unloaded voltage readings

[jeffdavison]

Doing some testing on my amp.

What should the unloaded (only the 5AR4 tube rectifier installed) voltage readings be for B+, B+1 and B+2 be?

I am getting high readings taking measurements and am a bit concerned by the high readings

I'm using a C.Chon (Tubes4pro) cap board, but the output should be in the same range as a standard Dyna ST70

Thanks

Jeff Davison

[TomMcNally]

Unloaded voltage readings will fool you - they are invalid, because the resistors in the circuit only work with a load. They can't drop the voltage unless the load is there to pull them down. I would venture to guess you're seeing about 480 to 490 volts at the rectifier ? Put the tubes in and fire up the amp with speakers or dummy load resistors (short thr inputs to ground) and you'll see everything fall into place.

... tom

[kheper]

My voltages seemed a bit high even
with tubes in and 8 ohm resistors
connected between 'C' and '8' of
the output screws. I am using
the 80-40-30-20 'upgrade'
multi-cap.

The amp works fine.

Measured at the multi-cap:

A = 320
B = 375
C = 453
D = 464


Measured at 5ar4:

pin 2 = 463
pin 4 = 379
pin 6 = 366
pin 8 = 463

[dcriner]

Jeff, why are you measuring the unloaded voltages? What is the significance?

Voltages with only the rectifier installed will be higher than normal and could put unnecessary stress on the filter caps.

Doug

[jeffdavison]

I was afraid I'd do damage to the NOS 6550s I'm using if they recieved too much, to keep them in for testing as the B+ , B+1, and B+2 voltages that are required by the driver board per their schematic are 428, 397 and 120 (Changed the 22k resistor to 220K as they required).

I'm a newby so I assumed that the numbers on the schematic were the voltages that were supposed to be put out at their points. I just "discovered" what this "loading" thing is. I'm just begining to get a grasp of regulated vs unregulted means as well

I have already blown a couple of 100r 1/2w screen resistors and CChong said to put in 200 ohm 2 watters in (there is also the triode mode put into play as well as the diode per a suggestion here (from pin 3 cathode towards pin 4 where the screen resistor is).

Jeff, why are you measuring the unloaded voltages? What is the significance?

Voltages with only the rectifier installed will be higher than normal and could put unnecessary stress on the filter caps.

Doug

[jeffdavison]

due to the design of th C.Chong cap board, I can not accsees the individual caps directly as his circuit board has the solder joints underneath between the board and the chassis. I can only take measurements from the B+, B+!, and B+2 points

JD

My voltages seemed a bit high even
with tubes in and 8 ohm resistors
connected between 'C' and '8' of
the output screws. I am using
the 80-40-30-20 'upgrade'
multi-cap.

The amp works fine.

Measured at the multi-cap:

A = 320
B = 375
C = 453
D = 464


Measured at 5ar4:

pin 2 = 463
pin 4 = 379
pin 6 = 366
pin 8 = 463

[erichayes]

Hi All,

Back in the good ol' days, when DVMs were things that occupied six rack spaces and a 20,000 Ω/V VOM was considered a luxury, voltages were routinely spec'd at ±20% for (at least) two reasons: that much deviation is generally acceptable, and nobody's meters read exactly the same.

A good rule of thumb to remember is: if the voltages are high, you're probably not drawing excessive current and everything's okay. If your voltages are low, you probably are drawing excessive current, and/or you've miswired something.

[dcriner]

Jeff, is this your cap board: http://cgi.ebay.com/NEW-CAP-BOARD-FOR-D ... dZViewItem

I don't have experience with such cap boards, but I would tend to be leery of them. This particular board has boosted the values of the original filter caps enormously, to 60/195/82/82uF (from 20/20/20/30).

Increasing the caps' value will increase the B+ voltages and will cause extra stress on the rectifier tube at turn-on.

Jacking up the value of the 22K dropping resistors will help lower the two lower B+ voltages, but not the top one that is the plate supply for the EL34s.

But, if you are blowing resistors here and there, I would suspect the possibility of a wiring error someplace.

I would be more worried about your filter caps than your 6550s. I have little doubt that you've already given the caps a torture test, particularly the two rated at 450V.

Doug

[TomMcNally]

My opinion ... unless the amplifier isn't working, there really isn't any need to measure anything. Turn it on, if it works, enjoy the music. The voltage and resistance charts Shannon supplies are for troubleshooting purposes, and are for the most part, like Eric said, +/- 20% ... tubes aren't critical. If you're getting fireworks inside tubes, they are probably bad, and if resistors are smoking, something is miswired. Back in the day this stuff was first built, capacitors had off the shelf tolerances of something like -20% to +100% ... it isn't all that important. As Eric and others have mentioned before, the 5AR4's are stressed in the ST-70's to begin with ... buy a couple of spares. If you see sparks inside more than a few times, toss it and put a new one in. Rectifier tubes have been fussy since they were invented. Again, back in the day, anyone who used tube equipment had a big cabinet full of spares for everything. Some people (like radio stations) changed the tubes in the critical equipment on a schedule, even if they were good.

