Power Transformer

[John]

Cool! Your PT sounds fine. What are you going to do next?

[taylorc315]

I was thinking that I would put the tubes in one at a time, just to make sure that I have them connected right.

[taylorc315]

Also I need to figure out the resistance on the DC side of the diodes, I had resistance the other night, and the next morning I didn't have any resistance.

[John]

If your voltages are good, don't worry about the resistance.... what are you taking resistance readings on anyway? Put your power tube in, plug into limiter, see what happens.

[Willabe]

If your voltages are good, don't worry about the resistance.... what are you taking resistance readings on anyway? Put your power tube in, plug into limiter, see what happens.

Yes, yes, yes!

Don't over think it.


                     Brad   

[taylorc315]

Tubes in power on, everything looks good. Do I need to check anything else as far as voltages? Or do I try to
see if I get any sound out of it? 

[taylorc315]

Hooked up to a speaker cab nothing. I'm going to check the input/output jacks they seem OK but I will go over them.

[Willabe]

Ok, no sound, but no smoke.

Now take voltage readings on every tube and post them.


                Brad     

[taylorc315]

V5..pin 1-0. Pin2-3.25vac. Pin3-505vdc.. Pin 4-510vdc. Pin 5-40.7vdc. Pin6-40.7vdc. Pin 7-3.25vac. Pin 8-0.. V4. Pin 1-0. Pin2-3.2vac. Pin3-518vdc. Pin4-517 vdc. Pin5-41.3vdc. Pin6-40.6vdc. Pin7-3.23vac. Pin8-0..V3. Pin1-593vdc. Pin2-598vdc. Pin3-573vdc. Pin4-3.2vac. Pin5-3.2vac. Pin6-601vdc. Pin7-628vdc. Pin8-566vdc. Pin9-3.15vac. V2. Pin1-605vdc. Pin2-594vdc. Pin3-0. Pin4&5-3.2vac. Pin6-595. Pin7-693vdc. Pin8-92.8vdc. Pin9-3.1vac. V1. Pin1570vdc. Pin2-357vdc. Pin3-8.9. Pin4&5-3.17vac. Pin6-578vdc. Pin7-353vdc. Pin8-29.2vdc. Pin9-3.2vac... I see some discrepancy in some of the voltages, does this tell you anything?                Thank you!

[Willabe]

V5.
Pin 1- 0.
Pin 2- 3.25vac.
Pin 3- 505vdc.
Pin 4- 510vdc.
Pin 5- 40.7vdc.
Pin 6- 40.7vdc.
Pin 7- 3.25vac.
Pin 8- 0.

V4.
Pin 1- 0.
Pin 2- 3.2vac.
Pin 3- 518vdc.
Pin 4- 517 vdc.
Pin 5- 41.3vdc.
Pin 6- 40.6vdc.
Pin 7- 3.23vac.
Pin 8- 0.

V3.
Pin 1- 593vdc.
Pin 2- 598vdc.
Pin 3- 573vdc.
Pin 4- 3.2vac.
Pin 5- 3.2vac.
Pin 6- 601vdc.
Pin 7- 628vdc.
Pin 8- 566vdc.
Pin 9- 3.15vac.

V2.
Pin 1- 605vdc.
Pin 2- 594vdc.
Pin 3- 0.
Pin 4&5- 3.2vac.
Pin 6- 595.
Pin 7- 693vdc.
Pin 8- 92.8vdc.
Pin 9- 3.1vac.

V1. 
Pin 1- 570vdc.
Pin 2- 357vdc.
Pin 3- 8.9.
Pin 4&5- 3.17vac.
Pin 6- 578vdc.
Pin 7- 353vdc.
Pin 8- 29.2vdc.
Pin 9- 3.2vac.

Edit; I rearranged the voltages into a column to read them a little easier.   Brad     

[Willabe]

If those voltage readings are correct.........

Turn the amp off, unplug it and don't plug it back in and turn it back on until you fix some wiring!

V3, V2 and V1 must be wired up wrong. Something is very wrong for the grids and K's to have the high plate B+ dcv on them.

On 12 _ _ 7 tubes pin's 1 and 6 are the plate, pin's 2 and 7 are the grid and pin's 3 and 8 are the cathode.

The grids and cathodes (K) should not have high dcv on them like the plates. (Cathode followers and concertina PI will have K dcv the same/very close too the plate dcv.)

And I take it you meant you have around negative -40.7dcv on V5/V4 pins 5 and 6?


                   Brad   

[taylorc315]

Thank You, I will go through the wiring on V1-V3 and see what is going on. Yes negative 40.7vdc

[sluckey]

I have 509-512 vdc coming from the filter caps, the DC side of the diodes is at 512volts.

This is what you measured 3 days ago with no tubes plugged in. The voltage reading at the rectifier diodes will be the highest voltage in the entire amp. 512V is very believable for a Hoffman Plexi 50 with no tubes.

