Ω resistor to pin 8; solder the other leg to the free leg of the cathode resistor (which used to go to pin 8). Now, if you measure d.c. volts across the 1Ω resistor, the number will directly convert to the # of d.c. milliamps of cathode current.

But power = voltage * current, and you need the voltage part of that. Place you meter's black lead on pin 8 of the 6V6, and place the red lead on pin 3. If you can clip these in place with the power off, so much the better. With power on, you will be measuring the voltage from plate to cathode, which the tube feels as its plate voltage.

Multiply that plate-to-cathode voltage by the number of milliamps you found across the 1Ω (in the form of 0.020, which equals the 20mA you would have if you measured 20mV across the 1Ω). This will be plate dissipation in watts.

[Boone]

Blueplates : I would rather not bother with this bias stuff either with cathode biased amp. I was just concerned about the 408 pin 3 , 187 pin 4 and .9V at pin 5 damaging tubes prematurely. I think the amp sounded fine as it was.....just wanted it back into specs so as to not damage tubes. Thanks for the lengthy bias direction, I will use that.

[sluckey]

I was just concerned about the 408 pin 3 , 187 pin 4 and .9V at pin 5 damaging tubes prematurely. I think the amp sounded fine as it was.....just wanted it back into specs so as to not damage tubes.

Those voltages are perfectly safe for the tubes. If you liked the sound of the amp with those voltages, then put the 100K resistor back in the B+ rail. The tubes will be running cool and probably last forever.

[HotBluePlates]

One day I'll post a picture of a Westinghouse 6V6 I have.

I got the tube in a lot of tubes off ebay. It actually has a hole melted through the plate of the tube. Not a small one either, maybe 1/8" x 1/4". I put it on a tube tester and it came up fine. I've played it in amps and it sounds and works just fine.

I will admit I've not monitored its plate current over time to see if it was unstable, but it sure seemed to perform alright. That experience makes me think your tubes are fine. Plus, the way they were being run before (with the 100k in the power supply) just reduced the 6V6 current and power output.

[Boone]

One last question: I bet you folks have had it with this. If tubes are cool with the 100K I may put it back or try 50K-75K to see what it sounds like. I take it that is ok as well? Btw- I know everyone has the tone of tone amps . But this thing is effin LOUD with the 100k in there. Go figure.....cold tube= much vol.  I have a 12" ceramic spk in cabinet. The tone/crunch is nice Sumlin type up until around 9 or so..... Go figure that out with cold tubes. A VERY BIG thank you everybody. Ps- anyone need some xtra cash? I need a (layout) drawn out for a 2x6V6  5y3. 12ax7 amp 1 vol 1 tone 2 input based on another amp. If anyone is interested we can correspond through email. Boone

[Boone]

Blueplates- please post pic of Westinghouse !! I'd like to see that. Thanks for all your time and effort folks.

[HotBluePlates]

... If tubes are cool with the 100K I may put it back or try 50K-75K to see what it sounds like. I take it that is ok as well? Btw- I know everyone has the tone of tone amps . But this thing is effin LOUD with the 100k in there. Go figure.....cold tube= much vol.  I have a 12" ceramic spk in cabinet. ...

3-4w is plenty loud, especially if you have an efficient speaker. A Champ into a 4x12 cabinet is surprisingly loud.

But yes, you can experiment with higher-than-stock resistors in the power supply all you like.

[labb]

Take a look at the Fender 5E3. http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/deluxe_5e3_schem.pdf

That will get you pretty close

[Boone]

Blueplates, are you referring to the 5000 and 22K? Is the 5000 a 50K? Thanks     

[Boone]

Labb, does the layout you suggest call for a 5000(50k?) first then the 22K?

[Boone]

I think the voltage issue may be the PT: pin 2 and 8 on 5Y3 read 382V. PT is a TF22772. Any suggestions on a better brand type?

[Boone]

Pin2 and 8 on PT are red wires

[Willabe]

Pin2 and 8 on PT are red wires

That's backwards.

Standard wire colors for PT B+ with a rectifier tube for high acv secondary are red/red and goes to rectifier tube pins 4 and 6. The red/yellow wire is the high acv secondary center tap (CT) and goes to ground.

Yellow wires are the 5acv for the rectifier tube heaters and go to pins 2 and 8. Take B+ dcv from pin 8.