... tom

[jeffdavison]

Wiring is 100% correct. double triple checked everything. I'm wondering if it's the diodes that were suggested by MeSherm. Tried again to take loaded reading. Put bias pot adjustment to the middle for startup. I installed 220K 2w screen resistors in

Smoked those puppies too

I am loosing stomach over this

JD


Jacking up the value of the 22K dropping resistors will help lower the two lower B+ voltages, but not the top one that is the plate supply for the EL34s.

But, if you are blowing resistors here and there, I would suspect the possibility of a wiring error someplace.

I would be more worried about your filter caps than your 6550s. I have little doubt that you've already given the caps a torture test, particularly the two rated at 450V.

Doug[/quote]

[jeffdavison]

i really really wish it was that easy

fried the 200R 2w screen resitors.

all wiring has been triple douple and double triple check for correctens.

JD

My opinion ... unless the amplifier isn't working, there really isn't any need to measure anything. Turn it on, if it works, enjoy the music. The voltage and resistance charts Shannon supplies are for troubleshooting purposes, and are for the most part, like Eric said, +/- 20% ... tubes aren't critical. If you're getting fireworks inside tubes, they are probably bad, and if resistors are smoking, something is miswired. Back in the day this stuff was first built, capacitors had off the shelf tolerances of something like -20% to +100% ... it isn't all that important. As Eric and others have mentioned before, the 5AR4's are stressed in the ST-70's to begin with ... buy a couple of spares. If you see sparks inside more than a few times, toss it and put a new one in. Rectifier tubes have been fussy since they were invented. Again, back in the day, anyone who used tube equipment had a big cabinet full of spares for everything. Some people (like radio stations) changed the tubes in the critical equipment on a schedule, even if they were good.

... tom

[TomMcNally]

Did you disconnect and tape the Ultra-Linear taps from the output transformers ? It sounds like they are still connected, or something else is strange. Have you tried the amp in stock configuration before messing with triode mode ? Is have you listened to the amp with shorted inputs and speakers on the output when you fire it up ? If it's howling, you forgot to cross the leads on the output of the driver board to the tubes.

Take some side closeup pix and we can help spot a problem.

[dcriner]

Wiring is 100% correct. double triple checked everything. I'm wondering if it's the diodes that were suggested by MeSherm

Well, I'm just as sure there is a wiring error, but either of us could be wrong.

What diodes are you talking about? Where are they in the ckt, and what are they for?

I'm sensing that you've made too many mods to the amp without checking intermediate steps for operability. Diodes, cap board, various resistors, etc.

If you can't find a wiring error, the only thing I can suggest is reversing your tracks, and restore the amp to a working condition, which I assume it was before you implemented all these mods. Was it, in fact, working before all the changes?

[jeffdavison]

pics were posted here:

http://www.diytube.com/phpBB2/viewtopic ... cd4db563ca

Got one channel not to blow screen resistors. got line voltage brought up slow to 117 with a variacc and using shorted input plugs and a 8R 50w dummy load.

loaded voltages B+ 468, 449 and 206

still higher than what I'm looking for at 428, 397 and 120.

bothe triode and ultralinear mode do the same thing.

got bias going to the tubes at lowest pot position. where one tube fluctuates between .39 and .42 and the other between .49 and .51
(a pot per tube w/ 10R pin 8 & ground)
Both power tubes flicker

wiring is good. everything where it should be.

JD

Did you disconnect and tape the Ultra-Linear taps from the output transformers ? It sounds like they are still connected, or something else is strange. Have you tried the amp in stock configuration before messing with triode mode ? Is have you listened to the amp with shorted inputs and speakers on the output when you fire it up ? If it's howling, you forgot to cross the leads on the output of the driver board to the tubes.

Take some side closeup pix and we can help spot a problem.

[jeffdavison]

The diode was recommended by MeSherm towards the end of this thread:

http://www.diytube.com/phpBB2/viewtopic ... cd4db563ca

Amp is not modded per se from something else. it was built from scratch so no "steps" were taken.

JD

Wiring is 100% correct. double triple checked everything. I'm wondering if it's the diodes that were suggested by MeSherm

Well, I'm just as sure there is a wiring error, but either of us could be wrong.

What diodes are you talking about? Where are they in the ckt, and what are they for?

I'm sensing that you've made too many mods to the amp without checking intermediate steps for operability. Diodes, cap board, various resistors, etc.

If you can't find a wiring error, the only thing I can suggest is reversing your tracks, and restore the amp to a working condition, which I assume it was before you implemented all these mods. Was it, in fact, working before all the changes?