But now you have several readings that are considerably higher than that. 598, 566, 628, even 693! These readings are impossible! You are doing something very wrong when you measure voltage. I suspect you may be misinterpreting your meter or your meter is just no good. Maybe your 693V reading is really .693V or 693mV. There's a big difference.

The point is these reading can mislead someone that is troubleshooting your amp. They are so badly wrong that you need to take another complete set of readings and be sure they are accurate. At this point the only thing that is clear is the amp is not wired correctly. Your only reliable symptom is "no sound" and that can be any of hundreds of problems. Probably even multiple problems.

[taylorc315]

Thanks Sluckey,
I will take the readings (using a different meter) again and repost, I'm sure it's operator error.

[shooter]

You have good "standards" out in the world to both calibrate a meter and it's operator, a car battery is good for DC, (12 - 14.8vdc), your house wiring for AC, (115 - 125Vac).  might start there to verify your equipment.

[John]

It's possible his meter is reading the DC and AC both. I had one do the same thing that had me scratching my head a while back.


A different meter is a good idea.

[taylorc315]

OK here we go....
V5
Pin 1. 32mvdc
Pin 2. 3vac
Pin 3. 429vdc
Pin 4. 424vdc
Pin 5&6.-39vdc
Pin 7. 3vac
Pin 8. 32mvdc
V4
Pin 1&8 30mvdc
Pin 2. 3.25vac
Pin 3. 424vdc
Pin 4. 419vdc
Pin 5&6 -39.8vdc
Pin 7. 3.28vac
V3
Pin 1.327vdc
Pin 2. 369vdc
Pin 3. 12.68vdc
Pin 4&5. 3vac
Pin 6. 328vdc
Pin 7. 372vdc
Pin 8. 12.6vdc
Pin 9. 3vac
V2
Pin 1.162vdc
Pin 2. 0v
Pin 3- .95vdc
Pin 4&5. 3.2 vac
Pin 6. 289vdc
Pin 7. 161vdc
Pin 8. 163vdc
Pin 9. 3vac
V1
Pin 1. 156vdc
Pin 2. 0v
Pin 3- .93vdc
Pin 4&5. 3.17vac
Pin 6. 203vdc
Pin 7. 0v
Pin 8. 1.78vdc
Pin 9. 3vdc
Pins 3 in V2&V3 are point 95 and point 93 vdc. Hopefully this looks better. Thanks again for your patience guys.

[sluckey]

Looks OK except for V3. Please recheck those pins, in particular, pin 2, 7, 3, and 8.

[taylorc315]

Recheck
V3
Pin 1. 340vdc
Pin 2. 388mvdc
Pin 3. 13.3 vdc
Pin 4&5. 3vac
Pin 6. 343 vdc
Pin 7. 389 mvdc
Pin 8. 13.15 vdc
Pin 9. 3 vac

[John]

Check the values of your cathode resistors on V3. (only a guess on my part)

[taylorc315]

I will do that, Thanks

[sluckey]

As a quick check... Turn power off. Measure resistance from V3 pin 8 to chassis ground. Should be about 15KΩ. What do you get?

[taylorc315]

Will do asap...Thanks

[taylorc315]

I've got .484 m ohm's

[AZJimC]

I've got .484 m ohm's

That indicates you have a 470K as bias resistor probably. It should be 470 Ohm, not 470,000 ohm. That is the resistor right off the cathodes, which connects to a 15K then to ground. That explains the 480K, or .480M. Check that resistor..

[taylorc315]

I'm on it I will check it out. Thanks

[taylorc315]

I changed out that resistor,  I now have 15k out of pin 8. Thanks guys is there any thing else I should look at?

[AZJimC]

I changed out that resistor,  I now have 15k out of pin 8. Thanks guys is there any thing else I should look at?

Make sure you have the grid leak resistors (1m) from pins 2 and 7 to the junction of 470ohm-15Kohm, and try it out.


That resistor was the reason for little to no sound. The Phase inverter was biased into shutoff, and if you really pushed it, it would likely moan and groan. The amp should make noise now...

[taylorc315]

Will do. Thank you

[taylorc315]

Still nothing.. The power tubes were getting pretty hot right away it doesn't seem right, darn it!

[AZJimC]

Still nothing.. The power tubes were getting pretty hot right away it doesn't seem right, darn it!

Did you set bias. If not the tubes can come up and immediately begin to red plate. You need to check every connection, and resistor value throughout. It is a common mistake to pop in a certain "k" resistor for a certain ohm resistor.