                  Brad    

[sluckey]

I think the voltage issue may be the PT: pin 2 and 8 on 5Y3 read 382V.

382??? How can that be. Your main concern was the 408V on pin 3 of the 6V6. But pin 8 of the 5Y3 is the ABSOLUTE HIGHEST VOLTAGE inside the amp.

I think you are obsessing over some very minor details.

[Boone]

Brad, the rec pins are marked as follows: I have a 5y3 tube that has 5 pins . Three on one side and two on the other. The #1 pin marked on socket is the ( middle) pin of the three on one side. Or the 1st pin just to the right or notch looking straight down on to  the socket. Looking at it again- my bad: 2&8 have yellow wires, 4&6 have red, 8 has red wire going to first filter. Any thoughts on the 387V ?

[Boone]

That what I assumed too. I have no idea Sluckey.......what is typical v at pin 2 on rec socket? Aren't most Tweed Prince PT putting out 360v at 2/8?

[punkykatt]

Boone, you have the right power transformer for this amp and it is performing as it should.  with the 10k resistor in the power supply all the voltages are normal. Pin 5 of the 6V6 you have 3.7 volts. Perhapes you ment to write 3.7mv?  It is very common to have a 680 ohm or higher cathode resistor to cool down the bias on these style amps. When these amps were designed and built the wall voltages were 110vac at best. Now a days in some places they are over 125vac. I have built many of these style amps and had to increase the cathode bias resistor on all of them.  Punky

[Willabe]

Any thoughts on the 387V ?

Yes, it's fine, listen to Sluckey and punkykatt.

If it were me, I'd put the 10K B+ dropping R back in and add a 470R screen grid stopper. Then if I had a bunch of 6V6's I'd try and find 1 that didn't red plate. Then if I couldn't find 1 that didn't red plate I'd do what punkykatt said, increase the KR.


                   Brad     

[Boone]

Pinky, I hear you on the 680. With the 10K in there pin 5 on 6V6 is 1.47V. Any ideas?

[Boone]

680 and 700 are being shipped. Should the 680 take pin 5 down to zero? Thanks  for your patience and help gentlemen and ladies.

[Willabe]

Boone, you have the right power transformer for this amp and it is performing as it should.  with the 10k resistor in the power supply all the voltages are normal.

When these amps were designed and built the wall voltages were 110vac at best. Now a days in some places they are over 125vac. I have built many of these style amps and had to increase the cathode bias resistor on all of them.  

This is why I suggested to try and find a tube that would run a little cooler. They run a little hotter with the higher wall voltage.

If you happen to put in a tube that draws current on the high side plus with the higher wall acv, it can red plate.


                     Brad      

[Boone]

Hi Willaby, I hear you as well ....I was concerned I may wear out tubes prematurely even if they are not red plating. Kind  of like running a 5 speed manual vehicle in 4th gear on the freeway...speed ok, temp ok but undue stress wear and tear on engine....... None of my NOS tubes red plate .....   This amp eats the current production Tung sol like popcorn. I use the Tung to practice with and the NOS for recording . You guys are whipping me into shape . I appreciate that. I feel I may have called in the Calvary for a very minor skirmish . Man, you guys are on this stuff like stink on a turd. A great VERY knowledgable group for sure. I will install the 680 and give you gents and ladies an update as soon as it arrives. I will figure bias as it sits now with the 470 and let you guys know soon. Thank you

[punkykatt]

Pinky, I hear you on the 680. With the 10K in there pin 5 on 6V6 is 1.47V. Any ideas?

It may be possible that the .022 coupling cap connected to pin 5 of the 6V6 is leaking DC voltage. That would be the cap also connected to pin 6 of the 12AX7. Unsolder the end that is connected to the 220k resistor on the board and lift it out so it is not connected to the circuit. with your meter set on DC connect the black (-) lead to the chassis and with the red probe see what DC voltage reading you get on the lifted end of the cap..

[Boone]

Punky, I will do that and get back to you. I have a friend who salvaged a toolbox full of old caps. I have a couple of red Sangamo in there right now. I will get back to you. Thanks for the efforts

[Boone]

Punky, started at 220 and quickly dropped and settled at 47.3V

[sluckey]

Punky, started at 220 and quickly dropped and settled at 47.3V

Replace that cap with a NEW cap. Give that toolbox full of old caps back to your friend.