[taylorc315]

I will go through everything make sure that I have all the right resistors in,check the bias. I'll post what I come up with. Thanks again

[taylorc315]

I did find another 470ohm in a 470k that is connected to V2 pin 2. Changed it to the correct resistor, now when I power up and switch off stand by the limiter slowly starts to glow??? If it ain't one thing its another

[AZJimC]

I did find another 470ohm in a 470k that is connected to V2 pin 2. Changed it to the correct resistor, now when I power up and switch off stand by the limiter slowly starts to glow??? If it ain't one thing its another

If that was a 470 and you replaced it with a correct 470K, that makes sense, 470 ohm would've not allowed enough signal on the V2a grid, so that would make a silent amp... now find out where the short is, look around your work where you changed the resistor. set bias on your output tubes, and that may stop the glowing, but a slight glow would be normal when the current draw rises to full.... if it isn't bright, it's just a small load, not a direct short.

[taylorc315]

OK great, it's just a very lite glow and it seems to start as the tubes warm up. I will double check everything and check voltages Thank you once again.

[AZJimC]

OK great, it's just a very lite glow and it seems to start as the tubes warm up. I will double check everything and check voltages Thank you once again.

That's very normal, it means the tubes are conducting. Time to try it off the limiter. then post results.

[taylorc315]

Will do. Thanks again

[Willabe]

Time to try it off the limiter. then post results.

Be ready, just in case, to pull the plug. (We all do the same with the 1st no limiter flight.)


               Brad   

[taylorc315]

Well powered up everything looks OK, no smoke : ) bias is set at 35vdc. Tubes still seem to get hot when I switch the stand by switch to on. I'd there any thing I should look at?

[AZJimC]

Well powered up everything looks OK, no smoke : ) bias is set at 35vdc. Tubes still seem to get hot when I switch the stand by switch to on. I'd there any thing I should look at?

Not sure what you're reading at 35vdc, but your bias setting should be done to adjust the milliamps of current read at the cathodes of the power tubes. The way most of us do this is by reading the voltage drop across a 1ohm resistor that goes from the cathodes to ground. That voltage drop, read in mvdc will equal the current, in milliamps dc, the tube is drawing through the resistor, since it's value is 1. that is the reason a 1 ohm is used, to make reading amps with a volt meter easy

[taylorc315]

OK I will check what I get across the resistor and post. Thanks

[taylorc315]

OK I checked across the 1ohm resistor's and set the bias at 35mvdc. I did plug back into the limiter just to be safe, it seems that the bias was the cause of the light glowing.

[AZJimC]

OK I checked across the 1ohm resistor's and set the bias at 35mvdc. I did plug back into the limiter just to be safe, it seems that the bias was the cause of the light glowing.

For reference, it is a good procedure to always adjust bias voltage to max (Which means as far negative as you can) before installing tubes to test. Then when the tubes are installed, they are at the least current draw setting, and then adjust them to proper idle current first thing. No need for the limiter to set bias, in fact that could make the setting incorrect. Check it again after you take it off. does the amp work? do you get output?

[taylorc315]

Well I did get a little output. Very little also noticed that the tube got very hot. I took the tubes out and set the voltage as far as I could around -56v and then set it at 35. I plugged into a speaker cab and did get some sound out of it. Right when I saw the tubes start glowing I shut it down.

[AZJimC]

Well I did get a little output. Very little also noticed that the tube got very hot. I took the tubes out and set the voltage as far as I could around -56v and then set it at 35. I plugged into a speaker cab and did get some sound out of it. Right when I saw the tubes start glowing I shut it down.

Do you mean you set it to 35ma per tube? How did you derive that number? What is your plate voltage? (varies with bias current). (Plate voltage) times (idle current), (Voltage dropped across 1 ohm) equals idle wattage of plate and screen combined.... so plate is less than this calculation, since screen current is not subtracted. The tube data will tell you how much wattage the plate can dissipate, if you run more than that, the tubes will redplate, because you are adding heat, or wattage, faster than the plate can get rid of it.

[taylorc315]

I checked the voltages across the 1ohm resistors to ground and set it at 35mvdc And I checked the voltages at pin's 6&5 which gave me -46vdc. I'm still learning how to do the math part of amp building.

[AZJimC]

I checked the voltages across the 1ohm resistors to ground and set it at 35mvdc And I checked the voltages at pin's 6&5 which gave me -46vdc. I'm still learning how to do the math part of amp building.

Good, now multiply that 35ma by plate voltage. that gives you wattage the tube is drawing. Think about this. .035A x 500V = 17.5W while .035A x 400V = 14W. Since the plate dissipates watts (directly related to heat), that same .035A setting can give quite a range of wattage. The watts is what matters in bias settings.

[taylorc315]

Will do, I will post what I get. Thank you

[taylorc315]

I have 487vdc at the plates and I have 30mvdc at resistor's I come up with 14.61 watts.

[AZJimC]

I have 487vdc at the plates and I have 30mvdc at resistor's I come up with 14.61 watts.

Those are EL34? if so 17.5W would be 70% of the rated 25W dissapation. 14.61 would be a bit cold, but certainly should make lots of sound... how is it sounding? if it's weak, then you have to trace down why it's getting attenuated somewhere in the circuit.